The axis upon which Uranus rotates is very much bowed towards the plane

of its orbit. Its seasons are hence abnormal; but their vicissitudes can

scarcely be sensible at a distance from the sun more than twice that of

Saturn. This, as Mr. Proctor noticed, is the only case in which the

ratio of one to two is exceeded in the radii of two adjacent planetary

orbits. The radius of the Uranian track, pursued at the leisurely pace

of 4⅕ miles a second, is 1,782 millions of miles, or more than 19

astronomical units. It consequently receives from the sun 370 times less

warmth and light than the earth does. Area for area, it is true, the sun

shines with the same intensity there as here; the difference lies in its

apparent size. Instead of the broad eye of day to which we are

accustomed, the luminary of Uranus presents a surface only 2¼ times that

of Jupiter, as seen from the earth at an _unfavourable_ opposition; and

although Uranus is 166 millions of miles nearer to the sun at perihelion

than at aphelion, no conspicuous difference would mark the passage from

one to the opposite point. This is accomplished in 42, the entire round

in 84 years.

In point of size, as Professor Young remarks, Uranus compares with the

earth very much as the earth compares with the moon. For its surface

exceeds the terrestrial surface about sixteen times, and its volume

amounts to sixty-six times the terrestrial volume. Its mass, however, is

less than fifteen times that of the earth, whence its density is

represented (in round numbers) by the fraction ¹⁵⁄₆₆. The large globe is

then nearly five times less dense than the small one, its materials

exceeding the weight of an equal bulk of water by only one-fifth.

Gravity is actually less at its surface than at the sea-level on the

earth. Every ton of coal, for instance, delivered in that remote globe

would fall short by two hundred pounds. The albedo of Uranus differs

little from that of Jupiter; if anything, it is somewhat higher, and is

nearly represented by the brilliancy of white paper.

The spectrum of Uranus indicates an emphatic departure from the

planetary conditions so far met with. This body is obviously surrounded

by a powerfully absorptive atmosphere, of a constitution foreign to our

experience. The greenish hue of the light which has traversed some of

its strata gives a preliminary indication of the manner in which it has

been affected. This its spectrum, first inspected by Secchi in 1869,

expounds in detail. He noticed a number of heavy dark bands in the red,

while the green and blue sections remaining open gave to the planet its

characteristic colour. A couple of years later, Huggins and Vogel

executed concordant measurements of six pronounced bands, besides some

faint streaks; and on June 3, 1889, the former obtained, with two hours’

exposure, a beautiful spectrographic impression extending far up into

the ultra-violet. A corroborative, though less comprehensive, photograph

was taken by Mr. Frost at Potsdam, April 23, 1892. Both included many

Fraunhofer lines, the presence of which demonstrates that the light of

Uranus, although more powerfully stamped with original absorption than

that of the rest of the planets, consists essentially of reflected solar

rays. Professor Keeler’s admirable series of visual observations with

the Lick refractor were undertaken in 1889 to test the truth of a

suggestion that this peculiar spectrum consisted of bright bands upon a

dark ground, and not of dark bands upon a bright ground. His decision in

favour of the latter alternative was without appeal.

Of the six principal dark bands representing the arresting action upon

light of the planetary atmosphere, four are quite distinctive; the fifth

is the “red star line” common to the spectra of Jupiter and Saturn; the

sixth is the hydrogen “F” (Hβ)—not definite and narrow as it is seen in

the solar spectrum, but hazy, and graduating in darkness towards the

middle, an undoubted outcome of native absorption.[78] Now, this is a

fact that implies a great deal. It gives direct evidence of a very high

temperature. Free hydrogen ceases to be present in a body upon which

water can form—given, of course, the presence of oxygen, which it would

be in the highest degree arbitrary to exclude. At one epoch of its

development, the earth must have been surrounded by immense volumes of

hydrogen. But with the diminution of heat, union with oxygen became

possible, and the gas vanished to reappear in the form of liquid oceans,

with their related hydrographic and cloud-systems. Uranus is

presumably—almost certainly—still too hot to permit the combination of

hydrogen and oxygen; and the absence from its spectrum of the slightest

trace of aqueous absorption strengthens this inference. Doubtless, the

time will come when the two elements will no longer be held at arms’

length; their affinities will come into play; the familiar,

all-important terrestrial liquid will be formed, and the geological

history of Uranus will begin.

Uranus is attended by four moons. They are named Ariel, Umbriel, Titania

and Oberon. Titania—the third in order of distance from the primary—is

the brightest of the group, and has a diameter of possibly one thousand

miles. Oberon is slightly inferior. Both were detected by Herschel in

1787. Ariel and Umbriel, captured by Lassell at Malta in 1851, are

insignificant bodies in themselves—their dimensions probably differing

but slightly from those of Hyperion, the seventh and least Saturnian

moon, estimated to measure five hundred miles across. They are among the

most difficult of telescopic objects, since they circulate about as

close to Uranus as Mimas and Enceladus do to Saturn, are physically

smaller, and more than twice as remote from the earth. Both were

believed variable by Lassell, and Newcomb obtained in 1875 plausible,

though not convincing, evidence that Ariel, at any rate, is subject to

light changes in the period of its orbital circulation, showing that,

here again, tidal friction has done its work of synchronising rotation

and revolution.[79] None of the four orbits are appreciably eccentric;

they all lie in the same plane, and are described in periods ranging

from 2½ to 13½ days.

The position of that plane is, however, exceedingly remarkable. It is

tilted at an angle of 98° to the ecliptic. This means that the

satellites move _backward_, against the succession of the zodiacal

signs. For direct becomes retrograde motion automatically, so to speak,

by turning the plane in which it is performed beyond the limit of the

vertical. The same fact is merely expressed in two different ways by

saying that the bodies in question travel from west to east at an angle

of 98°, or from east to west at an angle of 82° to the ecliptic. The

planes of the ecliptic and of the Uranian orbit deviate, it should be

mentioned, by only two-thirds of a degree. The disturbance by which the

Uranian system was set topsy-turvy did not in the least affect the

motion of Uranus itself.

Another unusual circumstance about that system is that the

satellite-plane departs widely from the equatorial plane. Our own moon,

it is true, is similarly circumstanced; but, on the Uranian scale, it is

nearly eight times farther from its primary than Ariel, and 2·6 times

farther than Oberon; while the enormous equatorial protuberance of

Uranus almost seems to impose conformity upon bodies revolving so close

to it. Conformity, none the less, is absent. The direction taken by the

equator of Uranus, as we have seen, is indicated in a two-fold manner:

first, by the trend of the belts; secondly, by the lie of the major

axis. And these indications agree. Supposed discrepancies between them

have been reconciled by improvements in the conditions of observation.

But with the equatorial line the plane of satellite-revolution cannot be

brought to coincide. The angle of divergence is uncertain, but may be

put roughly at 20°. This would give 78° for the inclination of the

Uranian equator, so that the rotation of the planet is likely to be

direct. If so, the extraordinary anomaly is here met with of a

satellite-system circulating in a direction opposite to that of its

primary’s rotation.

Uranus can at times be perceived with the naked eye. Indian traditions

of an eighth “dark” planet have been thought to refer to it, and its

slow course among the stars had been noted by savage tribes long before

Herschel singled it out from them by its tiny disc. It is about three

times brighter than Vesta; and Mr. Proctor stated that “in the summer of

1887 they were comparable under favourable conditions,” when both, in

the transparent skies of Florida, were “quite conspicuous without

telescopic aid.” Twenty chances of discovering Uranus were missed before

it came to Herschel’s turn. So many times it had been located or

catalogued as a fixed star by astronomers far from indifferent to

immortal fame.

Neptune is much nearer to the sun than it ought to be. Both Leverrier

and Adams assumed that Bode’s law would hold good for the planet still

below the horizon of knowledge; they could do no otherwise; yet the rule

played them false. Some have even asserted paradoxically that the planet

found was not the planet sought. In point of fact, the distance of the

theoretical Neptune is thirty-eight, that of the real Neptune thirty

astronomical units. The mean radius of its orbit measures 2,792 million

miles. Hence the sun is reduced to ¹⁄₉₀₀th its terrestrial brilliancy,

and could be replaced by 687 full moons. “As seen from Neptune,”

Professor Young remarks, “the sun would look very much like a large

electric arc lamp at a distance of a few feet. It would give about

forty-four millions the light of a first-magnitude star.”[80]

Accordingly, Neptune does not circulate by any means in outer darkness.

His orbit, although very slightly eccentric, brings him at perihelion

fifty millions of miles nearer to the sun than at aphelion. It makes an

angle of less than 2° with the ecliptic, and is traversed, at the rate

of 3⅓ miles a second, in a period of 165 years.

Neptune, being fainter than the eighth stellar magnitude, is quite

inaccessible to unaided vision. But a good telescope at once displays

the seeming star in the guise of a small planetary nebula with a

diameter of 2″·433. This mean value, reduced to the mean distance of the

planet from the sun, was afforded by Barnard’s measures in 1895 with a

power of 1,000 on the Lick refractor.[81] It corresponds to a linear

diameter of 32,900 miles. Neptune accordingly, although only 17 times

more massive than the earth, is 72 times more bulky, and composed of

materials 4·2 times specifically lighter. Gravity at its surface has

almost precisely its terrestrial power. The albedo of Neptune, combining

Zöllner’s with Müller’s results, is 0·65; and its spectrum appears

identical with that of Uranus. It may be inferred that this planet also

is too hot to contain water.

Its satellite is believed to be of about the size of the moon; but since

it is 12,000 times more distant, it can be distinguished only with the

most powerful telescopes as a star of the fourteenth magnitude. The

radius of its orbit measures 225,000, that of our moon 238,000 miles;

but Neptune’s attendant completes a circuit in 5 days 21 hours; and it

is through this rapidity of movement that the large mass of its primary

has been learned. It resembles the moon besides in being solitary, so

far as can be ascertained by the most diligent researches; and it is

beyond doubt that if any companion-bodies exist they are comparatively

small or obscure. That they do exist, appears probable on the face of

it.

The one Neptunian satellite emphasises the problems set by the Uranian

four. These problems are concerned with the origin and early mechanical

relations of the solar system. Here, at its utmost verge, we encounter a

decided reversal in the direction of systemic motion—a reversal prepared

for, as it might seem, by the nearly vertical position of the Uranian

plane of satellite-revolution. This diversity is in no sense

“accidental,” as some have unwisely asserted, invoking impacts of

comets, and such like futile devices, to account for it; it belongs

fundamentally to the design of planetary evolution. Laplace’s scheme has

no room for it; Faye’s, constructed expressly to include it, requires

that Uranus and Neptune, instead of being the first, should have been

the latest formed of all the solar train. And their obviously

rudimentary condition favours the suggestion. Neptune’s satellite

revolves from east to west in a quasi-circular path, inclined to the

ecliptic at an angle of 35°; or, putting it otherwise, it revolves from

west to east at an angle of 145°.

As the only member of the solar system exempt from perturbations by a

third body (the sun being too remote to cause perceptible deflections),

it seemed admirably fitted to discharge the functions of a standard

celestial clock, greatly needed, but nowhere to be found in our

system.[82] But in 1886 Mr. Marth drew attention to certain divagations

of this “ideal time-keeper” resulting from conspicuous changes in the

position and plane of its orbit. They were explained almost

simultaneously in 1888 by M. Tisserand,[83] late director of the Paris

Observatory, and by Professor Newcomb.[84] The disturbance, which, in

its mode of production, is analogous to the precession of the equinoxes,

results from the polar compression of the Neptunian globe combined with

a deviation of the satellite’s motion from its equatorial plane. By the

action of the protuberant girdle, a slow gyration of the secondary

body’s orbital plane is produced, its inclination to the primary’s

equator remaining unchanged. Viewed under a different aspect, the same

phenomenon may be described as a retrograde movement, in a period of at

least five hundred years, of the pole of the satellite’s orbit round the

pole of the planet’s equator. The radius of the circle described cannot

be less than 20°, implying a flattening of the Neptunian globe of

¹⁄₈₅th, and may easily amount to 30°, with which an ellipticity of ¹⁄₁₁₅

should be associated. But before the centre of this circle—that is, the

pole of Neptune’s axial movement—can be satisfactorily located, several

centuries must elapse. At present we may affirm with reasonable

certainty: first, that the rotation in question is retrograde, like the

satellite’s revolution; secondly, basing the inference upon the

comparatively slight ellipticity of Neptune’s figure, that it is much

slower than the vertiginous spinning of Jupiter, Saturn, and Uranus.

Uranus and Neptune are, as has been said, companion globes. In bulk and

density they differ very slightly; their albedoes are virtually the

same, their spectra indistinguishable. They seem perfectly alike in

chemical and physical constitution, and to be situated at precisely the

same stage of development. Both govern retrograde systems. In Uranus the

peculiarity appears as if in an incipient form; in Neptune, strongly

accentuated.

Viewed from the position of Neptune, all the planets are morning and

evening stars. They are tethered to the chariot-wheels of the sun,

instead of having the run of the sky. “The four terrestrial planets,”

Professor Young writes, “would be hopelessly invisible, unless with

powerful telescopes, and by carefully screening off sunlight. Mars would

never reach an elongation of three degrees from the sun; the maximum

elongation of the earth would be two, and that of Venus about one and a

half degrees. Jupiter, attaining an elongation of about ten degrees,

would probably be easily seen somewhat as we see Mercury. Saturn and

Uranus would be conspicuous, though the latter is the only planet of the

whole system that can be better seen from Neptune than it can be from

the earth.”[85]

To a spectator retreating with the velocity of light, all the planetary

cortège would in a few hours disappear, and the sun would shine alone.

No sign would remain that his office is purely ministerial—that he

exists only to enlighten, rule, and vivify the relatively minute globes

shred from his mass in the beginning, maintaining by his attractive

power the adjusted movements of the complicated piece of mechanism they

constitute. The skies perhaps hold millions of his stamp; every solitary

star telescopically visible may be the centre of a planetary scheme like

our own; or, on the other hand, our own may, quite conceivably, have no

counterpart in the wide universe.

CHAPTER XI.

FAMOUS COMETS.

In the fourth year of the 101st Olympiad (373 B.C.), the Greeks were

startled by a celestial portent. They did not, at that time, draw fine

distinctions, and posterity would have remained ignorant that the

terrifying object was a great comet but for the description of it left

by Aristotle, who saw it as a boy at Stagira. It was mid-winter when it

flared up from due west at sunset, its narrow, definite tail running

“like a road through the constellations” over a third of the heavens.

Diodorus relates that it cast shadows like the moon, which implies a

very unusual, yet not impossible, degree of brightness. The prompt

engulfment by an earthquake and its attendant tidal wave of the Achaean

towns, Helice and Bura, justified the apprehensions it aroused. It never

came back to retrieve its reputation. During at least two thousand

subsequent years, such objects lay under the ban of popular

superstition; and the counts upon which they were accused of malefic

influence were so many and so vague that acquittal was impossible. Their

respect of persons was notorious; nor were they consistent in their

dealings with the great, to whom alone they paid individual attention. A

comet marked the apotheosis of the great Julius; a comet announced the

death of Constantine; a comet illuminated the cradle of Napoleon.

The very word “comet” takes us back to the Stagyrite; for it is derived

from the Greek word κόμη, hair, and signifies a _hirsute_ star.

Shakspeare’s “crystal tresses” represent what we now, in homely fashion,

call the “tail,” while the “nucleus” and “coma” make up the “head.” The

nucleus, in great comets, shines like a star of the first magnitude,

sometimes indeed surpassing the brilliancy of Jupiter. It is usually of

measurable dimensions, often of granular texture. The planetary disc,

round which the filmy appendages of the comet of December 1618 were

displayed, was observed by Cysatus, a Jesuit astronomer at Ingolstadt,

to become transformed into the semblance of a star cluster; Hevelius

noticed a double nucleus in the comet of 1652; and modern instances of

the same kind abound. There is indeed no likelihood that substantial

globes are ever included in the construction of comets.

The coma is of immense volume, and extreme tenuity. The rays of faint

stars traverse, undimmed and unrefracted, strata of it tens of thousands

of miles in thickness. Yet strong lines of structure develop in it

through the influence of forces emanating from the sun. As they approach

our system out of the depths of space, comets are scarcely

distinguishable from round nebulæ, and they relapse into a similar

quiescent condition on leaving it. Their temperature must then be very

near the absolute zero of cold, since they cannot be supposed either to

contain stores of native heat, or to retain stores of borrowed heat.

Thus the rapidly augmenting power of solar radiation, as they rush with

accelerated velocity nearer and nearer to its source, produce upon them

stupendous effects. The nucleus blazes out into a coruscating star; the

coma, violently driven off from it, forms multiple envelopes like thin

gauze veils, one outside the other, flung round the nucleus on the side

next the sun, separated by intervening dark spaces, and diversified by

brilliant jets and sectors. The tail is the outcome of a double

repulsion. Matter expelled by the nucleus towards the sun is, at a

certain point, thrown back to form an immense, oppositely directed

appendage, usually convex on the forward side. Some tails resemble

hollow cones, being bright at the edges, and dark within: others are

traversed by a shining _backbone_; many, perhaps all, are composite. The

magnificent object first seen by Klinkenberg at Haarlem, December 9,

1743, was supplied with six, varying in length from 30° to 44°, each,

according to the extant representations, being separately _rooted_ in

the head. Grouped into a lustrous fan, they presented a very beautiful

and surprising appearance, not again to be displayed until the world and

humanity have undergone some unlooked-for changes. For the period of the

comet was computed to be one hundred thousand years! Tails, less

obviously and splendidly multiplex, are rather the rule than an

exception. Or rather, closer observations, chiefly photographic, have

made it manifest that the single efflux of nebulous stuff generally

designated as a comet’s tail can be analysed into bundles of fibres,

into straight rays and curved plumes of light, or into knotted and

branching emanations. Homogeneous outflows, such as are seen in

drawings, do not really exist. Tails pointing _towards_ the sun have

also been occasionally noticed; but they are always feeble. Olbers

recorded, however, that, during eight days of January, 1824, the comet

then visible had a solar tail of 7°, while its anti-solar tail was only

3½° long.

The great comet of 1680 will always be memorable for having had its

orbit calculated by Newton on gravitational principles. It was not

unworthy of the distinction. Approaching the sun almost in a straight

line, it penetrated the corona at the rate of 370 miles a second, and

passing within 140,000 miles of the photosphere, escaped by means of its

extraordinary velocity from those perilous precincts. Resulting internal

commotions became evident through the rapid development of a tail more

than a hundred million miles in length. Newton calculated that particles

from the head reached its extremity in two days. He assigned to the

comet a highly elliptical orbit traversed in six centuries. But, since

its speed might be called parabolic, millenniums may be nearer the mark

than centuries. It cannot, therefore, be identified with any earlier

apparition.

The comet of 1682 was Halley’s, the predicted return of which, in 1759,

was unprecedented and memorable. At its apparition in 1835, valuable

observations of a physical kind were made upon it by Bessel at

Königsberg, and by Sir John Herschel at the Cape. They were facilitated

by the circumstance that this far-travelling body, the perihelion

distance of which is 55 million miles, and the aphelion-distance 2½

times that of Neptune, approached the earth on this occasion within 4½

million miles. It was remarkable for singular and sudden changes of

aspect. To Bessel the nucleus seemed like a burning rocket. Divergent

flames issued from it towards the sun, and he took especial note of a

blazing “sector,” which swung like a pendulum to and fro, in a period of

4⅗ days. These emanations, accumulating at the surface where the solar

balanced the cometary repulsive force, were then swept back, as if by a

tempestuous wind, to form a tail, which, on October 15, measured at

least 24°. The conviction was forced upon him that the body in which

these wonderful processes were going on was affected by opposite

polarities; and he fully concurred with Olbers in the opinion that

tail-production was a purely electrical phenomenon.

During some time before and after its perihelion passage on November 16,

the comet wore the disguise of a star. All its hairy appendages had

vanished. On the 23rd of January, 1836, it was sharply stellar;

twenty-four hours later it had acquired, besides a twenty-fold increase

of light, a disc like that of the planet Neptune, enclosed in a nebulous

sheath of about fourfold breadth. Later in its career, Sir John

Herschel[86] observed the nucleus under the form of “a miniature comet,

having a nucleus, head, and tail of its own, perfectly distinct, and

considerably exceeding in intensity of light the nebulous disc or

envelope” containing it, which was, properly speaking, the “head” of the

comet. At last, on May 5, through the progress of distension, the last

thin shred of its substance melted into the sky. The next return of

Halley’s comet, somewhat accelerated by Jupiter’s influence, is looked

for in the year 1910.

The “vintage comet” lingered in northern skies during 510 days—from

March 26, 1811, until August 17, 1812. It was attentively observed by

Sir William Herschel, who gathered from it the then new truth that

comets are self-luminous bodies. “The quality of giving out light,” he

acutely remarked, “is immensely increased by an approach to the sun.”

But he failed to persuade his contemporaries or successors. His

inference had to wait for spectroscopic demonstration. The nucleus of

the comet of 1811 he found to measure 428 miles. It showed a ruddy hue,

and was eccentrically placed within a greenish-blue “planetary body”

127,000 miles in diameter. This was again enclosed in a shining

atmosphere about four times as wide, round which was flung an envelope

of a yellow tint, forming a thin hemispherical shell on the side next

the sun, and continued indefinitely away from the sun as the hollow cone

of the tail. Owing to this mode of construction, the space between the

head and the hemispherical sheath, as well as the central part of the

tail, appeared dark. The latter extended, in October, over 100 million

miles of space, and was 15 million miles broad. Its soft radiance

resembled that of the Milky Way, side by side with which it ran on

November 9, 1811. The comet’s path lay entirely outside the earth’s

orbit, and Argelander assigned to it a period of 3,065 years. The

restriction was needless. Between a period of infinite length, and one

of 3,000, or 1,000 years, no valid distinction can, where comets are in

question, be drawn. The short sections of their tracks observable from

the earth might belong equally well to parabolas or to the

far-stretching ellipses which such protracted periods imply.

The apparition of 1811 suggested to Olbers the “electrical theory” of

comets’ tails. The uncommon impressiveness with which it displayed not

uncommon phenomena, was perhaps a result of its considerable distance

from the sun, owing to which the _interior_ force obtained an advantage

over the _exterior_, and the locus of equilibrium between solar and

cometary repulsion was pushed back further than usual from the

nucleus.[87] He calculated that the materials of the tail spent 11

minutes in the journey from its root to its tip, indicating ejection by

a force greatly more powerful than the opposing force of gravity. Olbers

anticipated the modern view that chemical differences determine the

shapes of comets’ tails, the various species of matter being diversely

acted upon by electrical repulsion. The long, straight ray, for

instance, issuing from the comet of 1807, must, he perceived, have been

composed of particles much more energetically repelled than those

aggregated in the inflected plume with which it was associated. The

curvature of these appendages, in fact, depends upon the relation

between the orbital velocity of the comet and the velocity of ejection

imparted to their constituent molecules. It has to be borne in mind,

however, that while curved tails may appear straight in projection,

straight tails can never appear curved

Olbers’ classification of comets is still of great significance. He

divided them into:

(1.) Comets which develop no matter subject to solar repulsion. These

are without tails, and may be regarded as simple nebulosities devoid of

solid nuclei.

(2.) Comets showing no trace of nuclear, while subject to solar

repulsion. They throw out no matter _towards_ the sun; the heads are

consequently left bare of envelopes, and are of simple structure. The

comet of 1807 was of this kind.

(3.) Comets manifesting the effects of both species of action. They are

characterised by the presence of a dark hoop round the head, and of a

dark rift in the tail, by which it may be judged to be a hollow conoid.

On February 28, 1843, a “short, dagger-like object” blazed out at an

interval of only fifty-two minutes of arc from the sun’s limb. It was

viewed with amazement in various parts of the world; and spectators in

Italy, by shielding their eyes from the direct mid-day glare, were able

to discern a tail already several degrees long. The proportions of the

appendage rapidly grew. On March 3, it measured twenty-five degrees; on

March 11, an adjunct to it shot out, within twenty-four hours, to nearly

twice the apparent length of the main structure, conveying, as Sir John

Herschel said, “an astounding impression of the intensity of the forces

at work.” It was first seen in this country after sunset on March 17, as

“a perfectly straight, narrow band of white cloud, thirty degrees in

length, and about one and a half in width.” On the following night, Sir

John identified this “luminous appearance” as the tail of a grand comet,

stretching over an extent of space (as it afterwards proved) of no less

than two hundred millions of miles.

The movements of this body were as surprising as its aspect. It rushed

past perihelion with a speed of 366 miles a second, leaving an interval

of 100,000 miles between its centre and the sun’s surface, and swinging

through two right angles in two hours and eleven minutes. The northern

part of its course was finished in two hours and a half; hence, it was a

“southern” comet. Very curiously, it seems to have remained obscure

throughout its journey towards the sun, reserving its outburst for the

day _after_ perihelion. Periods were assigned to it ranging from seven

to six hundred years.

Strangest of all, it turned out to be but one member of a whole family

of similarly-conditioned bodies. The “great southern comet” of February,

1880, seemed like its ghost. It had no perceptible nucleus, but an

inordinately extended train, which rapidly faded; and it scarcely

deviated by a hair’s breadth from the track of its predecessor. That is

to say, so far as could be ascertained; for the object was so indefinite

as to elude exact observation. Its period could not even be conjectured.

The nature of the relationship between the comets was thus left

uncertain.

But after the lapse of two years and a half, the question was reopened

by the appearance of the leading constituent of the group. Like the

comet of 1843, the “great September comet” of 1882, was first seen close

beside the sun. At Ealing, shortly before noon, on September 17, Dr.

Common was struck with the astonishing spectacle of a brilliant comet

hurrying up to perihelion. A transit was evidently imminent, but clouds

veiled the scene. Its completion was, however, fortunately witnessed six

thousand miles away by Mr. Finlay and Dr. Elkin at the Cape Observatory.

The comet was watched by them “right into the boiling of the limb,”

which it had no sooner touched, than it utterly disappeared. This cannot

have been through the absence of contrast; for although its intrinsic

brilliancy was excessive, it must either have shown bright against the

sun’s dusky margin, or dark when projected upon his dazzling centre.

Since neither effect was produced, it can only be inferred that the

object was translucent owing to insubstantiality. That it had not passed

_behind_ the sun was later fully ascertained. During three subsequent

days the “blazing star near the sun” drew popular attention in the

southern hemisphere, and many parts of Europe. Nothing quite so

extraordinary had ever been seen before. The spectacle of 1843 was

renewed, but outdone.

Meanwhile, an astonished public hung on the dicta of perplexed

astronomers. The speculation which obtained most currency was that the

three successive southern comets were accelerated returns of the same

body, destined, after a few short, spiral circuits, to make fiery

shipwreck in the glowing solar ocean. The effects upon terrestrial life

were unwarrantably described as likely to prove disastrous; but only an

abortive panic ensued. Data, however, to serve as the basis of a

determinate conclusion, were on this occasion collected in abundance.

The comet of 1882 was not lost sight of until June 1, 1883, when its

distance from the earth was more than five astronomical units—the

greatest at which any previous comet except that of 1729 had been

observed. Hence the general character of its orbit became thoroughly

known. It proved to deviate somewhat from the tracks pursued by the

comets of 1843 and 1880; it gave the sun a slightly wider berth; above

all, its period had unmistakably a duration of several centuries. There

could then be no further question of its being a return of either, or

both of those bodies, although its close connexion with them was

assured. This can be most rationally explained by supposing them to have

primitively constituted a single body. According to Professor Kreutz’s

able and exhaustive research, the period of the September comet is 772,

that of the comet of 1843, between five and six hundred years; and the

relative situation of their orbits indicates that the supposed

catastrophe of their disruption took place at perihelion, where a large

incoherent mass could scarcely fail to yield to the strain of the sun’s

unequal attraction at the excessively close quarters it was brought into

by the conditions of its movement. The comet of 1880 is another splinter

from the same trunk; and yet one more fragment presented itself to M.

Thome at Cordoba, January 18, 1887, when he observed literally a “nine

days’ wonder” in the guise of a shadowy ray, thirty-five degrees in

extent, following the lead of the other “southern comets,” and taking

rank (so far) as the last and least of their company.

A tendency to still further disaggregation was evident in the comet of

1882. It did not pass with impunity through the fiery ordeal of its

visit to the sun; internal agitations supervened; abnormal appendages of

rarefied texture, but prodigious dimensions, issued from it sunward; the

nucleus broke up into six spherules like strung pearls; and it was

noticed in October to be surrounded by detached nebulous masses, just

launched perhaps on independent cometary careers. The tail was two-fold.

It consisted of a dim, straight ray which temporarily attained a length

of a couple of hundred millions of miles, and a massive forked

appendage, strongly luminous and unusually permanent. Fig. 19 shows one

of a series of photographs of this comet taken with an ordinary portrait

lens under Dr. Gill’s direction in October, 1882. The observations of

its transit proved to be of great importance. Having been made just

before perihelion, they availed to demonstrate that no loss of motion

had been suffered in its plunge through the corona. This

incontrovertible fact implies an inconceivable degree of rarity in the

solar surroundings.

[Illustration:

FIG. 19.—_Great Comet of September, 1882. Photographed at the Royal

Observatory, Cape of Good Hope._ (From Clerke’s “History of

Astronomy,” 3rd ed.)

]

So long ago as 1831, Clausen pointed out that many comets are grouped

together after the manner incomparably exemplified later by the southern

comets. An analogous system, composed of only two known members, is

formed by the comet of 1807, and Tebbutt’s comet of 1881. The former,

made by Bessel the subject of a masterly investigation, was not again

due at perihelion until the remote epoch 3346 A.D., so that the

announcement of a reappearance so exceedingly premature was startling.

But when the new comet was also found to have a period of several

thousand years, it became clear that no return had been observed, but

only a companion recognised. Tebbutt’s comet was a beautiful object. Its

head, adorned with interlacing arcs of light, was an overmatch for

Capella, while so translucent that a star of the seventh magnitude

seemed rather to gain than to lose brightness by shining centrally

through it. As the upshot of these singular experiences, the difficulty

of identifying comets has been increased tenfold. Their aspects were

always perceived to be well-nigh interchangeable, but their movements

were held to be distinctive; now their very orbits are found to be, to a

considerable extent, common property.

A small, glimmering nebulosity descried at Florence by Donati, June 2,

1858, gave little promise of coming splendour. Yet few more picturesque

celestial effects have been witnessed than it presented, October 5, when

Arcturus blazed undimmed through the denser part of the tail, in

brilliant conjunction with the equal splendour of the nucleus. The

ineffable grace with which the comet spread its luminous plumage was set

off by the juxtaposition, as if for the purpose of determining the

amount of its curvature, of a long, perfectly straight ray. The aspect

of this beautiful object on October 3, is represented in Fig. 20; some

idea of its rapid development in size and brilliancy can be gathered

from an inspection of the Frontispiece to this Section. The apparition

lasted, to the naked eye, for 112 days, and will not again be visible

for 2,000 years. So that Donati’s comet may be reckoned an “irrevocable

traveller.”

[Illustration:

FIG. 20.—_Donati’s Comet, October 3, 1858._ (From Langley’s, “New

Astronomy.”) _The Star to the left of the Comet’s head is Arcturus._

]

Twice during the present century the earth has traversed, with impunity,

the tail of a comet. First, on June 26, 1819, when a comet passed

invisibly between us and the sun, sending its tail our way. Again on

June 30, 1861. The sun had scarcely set that evening when a yellowish

disc became apparent at the horizon, from which issued an enormous

double train, enclosing our planet within its folds. The closing-up and

withdrawal of the “outspread fan” to which they were compared was

accomplished in a few hours. The head of the comet had as many envelopes

as a Chinese puzzle.

The first recognised “short-period” comet approached within one and a

half million miles of the earth, July 1, 1770. Had it possessed ¹⁄₅₀₀₀th

the mass of the globe which rushed by it with entire indifference, a

perceptible lengthening of the year should have ensued; and its

gravitational insignificance was confirmed by the fact that it passed,

in 1779, right through the Jovian system without troubling the mutual

relations of its members. Lexell (with whose name it has continued to be

associated) fixed its period of revolution at five and a half years; yet

it had never been seen before. Astronomers, in fact, caught it on its

trial trip along a fresh orbit to which it had been transported in 1767

by the disturbing power of Jupiter, and whence it was removed by the

same influence in 1779. An intermediate return in 1776 had doubtless

occurred; but circumstances precluded its observation. Further

encounters with the giant planet may, however, bring back the vagrant,

and the possibility was thought to have been realised when the history

of a comet discovered by Mr. Brooks of Geneva, N.Y., July 6, 1889, came

to be inquired into. Its return about the predicted time in 1896

afforded an opportunity for revising the laborious inquiry, with the

result of disproving the case for identity.

A comet, lost under very different circumstances, was picked up February

27, 1826, by an Austrian officer, Wilhelm von Biela. His calculations

led him to the unlooked-for discovery that it travelled in an orbit with

a period of 6½ years, and had already been observed in 1772 and in 1805.

On its return in 1832, when it had become reduced to the status of a

telescopic object, Sir John Herschel watched its conjunction with a knot

of minute stars, the rays of which traversed it without the smallest

obstruction. It had neither tail nor nucleus; its aspect was that of the

commonest type of nebula. On December 29, 1845, however, a curious

change was seen to have affected it. The comet had split into two, each

of which immediately assumed the characteristic cometary shape, by

providing itself with a tail and bright nucleus. Thus divided and

regenerated, the pair advanced side by side, 157,000 miles apart,

without the least trace of mutual action through gravity, but displaying

vivid interchanges of brightness, reasonably attributed to the play of

electrical forces.[88] They re-visited the sun in 1852, but have never

since, and most probably will never again, be seen. Their end came

through senile decay. It was that predicted by Newton for all such

bodies. _Diffundi tandem et spargi per universos cœlos._

The most rapidly-revolving comet of our acquaintance was investigated in

1819 by Johann Franz Encke, of the Seeberg Observatory, who assigned to

it a period of 3½ years, and predicted its return in May, 1822. It was

punctually recaptured at Sir Thomas Brisbane’s Observatory in New South

Wales. Encke traced back its appearances to 1786, and identified it with

a comet detected by Caroline Herschel in 1795. At its last return in

1894–5, it was just at the limit of naked eye visibility. It fluctuates,

however, considerably, at successive apparitions. M. Berberich[89] has

sought to associate these perplexing changes with solar vicissitudes;

but his arguments are not entirely convincing. Encke’s comet, even if

45,000 billion times less dense than air at atmospheric pressure—the

consistence attributed by Babinet to cometary matter—would still weigh

twelve hundred tons.[90] Its excessive rarefaction is a matter of ocular

proof. On October 21, 1881, Barnard observed a central passage of this

comet, then more than usually bright and condensed, over a ninth

magnitude star, which “remained so remarkably distinct during the entire

progress of occultation, that it formally impressed me with the idea of

a transit of the star _across_ the comet—a pearly point floating between

me and the bright mass of vapour.”[91]

This object signally exemplifies the cometary peculiarity of contracting

near perihelion, and re-expanding after the critical point has been

passed. Thus, it measured 312,000 miles across, October 28, 1828, when

135 million miles from the sun, but only 14,000 on December 24, when its

distance had been reduced to 50 millions; and in passing perihelion,

December 17, 1838, at an interval of 32 millions, its diameter had

shrunk to 3,000 miles. It fulfils, as regards Mercury, the function of

spying upon the planets, assigned to comets by Airy; for, only through

the Mercurian disturbances of its motion has the Mercurian mass been at

all definitely ascertained; and a residual acceleration, which, at each

circuit, brings it back to perihelion a couple of hours before the

appointed time, has long been regarded as an index to the condition of

planetary space. Encke explained this shortening of period by the action

of an hypothetical “resisting medium” augmenting in density towards the

sun; but accumulated facts have swept it out of existence. The southern

comets performed for our benefit, one after the other, an _experimentum

crucis_ in the matter. The chief of them, on September 17, 1882, swept

through a region where Encke’s medium should be _two hundred thousand_

times denser than it is at the perihelion distance of Encke’s comet; yet

suffered no appreciable loss of motion. Nor has the comet itself of late

complied with the requirements of the theory it suggested. At its return

to the sun in 1868, the acceleration had fallen to one-half its

customary, and until then, constant value. And the change has proved to

be permanent. But the influence of the postulated medium is evidently

incapable of diminution. Thus, the movements of Encke’s comet still

remain problematical.

CHAPTER XII.

NATURE AND ORIGIN OF COMETS.

Comets reflect sunlight, and also emit light of their own. But the

combination was scarcely thought of as possible until the spectroscope

gave its verdict. The first analysis of cometary rays was made by Donati

at Florence, August 5, 1864. They were dispersed by his prisms into a

yellow, a green, and a blue band, with wide intervals between. Their

chemical interpretation was afforded by Dr. Huggins in 1868. The subject

of his experiments was Winnecke’s comet, an insignificant object with a

period of five and a half years. He found it to be composed—at least in

part—of acetylene, or some other hydro-carbon gas. The coloured bands

agreed precisely in position with those in the spectrum of the blue

light at the base of a candle-flame, or of a gas-jet. The spectra of the

immense majority of comets is of this pattern, with more or less of

continuous light added. A portion of this is borrowed, a portion

inherent. A photograph of the spectrum of Tebbutt’s comet (1881, III.),

taken by Dr. Huggins, June 24, 1881, demonstrated by its distinct

impression with several Fraunhofer lines the presence of solar radiance;

the association of which with native emissions of the continuous sort

has been made evident in various comets by sudden outbursts of white

light.

Comets do not then consist entirely of carbon-compounds; but their

remaining constituents make no distinctive show in their spectra unless

when sun-raised agitation is particularly vehement. Thus, an approach

within five million miles of the sun evoked in comet Wells (1882, I.),

sodium-luminosity, detected by Dr. Copeland at Dunecht, June 17, 1882.

The blaze was so vivid that a crocus-tinted image of the entire head

with the beginning of the tail was visible, like a solar prominence,

through the open slit of the spectroscope. The same observer witnessed

an outbreak of both sodium and iron lines in the September comet (1882,

II.). In both cases, the newly-kindled emissions effaced the old, and,

after a time, were replaced by them. This mode of procedure is

characteristic of electrical action, and combines with other symptoms to

assure us that cometary illumination is produced by interior electrical

disruptive discharges due to solar induction.

Olbers’s felicitous conjecture has been developed into a plausible

theory of comets’ tails by M. Bredichin, late director of the Pulkowa

Observatory. He divided them into three “types,” distinguished by the

values of the repulsive forces employed severally in their production.

Those belonging to type I. imply the exertion of a counter-influence

fourteen times stronger than gravity. They are long, straight rays, the

constituent particles of which are carried, in a torrent too swift to be

deflected, to the observed extraordinary distances. Their outward

velocity of five miles a second to start with is, we must remember,

constantly accelerated, and finally becomes enormous. Halley’s comet and

the great comets of 1811 and 1861 had tails of this type. Donati’s great

plume exemplified the second, in which the average strength of repulsion

exceeds that of gravity one and a half times. Tails of the third type

correspond to a ratio varying from three-tenths to one-tenth. Solar

attraction is, in them, only partially neutralised. They are short,

strongly-bent, brush-like appendages, seldom seen apart from those of a

more striking kind.

These three types have a physical meaning of great interest. The

attractive force of gravity varies as the mass, the repulsive force of

electricity as the surface of the molecules they sway; hence the ratio

of repulsion is inversely as the ratio of molecular weight, the lightest

particles being the most violently driven away from the sun. Assuming

them to be hydrogen-molecules, Bredichin found that the atomic weights

of hydro-carbon gases and iron would correspond fairly well with the

speed of projection signified respectively by the curvatures of the

second and third types of tail. Materials of other kinds are not

excluded; their presence is, indeed, demanded by the width of these

appendages, which obviously consist of bundles of emanations differently

influenced, and presumably of a different chemical nature. Bredichin’s

theory works admirably from a geometrical point of view. All the

varieties of cometary trains can be constructed by strict calculation

from the basis it supplies. Yet there are spectroscopic difficulties in

the way of accepting it unreservedly. No evidence is at present

forthcoming of any connexion between the chemistry of tails and their

shapes; and hydrogen rays are conspicuously absent from cometary

spectra.

“Short period,” or “planetary” comets may be defined as those revolving

in periods of less than eight years. They have much more in common,

however, than the quickness of their successive returns to the sun. All

move from west to east; they show some preference for the plane of the

ecliptic; and none of their orbits are excessively elongated. Thus, they

tend towards conformity with the regular ordinances of the solar system,

which its less accustomed visitants completely ignore. All, too, have a

_used-up_ appearance. This is easily understood. They have wasted their

substance spinning out nebulous appendages—_sicut bombyces filo

fundendo_, as Kepler said—at their frequent returns to perihelion. They

are thus visibly effete bodies. Before long, they will drop out of

individual existence, and survive obscurely, reduced to the “dust of

death.” Yet the supply is not likely to become exhausted. Discovery

proceeds faster than disappearance.

“Lost comets” belong, without exception, to this class. Two typical

instances have already been mentioned in the disaggregation of Biela’s,

and the removal of Lexell’s comet. The fate of Biela may have been

shared by Brorsen’s, a comet with an established period of five and a

half years, which has, nevertheless, remained submerged since 1879. It

is believed by Dr. Lamp to have exploded through internal forces in

1881, and he recognises as one of its fragments a faint comet detected

by Mr. Denning at Bristol, March 26, 1894. The adventures of displaced

comets, such as Lexell’s can be traced only by arduous and delicate

inquiries. They depend upon a single cause. Unsettled comets are those

which pass near Jupiter’s orbit, and are subject to encounters with his

mighty mass. And since they must necessarily return to the point of

disturbance, the series of their vicissitudes can come to an end only by

their being driven off finally from the solar system along a hyperbolic

path.

The condition of these bodies might be described by saying that, in the

regular course of things, they revolve round the sun disturbed by

Jupiter; while, during brief but energetic crises, they revolve round

Jupiter disturbed by the sun. Their abnormal condition results from the

situation of their aphelia close to the Jovian track. This is the case,

in a minor degree, with many comets of comparatively settled habits.

They escape eviction and exile, and suffer only disquietment. Such are

Winnecke’s, D’Arrest’s, Faye’s comets, which, having been continuously

observed during half a century, are, as Mr. Plummer expresses it, “well

under control.”[92]

Short-period comets, with the solitary exception of Encke’s, appear to

be inevitably connected with Jupiter. The peculiarity is rendered more

significant by the circumstance that the other great planets are also

provided with cometary clients. The Jovian group is the largest; it

includes more than two dozen recognised individuals. Saturn claims nine,

Uranus eight, and Neptune five. Halley’s comet belongs to the Neptunian

family. Another of its members was discovered by Pons in 1812, and

re-discovered by Brooks in 1883, so that it has a period of 71 years.

And the reappearance in 1887 of a comet first seen by Olbers in 1815,

bore reassuring testimony to the regularity with which Neptune’s comets

conduct themselves during their long periods of invisibility.

The nature of these planetary relationships was at once conjectured. It

seemed an open secret that the comets had been taken prisoners by the

attractive force of the great globes they flitted past on their way to

the sun. But astronomers can take nothing for granted; and preliminary

mathematical inquiries served rather to discredit the first and easy

surmise. The case had to be thoroughly sifted; and it was only through

the profound researches of Tisserand, Callandreau, and Newton of Yale,

that the “capture-theory” has taken its place as a highly probable

truth. With an unstinted allowance of time and _comets_, it can perform

all that is required of it. “Captures” are not effected all at once; the

lasso is thrown many times over the escaping body before it is

definitively secured. Moreover, at each such effort, the chances are

even of its being made in the wrong direction. We observe only the

outcome of the hits; the misses are beyond our reckoning. A multitude of

happy accidents have led to the domestication in our system of Faye’s,

Tuttle’s, Winnecke’s, D’Arrest’s comets. Mr. Plummer has adverted to the

likelihood that we are indebted to some slight but well-directed pulls

from Mercury for the permanent addition of Encke to the solar company;

and Neptune exerted itself ages ago with similar success as regards

Halley’s comet, yet under great difficulties, since retrograde comets,

and those with highly inclined orbits are, as a rule, exempt from

capture. This is one of the reasons why short-period comets show some

degree of conformity to planetary modes of motion.

These investigations remove all doubt as to the foreign origin of

comets. Those that are in the solar system are not of it. They assuredly

remained unaffected by the gradual processes of its development. Yet

they, as well as the multitude of parabolic comets, belong to it in a

wider sense. That is to say, they accompany its march through space.

Otherwise, as M. Fabry has demonstrated, most of their orbits should be

strongly hyperbolic; and no such cometary orbits are known. They should,

besides, if casually encountered, present themselves chiefly along the

line of the sun’s way; they arrive, on the contrary, indifferently from

all quarters of the heavens. They are then subject to the same

mysterious influences which govern his motion, and drift with the cosmic

current which bears the solar family along, we know not how or whither.

[Illustration:

FIG. 21.—_Photograph of Swift’s Comet. Taken by Prof. Barnard, April

6, 1892. Exposure, 1h. 5m._

]

[Illustration:

FIG. 22.—_Photograph of Swift’s Comet. Taken by Prof. Barnard 24h.

later. Exposure, 50m._

]

Comet-photography became possible only through the introduction of

highly-sensitive gelatine plates; and even with them, exposures of an

hour and upwards are necessary in order to obtain the desired results.

But these results are of such importance as to deserve the closest

attention. For investigating either the forms or the spectra of comets,

the camera is unrivalled. Its systematic employment for these purposes

dates from 1892. It can also serve as an engine of discovery. On October

12, 1892, a comet so faint that, had it not been photographed, it would

most likely never have been seen, appeared as a nebulous trail on a

plate exposed by Professor Barnard to the Milky Way in Aquila. It proved

to be one of Jupiter’s dependents, pursuing, in a period of 6·3 years, a

track so closely resembling the orbit of Wolf’s comet in 1884, that

Schulhof regarded them as the offspring of one parent body.

In the year 1892, seven comets were detected; and all, by one of those

picturesque coincidences with which nature loves to entertain her

devotees, were, towards its close, visible in the sky together. One of

them was first noticed by Lewis Swift—a specialist in that line—and

passed perihelion April 6.[93] The head competed in brightness with a

third-magnitude star; the tail was 20° long, and came out, in a

photograph taken by Mr. Russell at Sydney, on March 22, self-analysed

into eight perfectly distinct rays. _No such structure could be seen

with the telescope._ Figs. 21 and 22 reproduce two pictures of this

object obtained by Professor Barnard, April 6 and 7 respectively. During

the interval, a striking change had occurred. In the first photograph,

the tail is sharply separated into two branches, and shows traces of

further indefinite subdivisions. The uneven, knotty texture of the main

stream is obvious. The matter composing it seems as if it had rushed in

a torrent over a rocky bed, whirling and foaming round the obstacles it

encountered. Twenty-four hours later, this powerful emanation left

scarcely a trace on the plate. Its dwindled remnant had split up into

two faint streaks, while the almost negligeable offset of the previous

night had sprung into unlooked-for prominence. A unique feature was

added in the apparent development of a secondary comet two degrees

behind the head. The anomalous enlargement brightened gradually inwards,

and can readily be seen upon the plate to be the centre of an entirely

new system of tails.[94]

Owing to moonlight and clouds, the autobiography of this planetary _bud_

unfortunately remained a fragment; and since Swift’s comet has an

indefinitely long period, it will never again exhibit for our benefit

any of its caprices of change.

[Illustration:

FIG. 23.—_Photograph by Prof. Barnard of Holmes’ Comet near the

Andromeda Nebula._

]

On November 8, 1892, Professor Barnard secured a very perfect

representation (shown in Fig. 23) of a peculiar-looking comet grouped

with the great Andromeda and its attendant nebula. Discovered only two

days previously by Mr. Edwin Holmes of London, it presented a great

round disc with definite edges visible to the naked eye. This contained

a tail in embryo, which subsequently opened out into a feeble brush, the

head being then pear-shaped, and granulated like a remote star

cluster.[95] A strictly continuous spectrum was derived from it. “Its

appearance,” Professor Barnard wrote, “was absolutely different from

that of any comet I had ever seen. It was a perfectly circular and

clean-cut disc of dense light, almost planetary in outline. There was a

faint, hazy nucleus.”[96] A photograph taken by him, November 10,

showed, distant about one degree to the south-east, “a large irregular

mass of nebulosity covering an area of one square degree or more, and

noticeably connected with the comet by a short, hazy tail.”

This object underwent extraordinary vicissitudes of aspect. From a

seeming planet it quickly degenerated by distension into the thinnest of

nebulosities; then suddenly, on January 16, 1893, gathered itself

together into an ill-defined star of the eighth magnitude. This

evanescent outburst was simultaneously observed in several parts of the

world. After some minor rallies and relapses, the comet finally, on

April 6, 1893, melted into the sky-ground. Jupiter is responsible for

its introduction into the solar system, and it will again be due at

perihelion in May, 1899. Yet its reappearance is considered doubtful.

It was perhaps caught sight of during a temporary crisis of internal

agitation, which may not recur. Certainly it could not, if as bright as

when discerned by Mr. Holmes, have remained many nights unnoticed.

Nevertheless, it had passed the sun five months previously. Its orbit is

more nearly circular than that of any previously observed comet, and it

revolves wholly within the asteroidal zone. That is to say, its

perihelion lies outside the orbit of Mars, its aphelion inside that of

Jupiter. Hence, it ought to be visible like a planet, at every

opposition. Professor Barnard, however, sought vainly for it, when thus

situated. The apparition was in many ways enigmatical.

A comet discovered by Brooks, October 16, 1893, was photographed by

Barnard three nights later, when a tail was disclosed, 3½° long, and

flowing off in two branches with a spine-like ray attached to each. A

series of impressions were fortunately taken, and that of October 21

(reproduced in Fig. 24) proved to be of peculiar interest. Since the

night before, the tail had apparently met with an accident. It imprinted

itself upon the plate shattered, deformed, and affected by a double

curvature. A collision with some external body was at first suggested as

the cause of this untoward state of things; but, knowing all that we do

about the violent interior paroxysms of comets, it seems more rational

to attribute it to extreme irregularities in the quantity and direction

of effluences from the nucleus. The following night’s photograph gave

evidence of a partial return to normal conditions. Yet the appendage

still looked badly damaged; and an elliptical fragment, wrenched from it

during the convulsion, showed no tendency towards reunion. At the time

of this incident, Brooks’ comet was situated well outside the orbit of

the earth.

The facts already collected by the photographic study of comets are

concordant, and easily interpreted. One obvious inference from them is

“that the matter of a comet’s tail is driven away from the nucleus in a

very irregular and spasmodic manner.”[97] At certain crises, outflows

are only accomplished by convulsions, compared by Mr. Ranyard to the

explosions of terrestrial volcanoes, or solar prominences. Moreover,

capricious as cometary forms are to the eye, they are still more

inconstant as recorded chemically. “The appearance one day,” Professor

Hussey says, “affords no indication as to what it may be the next. The

most radical changes of form have been observed in almost every

reasonably bright comet that has been photographed; and they sometimes

take place so rapidly as to become conspicuous in an hour or two.”[98]

[Illustration:

FIG. 24.—_Brooks’ Comet, photographed by Prof. Barnard, October 21,

1893. Exposure, 35m._

]

Comets’ tails appear very different in structure photographically and

visually. On the sensitive plate, they are perceived to be composed of

innumerable, distinct filaments, sometimes tied up, as it were, into

sheaves. The filaments, or streamers may, however, according to the same

authority, “leave the coma in a single compressed bundle, or they may

spring from it in widely divergent and loosely connected groups; they

may be smooth, and straight, and distinct, or they may be lumpy,

crooked, interlacing, and spirally twisted; or again, they may be broken

into fragments, and scattered as though they were smoke driven by the

wind.” And these effects often swiftly succeed each other in the same

comet.

In photographs of Swift’s and Rordame’s comets in 1892 and 1893 (taken

by Barnard and Hussey respectively), the effects of a spiral outward

movement in the grouped streamers of the tail can be plainly recognised.

They are indistinguishable from “the twisted forms produced by an

electrical discharge in a magnetic field.”[99] Another much more common

peculiarity of such appendages brought into prominence by chemical

portraiture, is the occurrence upon them of knots, or condensations.

These are evidently accumulations of outflowing matter. Again, in most

of the comets recently photographed, the tails start directly from the

nuclei, which appear destitute of genuine envelopes. This is the precise

criterion of Olbers’ first cometary division, in which solar repulsion

acts alone, nuclear repulsion being ineffective, or non-existent. It

comes out remarkably in Barnard’s photographs of Gale’s comet in 1894.

We may now resume in a few words what we have learned about comets. To

begin with, they are of such small mass that no gravitational effects

from their closest vicinity have ever yet been detected. Their bulk, on

the other hand, is enormous. The great comet of 1811 comprised a

nebulous globe 2½ times larger than the sun, with a tail many thousand

times more voluminous. Hence the extraordinary tenuity of such bodies.

They must indeed contain solid matter; otherwise they could not hold

together even in the imperfect way that they do; but it is probably in a

state of very loose aggregation. Their permeability to light may thus be

accounted for. The visibly granular texture of their nuclei is

confirmatory of the supposition. If, then, the nuclei of comets are

essentially “meteor-swarms,” all the constituent particles must revolve

round the centre of gravity of the whole, in a common period, but with a

velocity directly proportional to distance from the centre—that is,

increasing outward. And the joint mass being so small, the utmost speed

attained would perhaps rarely exceed a couple of hundred yards a second.

Moreover, towards the centre, where the components of the swarm would

crowd most closely together, motion would become so slow as to be

scarcely perceptible. Hence collisions would be infrequent and of slight

effect; while the probability of their occurrence should diminish with

the comet’s approach to the sun, which, by its unequal attraction, would

draw the revolving particles asunder, and amplify their allowance of

space. Internal collisions may then fairly be left out of the account in

considering the phenomena of comets. The expansion of their nuclear

parts, due to tidal forces, is, however, usually disguised by the

contraction, near perihelion, of their nebulous surroundings. The latter

effect can be explained by the immense predominance at that conjuncture

of solar over cometary electrical repulsion.

That the light-emissions of comets are largely of electrical origin is

no longer doubtful; so that the present rush-ahead in this branch of

knowledge cannot but help to elucidate many of the still mysterious

circumstances connected with these strange visitants from the uttermost

verge of the sun’s empire. The tie of allegiance hangs loosely there;

but by the persevering efforts of the great planets it is sometimes

drawn closer, with the result of domiciling under their control a train

of dilapidated comets, verging towards dissolution.

Carbon, sodium, and iron, are the only substances directly known to

exist in these bodies. Spectroscopic evidence also suggests the presence

of nitrogen or hydrogen; and a number of chemical elements which make no

show in their light doubtless enter into their composition. The state of

comets when remote from the sun can only be surmised. Their gaseous

constituents may be solidified by cold. They can, in any case, scarcely

be other than obscure and inert bodies.

CHAPTER XIII.

METEORITES AND SHOOTING STARS.

At Madrid, on the morning of February 10, 1896, the sunshine was at 9.30

overpowered by a vivid flash of bluish light, succeeded by a violent

explosion. Much glass was broken, and other devastation of a minor kind

wrought; above all, some hundreds of thousands of people were thoroughly

frightened. The origin of the commotion was visible in a white cloud

rushing across the sky, and leaving behind a dusty train. Of this

débris, scattered from a height of fifteen miles, some fragments were

picked up and analysed. They were composed of silicates of magnesia and

iron, with very small quantities of aluminium, nickel, and calcium.

These specimens were strictly “aerolites,” a term used to designate any

solid meteoritic matter that reaches the earth.

Equally conspicuous apparitions of the sort are not always equally

clamorous. There are silent, as well as detonating fire-balls. The cause

of the difference cannot certainly be assigned. It resides, perhaps, in

the diverse constitution of the exploding bodies; it is, beyond doubt,

unconnected with their height in the atmosphere. Thus, a remarkable

meteor was seen, but not heard, by Dr. Rambaud, the astronomer-royal for

Ireland, at Dunsink, February 8, 1894. The object, he says, “suddenly

burst into view with an intense brilliance, and shone out against the

cloudless blue sky with a greenish metallic lustre. It fell in a

vertical direction until it disappeared behind some trees. In shape it

resembled a very elongated pear, like most fire-balls of the sort. It

emitted no visible sparks, and disappeared quite noiselessly.” When

first observed, it was at a height of about 87 miles above the Irish

Channel; then crossing Lancashire, it descended so rapidly on its way,

probably, to engulfment in the North Sea, that, when last noticed, it

was scarcely, if at all, higher above the earth’s surface than the

Madrid meteorite at the moment of its formidable disruption. Astonished

rustic beholders at Kingswood and Dudley averred that it burst “in the

next field”; but this is a common illusion. Professor Langley relates

that some witnesses of a marvellously swift meteor at a presumable

elevation of some fifty miles, sallied out of their houses next day to

make sure that it had not struck their chimneys.

Such phenomena are tolerably frequent, and have been recorded from the

remotest antiquity. Homer lends a meteoric aspect to Athene, when she

descends from Olympus to take the war-path by the shore of Scamander.

Chronicles abound with accounts substantially identical with the

telegrams supplied by Reuter’s Agency on February 10, 1896. The fall of

the “Crema meteorite” has a special interest as having been depicted by

Raphael in his “Madonna di Foligno.”[100] A multitude of stones were

discharged by it on the banks of the Adda, six of which weighed each one

hundred pounds and upwards; the sulphurous smell characteristic of

fresh-fallen aerolites is mentioned in contemporary accounts of the

event, which occurred September 4, 1511; and it is further said that

“sheep were killed in the fields, birds in the air, and fishes in the

streams.” No specimen of this sky-volley is known to exist. In elder

times, objects of this class were worshipped; and Professor Newton[101]

has collected many curious facts about the meteoric cult traceable in

classical history. To this day, indeed, the central sanctuary of

Mahometanism—the Kaaba—owes its sacredness to the embedment in its

masonry of a blackened aerolite.

Until the beginning of the present century, only the ignorant believed

it possible that stones could come from heaven; philosophers regarded

them as generated in the clouds. They were at last convinced that the

popular view was correct by Biot’s investigation of the meteoric tempest

which broke over L’Aigle, in the department of the Orne, April 26, 1803.

He estimated at two thousand the number of fragments scattered over an

area six by two and a half miles, one of which, weighing five pounds, is

now in the South Kensington Museum. And at Pultulsk, January 30, 1869,

one hundred thousand stones were reported to have been showered upon the

earth. It is not often, indeed, that largesse from space is so lavishly

made. Yet all meteors (with the rarest exceptions) rendered luminous by

the resistance of its atmosphere, become, in one way or another,

incorporated with its mass. Their materials are no doubt often reduced

to fine dust and gas; yet six or seven hundred solid masses per annum

are computed to reach the surface of sea or land, for the most part

“unrecked-of and in vain.” Of late, the scientific demand for them has

grown keen, and their enhanced value has raised the legal question of

their ownership. The decision of the American courts is that aerolites

are not “wild game,” but “real estate,” and, as such, belong to the

owner of the land upon which they fall.

No wonder they should be at a premium, those blackened and wasted

samples of immeasurably distant globes. The velocities with which they

entered our atmosphere alone suffice to prove their cosmical origin. Had

it not trapped them, many, circuiting the sun in a hyperbolic curve,

would have escaped for ever from our system. Their primitive

disconnexion from it is implied by their swift motions, which

considerably exceed, on an average, those of comets, and point to

interstellar space as their proper habitat. The earth’s orbital pacing

has, however, to be added or subtracted as the case may be; so that the

actual rate of encounter varies from ten to forty-five miles a second.

Most of this is spent before the earth’s surface is reached. Only

considerable masses travelling at express speed bring any sensible

proportion of it with them to the ground. But what is lost as motion

reappears in other forms of energy, as light, heat, and sound. In front

of the rushing body, the air—despite its inconceivable tenuity at

elevations of fully one hundred miles—is suddenly compressed and raised

to an exceedingly high temperature, while a corresponding vacuum behind

gives rise to violent reactive currents. Professor Dewar calculated, by

way of example, in 1887, that a body, three feet in diameter, moving

eighteen miles a second at an altitude of twenty-three miles, where

barometric pressure is reduced to one-fifth of an inch, would compress

the air in its path 5,600 times, the resistance offered to its passage

thus equalling that of thirty-seven atmospheres. The abrupt increase of

heat accompanying compressions of this order amounts to thousands of

degrees, and tends to rend in pieces a body arriving from frigid abysses

where matter can only exist in a stark and, so to speak, lifeless state.

Explosions of occluded gases ensue; vaporised and incandescent particles

are blown behind in a luminous train; and, at the most, some shattered

solid remnants tumble to our continents, or plunge into our oceans. The

few that are rescued for examination look much the worse for their final

adventure. The signs of the furnace and the hurricane (both

self-created), are visible in their jetty and fused surfaces,

“thumb-marked,” probably through the continual and irregular changes in

the pressure exerted upon them. The crust is, however, a mere varnish,

the interior, which is usually of a greyish hue, being entirely

unaffected by heat. It remains, on the contrary, sunk in the depths of

cold. Agassiz compared the aerolite which fell at Dhurmsala in India, in

1860, to the Chinese _chef d’œuvre_, a “fried ice”;[102] and a large

fragment of it, which fell in moist earth, was found coated with

ice.[103]

Aerolites, or meteorites, as they may equally well be called, are

roughly divided into “stones” and “irons”; the former being composed of

various and peculiar minerals, the latter of iron, with a considerable

percentage of nickel.[104] All show a more or less distinctive

crystalline structure. Meteoric chemistry includes about thirty of the

seventy or so terrestrial elements. The chief of them are: iron, nickel,

carbon, oxygen, silicon, magnesium, sulphur, aluminium, phosphorus, with

smaller quantities of chromium, cobalt, tin, copper, titanium,

manganese, antimony, arsenic, lithium, hydrogen, nitrogen, argon, and

helium. Argon and helium were expelled by heat from a piece of meteoric

iron picked up in Augusta County, Virginia, the former coming off nearly

a hundred times more plentifully than the latter. As the light of argon

makes no show in the spectrum of any heavenly body, the proof of its

cosmical diffusion thus obtained by Professor Ramsay is of great value.

Besides argon and helium, hydrogen, carbonic acid, and carbonic oxide

gases are found included in meteorites. They seem, as it were, to

hybernate in the stony or metallic enclosures from which they can only

be _boiled out_.

Although these wind-falls from space contain no strange elements, the

manner of their composition is special to themselves. Their study

constitutes a separate branch of mineralogy. They are certainly of

igneous origin. They show no sign of water-action, and but little of

oxidation. The nearest affinities of the minerals aggregated in them are

with volcanic products from great depths. Thus meteorites seem broken up

fragments of the interior parts of globes like our own. A few among them

contain solid carbon, either amorphous, or in the shape of graphite, or

even crystallised into minute diamonds. In the Cañon Diablo siderite, or

meteoric iron, all three varieties occurred together, some of the

translucent particles proving, when put to the test of actual

combustion, to be indeed “gems of purest ray serene,” dwelling incognito

in a strange environment!

The thin streaks of light called “shooting stars” differ in several

respects from explosive meteorites. In the first place, they—probably

without exception—form systems. Innumerable multitudes of them travel in

the same paths round the sun. Moreover, those paths resemble cometary

orbits; they are very elongated ellipses, inclined at all angles to the

plane of the ecliptic, and traversed indifferently in either direction.

Their velocities are thus sensibly parabolic, while fire-balls commonly

attain hyperbolic speed. Finally, they are soundless. They slide by in

ghostly silence. Most of them are probably not larger than a pea, yet

were the shield of its atmosphere withdrawn, the earth would be rendered

well-nigh uninhabitable by their pelting. Incredible numbers of them are

encountered. They come by the million daily to be burnt, visibly to the

naked eye, in the thin upper air. Kleiber’s allowance is eleven,

Newton’s twenty millions; and these figures should be multiplied a score

of times to include telescopic fire-specks. Now, the combined mass of

all these particles goes to reinforce the mass of the earth; but it is

relatively so small that ages must elapse before the contribution can

become sensible. Our defeated meteoric assailants surrender to us also

the heat of their arrested motion; which is, however, only as a spark

added to the furnace of our supply from the sun.

Shooting stars, as we have seen, move in closed orbits. They are, then,

a periodical phenomenon. Not that we ever see the same individual twice;

its visibility implies its dissolution, but its companions are as the

sands of the seashore. Their association is recognised by their

agreement in direction and date. Unless their orbits intersected that of

the earth, nothing could be known of them terrestrially; they come to

our notice only through actual encounters, and encounters are possible

only at the time of year when our planet is passing through the node.

This is the given rendezvous, different, speaking generally, for each

system; although, speaking particularly, many meteoric streams are so

wide that the earth takes days, even weeks, to cut its way through them,

and so may be overtaken by fresh onsets before the original one is

exhausted. Each community is distinguished by the lie of its orbit—that

is, by the point in the sky from which the flying arrows of light seem

to diverge. This is known as the “radiant-point” of the system, and is

its special characteristic.

The August meteors are a familiar example of such an association. Their

annual recurrence is no new discovery. Long ago, in mediæval times, they

were called the “tears of Saint Lawrence,” because never looked for

vainly on the 10th of August. But they are so far from being limited to

that particular night, that Mr. Denning has picked up skirmishers and

stragglers from the main body all the way from July 8 to August 22. They

are distributed with tolerable evenness along an immensely long ellipse,

traversed in 120 years; and, because they radiate from near the star η

Persei, are known to science as the “Perseids.”

The scattering of the November meteors—or “Leonids,” since their point

of emanation is marked by ζ Leonis—is on the same plan, with a

difference: the Perseids might be compared to a plain gold ring; the

Leonids, to a ring with a gem on it They send us some shots every year

on the 13th and 14th of November; but three times in a century they open

fire for a regular bombardment. An early Leonid display took place in

902 A.D., noted in old chronicles as “the year of the stars.” All night

long on October 19—the node advances 14½ degrees in a thousand

years—while the tyrant Ibrahim lay dying “by the judgment of God” before

Cosenza, beholders far and near viewed with consternation the stars

precipitating themselves from the sky. Recurrences of the phenomenon

every thirty-three years received curiously little attention until

Humboldt described, and insisted on the periodic nature of the meteoric

tempest witnessed by him at Cumana on the morning of November 12, 1799.

One scarcely less violent broke over Europe and Asia in 1832, and the

American continent in 1833. From the Gulf of Mexico to Halifax the stars

were seen to fall as silently as snow-flakes, and almost as thickly, yet

after a less undirected fashion. Rather they darted and swooped, like

falcons, with a purpose; and it was noticed that the lines of their

flight could, with essential invariability, be traced back to one point,

or small area in the heavens. This remark gave the clue to their nature.

They were perceived to be necessarily cosmical bodies. For since the

focus of the meteors remained unaffected by the earth’s rotation, they

showed themselves plainly extraneous to its domestic arrangements. “A

new planetary world,” exclaimed Arago, “has been disclosed to us!”

The anticipated repetition, in 1866, of the November shower of 1833,

came off with _éclat_. Many still remember the amazing spectacle

presented by the heavens in the early morning of November 14, in that

year. In 1867, when the earth came round again to the same point of its

orbit, the star-rain was still falling heavily; and even in 1868 it

amounted to a fair sprinkle. Thus the swarm was, thirty years ago,

already so extended that it spent three years in sweeping past the node,

at the rate of twenty-seven miles a second. “The meteors themselves,”

according to Dr. Johnstone Stoney,[105] “are probably little pebbles,

the larger about an ounce, or perhaps two ounces, in weight, and spaced

in the densest part of the swarm at intervals of one or two miles

asunder every way. The thickness of the stream is about 100,000 miles,

which, however, is a mere nothing compared with its enormous length. The

width is such that the earth, when it passes obliquely through the

stream, is exposed to the downpour of meteors for about five hours.”

Each “pebble” revolves round the sun, and suffers planetary

perturbation, in complete independence of its fellows, their orbits

being only alike, not identical. The next full encounter with them will

take place November 14, 1899; but avant-couriers may be looked for at

the critical dates in 1897 and 1898, as well as a strong rear-guard in

1900.

The orbit of the November meteors is roughly bounded by the orbits of

the earth and of Uranus. They pass perihelion very near our

meeting-place with them; and since they run counter to the earth’s

motion, the velocity of collision is nearly equal to the sum of the two

orbital velocities, or forty-four miles a second. They are almost the

swiftest shooting stars of our acquaintance.

The successful calculation of meteoric orbits by Adams, Schiaparelli,

and Leverrier, promptly led to a discovery as important as it was

unexpected. Late in 1866, Schiaparelli announced that the August meteors

follow precisely the same track with a bright comet (1862, III.)

discovered in 1862 by Tuttle, an American astronomer; and the reality of

this singular relationship was, in the following year, verified by the

detection of three similar examples. The Leonids, with a period of 33¼

years, proved to be close associates of Tempel’s comet (1866, I.); a

meteoric stream flowing down upon the earth annually on April 20, from

the direction of the constellation Lyra, was perceived to move in the

vast ellipse traced out in 415 years by the comet 1861, I.; finally a

star-drift, first noticed December 6, 1798, was rightfully claimed as an

appurtenance of Biela’s comet.

Thus the fact of a close connexion between comets and meteors was at

once rendered patent; and as to the nature of the connexion, the history

of Biela’s comet is particularly instructive. Since its disappearance,

the meteor-swarm sharing its orbit has received a notable accession. The

comet seems to have broken up into meteors. And this, we can scarcely

doubt, is what has really occurred. Hence, when the earth passes

moderately, near where the comet _would_ have been, had it survived in

cometary shape (a conjuncture happening once in thirteen years), a

vehement outburst of shooting stars is observed. On November 27, 1872,

the “Bielids,” or “Andromedes,” came in tens of thousands from near γ

Andromedæ, the very point whence the track of the disaggregated comet

intersects the earth’s orbit at an angle of twelve degrees. Their

movements were leisurely; for they came up with our globe, instead of,

like the Leonids, rushing to meet it. They seemed to sail, rather than

shoot, across the sky. The calculated position of the originating body

was, at this date, two hundred millions of miles _in advance_ of the

node, and it was three hundreds of miles _behind_ the same point when

the display was renewed in 1885. It is then certain[106] that at least

five hundred millions of miles of Biela’s route are densely strewn with

meteoric fragments. The entire multitude, moreover, necessarily

separated from the comet subsequently to an episode of disturbance by

Jupiter in 1841. This is plainly shown by the fact that the members of

the associated company pursue the modified track. The perturbation of

1841 was exerted upon them no less than upon the comet, with which,

accordingly, they must then have formed one mass.

Biela’s comet has thus taught us that such bodies meet their end by

getting pulverised into meteoric particles; and further, that the

particles disperse with extraordinary rapidity along the length of their

orbits. Solar and planetary _differential_ action produce this kind of

effect, although they hardly explain its amount. Subordinate swarms are

also created by disturbance. Such an one met the earth November 23,

1892, when Professor Young estimated that at least 30,000 Andromedes

furrowed the sky at Princeton. Heavy star-showers, however, are

perishable phenomena. They thin out with comparative rapidity into a

continuous drizzle. At each recurrence, diffusion is seen to have made

progress, until at last the “gem on the ring” has vanished. With the

Perseids this is already the case. The stream flows without material

interruption over a bed a hundred times wider than that of the Leonids.

These meteors, too, will no doubt eventually reach a similar condition.

In the course of a couple of centuries, their thirty-three year period

will be completely effaced. In 1799, the main body of them crossed the

node in less than a year; at the close of the present century, the earth

will probably make her annual round at least four times, before the

march-past comes to an end. Obviously, it is about to become perennial.

Leverrier concluded from his researches that the Leonid comet and the

Leonid meteors, which then made part of its substance, were “captured”

by Uranus in 126 A.D., and so introduced into the solar domain. The

truth of the supposition may still be tested; should it be established,

this remarkable system affords yet another example of the rapidity with

which cometary materials become disintegrated and scattered.

The number of meteoric radiants now distinctly known is estimated by Mr.

Denning at about three thousand; and we need not hesitate to ascribe to

all these streams a cometary origin. It is true that the three thousand

generating comets have, all but three, “gone over to the majority.” But

we have witnessed the obsequies of Biela, and it seems only logical to

infer that those of its 2996 congeners were, in old times, celebrated

after the same fashion, and are still kept in mind by the annual blaze,

in their honour, of a few representative sky-rockets.

No component of a star-burst has so far _undoubtedly_ come to the

ground. The fire-works shown are of the most innocuous kind. Two

_possible_ exceptions are, however, on record. On April 4, 1095, a

shower of Lyraids was visible in Western Europe. The stars, according to

the Saxon Chronicle,[107] crowded “so thickly that no man could count

them.” And in France, one of the throng fell so accessibly that a

bystander, having noted the spot, “cast water upon it, which was raised

in steam with a great noise of boiling.” But, unless the aerolite came

from the same radiant as the stars, their simultaneous arrival was an

unmeaning coincidence. It implied no connexion, physical or dynamical,

between them. The same coincidence was renewed during the Andromede

shower of November 27, 1885. Just before it began, a “ball of fire”

struck the ground at Mazapil in Mexico, and proved to be a substantial

piece of iron containing nodules of graphite. It weighed eight pounds.

Yet here again that essential circumstance, the direction of its fall,

remained unknown. We must then, for the present, suspend our judgment as

to whether aerolites may be regarded, like shooting stars, as actual

cometary débris.

Mr. Denning’s patient watch of thirty years has led him to the singular

discovery of “stationary radiants.” The direction in which meteors

appear to approach the earth is determined by the combination of theirs

with the earth’s movements. The effect is strictly analogous to the

aberration of light. Meteoric radiants ought accordingly to shift on the

sphere just as the heavenly bodies change their apparent places by the

prescribed measure of aberration. And most do in this respect conform to

theory, the Perseid radiant notably. On the other hand, certain

well-known radiants continue fixed night after night in seeming

independence of the earth’s orbital advance; and there are a good many

points in the sky whence shooting stars continue to _dribble_ without

sensible interruption during many months of each year. The fact is

undeniable, although inexplicable.

The future progress of meteoric astronomy depends largely upon the

introduction of the photographic mode of observation. Only by its aid

can the precise determination of radiant-points be effected; and this is

the chief desideratum. Its realisation before the close of the century

may safely be predicted. Dr. Elkin, director of Yale College

Observatory, had a “meteorograph” constructed for the purpose in 1894,

and hopes to use it for the registration of the Leonids now hastening to

meet us. Hitherto, only casual fire-balls have printed their tracks on

sensitive plates. Success in obtaining permanent records of shooting

stars diverging from a radiant will mark a turning-point in meteoric

investigations.

ASTRONOMY

[Illustration:

NEBULA IN ANDROMEDA. 31 MESSIER.

(_From a Photograph, by Dr. Roberts._)

]

SECTION IV.—THE SIDEREAL HEAVENS.

BY J. E. GORE, F.R.A.S.

CHAPTER I.

THE STARS AND CONSTELLATIONS.

The study of the sidereal heavens is one of surpassing interest, and

tends to raise our minds above the sordid things of time and the petty

affairs of the little planet on which we dwell,—a globe absolutely

large, of course, when compared with objects around us, but relatively

very small in comparison with the vast stellar universe which surrounds

us on all sides, a universe so vast that even the largest telescopes can

only partially fathom its immeasurable depths.

For the study of the sidereal heavens, as revealed to us by the giant

telescopes of modern times, it will be advisable to begin by a

consideration of the starry sky as seen by the naked eye, without

optical assistance of any kind. On a clear and moonless night, when the

vault of heaven is spangled over with shining points of light, some

bright, others fainter, and many more barely perceptible to the unaided

vision, we are inclined to imagine that the stars visible to the naked

eye are innumerable, and that any attempt to count them would be a

hopeless task. This idea, however, is quite a mistake, and, indeed,

merely an optical illusion, due partly to the scintillation or twinkling

of the brighter stars, and stars near the limit of vision, and partly to

their irregular distribution over the surface of the heavens. As a

matter of fact, the stars visible to the naked eye can be easily

counted; and they have been counted and catalogued. As every book in the

catalogue of a large library can be identified, so every star visible to

the unaided vision—and thousands even fainter, and only visible in

telescopes—have been mapped, and their exact positions are as well known

to astronomers as those of every town and village in Great Britain are

known to geographers. The number of stars which can be seen with

ordinary eyesight is, in fact, very limited, and does not exceed the

number of inhabitants in a small town. Some years ago, a German

astronomer, Heis, who was gifted with excellent eyesight carefully

mapped down all the stars visible to his eye without optical aid, and

found the total number visible in the middle of Europe to be only 3,903.

A similar work was undertaken for the Southern Hemisphere by Behrmann,

another German astronomer, and the total number distinctly seen by both

astronomers in both hemispheres of the star sphere is 7,249. Of course,

at any given time and place only one half the star sphere is visible,

the other half being below the horizon. It follows, therefore, that

about 3,600 stars are visible at one time from any point on the earth’s

surface. As, however, everyone does not possess the keen vision of the

astronomers referred to above, we may safely say that not more than

3,000 stars are, on the average, visible at a time to ordinary eyesight.

On the other hand, persons gifted with exceptionally keen vision may

possibly see even more than Heis and Behrmann did; but even to such

eyes, the total number distinctly visible on a clear night without a

moon would probably not exceed 5,000. We may easily satisfy ourselves as

to the truth of this statement by taking a portion of the sky, and

counting the number of stars which can be steadily seen. Everybody knows

the Great Bear, sometimes called the “Plough,” or “Charles’ Wain.” Four

of the well-known stars in this remarkable group form a four-sided

figure. Well, let the reader look carefully at this figure, and see how

many stars can be detected within the space formed by imaginary lines

joining the bright stars. Probably surprise will be felt at the small

number which can be distinctly seen. Heis, with his keen vision, only

shows eight on his map, and of these, four are very faint, and near the

limit of even good eyesight. Probably very few eyes will see more than

eight, and perhaps most persons will fail to see so many. As the whole

hemisphere is roughly five hundred times larger than this spot, the

number seen by Heis in the quadrilateral of the Plough would give a

total of 4,000 stars visible at one time. Of course, some portions of

the sky are much richer in stars than the spot selected; but, on the

other hand, others are much poorer, so that perhaps this may be taken as

a spot of average richness. From this single example it will be seen

that the idea of countless multitudes of stars visible to the naked eye

is a mistake. Probably the effect of a great number is partly due to our

catching glimpses by “averted vision” of still fainter stars, which

cannot, however, be seen steadily when the eye is turned directly

towards them.

[Illustration:

FIG. 1.—_Stars visible in the Northern Hemisphere._

(From “Visible Universe.”)

]

In speaking of stars visible to the naked eye, we do not, of course,

include the stars in the Milky Way, that arch of cloudy light which

spans the heavens; for although this wonderful zone is composed of faint

stars, these stars are not individually visible without a telescope.

Notwithstanding the limited number of the visible, or lucid, stars, as

they are called, the aspect of the starry sky still presents a spectacle

of marvellous beauty and interest, and may be viewed with pleasure and

profit even without a telescope. There are many interesting objects

which may be seen without optical assistance of any kind. Look at the

middle star of the three forming the “tail” of the Great Bear, or

“handle” of the Plough. This star was called Mizar by the old Arabian

astronomers. Close to it, good eyesight will see a small star, known as

Alcor. This little star was called by the Arabians Alsuha, which means

“the neglected small star.” The name Alcor means the “test,” and is

supposed to indicate that the old astronomers considered it a test for

keen vision; but the Arabians had a proverb, “I show him Alsuha, and he

shows me the moon,” a saying which seems to imply that it could be

easily seen by these old astronomers. The faintest star of the seven,

the one at the root of the tail, was called Megrez by the Arabian

astronomers. This star is supposed to have diminished in brightness

since ancient times, as it was rated of the third magnitude by Ptolemy,

and of the second by Tycho Brahé, while at present it is not much above

the fourth magnitude. It may possibly be variable in its light, like

many other stars in the heavens.

[Illustration:

FIG. 2.—_Stars visible in the Southern Hemisphere._

(From “Visible Universe.”)

]

Here it may be mentioned that the stars were divided into magnitudes or

classes according to their brightness by the ancient astronomers, all

the brightest stars being placed in the first magnitude, those

considerably fainter being called second magnitude, those fainter still

third magnitude, and so on to the sixth magnitude, or those just visible

to ordinary eyesight. This classification has been practically retained

by modern astronomers, but, of course, there are stars of all degrees of

brightness from Sirius down to the faintest stars visible in the largest

telescopes. Sirius is the brightest star in the heavens, and is equal to

about six average stars of the first magnitude, such as Altair or

Aldebaran. According to the Harvard photometric measures, the following

are the brightest stars in the heavens in order of magnitude:—(1)

Sirius, (2) Canopus, (3) Arcturus, (4) Capella, (5) Vega, (6) Alpha

Centauri, (7) Rigel, (8) Procyon, (9) Achernar, (10) Beta Centauri, (11)

Betelgeuse (slightly variable), (12) Altair, and (13) Aldebaran. Of

these Canopus, Alpha, and Beta Centauri, and Achernar, do not rise above

the horizon of London. Of those brighter than the second magnitude, the

following are north of the Equator: Alpha Cygni, Pollux, Castor, Eta

Ursæ Majoris, Gamma Orionis, Beta Tauri, Epsilon Ursæ Majoris, Alpha

Ursæ Majoris, Alpha Persei, and Beta Aurigæ; and south of the Equator:

Alpha Crucis, Fomalhaut, Antares, Spica, Beta Crucis, Gamma Crucis,

Epsilon Orionis, Zeta Orionis, Epsilon Canis Majoris, Beta Carinæ,

Epsilon Carinæ, Lambda Scorpii, Alpha Triangulum Australis, Gamma Argûs,

Alpha Gruis, Epsilon Sagittarii, Alpha Hydræ, Theta Scorpii, and Delta

Velorum. Of those below the second magnitude, and brighter than the

third, there are about 34 in the Northern Hemisphere, and 61 in the

Southern. As the brightness decreases, the numbers increase rapidly.

Indeed, the increase is in geometrical progression, the number in each

class of magnitude being about three times as many as those in the class

one magnitude brighter. The exact magnitudes of all stars visible to the

naked eye in both hemispheres have now been determined by the aid of

photometers. These instruments are described in Section II. of the

present work, Chapter XVII.

The stars were divided by the ancient astronomers into groups called

constellations. Some of these were formed in the earliest ages of

antiquity. Orion and the Pleiades are mentioned in Job (Chapter

XXXVIII.), which is believed to be one of the oldest books in existence.

Josephus ascribes the division of the stars into constellations to the

family of Seth, the son of Adam; and according to the Book of Enoch the

constellations were already known and named in the time of that

patriarch. The brightest stars of each constellation are designated by

the letters of the Greek alphabet, which were assigned to them by Bayer

in the year 1603, Alpha generally denoting the brightest star, Beta the

next in lustre, and so on. This is not, however, invariably the case,

and Bayer seems in many cases to have followed the outline of the

imaginary figure from which the constellation derives its name, rather

than the relative brightness of the stars composing the constellation.

For example, the seven stars in the Plough are known as Alpha, Beta,

Gamma, Delta (the faint one), Epsilon, Zeta, and Eta, beginning with the

northern of the two in the square farthest from the tail, thus evidently

following the shape of the figure, and not the order of relative

brightness. When the letters of the Greek alphabet are exhausted,

recourse is had to numbers, those in Flamsteed’s catalogue being usually

employed. Those only visible in telescopes are known by their numbers in

various catalogues. The exact positions of the stars are fixed by

determining their right ascensions and declinations, terms which on the

celestial sphere correspond to longitude and latitude on the earth.

The stars Alpha and Beta of the Plough are called “the pointers,”

because a line drawn from Beta through Alpha points nearly to a star of

the second magnitude, called the Pole Star, which lies near the pole of

the celestial sphere, or the point round which the whole star sphere

seems to rotate, owing to the rotation of the earth on its axis, in

twenty-four hours. The distance from Alpha to the Pole Star is about

five times the distance between Alpha and Beta.

If we draw an imaginary line from the star Epsilon through the Pole

Star, and produce it to about the same distance on the opposite side of

the Pole, it will pass through a well-known group called Cassiopeia’s

Chair. This consists of five fairly bright stars arranged in the form of

an irregular W. A sixth star, much fainter than the others, forms with

three of them a quadrilateral figure. It was near this faint star—known

to astronomers as Kappa—that the famous “new,” or temporary, star of

Tycho Brahé, sometimes called the “Pilgrim Star,” suddenly appeared in

November, 1572, of which more hereafter.

If we continue the curve formed by the three stars in the tail of the

Great Bear, it will pass near a very bright star of an orange colour.

This is Arcturus, one of the brightest stars in the sky. If we can rely

on the measures of distance which have been made of this brilliant star,

it must be one of the largest bodies in the universe, much larger than

our sun, which, placed at the distance assigned to Arcturus, would only

shine as a small star, quite invisible indeed to the naked eye.

Returning again to the Great Bear, if we draw a line from Gamma to Beta

and produce it, it will pass near a bright star of a yellow colour. This

is Capella. It was called by the Arabian astronomers the “Guardian of

the Pleiades.” It is the brightest star of the constellation Auriga or

“the Charioteer,” referred to by Tennyson in the lines:

“And the shining daffodil dies, and the Charioteer

And starry Gemini hang like glorious crowns

Over Orion’s grave low down in the West,”

evidently referring to the disappearance of Orion below the western

horizon in the evening sky of April. “Starry Gemini” is marked by two

bright stars, Castor and Pollux, which may be found by drawing a line

from Delta to Beta of the Great Bear, and producing it. Another line

drawn from Delta to Gamma, and produced towards the south, will pass

near a bright star called Regulus, the brightest star in the well-known

“Sickle” in Leo or the Lion. Again, a line drawn from Regulus to Gamma

in the Great Bear, and produced, will pass near another bright star,

Vega in the Lyre. This is one of the brightest stars in the Northern

Hemisphere, the three, Arcturus, Capella, and Vega, being nearly equal

in brightness. The name Vega seems to be a corruption of the Arabic name

_vaki_, or _al-nasr al-vaki_, “the falling eagle,” the wings of the bird

being represented by the stars Epsilon and Zeta Lyræ, which form, with

Vega, a small triangle, called by the Arabians _al-alsafi_, “the

trivet.” But what relation exists between a “falling eagle” and the

musical instrument known as the Lyre (Persian _al-lûra_) is not very

obvious. Possibly, however, as suggested by Schjellerup, the Arabic

word, _al-schalzâk_ “a goose,”—also applied to the constellation—refers

to the resemblance in shape between a plucked goose and a Greek lyre.

The Greeks called the constellation χέλυς, a tortoise, which also

somewhat resembles a lyre in shape.

Of the two stars which form a triangle with Vega, the northern, Epsilon,

is a double star, which is said to have been seen double with the naked

eye by several astronomers, but, probably, most people would fail to see

it as anything but a single star, as the component stars are very close.

An opera-glass will, however, show it distinctly. Each of the components

is again double, so that the object forms a most interesting quadruple

star when viewed with a good telescope.

To the east of Vega lies Cygnus, or the Swan, one of the finest of the

constellations. It may be distinguished by the long cross formed by the

principal stars which are known to astronomers as Alpha, Beta, Gamma,

Delta, and Epsilon; Alpha, or Deneb, being the brightest and most

northern of the five, and Beta the most southern and faintest. The name

Deneb is derived from the Arabic word _dzanab al-dadjâdja_, or “the tail

of the hen,” referring to its position in the ancient figure, which

represents a hen or swan flying towards the south.

To the south-east of Cassiopeia’s Chair, we find the well-known festoon

of stars which marks the constellation Perseus. Its brightest star is

sometimes called Mirfak, a name derived from the Arabic word _marfik_,

the elbow, referring, perhaps, to its position in the curved line of

stars. South of Perseus, and the nearest bright star to Mirfak in that

direction, is Algol, the famous variable star. Further south, we come to

the constellation of Taurus, or the Bull, with the well-known groups of

the Pleiades and Hyades. The Pleiades form a remarkable cluster, and

when once recognised can never be mistaken. To ordinary eyesight six

stars are visible, but those having keener vision can see more. A little

south of the Pleiades is a V-shaped figure, the Hyades, with a bright

star of a reddish colour. This is Aldebaran, a name derived from the

Arabic _al-dabarân_, the attendant or follower, because it appears to

follow the Pleiades in the diurnal motion. It was also called _aïn

al-tsaur_, “the eye of the bull,” and by several other names such as

_al-fanîk_, “the great camel,” the other smaller stars forming the

Hyades being called _al-kilas_, “the young camels!”

South of Taurus and Gemini comes the magnificent constellation of Orion,

perhaps the most splendid collection of stars in the sky. This brilliant

asterism contains many fine objects. Looking at it when it is visible in

the winter sky, we notice a large quadrilateral figure formed by four

conspicuous stars. The upper one to the left is called Betelgeuse, and

is decidedly reddish in colour—very much resembling Aldebaran both in

tint and brightness. Its name is derived from our Arabic word meaning

the shoulder, because it is situated on the right shoulder of the giant

Orion on the old celestial globes. The upper one to the right is called

Bellatrix, or the female warrior! The real significance of some of those

old names is sometimes difficult to understand. Of the lower stars, the

one on the right is a fine white star of the first magnitude known as

Rigel. It is situated on the left foot of the ancient figure of Orion,

and its name is derived from the first part of the compound Arabic name

_ridjl-al-djauzâ_, “the leg of the giant.” The lower star on the left is

known to astronomers by the Greek letter Kappa.

In the middle of the four-sided figure referred to above are three stars

of the second magnitude, nearly in a straight line, forming “Orion’s

Belt.” The upper one of the three is slightly fainter than the others,

and has been suspected of being slightly variable in its light, but the

variability is doubtful. South of these three conspicuous stars are

three fainter stars, forming a nearly vertical line. This is “the Sword

of Orion.” The middle star of the three marks the position of “the great

nebula in Orion,” one of the finest objects in the heavens, of which

more hereafter. To some eyes a nebulous glow is visible round this star.

Even in a small telescope the nebula is an interesting object. On a very

clear night the southern star of the three may be seen double with good

eyesight. The stars forming Orion’s Belt were called by the Arabian

astronomers _mintakat al-djauza_, “the Belt of the Giant”; and the stars

forming the “sword,” _al-lakat_, the “gleaned ears of corn,” and also

_saif-al-djabbâr_, “the Sword of the Giant.” Perhaps the latter word is

the origin of the name Algebar, formerly applied to Rigel.

The three bright stars in Orion’s Belt nearly point (to the south-east)

to Sirius, the brightest star in the heavens. This is a splendid white

star, and is so much brighter than any other fixed star that its

identity cannot be mistaken.

If we draw a line from the star Gamma in the Plough to the Pole Star,

and produce it, it will pass through a somewhat similar four-sided

figure, but of much larger size, and the stars rather fainter. This is

known as “the Square of Pegasus.” The upper stars are known as Beta

Pegasi (the one to the right) and Alpha Andromeda. To the east of Alpha

Andromedæ is a star of the third magnitude, Delta, and to the east of

Delta, a star of the second magnitude called Beta Andromedæ. A little

north of Beta are two small stars, Mu and Nu, nearly in a line with

Beta, and to the north of Nu is the famous “nebula in Andromeda” “the

queen of the nebulæ,” as it has been termed. It is just visible to the

naked eye as a hazy spot of light, and it may be well seen in a good

opera-glass or binocular. Even in a small telescope it is a really

splendid object. The reader should fix its exact position carefully, as

it has been frequently mistaken for a comet by observers whose knowledge

of the heavens is not very accurate.

The following alignments may be found useful by beginners in the study

of the starry sky:—

Castor and Pollux, already mentioned, nearly point south to the star

Alpha Hydræ, an isolated reddish star of the second magnitude. It is

also called Alphard, from the Arabic _al-fard_, “the solitary one,”

because there is no other bright star near it. It is described by

Al-Sûfi, the Persian astronomer, as red in the tenth century. In the

Chinese annals it is called “the Red Bird.”

An isosceles triangle is formed by Castor (at the vertex), Alphard and

Sirius. Procyon is nearly in the centre of this triangle. Two other

roughly isosceles triangles are formed, having Aldebaran at the vertex

of each, namely: Aldebaran, Castor, and Procyon, and Aldebaran, Procyon,

and Sirius.

Castor, Alpha, Delta, and Beta Orionis are nearly in a straight line;

also Beta Pegasi, Alpha Pegasi and Fomalhaut. A right-angled triangle is

formed by Arcturus, Spica, and Regulus, Spica being at the right angle.

In the Southern Hemisphere, the most remarkable group of stars is the

well-known Southern Cross. It consists of four stars, known as Alpha,

Beta, Gamma and Delta—Gamma being at the top of the cross, and Alpha at

the bottom. These stars are popularly supposed to be of great

brilliancy, but this is a mistake; their magnitudes, according to recent

photometric measures, being Alpha, first magnitudes, Beta 1½, Gamma,

second magnitude, and Delta, third magnitude. A little south of Delta is

Epsilon, a star of the fourth magnitude, which rather spoils the

symmetry of the cross-shaped figure. A little to the east of the

Southern Cross are Alpha and Beta Centauri, two of the brightest stars

in the sky. Another fine group of stars is Scorpio, or the Scorpion, of

which the brightest star is Antares, a reddish star of about magnitude

1½, which is visible near the southern horizon in the months of June and

July in England.

When the positions of the principal stars are known, it will be easy to

find any other required object by means of star maps.

CHAPTER II.

DOUBLE, MULTIPLE, AND COLOURED STARS.

Many of the stars when examined with a good telescope are seen to be

double, some triple, and a few quadruple, and even multiple. These when

viewed with the naked eye, or even a powerful binocular, seem to be

single, and show no sign of consisting of two components. These

telescopic double stars should be carefully distinguished from those

which appear very close together with the naked eye, and which in

opera-glasses or telescopes of small power might be mistaken for wide

double stars by the inexperienced observer. These latter stars, such as

Mizar—the middle star in the tail of the Great Bear, and its small

companion, Alcor, referred to in the last chapter—have been called

“naked eye doubles,” but they are not, properly speaking, double stars

at all. Telescopic double stars are far closer, and even the widest of

them could not possibly be seen double without optical aid, even by

those who are gifted with the keenest vision. Of these so-called “naked

eye doubles,” we may mention Alpha Capricorni, which on a very clear

night may be seen with the naked eye to consist of two stars. On a very

fine night two stars may be seen in Iota Orionis, the most southern star

in Orion’s Sword. The star Zeta Ceti has near it a fifth magnitude star,

Chi, which may be easily seen with the unaided vision. The star Epsilon

Lyræ (near Vega), is, as mentioned in the last chapter, a severe test

for naked eye vision. Bessel, the famous German astronomer, is said to

have seen it when thirteen years of age. Omicron Cygni (north of Alpha

and Delta Cygni) forms another naked eye double, and other objects of

this class may be noticed by a sharp-eyed observer.

The star Mizar, already referred to, is itself a wide telescopic double,

and it seems to have been the first double star discovered with the

telescope (by Riccioli in 1650). It consists of two components, of which

one is considerably brighter than the other. It will give an idea of the

closeness of even a “wide” telescopic double when we say that the

apparent distance between Mizar and Alcor is nearly forty times the

distance which separates the close components of the bright star. From

this it will be seen that even a powerful binocular field-glass would

fail to show Mizar as anything but a single star. The components may,

however, be well seen with a 3-inch telescope, or even with a good

2-inch. The colours of the two stars are pale green and white. Between

Mizar and Alcor is a star of the eighth magnitude, and others fainter.

Mizar was the first double star photographed by Bond.

The Pole Star has a small companion at a little greater distance than

that which separates the components of Mizar, but owing to the faintness

of this small star, the object is not so easy as Mizar. A telescope with

a good 3-inch aperture should, however, show it readily. Dawes saw it

with a small telescope of 1³⁄₁₀-inch aperture, and Ward, who has

wonderful vision, with only 1¼-inch.

The star Beta Cygni is composed of a large and small star, of which the

colours are described as “golden-yellow and smalt-blue.” This is a very

wide double, and may be seen with quite a small telescope. Another fine

double star is that known to astronomers as Gamma Andromedæ. The

magnitudes of the components are about the same as those of Mizar, but a

little closer. Their colours are beautiful (“gold and blue”). This is

one of the prettiest double stars in the heavens. It is really a triple

star, the fainter of the pair being a very close double star; but this

is beyond the reach of all but the largest telescopes. The star Gamma

Delphini is another beautiful object, the components being a little more

unequal in magnitude, but the distance between them about the same as in

Gamma Andromedæ. I have noted the colours with a 3-inch telescope as

“reddish-yellow and greyish-lilac.” Gamma Arietis, the faintest of the

three well-known stars in the head of Aries, is another fine double

star, a little closer than Gamma Delphini. This is an interesting

object, from the fact that it was one of the first double stars

discovered with the telescope—by Hooke, in 1664, when following the

comet of that year. He says:—“I took notice that it consisted of two

small stars very near together, a like instance of which I have not else

met with in all the heaven.” Eight years previous to this, however, in

1656, Huygens is said to have seen three stars in Theta Orionis, the

well-known multiple star in the Orion nebula; and in 1650, Riccioli, at

Bologne, saw Zeta Ursæ Majoris (Mizar) double, as already stated.

Another beautiful double star is Eta Cassiopeiæ, the components being

about equal in brightness to those of Gamma Delphini, but the distance

less than one half, so that a higher magnifying power will be required

to see them well. The colours are, according to Webb, yellow and purple;

but other observers have found the smaller star garnet or red. This is a

very interesting object, the components revolving round each other, and

forming what is called a binary star.

Another fine double star is Castor, which is composed of two nearly

equal stars separated by a distance about half that between the

components of Gamma Andromedæ. This is also a binary or revolving double

star, but the period is long. Gamma Virginis is another fine double

star, with components at about the same distance as those of Castor, and

the colours very similar. It is also a remarkable binary star, and

further details respecting it will be given when we come to speak of the

binary stars.

Among double stars of which the components are closer than those

mentioned above, but which are within the reach of a good 3-inch

telescope—a common size with amateur observers—the following may be

noticed:—Alpha Herculis, colours, orange or emerald green; the light of

this star is slightly variable. Gamma Leonis, another binary star with a

long period; colours, pale yellow and purple. Epsilon Boötis, a lovely

double star, the colours of which Secchi described as “most beautiful

yellow, superb blue.” This has been well seen with a 2¼-inch achromatic.

For observers in the Southern Hemisphere, the following fine double

stars may be seen with a 3-inch telescope:—Alpha Centauri; this famous

star, the nearest of all the fixed stars to the earth, is also a

remarkable binary; its period, as recently computed by Dr. See, is 81

years, and the component stars are now at nearly their greatest distance

apart, the distance being greater than that between the components of

Mizar, so that any small telescope will show them. Theta Eridani is a

splendid pair, but closer than Alpha Centauri. It is, however, an easy

object with a 3-inch telescope, and with a telescope of this size I

noted the colours in India as light yellow and dusky yellow. The star

known as _f_ Eridani is a very similar double to Theta, but the

components are fainter. I noted the colours in India as yellowish-white

and very light green. There are, of course, many other double stars in

both hemispheres within the reach of small telescopes; but those

described above are perhaps the finest examples.

In addition to these comparatively wide double stars, there are many of

which the components are so close that they are quite beyond the reach

of a 3-inch or even a 4-inch telescope. Some, indeed, are so excessively

close as to tax the highest powers of the largest telescopes yet

constructed.

Of triple, quadruple, and multiple stars, there are several which may be

well seen with a small telescope. Of these may be mentioned Iota

Orionis, the lowest star in the Sword of Orion, which consists of a

bright star accompanied by two small companions. In Theta Orionis, the

middle star of the Sword, four stars may be seen forming a quadrilateral

figure, known to observers as the “trapezium.” I have seen these in

India—where the star is higher in the sky than in this country—with a

3-inch refractor reduced by a “stop” over the object-glass to 1½ inch.

There are two fainter stars in this curious object, which lie in the

midst of the Orion nebula, but a somewhat larger telescope is required

to see them. Within the trapezium are two very faint stars, which are

only visible in the largest telescopes. In Sigma Orionis—a star closely

south of Zeta, the lowest star in Orion’s Belt—six stars may be seen

with a 3-inch telescope. Indeed, Ward has seen ten with a slightly

smaller telescope. Epsilon Lyræ may be seen double with a low power, and

each star of the pair again double with a high power; but this is more

difficult than the other close stars mentioned above.

When carefully examined, many of the stars show differences in colour.

Among the brightest stars it will be noticed that Sirius, Rigel, and

Vega, shine with a white or bluish-white light; Capella is distinctly

yellowish; Arcturus yellow or orange; and Aldebaran and Betelgeuse have

a well-marked reddish hue. There are no stars of a decided blue colour

visible to the naked eye, at least in the Northern Hemisphere. The third

magnitude star, Beta Lyræ, is said to be greenish, but its colour is not

conspicuous. Betelgeuse is perhaps the ruddiest of the brighter stars,

and its reddish tint contrasts strongly with the white light of Rigel,

in the same constellation. Aldebaran, which lies not far from

Betelgeuse, is of nearly the same hue. But the reddest star visible to

the naked eye in the Northern Hemisphere is the fourth magnitude star,

Mu Cephei. It is not, however, sufficiently bright to enable its colour

to be well seen without optical aid, but with an opera-glass its reddish

hue is beautiful and striking when compared with other stars in its

immediate vicinity. It was called by Sir William Herschel the “garnet

star,” and its colour is certainly remarkable. Like so many of the red

stars, it is variable in light, but numerous observations by the present

writer seem to show that there is no regular period, and its light often

remains for many weeks with little or no perceptible change.

Among other stars visible to the naked eye, the reddish colour is also

conspicuous in Antares, Alphard, Eta, and Mu Geminorum, Mu and Nu Ursæ

Majoris, Beta Ophiuchi, Gamma Aquilæ, and others in the Southern

Hemisphere· Alphard was noted as red by the Persian astronomer, Al-Sûfi,

in the tenth century, and it was called “the Red Bird,” by the old

Chinese observers.

Ptolemy, in his catalogue, calls the following stars “fiery red”:

Arcturus, Aldebaran, Pollux, Antares, Betelgeuse, and, curious to say,

Sirius, which is now white. There is some little doubt as to the reality

of this change of hue in Sirius, but Al-Sûfi distinctly describes the

variable star, Algol, as red, whereas it is now white, or only slightly

yellowish.

The finest examples of red stars are, however, found among those only

visible with a telescope. Of these may be mentioned the star numbered

713 in Espin’s edition of Birmingham’s “Catalogue of Red Stars,” which

Franks describes as “orange vermilion,” and the star Birmingham 248,

which Espin notes as “magnificent blood-red.” Another very fine red star

is the variable R Crateris, which Sir John Herschel described as

“scarlet, almost blood colour,” Birmingham “crimson,” and Webb “very

intense ruby.” Observing it in India with a 3-inch telescope, I noted it

as “full scarlet.” It has near it a star of the ninth magnitude of a

pale bluish tint. No. 4 of Birmingham’s “Catalogue” is described by

Espin as of an “intense red colour, most wonderful.” The variable star U

Cygni is very red, and is described by Webb as showing “one of the

loveliest hues in the sky.” Another red star is the remarkable, variable

R Leonis, whose fluctuations in light will be described in the chapter

on Variable Stars. Hind says: “It is one of the most fiery-looking

variables on our list—fiery in every stage from maximum to minimum, and

is really a fine telescopic object in a dark sky about the time of

greatest brilliancy, when its colour forms a striking contrast with the

steady white light of the sixth magnitude, a little to the north.”

In the Southern Hemisphere there are some fine red stars. Epsilon

Crucis, one of the stars of the Southern Cross, is said to be very red,

and so are Mu Muscæ and Delta Gruis, the southern star of a naked eye

double. Pi Gruis is also a wide double star, and Dr. Gould describes one

of the pair as “deep crimson,” while the other is “conspicuously white.”

The variable R Sculptoris is another fine red star, which Gould

describes as “intense scarlet,” and Miss Clerke says it “glows like a

live coal in the field,” a good description of these telescopic red

stars. With reference to a small star in the field of view with Beta

Crucis, one of the brightest stars in the Southern Cross, Sir John

Herschel says: “The fullest and deepest maroon-red, the most intense

blood-red of any star I have seen. It is like a drop of blood when

contrasted with the whiteness of Beta Crucis.”

Among the double stars there are numerous examples of coloured suns. Of

these may be mentioned Alpha Herculis, the components of which are

orange and emerald, or bluish-green, and described by Smith as “a lovely

object, one of the finest in the heavens”; Epsilon Boötis, of which the

colours are described by Secchi as “most beautiful yellow, superb blue”;

Beta Cygni, “golden-yellow and smalt-blue”; Beta Cephei, “yellow and

violet”; Delta Cephei, “yellow and blue”; Gamma Andromedæ, “gold and

blue”; and Beta Piscis Australis, of which the colours were noted by the

present writer in India as white and reddish-lilac.