This brings us directly back to “Nova Persei,” for among the many

suggestions offered to explain its outburst, as well as those of other

temporary stars, one of the most fruitful is that of a collision

between a star and a vast invisible nebula. Professor Seeliger, of

Munich, first proposed this theory, but it afterward underwent some

modifications from others. Stated in a general form, the idea is that a

huge dark body, perhaps an extinguished sun, encountered in its

progress through space a widespread flock of small meteors forming a

dark nebula. As it plunged into the swarm the friction of the

innumerable collisions with the meteors heated its surface to

incandescence, and being of vast size it then became visible to us as a

new star. Meanwhile the motion of the body through the nebula, and its

rotation upon itself, set up a gyration in the blazing atmosphere

formed around it by the vaporized meteors; and as this atmosphere

spread wider, under the laws of gyratory motion a rotation in the

opposite direction began in the inflamed meteoric cloud outside the

central part of the vortex. Thus the spectral lines were caused to show

motion in opposite directions, a part of the incandescent mass

approaching the earth simultaneously with the retreat of another part.

So the curious spectroscopic observations before mentioned were

explained. This theory might also account for the appearance of the

nebulous spiral first seen some six months after the original outburst.

The sequent changes in the spectrum of the “nova” are accounted for by

this theory on the assumption, reasonable enough in itself, that at

first the invading body would be enveloped in a vaporized atmosphere of

relatively slight depth, producing by its absorption the fine dark

lines first observed; but that as time went on and the incessant

collisions continued, the blazing atmosphere would become very deep and

extensive, whereupon the appearance of the spectral lines would change,

and bright lines due to the light of the incandescent meteors

surrounding the nucleus at a great distance would take the place of the

original dark ones. The vortex of meteors once formed would protect the

flying body within from further immediate collisions, the latter now

occurring mainly among the meteors themselves, and then the central

blaze would die down, and the original splendor of the phenomenon would

fade.

But the theories about Nova Persei have been almost as numerous as the

astronomers who have speculated about it. One of the most startling of

them assumed that the outburst was caused by the running amuck of a

dark star which had encountered another star surrounded with planets,

the renewed outbreaks of light after the principal one had faded being

due to the successive running down of the unfortunate planets! Yet

another hypothesis is based on what we have already said of the tidal

influence that two close approaching suns would have upon each other.

Supposing two such bodies which had become encrusted, but remained

incandescent and fluid within, to approach within almost striking

distance; they would whirl each other about their common center of

gravity, and at the same time their shells would burst under the tidal

strain, and their glowing nuclei being disclosed would produce a great

outburst of light. Applying this theory to a “nova,” like that of 1866

in the “Northern Crown,” which had been visible as a small star before

the outbreak, and which afterward resumed its former aspect, we should

have to assume that a yet shining sun had been approached by a dark

body whose attraction temporarily burst open its photosphere. It might

be supposed that in this case the dark body was too far advanced in

cooling to suffer the same fate from the tidal pull of its victim. But

a close approach of that kind would be expected to result in the

formation of a binary system, with orbits of great eccentricity,

perhaps, and after the lapse of a certain time the outburst should be

renewed by another approximation of the two bodies. A temporary star of

that kind would rather be ranked as a variable.

The celebrated French astronomer, Janssen, had a different theory of

Nova Persei, and of temporary stars in general. According to his idea,

such phenomena might be the result of chemical changes taking place in

a sun without interference by, or collision with, another body. Janssen

was engaged for many years in trying to discover evidence of the

existence of oxygen in the sun, and he constructed his observatory on

the summit of Mount Blanc specially to pursue that research. He

believed that oxygen must surely exist in the sun since we find so many

other familiar elements included in the constitution of the solar

globe, and as he was unable to discover satisfactory evidence of its

presence he assumed that it existed in a form unknown on the earth. If

it were normally in the sun’s chromosphere, or coronal atmosphere, he

said, it would combine with the hydrogen which we know is there and

form an obscuring envelope of water vapor. It exists, then, in a

special state, uncombined with hydrogen; but let the temperature of the

sun sink to a critical point and the oxygen will assume its normal

properties and combine with the hydrogen, producing a mighty outburst

of light and heat. This, Janssen thought, might explain the phenomena

of the temporary stars. It would also, he suggested, account for their

brief career, because the combination of the elements would be quickly

accomplished, and then the resulting water vapor would form an

atmosphere cutting off the radiation from the star within.

This theory may be said to have a livelier human interest than some of

the others, since, according to it, the sun may carry in its very

constitution a menace to mankind; one does not like to think of it

being suddenly transformed into a gigantic laboratory for the explosive

combination of oxygen and hydrogen! But while Janssen’s theory might do

for some temporary stars, it is inadequate to explain all the phenomena

of Nova Persei, and particularly the appearance of the great spiral

nebula that seemed to exhale from the heart of the star. Upon the

whole, the theory of an encounter between a star and a dark nebula

seems best to fit the observations. By that hypothesis the expanding

billow of light surrounding the core of the conflagration is very well

accounted for, and the spectroscopic peculiarities are also explained.

Dr Gustov Le Bon offers a yet more alarming theory, suggesting that

temporary stars are the result of _atomic explosion;_ but we shall

touch upon this more fully in Chapter 14.

Twice in the course of this discussion we have called attention to the

change of color invariably undergone by temporary stars in the later

stages of their career. This was conspicuous with Nova Persei which

glowed more and more redly as it faded, until the nebulous light began

to overpower that of the stellar nucleus. Nothing could be more

suggestive of the dying out of a great fire. Moreover, change of color

from white to red is characteristic of all variable stars of long

period, such as “Mira” in Cetus. It is also characteristic of stars

believed to be in the later stages of evolution, and consequently

approaching extinction, like Antares and Betelgeuse, and still more

notably certain small stars which “gleam like rubies in the field of

the telescope.” These last appear to be suns in the closing period of

existence as self-luminous bodies. Between the white stars, such as

Sirius and Rigel, and the red stars, such as Aldebaran and Alpha

Herculis, there is a progressive series of colors from golden yellow

through orange to deep red. The change is believed to be due to the

increase of absorbing vapors in the stellar atmosphere as the body

cools down. In the case of ordinary stars these changes no doubt occupy

many millions of years, which represent the average duration of solar

life; but the temporary stars run through similar changes in a few

months: they resemble ephemeral insects—born in the morning and doomed

to perish with the going down of the sun.

VI

Explosive and Whirling Nebulæ

One of the most surprising triumphs of celestial photography was

Professor Keeler’s discovery, in 1899, that the great majority of the

nebulæ have a distinctly spiral form. This form, previously known in

Lord Rosse’s great “Whirlpool Nebula,” had been supposed to be

exceptional; now the photographs, far excelling telescopic views in the

revelation of nebular forms, showed the spiral to be the typical shape.

Indeed, it is a question whether all nebulæ are not to some extent

spiral. The extreme importance of this discovery is shown in the effect

that it has had upon hitherto prevailing views of solar and planetary

evolution. For more than three-quarters of a century Laplace’s

celebrated hypothesis of the manner of origin of the solar system from

a rotating and contracting nebula surrounding the sun had guided

speculation on that subject, and had been tentatively extended to cover

the evolution of systems in general. The apparent forms of some of the

nebulæ which the telescope had revealed were regarded, and by some are

still regarded, as giving visual evidence in favor of this theory.

There is a “ring nebula” in Lyra with a central star, and a “planetary

nebula” in Gemini bearing no little resemblance to the planet Saturn

with its rings, both of which appear to be practical realizations of

Laplace’s idea, and the elliptical rings surrounding the central

condensation of the Andromeda Nebula may be cited for the same kind of

proof.

[Illustration: Lord Rosse’s nebula]

But since Keeler’s discovery there has been a decided turning away of

speculation another way. The form of the spiral nebulæ seems to be

entirely inconsistent with the theory of an originally globular or

disk-shaped nebula condensing around a sun and throwing or leaving off

rings, to be subsequently shaped into planets. Some astronomers,

indeed, now reject Laplace’s hypothesis _in toto,_ preferring to think

that even our solar system originated from a spiral nebula. Since the

spiral type prevails among the existing nebulæ, we must make any

mechanical theory of the development of stars and planetary systems

from them accord with the requirements which that form imposes. A

glance at the extraordinary variations upon the spiral which Professor

Keeler’s photographs reveal is sufficient to convince one of the

difficulty of the task of basing a general theory upon them. In truth,

it is much easier to criticize Laplace’s hypothesis than to invent a

satisfactory substitute for it. If the spiral nebulæ seem to oppose it

there are other nebulæ which appear to support it, and it may be that

no one fixed theory can account for all the forms of stellar evolution

in the universe. Our particular planetary system may have originated

very much as the great French mathematician supposed, while others have

undergone, or are now undergoing, a different process of development.

There is always a too strong tendency to regard an important new

discovery and the theories and speculations based upon it as

revolutionizing knowledge, and displacing or overthrowing everything

that went before. Upon the plea that “Laplace only made a guess” more

recent guesses have been driven to extremes and treated by injudicious

exponents as “the solid facts at last.”

[Illustration: Wonderful spiral in triangulum]

Before considering more recent theories than Laplace’s, let us see what

the nature of the photographic revelations is. The vast celestial

maelstrom discovered by Lord Rosse in the “Hunting Dogs” may be taken

as the leading type of the spiral nebulæ, although there are less

conspicuous objects of the kind which, perhaps, better illustrate some

of their peculiarities. Lord Rosse’s nebula appears far more wonderful

in the photographs than in his drawings made with the aid of his giant

reflecting telescope at Parsonstown, for the photographic plate records

details that no telescope is capable of showing. Suppose we look at the

photograph of this object as any person of common sense would look at

any great and strange natural phenomenon. What is the first thing that

strikes the mind? It is certainly the appearance of violent whirling

motion. One would say that the whole glowing mass had been spun about

with tremendous velocity, or that it had been set rotating so rapidly

that it had become the victim of “centrifugal force,” one huge fragment

having broken loose and started to gyrate off into space. Closer

inspection shows that in addition to the principal focus there are

various smaller condensations scattered through the mass. These are

conspicuous in the spirals. Some of them are stellar points, and but

for the significance of their location we might suppose them to be

stars which happen to lie in a line between us and the nebula. But when

we observe how many of them follow most faithfully the curves of the

spirals we cannot but conclude that they form an essential part of the

phenomenon; it is not possible to believe that their presence in such

situations is merely fortuitous. One of the outer spirals has at least

a dozen of these star-like points strung upon it; some of them sharp,

small, and distinct, others more blurred and nebulous, suggesting

different stages of condensation. Even the part which seems to have

been flung loose from the main mass has, in addition to its central

condensation, at least one stellar point gleaming in the half-vanished

spire attached to it. Some of the more distant stars scattered around

the “whirlpool” look as if they too had been shot out of the mighty

vortex, afterward condensing into unmistakable solar bodies. There are

at least two curved rows of minute stars a little beyond the periphery

of the luminous whirl which clearly follow lines concentric with those

of the nebulous spirals. Such facts are simply dumbfounding for anyone

who will bestow sufficient thought upon them, for these are _suns,_

though they may be small ones; and what a birth is that for a sun!

Look now again at the glowing spirals. We observe that hardly have they

left the central mass before they begin to coagulate. In some places

they have a “ropy” aspect; or they are like peascods filled with

growing seeds, which eventually will become stars. The great focus

itself shows a similar tendency, especially around its circumference.

The sense that it imparts of a tremendous shattering force at work is

overwhelming. There is probably more matter in that whirling and

bursting nebula than would suffice to make a hundred solar systems! It

must be confessed at once that there is no confirmation of the

Laplacean hypothesis here; but what hypothesis will fit the facts?

There is one which it has been claimed does so, but we shall come to

that later. In the meanwhile, as a preparation, fix in the memory the

appearance of that second spiral mass spinning beside its master which

seems to have spurned it away.

[Illustration: Spiral in Ursa Major]

For a second example of the spiral nebulæ look at the one in the

constellation Triangulum. _God, how hath the imagination of puny man

failed to comprehend Thee!_ Here is creation through destruction with a

vengeance! The spiral form of the nebula is unmistakable, but it is

half obliterated amid the turmoil of flying masses hurled away on all

sides with tornadic fury. The focus itself is splitting asunder under

the intolerable strain, and in a little while, as time is reckoned in

the Cosmos, it will be gyrating into stars. And then look at the

cyclonic rain of already finished stars whirling round the outskirts of

the storm. Observe how scores of them are yet involved in the fading

streams of the nebulous spirals; see how they have been thrown into

vast loops and curves, of a beauty that half redeems the terror of the

spectacle enclosed within their lines—like iridescent cirri hovering

about the edges of a hurricane. And so again are suns born!

Let us turn to the exquisite spiral in Ursa Major; how different its

aspect from that of the other! One would say that if the terrific coil

in Triangulum has all but destroyed itself in its fury, this one on the

contrary has just begun its self-demolition. As one gazes one seems to

see in it the smooth, swift, accelerating motion that precedes

catastrophe. The central part is still intact, dense, and uniform in

texture. How graceful are the spirals that smoothly rise from its oval

rim and, gemmed with little stars, wind off into the darkness until

they have become as delicate as threads of gossamer! But at bottom the

story told here is the same—creation by gyration!

Compare with the above the curious mass in Cetus. Here the plane of the

whirling nebula nearly coincides with our line of sight and we see the

object at a low angle. It is far advanced and torn to shreds, and if we

could look at it perpendicularly to its plane it is evident that it

would closely resemble the spectacle in Triangulum.

Then take the famous Andromeda Nebula (see Frontispiece), which is so

vast that notwithstanding its immense distance even the naked eye

perceives it as an enigmatical wisp in the sky. Its image on the

sensitive plate is the masterpiece of astronomical photography; for

wild, incomprehensible beauty there is nothing that can be compared

with it. Here, if anywhere, we look upon the spectacle of creation in

one of its earliest stages. The Andromeda Nebula is apparently less

advanced toward transformation into stellar bodies than is that in

Triangulum. The immense crowd of stars sprinkled over it and its

neighborhood seem in the main to lie this side of the nebula, and

consequently to have no connection with it. But incipient stars (in

some places clusters of them) are seen in the nebulous rings, while one

or two huge masses seem to give promise of transformation into stellar

bodies of unusual magnitude. I say “rings” because although the loops

encompassing the Andromeda Nebula have been called spirals by those who

wish utterly to demolish Laplace’s hypothesis, yet they are not

manifestly such, as can be seen on comparing them with the undoubted

spirals of the Lord Rosse Nebula. They look quite as much like circles

or ellipses seen at an angle of, say, fifteen or twenty degrees to

their plane. If they are truly elliptical they accord fairly well with

Laplace’s idea, except that the scale of magnitude is stupendous, and

if the Andromeda Nebula is to become a solar system it will surpass

ours in grandeur beyond all possibility of comparison.

[Illustration: Nebula in Cetus]

There is one circumstance connected with the spiral nebulæ, and

conspicuous in the Andromeda Nebula on account of its brightness, which

makes the question of their origin still more puzzling; they all show

continuous spectra, which, as we have before remarked, indicate that

the mass from which the light comes is either solid or liquid, or a gas

under heavy pressure. Thus nebulæ fall into two classes: the “white”

nebulæ, giving a continuous spectrum; and the “green” nebulæ whose

spectra are distinctly gaseous. The Andromeda Nebula is the great

representative of the former class and the Orion Nebula of the latter.

The spectrum of the Andromeda Nebula has been interpreted to mean that

it consists not of luminous gas, but of a flock of stars so distant

that they are separately indistinguishable even with powerful

telescopes, just as the component stars of the Milky Way are

indistinguishable with the naked eye; and upon this has been based the

suggestion that what we see in Andromeda is an outer universe whose

stars form a series of elliptical garlands surrounding a central mass

of amazing richness. But this idea is unacceptable if for no other

reason than that, as just said, all the spiral nebulæ possess the same

kind of spectrum, and probably no one would be disposed to regard them

all as outer universes. As we shall see later, the peculiarity of the

spectra of the spiral nebulæ is appealed to in support of a modern

substitute for Laplace’s hypothesis.

Finally, without having by any means exhausted the variety exhibited by

the spiral nebulæ, let us turn to the great representative of the other

species, the Orion Nebula. In some ways this is even more marvelous

than the others. The early drawings with the telescope failed to convey

an adequate conception either of its sublimity or of its complication

of structure. It exists in a nebulous region of space, since

photographs show that nearly the whole constellation is interwoven with

faintly luminous coils. To behold the entry of the great nebula into

the field even of a small telescope is a startling experience which

never loses its novelty. As shown by the photographs, it is an

inscrutable chaos of perfectly amazing extent, where spiral bands,

radiating streaks, dense masses, and dark yawning gaps are strangely

intermingled without apparent order. In one place four conspicuous

little stars, better seen in a telescope than in the photograph on

account of the blurring produced by over-exposure, are suggestively

situated in the midst of a dark opening, and no observer has ever felt

any doubt that these stars have been formed from the substance of the

surrounding nebula. There are many other stars scattered over its

expanse which manifestly owe their origin to the same source. But

compare the general appearance of this nebula with the others that we

have studied, and remark the difference. If the unmistakably spiral

nebulæ resemble bursting fly-wheels or grindstones from whose

perimeters torrents of sparks are flying, the Orion Nebula rather

recalls the aspect of a cloud of smoke and fragments produced by the

explosion of a shell. This idea is enforced by the look of the outer

portion farthest from the bright half of the nebula, where sharply

edged clouds with dark spaces behind seem to be billowing away as if

driven by a wind blowing from the center.

[Illustration: The Orion nebula]

Next let us consider what scientific speculation has done in the effort

to explain these mysteries. Laplace’s hypothesis can certainly find no

standing ground either in the Orion Nebula or in those of a spiral

configuration, whatever may be its situation with respect to the grand

Nebula of Andromeda, or the “ring” and “planetary” nebulæ. Some other

hypothesis more consonant with the appearances must be found. Among the

many that have been proposed the most elaborate is the “Planetesimal

Hypothesis” of Professors Chamberlin and Moulton. It is to be remarked

that it applies to the spiral nebulæ distinctively, and not to an

apparently chaotic mass of gas like the vast luminous cloud in Orion.

The gist of the theory is that these curious objects are probably the

result of close approaches to each other of two independent suns,

reminding us of what was said on this subject when we were dealing with

temporary stars. Of the previous history of these appulsing suns the

theory gives us no account; they are simply supposed to arrive within

what may be called an effective tide-producing distance, and then the

drama begins. Some of the probable consequences of such an approach

have been noticed in Chapter 5; let us now consider them a little more

in detail.

Tides always go in couples; if there is a tide on one side of a globe

there will be a corresponding tide on the other side. The cause is to

be found in the law that the force of gravitation varies inversely as

the square of the distance; the attraction on the nearest surface of

the body exercised by another body is greater than on its center, and

greater yet than on its opposite surface. If two great globes attract

each other, each tends to draw the other out into an ellipsoidal

figure; they must be more rigid than steel to resist this—and even then

they cannot altogether resist. If they are liquid or gaseous they will

yield readily to the force of distortion, the amount of which will

depend upon their distance apart, for the nearer they are the greater

becomes the tidal strain. If they are encrusted without and liquid or

gaseous in the interior, the internal mass will strive to assume the

figure demanded by the tidal force, and will, if it can, burst the

restraining envelope. Now this is virtually the predicament of the body

we call a sun when in the immediate presence of another body of

similarly great mass. Such a body is presumably gaseous throughout, the

component gases being held in a state of rigidity by the compression

produced by the tremendous gravitational force of their own aggregate

mass. At the surface such a body is enveloped in a shell of relatively

cool matter. Now suppose a great attracting body, such as another sun,

to approach near enough for the difference in its attraction on the two

opposite sides of the body and on its center to become very great; the

consequence will be a tidal deformation of the whole body, and it will

lengthen out along the line of the gravitational pull and draw in at

the sides, and if its shell offers considerable resistance, but not

enough to exercise a complete restraint, it will be violently burst

apart, or blown to atoms, and the internal mass will leap out on the

two opposite sides in great fiery spouts. In the case of a sun further

advanced in cooling than ours the interior might be composed of molten

matter while the exterior crust had become rigid like the shell of an

egg; then the force of the “tidal explosion” produced by the appulse of

another sun would be more violent in consequence of the greater

resistance overcome. Such, then, is the mechanism of the first phase in

the history of a spiral nebula according to the Planetesimal

Hypothesis. Two suns, perhaps extinguished ones, have drawn near

together, and an explosive outburst has occured in one or both. The

second phase calls for a more agile exercise of the imagination.

To simplify the case, let us suppose that only one of the tugging suns

is seriously affected by the strain. Its vast wings produced by the

outburst are twisted into spirals by their rotation and the contending

attractions exercised upon them, as the two suns, like battleships in

desperate conflict, curve round each other, concentrating their

destructive energies. Then immense quantities of débris are scattered

about in which eddies are created, and finally, as the sun that caused

the damage goes on its way, leaving its victim to repair its injuries

as it may, the dispersed matter cools, condenses, and turns into

streams of solid particles circling in elliptical paths about their

parent sun. These particles, or fragments, are the “planetesimals” of

the theory. In consequence of the inevitable intersection of the orbits

of the planetesimals, nodes are formed where the flying particles meet,

and at these nodes large masses are gradually accumulated. The larger

the mass the greater its attraction, and at last the nodal points

become the nuclei of great aggregations from which planets are shaped.

This, in very brief form, is the Planetesimal Hypothesis which we are

asked to substitute for that based on Laplace’s suggestion as an

explanation of the mode of origin of the solar system; and the

phenomena of the spiral nebulæ are appealed to as offering evident

support to the new hypothesis. We are reminded that they are elliptical

in outline, which accords with the hypothesis; that their spectra are

not gaseous, which shows that they may be composed of solid particles

like the planetesimals; and that their central masses present an oval

form, which is what would result from the tidal effects, as just

described. We also remember that some of them, like the Lord Rosse and

the Andromeda nebulæ, are visually double, and in these cases we might

suppose that the two masses represent the tide-burst suns that ventured

into too close proximity. It may be added that the authors of the

theory do not insist upon the appulse of two suns as the _only_ way in

which the planetesimals may have originated, but it is the only

supposition that has been worked out.

But serious questions remain. It needs, for instance, but a glance at

the Triangulum monster to convince the observer that it cannot be a

solar system which is being evolved there, but rather a swarm of stars.

Many of the detached masses are too vast to admit of the supposition

that they are to be transformed into planets, in our sense of planets,

and the distances of the stars which appear to have been originally

ejected from the focal masses are too great to allow us to liken the

assemblage that they form to a solar system. Then, too, no nodes such

as the hypothesis calls for are visible. Moreover, in most of the

spiral nebulæ the appearances favor the view that the supposititious

encountering suns have not separated and gone each rejoicing on its

way, after having inflicted the maximum possible damage on its

opponent, but that, on the contrary, they remain in close association

like two wrestlers who cannot escape from each other’s grasp. And this

is exactly what the law of gravitation demands; stars cannot approach

one another with impunity, with regard either to their physical make-up

or their future independence of movement. The theory undertakes to

avoid this difficulty by assuming that in the case of our system the

approach of the foreign body to the sun was not a close one—just close

enough to produce the tidal extrusion of the relatively insignificant

quantity of matter needed to form the planets. But even then the effect

of the appulse would be to change the direction of flight, both of the

sun and of its visitor, and there is no known star in the sky which can

be selected as the sun’s probable partner in their ancient _pas deux._

That there are unconquered difficulties in Laplace’s hypothesis no one

would deny, but in simplicity of conception it is incomparably more

satisfactory, and with proper modifications could probably be made more

consonant with existing facts in our solar system than that which is

offered to replace it. Even as an explanation of the spiral nebulæ, not

as solar systems in process of formation, but as the birthplaces of

stellar clusters, the Planetesimal Hypothesis would be open to many

objections. Granting its assumptions, it has undoubtedly a strong

mathematical framework, but the trouble is not with the mathematics but

with the assumptions. Laplace was one of the ablest mathematicians that

ever lived, but he had never seen a spiral nebula; if he had, he might

have invented a hypothesis to suit its phenomena. His actual hypothesis

was intended only for our solar system, and he left it in the form of a

“note” for the consideration of his successors, with the hope that they

might be able to discover the full truth, which he confessed was hidden

from him. It cannot be said that that truth has yet been found, and

when it is found the chances are that intuition and not logic will have

led to it.

The spiral nebulæ, then, remain among the greatest riddles of the

universe, while the gaseous nebulæ, like that of Orion, are no less

mysterious, although it seems impossible to doubt that both forms give

birth to stars. It is but natural to look to them for light on the

question of the origin of our planetary system; but we should not

forget that the scale of the phenomena in the two cases is vastly

different, and the forces in operation may be equally different. A hill

may have been built up by a glacier, while a mountain may be the

product of volcanic forces or of the upheaval of the strata of the

planet.

VII

The Banners of the Sun

As all the world knows, the sun, a blinding globe pouring forth an

inconceivable quantity of light and heat, whose daily passage through

the sky is caused by the earth’s rotation on its axis, constitutes the

most important phenomenon of terrestial existence. Viewed with a dark

glass to take off the glare, or with a telescope, its rim is seen to be

a sharp and smooth circle, and nothing but dark sky is visible around

it. Except for the interference of the moon, we should probably never

have known that there is any more of the sun than our eyes ordinarily

see.

But when an eclipse of the sun occurs, caused by the interposition of

the opaque globe of the moon, we see its immediate surroundings, which

in some respects are more wonderful than the glowing central orb. These

surroundings, although not in the sense in which we apply the term to

the gaseous envelope of the earth, may be called the sun’s atmosphere.

They consist of two very different parts—first, the red “prominences,”

which resemble tongues of flame ascending thousands of miles above the

sun’s surface; and, second, the “corona,” which extends to distances of

millions of miles from the sun, and shines with a soft, glowing light.

The two combined, when well seen, make a spectacle without parallel

among the marvels of the sky. Although many attempts have been made to

render the corona visible when there is no eclipse, all have failed,

and it is to the moon alone that we owe its revelation. To cover the

sun’s disk with a circular screen will not answer the purpose because

of the illumination of the air all about the observer. When the moon

hides the sun, on the other hand, the sunlight is withdrawn from a

great cylinder of air extending to the top of the atmosphere and

spreading many miles around the observer. There is then no glare to

interfere with the spectacle, and the corona appears in all its

surprising beauty. The prominences, however, although they were

discovered during an eclipse, can now, with the aid of the

spectroscope, be seen at any time. But the prominences are rarely large

enough to be noticed by the naked eye, while the streamers of the

corona, stretching far away in space, like ghostly banners blown out

from the black circle of the obscuring moon, attract every eye, and to

this weird apparition much of the fear inspired by eclipses has been

due. But if the corona has been a cause of terror in the past it has

become a source of growing knowledge in our time.

[Illustration: The Corona]

The story of the first scientific observation of the corona and the

prominences is thrillingly interesting, and in fact dramatic. The

observation was made during the eclipse of 1842, which fortunately was

visible all over Central and Southern Europe so that scores of

astronomers saw it. The interest centers in what happened at Pavia in

Northern Italy, where the English astronomer Francis Baily had set up

his telescope. The eclipse had begun and Bailey was busy at his

telescope when, to quote his own words in the account which he wrote

for the _Memoirs of the Royal Astronomical Society:_

I was astounded by a tremendous burst of applause from the streets

below, and at the same moment was electrified by the sight of one of

the most brilliant and splendid phenomena that can well be imagined;

for at that instant the dark body of the moon was suddenly surrounded

with a corona, or kind of bright glory, similar in shape and magnitude

to that which painters draw round the heads of saints...

Pavia contains many thousand inhabitants, the major part of whom were

at this early hour walking about the streets and squares or looking out

of windows in order to witness this long-talked-of phenomenon; and when

the total obscuration took place, which was _instantaneous,_ there was

a universal shout from every observer which “made the welkin ring,” and

for the moment withdrew my attention from the object with which I was

immediately occupied. I had, indeed, expected the appearance of a

luminous circle round the moon during the time of total obscurity; but

I did not expect, from any of the accounts of preceding eclipses that I

had read, to witness so magnificent an exhibition as that which took

place...

Splendid and astonishing, however, as this remarkable phenomenon really

was, and although it could not fail to call forth the admiration and

applause of every beholder, yet I must confess that there was at the

same time something in its singular and wonderful appearance that was

appalling...

But the most remarkable circumstance attending the phenomenon was the

appearance of _three large protuberances_ apparently emanating from the

circumference of the moon, but evidently forming a portion of the

corona. They had the appearance of mountains of a prodigious elevation;

their color was red tinged with lilac or purple; perhaps the color of

the peach-blossom would more nearly represent it. They somewhat

resembled the tops of the snowy Alpine mountains when colored by the

rising or the setting sun. They resembled the Alpine mountains in

another respect, inasmuch as their light was perfectly steady, and had

none of that flickering or sparkling motion so visible in other parts

of the corona...

The whole of these protuberances were visible even to the last moment

of total obscuration, and when the first ray of light was admitted from

the sun they vanished, with the corona, altogether, and daylight was

instantly restored.

I have quoted nearly all of this remarkable description not alone for

its intrinsic interest, but because it is the best depiction that can

be found of the general phenomena of a total solar eclipse. Still, not

every such eclipse offers an equally magnificent spectacle. The

eclipses of 1900 and 1905, for instance, which were seen by the writer,

the first in South Carolina and the second in Spain, fell far short of

that described by Bailey in splendor and impressiveness. Of course,

something must be allowed for the effect of surprise; Bailey had not

expected to see what was so suddenly disclosed to him. But both in 1900

and 1905 the amount of scattered light in the sky was sufficient in

itself to make the corona appear faint, and there were no very

conspicuous prominences visible. Yet on both occasions there was

manifest among the spectators that mingling of admiration and awe of

which Bailey speaks. The South Carolinians gave a cheer and the ladies

waved their handkerchiefs when the corona, ineffably delicate of form

and texture, _melted_ into sight and then in two minutes melted away

again. The Spaniards, crowded on the citadel hill of Burgos, with their

king and his royal retinue in their midst, broke out with a great

clapping of hands as the awaited spectacle unfolded itself in the sky;

and on both occasions, before the applause began, after an awed silence

a low murmur ran through the crowds. At Burgos it is said many made the

sign of the cross.

It was not long before Bailey’s idea that the prominences were a part

of the corona was abandoned, and it was perceived that the two

phenomena were to a great extent independent. At the eclipse of 1868,

which the astronomers, aroused by the wonderful scene of 1842, and

eager to test the powers of the newly invented spectroscope, flocked to

India to witness, Janssen conceived the idea of employing the

spectroscope to render the prominences visible when there was no

eclipse. He succeeded the very next day, and these phenomena have been

studied in that way ever since.

There are recognized two kinds of prominences—the “erruptive” and the

“quiescent.” The latter, which are cloud-like in form, may be seen

almost anywhere along the edge of the sun; but the former, which often

shoot up as if hurled from mighty volcanoes, appear to be associated

with sun-spots, and appear only above the zones where spots abound.

Either of them, when seen in projection against the brilliant solar

disk, appears white, not red, as against a background of sky. The

quiescent prominences, whose elevation is often from forty thousand to

sixty thousand miles, consist, as the spectroscope shows, mainly of

hydrogen and helium. The latter, it will be remembered, is an element

which was known to be in the sun many years before the discovery that

it also exists in small quantities on the earth. A fact which may have

a significance which we cannot at present see is that the emanation

from radium gradually and spontaneously changes into helium, an

alchemistical feat of nature that has opened many curious vistas to

speculative thinkers. The eruptive prominences, which do not spread

horizontally like the others, but ascend with marvelous velocity to

elevations of half a million miles or more, are apparently composed

largely of metallic vapors—_i.e._ metals which are usually solid on the

earth, but which at solar temperatures are kept in a volatilized state.

The velocity of their ascent occasionally amounts to three hundred or

four hundred miles per second. It is known from mathematical

considerations that the gravitation of the sun would not be able to

bring back any body that started from its surface with a velocity

exceeding three hundred and eighty-three miles per second; so it is

evident that some of the matter hurled forth in eruptive prominences

may escape from solar control and go speeding out into space, cooling

and condensing into solid masses. There seems to be no reason why some

of the projectiles from the sun might not reach the planets. Here,

then, we have on a relatively small scale, _explosions_ recalling those

which it has been imagined may be the originating cause of some of the

sudden phenomena of the stellar heavens.

[Illustration: A solar “prominence.” Photographed May 21, 1907]

Of the sun-spots it is not our intention here specifically to speak,

but they evidently have an intimate connection with eruptive

prominences, as well as some relation, not yet fully understood, with

the corona. Of the real cause of sun-spots we know virtually nothing,

but recent studies by Professor Hale and others have revealed a strange

state of things in the clouds of metallic vapors floating above them

and their surroundings. Evidences of a cyclonic tendency have been

found, and Professor Hale has proved that sun-spots are strong magnetic

fields, and consist of columns of ionized vapors rotating in opposite

directions in the two hemispheres. A fact which may have the greatest

significance is that titanium and vanadium have been found both in

sun-spots and in the remarkable variable Mira Ceti, a star which every

eleven months, or thereabout, flames up with great brilliancy and then

sinks back to invisibility with the naked eye. It has been suggested

that sun-spots are indications of the beginning of a process in the sun

which will be intensified until it falls into the state of such a star

as Mira. Stars very far advanced in evolution, without showing

variability, also exhibit similar spectra; so that there is much reason

for regarding sunspots as emblems of advancing age.

The association of the corona with sun-spots is less evident than that

of the eruptive prominences; still such an association exists, for the

form and extent of the corona vary with the sun-spot period of which we

shall presently speak. The constitution of the corona remains to be

discovered. It is evidently in part gaseous, but it also probably

contains matter in the form of dust and small meteors. It includes one

substance altogether mysterious—“coronium.” There are reasons for

thinking that this may be the lightest of all the elements, and

Professor Young, its discoverer, said that it was “absolutely unique in

nature; utterly distinct from any other known form of matter,

terrestial, solar, or cosmical.” The enormous extent of the corona is

one of its riddles. Since the development of the curious subject of the

“pressure of light” it has been proposed to account for the

sustentation of the corona by supposing that it is borne upon the

billows of light continually poured out from the sun. Experiment has

proved, what mathematical considerations had previously pointed out as

probable, that the waves of light exert a pressure or driving force,

which becomes evident in its effects if the body acted upon is

sufficiently small. In that case the light pressure will prevail over

the attraction of gravitation, and propel the attenuated matter away

from the sun in the teeth of its attraction. The earth itself would be

driven away if, instead of consisting of a solid globe of immense

aggregate mass, it were a cloud of microscopic particles. The reason is

that the pressure varies in proportion to the _surface_ of the body

acted upon, while the gravitational attraction is proportional to the

_volume,_ or the total amount of matter in the body. But the surface of

any body depends upon the _square_ of its diameter, while the volume

depends upon the _cube_ of the diameter. If, for instance, the diameter

is represented by 4, the surface will be proportional to 4 × 4, or 16,

and the volume to 4 × 4 × 4, or 64; but if the diameter is taken as 2,

the surface will be 2 × 2, or 4, and the volume 2 × 2 × 2, or 8. Now,

the ratio of 4 to 8 is twice as great as that of 16 to 64. If the

diameter is still further decreased, the ratio of the surface to the

volume will proportionally grow larger; in other words, the pressure

will gain upon the attraction, and whatever their original ratio may

have been, a time will come, if the diminution of size continues, when

the pressure will become more effective than the attraction, and the

body will be driven away. Supposing the particles of the corona to be

below the critical size for the attraction of a mass like that of the

sun to control them, they would be driven off into the surrounding

space and appear around the sun like the clouds of dust around a mill.

We shall return to this subject in connection with the Zodiacal Light,

the Aurora, and Comets.

On the other hand, there are parts of the corona which suggest by their

forms the play of electric or magnetic forces. This is beautifully

shown in some of the photographs that have been made of the corona

during recent eclipses. Take, for instance, that of the eclipse of

1900. The sheaves of light emanating from the poles look precisely like

the “lines of force” surrounding the poles of a magnet. It will be

noticed in this photograph that the corona appears to consist of two

portions: one comprising the polar rays just spoken of, and the other

consisting of the broader, longer, and less-defined masses of light

extending out from the equatorial and middle-latitude zones. Yet even

in this more diffuse part of the phenomenon one can detect the presence

of submerged curves bearing more or less resemblance to those about the

poles. Just what part electricity or electro-magnetism plays in the

mechanism of the solar radiation it is impossible to say, but on the

assumption that it is a very important part is based the hypothesis

that there exists a direct solar influence not only upon the magnetism,

but upon the weather of the earth. This hypothesis has been under

discussion for half a century, and still we do not know just how much

truth it represents. It is certain that the outbreak of great

disturbances on the sun, accompanied by the formation of sun-spots and

the upshooting of eruptive prominences (phenomena which we should

naturally expect to be attended by action), have been instantly

followed by corresponding “magnetic storms” on the earth and brilliant

displays of the auroral lights. There have been occasions when the

influence has manifested itself in the most startling ways, a great

solar outburst being followed by a mysterious gripping of the cable and

telegraph systems of the world, as if an invisible and irresistible

hand had seized them. Messages are abruptly cut off, sparks leap from

the telegraph instruments, and the entire earth seems to have been

thrown into a magnetic flurry. These occurrences affect the mind with a

deep impression of the dependence of our planet on the sun, such as we

do not derive from the more familiar action of the sunlight on the

growth of plants and other phenomena of life depending on solar

influences.

Perhaps the theory of solar magnetic influence upon the weather is best

known in connection with the “sun-spot cycle.” This, at any rate, is,

as already remarked, closely associated with the corona. Its existence

was discovered in 1843 by the German astronomer Schwabe. It is a period

of variable length, averaging about eleven years, during which the

number of spots visible on the sun first increases to a maximum, then

diminishes to a minimum, and finally increases again to a maximum. For

unknown reasons the period is sometimes two or three years longer than

the average and sometimes as much shorter. Nevertheless, the phenomena

always recur in the same order. Starting, for instance, with a time

when the observer can find few or no spots, they gradually increase in

number and size until a maximum, in both senses, is reached, during

which the spots are often of enormous size and exceedingly active.

After two or three years they begin to diminish in number, magnitude,

and activity until they almost or quite disappear. A strange fact is

that when a new period opens, the spots appear first in high northern

and southern latitudes, far from the solar equator, and as the period

advances they not only increase in number and size, but break out

nearer and nearer to the equator, the last spots of a vanishing period

sometimes lingering in the equatorial region after the advance-guard of

its successor has made its appearance in the high latitudes. Spots are

never seen on the equator nor near the poles. It was not very long

after the discovery of the sun-spot cycle that the curious observation

was made that a striking coincidence existed between the period of the

sun-spots and another period affecting the general magnetic condition

of the earth. When a curved line representing the varying number of

sun-spots was compared with another curve showing the variations in the

magnetic state of the earth the two were seen to be in almost exact

accord, a rise in one curve corresponding to a rise in the other, and a

fall to a fall. Continued observation has proved that this is a real

coincidence and not an accidental one, so that the connection, although

as yet unexplained, is accepted as established. But does the influence

extend further, and directly affect the weather and the seasons as well

as the magnetic elements of the earth? A final answer to this question

cannot yet be given, for the evidence is contradictory, and the

interpretations put upon it depend largely on the predilections of the

judges.

But, in a broad sense, the sun-spots and the phenomena connected with

them _must_ have a relation to terrestial meteorology, for they prove

the sun to be a variable star. Reference was made, a few lines above,

to the resemblance of the spectra of sun-spots to those of certain

stars which seem to be failing through age. This in itself is extremely

suggestive; but if this resemblance had never been discovered, we

should have been justified in regarding the sun as variable in its

output of energy; and not only variable, but probably increasingly so.

The very inequalities in the sun-spot cycle are suspicious. When the

sun is most spotted its total light may be reduced by one-thousandth

part, although it is by no means certain that its outgiving of thermal

radiations is then reduced. A loss of one-thousandth of its luminosity

would correspond to a decrease of .0025 of a stellar magnitude,

considering the sun as a star viewed from distant space. So slight a

change would not be perceptible; but it is not alone sun-spots which

obscure the solar surface, its entire globe is enveloped with an

obscuring veil. When studied with a powerful telescope the sun’s

surface is seen to be thickly mottled with relatively obscure specks,

so numerous that it has been estimated that they cut off from one-tenth

to one-twentieth of the light that we should receive from it if the

whole surface were as brilliant as its brightest parts. The condition

of other stars warrants the conclusion that this obscuring envelope is

the product of a process of refrigeration which will gradually make the

sun more and more variable until its history ends in extinction.

Looking backward, we see a time when the sun must have been more

brilliant than it is now. At that time it probably shone with the

blinding white splendor of such stars as Sirius, Spica, and Vega; now

it resembles the relatively dull Procyon; in time it will turn ruddy

and fall into the closing cycle represented by Antares. Considering

that once it must have been more radiantly powerful than at present,

one is tempted to wonder if that could have been the time when tropical

life flourished within the earth’s polar circles, sustained by a

vivific energy in the sun which it has now lost.

The corona, as we have said, varies with the sun-spot cycle. When the

spots are abundant and active the corona rises strong above the spotted

zones, forming immense beams or streamers, which on one occasion, at

least, had an observed length of _ten million miles._ At the time of a

spot minimum the corona is less brilliant and has a different outline.

It is then that the curved polar rays are most conspicuous. Thus the

vast banners of the sun, shaken out in the eclipse, are signals to tell

of its varying state, but it will probably be long before we can read

correctly their messages.

VIII

The Zodiacal Light Mystery

There is a singular phenomenon in the sky—one of the most puzzling of

all—which has long arrested the attention of astronomers, defying their

efforts at explanation, but which probably not one in a hundred, and

possibly not one in a thousand, of the readers of this book has ever

seen. Yet its name is often spoken, and it is a conspicuous object if

one knows when and where to look for it, and when well seen it exhibits

a mystical beauty which at the same time charms and awes the beholder.

It is called “The Zodiacal Light,” because it lies within the broad

circle of the Zodiac, marking the sun’s apparent annual path through

the stars. What it is nobody has yet been able to find out with

certainty, and books on astronomy usually speak of it with singular

reserve. But it has given rise to many remarkable theories, and a true

explanation of it would probably throw light on a great many other

celestial mysteries. The Milky Way is a more wonderful object to look

upon, but its nature can be comprehended, while there is a sort of

uncanniness about the Zodiacal Light which immediately impresses one

upon seeing it, for its part in the great scheme of extra-terrestrial

affairs is not evident.

If you are out-of-doors soon after sunset—say, on an evening late in

the month of February—you may perceive, just after the angry flush of

the dying winter’s day has faded from the sky, a pale ghostly presence

rising above the place where the sun went down. The writer remembers

from boyhood the first time it was pointed out to him and the unearthly

impression that it made, so that he afterward avoided being out alone

at night, fearful of seeing the spectral thing again. The phenomenon

brightens slowly with the fading of the twilight, and soon distinctly

assumes the shape of an elongated pyramid of pearly light, leaning

toward the south if the place of observation is in the northern

hemisphere. It does not impress the observer at all in the same manner

as the Milky Way; that looks far off and is clearly among the stars,

but the Zodiacal Light seems closer at hand, as if it were something

more intimately concerning the earth. To all it immediately suggests a

connection, also, with the sunken sun. If the night is clear and the

moon absent (and if you are in the country, for city lights ruin the

spectacles of the sky), you will be able to watch the apparition for a

long time. You will observe that the light is brightest near the

horizon, gradually fading as the pyramidal beam mounts higher, but in

favorable circumstances it may be traced nearly to the meridian south

of the zenith, where its apex at last vanishes in the starlight. It

continues visible during the evenings of March and part of April, after

which, ordinarily, it is seen no more, or if seen is relatively faint

and unimpressive. But when autumn comes it appears again, this time not

like a wraith hovering above the westward tomb of the day-god, but

rather like a spirit of the morning announcing his reincarnation in the

east.

The reason why the Zodiacal Light is best seen in our latitudes at the

periods just mentioned is because at those times the Zodiac is more

nearly perpendicular to the horizon, first in the west and then in the

east; and, since the phenomenon is confined within the borders of the

Zodiac, it cannot be favorably placed for observation when the zodiacal

plane is but slightly inclined to the horizon. Its faint light requires

the contrast of a background of dark sky in order to be readily

perceptible. But within the tropics, where the Zodiac is always at a

favorable angle, the mysterious light is more constantly visible.

Nearly all observant travelers in the equatorial regions have taken

particular note of this phenomenon, for being so much more conspicuous

there than in the temperate zones it at once catches the eye and holds

the attention as a novelty. Humboldt mentions it many times in his

works, for his genius was always attracted by things out of the

ordinary and difficult of explanation, and he made many careful

observations on its shape, its brilliancy, and its variations; for

there can be no doubt that it does vary, and sometimes to an

astonishing degree. It is said that it once remained practically

invisible in Europe for several years in succession. During a trip to

South Africa in 1909 an English astronomer, Mr E. W. Maunder, found a

remarkable difference between the appearance of the Zodiacal Light on

his going and coming voyages. In fact, when crossing the equator going

south he did not see it at all; but on returning he had, on March 6th,

when one degree south of the equator, a memorable view of it.

It was a bright, clear night, and the Zodiacal Light was

extraordinarily brilliant—brighter than he had ever seen it before. The

Milky Way was not to be compared with it. The brightest part extended

75° from the sun. There was a faint and much narrower extension which

they could just make out beyond the Pleiades along the ecliptic, but

the greater part of the Zodiacal Light showed as a broad truncated

column, and it did not appear nearly as conical as he had before seen

it.

When out of the brief twilight of intertropical lands, where the sun

drops vertically to the horizon and night rushes on like a wave of

darkness, the Zodiacal Light shoots to the very zenith, its color is

described as a golden tint, entirely different from the silvery sheen

of the Milky Way. If I may venture again to refer to personal

experiences and impressions, I will recall a view of the Zodiacal Light

from the summit of the cone of Mt Etna in the autumn of the year 1896

(more briefly described in _Astronomy with the Naked Eye_). There are

few lofty mountains so favorably placed as Etna for observations of

this kind. It was once resorted to by Prof. George E. Hale, in an

attempt to see the solar corona without an eclipse. Rising directly

from sea-level to an elevation of nearly eleven thousand feet, the

observer on its summit at night finds himself, as it were, lost in the

midst of the sky. But for the black flanks of the great cone on which

he stands he might fancy himself to be in a balloon. On the occasion to

which I refer the world beneath was virtually invisible in the moonless

night. The blaze of the constellations overhead was astonishingly

brilliant, yet amid all their magnificence my attention was immediately

drawn to a great tapering light that sprang from the place on the

horizon where the sun would rise later, and that seemed to be blown out

over the stars like a long, luminous veil. It was the finest view of

the Zodiacal light that I had ever enjoyed—thrilling in its

strangeness—but I was almost disheartened by the indifference of my

guide, to whom it was only a light and nothing more. If he had no

science, he had less poetry—rather a remarkable thing, I thought, for a

child of his clime. The Light appeared to me to be distinctly brighter

than the visible part of the Milky Way which included the brilliant

stretches in Auriga and Perseus, and its color, if one may speak of

color in connection with such an object, seemed richer than that of the

galactic band; but I did not think of it as yellow, although Humboldt

has described it as resembling a golden curtain drawn over the stars,

and Du Chaillu in Equatorial Africa found it of a bright yellow color.

It may vary in color as in conspicuousness. The fascination of that

extraordinary sight has never faded from my memory. I turned to regard

it again and again, although I had never seen the stellar heavens so

brilliant, and it was one of the last things I looked for when the

morning glow began softly to mount in the east, and Sicily and the

Mediterranean slowly emerged from the profound shadow beneath us.

The Zodiacal Light seems never to have attracted from astronomers in

general the amount of careful attention that it deserves; perhaps

because so little can really be made of it as far as explanation is

concerned. I have referred to the restraint that scientific writers

apparently feel in speaking of it. The grounds for speculation that it

affords may be too scanty to lead to long discussions, yet it piques

curiosity, and as we shall see in a moment has finally led to a most

interesting theory. Once it was the subject of an elaborate series of

studies which carried the observer all round the world. That was in

1845—46, during the United States Exploring Expedition that visited the

then little known Japan. The chaplain of the fleet, the Rev. Mr Jones,

went out prepared to study the mysterious light in all its phases. He

saw it from many latitudes on both sides of the equator, and the

imagination cannot but follow him with keen interest in his

world-circling tour, keeping his eyes every night fixed upon the

phantasm overhead, whose position shifted with that of the hidden sun.

He demonstrated that the flow extends at times completely across the

celestial dome, although it is relatively faint directly behind the

earth. On his return the government published a large volume of his

observations, in which he undertook to show that the phenomenon was due

to the reflection of sunlight from a ring of meteoric bodies encircling

the earth. But, after all, this elaborate investigation settled

nothing.

Prof. E. E. Barnard has more recently devoted much attention to the

Zodiacal Light, as well as to a strange attendant phenomenon called the

“Gegenschein,” or Counterglow, because it always appears at that point

in the sky which is exactly opposite the sun. The Gegenschein is an

extremely elusive phenomenon, suitable only for eyes that have been

specially trained to see it. Professor Newcomb has cautiously remarked

that

it is said that in that point of the heavens directly opposite the sun

there is an elliptical patch of light... This phenomenon is so

difficult to account for that its existence is sometimes doubted; yet

the testimony in its favor is difficult to set aside.

It certainly cannot be set aside at all since the observations of

Barnard. I recall an attempt to see it under his guidance during a

visit to Mount Hamilton, when he was occupied there with the Lick

telescope. Of course, both the Gegenschein and the Zodiacal Light are

too diffuse to be studied with telescopes, which, so to speak, magnify

them out of existence. They can only be successfully studied with the

naked eye, since every faintest glimmer that they afford must be

utilized. This is especially true of the Gegenschein. At Mount

Hamilton, Mr Barnard pointed out to me its location with reference to

certain stars, but with all my gazing I could not be sure that I saw

it. To him, on the contrary, it was obvious; he had studied it for

months, and was able to indicate its shape, its boundaries, its

diameter, and the declination of its center with regard to the

ecliptic. There is not, of course, the shadow of a doubt of the

existence of the Gegenschein, and yet I question if one person in a

million has ever seen or ever will see it. The Zodiacal Light, on the

other hand, is plain enough, provided that the time and the

circumstances of the observation are properly chosen.

In the attempts to explain the Zodiacal Light, the favorite hypothesis

has been that it is an appendage of the sun—perhaps simply an extension

of the corona in the plane of the ecliptic, which is not very far from

coinciding with that of the sun’s equator. This idea is quite a natural

one, because of the evident relation of the light to the position of

the sun. The vast extension of the equatorial wings of the corona in

1878 gave apparent support to this hypothesis; if the substance of the

corona could extend ten million miles from the sun, why might it not

extend even one hundred million, gradually fading out beyond the orbit

of the earth? A variation of this hypothesis assumes that the

reflection is due to swarms of meteors circling about the sun, in the

plane of its equator, all the way from its immediate neighborhood to a

distance exceeding that of the earth. But in neither form is the

hypothesis satisfactory; there is nothing in the appearance of the

corona to indicate that it extends even as far as the planet Mercury,

while as to meteors, the orbits of the known swarms do not accord with

the hypothesis, and we have no reason to believe that clouds of others

exist traveling in the part of space where they would have to be in

order to answer the requirements of the theory. The extension of the

corona in 1878 did not resemble in its texture the Zodiacal Light.

Now, it has so often happened in the history of science that an

important discovery in one branch has thrown unexpected but most

welcome light upon some pending problem in some other branch, that a

strong argument might be based upon that fact alone against the too

exclusive devotion of many investigators to the narrow lines of their

own particular specialty; and the Zodiacal Light affords a case in

point, when it is considered in connection with recent discoveries in

chemistry and physics. From the fact that atoms are compound bodies

made up of corpuscles at least a thousand times smaller than the

smallest known atom—a fact which astounded most men of science when it

was announced a few years ago—a new hypothesis has been developed

concerning the nature of the Zodiacal Light (as well as other

astronomical riddles), and this hypothesis comes not from an

astronomer, but from a chemist and physicist, the Swede, Svante

Arrhenius. In considering an outline of this new hypothesis we need

neither accept nor reject it; it is a case rather for suspension of

judgment.

To begin with, it carries us back to the “pressure of light” mentioned

in the preceding chapter. The manner in which this pressure is believed

generally to act was there sufficiently explained, and it only remains

to see how it is theoretically extended to the particles of matter

supposed to constitute the Zodiacal Light. We know that corpuscles, or

“fragments of atoms” negatively electrified, are discharged from hot

bodies. Streams of these “ions” pour from many flames and from molten

metals; and the impact of the cathode and ultra-violet rays causes them

to gush even from cold bodies. In the vast laboratory of the sun it is

but reasonable to suppose that similar processes are taking place. “As

a very hot metal emits these corpuscles,” says Prof. J. J. Thomson, “it

does not seem an improbable hypothesis that they are emitted by that

very hot body, the sun.” Let it be assumed, then, that the sun does

emit them; what happens next? Negatively charged corpuscles, it is

known, serve as nuclei to which particles of matter in the ordinary

state are attracted, and it is probable that those emitted from the sun

immediately pick up loads in this manner and so grow in bulk. If they

grow large enough the gravitation of the sun draws them back, and they

produce a negative charge in the solar atmosphere. But it is probable

that many of the particles do not attain the critical size which,

according to the principles before explained, would enable the

gravitation of the sun to retain them in opposition to the pressure of

the waves of light, and with these particles the light pressure is

dominant. Clouds of them may be supposed to be continually swept away

from the sun into surrounding space, moving mostly in or near the plane

of the solar equator, where the greatest activity, as indicated by

sunspots and related phenomena, is taking place. As they pass outward

into space many of them encounter the earth. If the earth, like the

moon, had no atmosphere the particles would impinge directly on its

surface, giving it a negative electric charge. But the presence of the

atmosphere changes all that, for the first of the flying particles that

encounter it impart to it their negative electricity, and then, since

like electric charges repel like, the storm of particles following will

be sheered off from the earth, and will stream around it in a maze of

hyperbolic paths. Those that continue on into space beyond the earth

may be expected to continue picking up wandering particles of matter

until their bulk has become so great that the solar attraction prevails

again over the light pressure acting upon them, and they turn again

sunward. Passing the earth on their return they will increase the

amount of dust-clouds careering round it; and these will be further

increased by the action of the ultra-violet rays of the sunlight

causing particles to shoot radially away from the earth when the

negative charge of the upper atmosphere has reached a certain amount,

which particles, although starting sunward, will be swept back to the

earth with the oncoming streams. As the final result of all this

accumulation of flying and gyrating particles in the earth’s

neighborhood, we are told that the latter must be transformed into the

semblance of a gigantic solid-headed comet provided with streaming

tails, the longest of them stretching away from the direction of the

sun, while another shorter one extends toward the sun. This shorter

tail is due to the particles that we have just spoken of as being

driven sunward from the earth by the action of ultra-violet light. No

doubt this whole subject is too technical for popular statement; but at

any rate the general reader can understand the picturesque side of the

theory, for its advocates assure us that if we were on the moon we

would doubtless be able to see the comet-like tails of the earth, and

then we could appreciate the part that they play in producing the

phenomenon of the Zodiacal Light.

That the Light as we see it could be produced by the reflection of

sunlight from swarms of particles careering round the earth in the

manner supposed by Arrhenius’ hypothesis is evident enough; and it will

be observed that the new theory, after all, is only another variant of

the older one which attributes the Zodiacal Light to an extension of

the solar corona. But it differs from the older theory in offering an

explanation of the manner in which the extension is effected, and it

differentiates between the corona proper and the streams of negative

particles shot away from the sun. In its details the hypothesis of

Arrhenius also affords an explanation of many peculiarities of the

Zodiacal Light, such as that it is confined to the neighborhood of the

ecliptic, and that it is stronger on the side of the earth which is

just turning away from a position under the sun than on the other side;

but it would carry us beyond our limits to go into these particulars.

The Gegenschein, according to this theory, is a part of the same

phenomenon as the Zodiacal Light, for by the laws of perspective it is

evident that the reflection from the streams of particles situated at a

point directly opposite to the sun would be at a maximum, and this is

the place which the Gegenschein occupies. Apart from its geometrical

relations to the position of the sun, the variability of the Zodiacal

Light appears to affirm its solar dependence, and this too would be

accounted for by Arrhenius’ hypothesis better than by the old theory of

coronal extension. The amount of corpuscular discharge from the sun

must naturally be governed by the state of relative activity or

inactivity of the latter, and this could not but be reflected in the

varying splendor of the Zodiacal Light. But much more extended study

than has yet been given to the subject will be required before we can

feel that we know with reasonable certainty what this mysterious

phenomenon really is. By the hypothesis of Arrhenius every planet that

has an atmosphere must have a Zodiacal Light attending it, but the

phenomenon is too faint for us to be able to see it in the case, for

instance, of Venus, whose atmosphere is very abundant. The moon has no

corresponding “comet’s tail” because, as already explained, of the lack

of a lunar atmosphere to repel the streams by becoming itself

electrified; but if there were a lunar Zodiacal Light, no doubt we

could see it because of the relative nearness of our satellite.

IX

Marvels of the Aurora

One of the most vivid recollections of my early boyhood is that of

seeing my father return hastily into the house one evening and call out

to the family: “Come outside and look at the sky!” Ours was a country

house situated on a commanding site, and as we all emerged from the

doorway we were dumbfounded to see the heavens filled with pale flames

which ran licking and quivering over the stars. Instantly there sprang

into my terrified mind the recollection of an awful description of “the

Day of Judgment” (the _Dies Iræ_), which I had heard with much

perturbation of spirit in the Dutch Reformed church from the lips of a

tall, dark-browed, dreadfully-in-earnest preacher of the old-fashioned

type. My heart literally sank at sight of the spectacle, for it

recalled the preacher’s very words; it was just as he had said it would

be, and it needed the assured bearing of my elders finally to convince

me that

That Day of Wrath, O dreadful day,

When Heaven and Earth shall pass away,

As David and the Sibyl say

had not actually come upon us. And even the older members of the

household were not untouched with misgivings when menacing spots of

crimson appeared, breaking out now here, now there, in the shuddering

sky. Toward the north the spectacle was appalling. A huge arch spanned

an unnaturally dark segment resting on the horizon, and above this arch

sprang up beams and streamers in a state of incessant agitation,

sometimes shooting up to the zenith with a velocity that took one’s

breath, and sometimes suddenly falling into long ranks, and _marching,

marching, marching,_ like an endless phalanx of fiery specters, and

moving, as I remember, always from east to west. The absolute silence

with which these mysterious evolutions were performed and the quavering

reflections which were thrown upon the ground increased the awfulness

of the exhibition. Occasionally enormous curtains of lambent flame

rolled and unrolled with a majestic motion, or were shaken to and fro

as if by a mighty, noiseless wind. At times, too, a sudden billowing

rush would be made toward the zenith, and for a minute the sky overhead

would glow so brightly that the stars seemed to have been consumed. The

spectacle continued with varying intensity for hours.

[Illustration: Auroral beams seen in England]

This exhibition occurred in Central New York, a latitude in which the

Aurora Borealis is seldom seen with so much splendor. I remember

another similar one seen from the city of New York in November, 1882.

On this last occasion some observers saw a great upright beam of light

which majestically moved across the heavens, stalking like an

apparition in the midst of the auroral pageant, of whose general

movements it seemed to be independent, maintaining always its upright

posture, and following a magnetic parallel from east to west. This

mysterious beam was seen by no less than twenty-six observers in

different parts of the country, and a comparison of their observations

led to a curious calculation indicating that the apparition was about

_one hundred and thirty-three miles tall_ and moved at the speed of ten

miles per second!

[Illustration: Auroral arches of an eliptic form seen in the Arctic

regions]

But, as everybody knows, it is in the Arctic regions that the Aurora,

or the “Northern Lights,” can best be seen. There, in the long polar

night, when for months together the sun does not rise, the strange

coruscations in the sky often afford a kind of spectral daylight in

unison with the weird scenery of the world of ice. The pages in the

narratives of Arctic exploration that are devoted to descriptions of

the wonderful effects of the Northern Lights are second to none that

man has ever penned in their fascination. The lights, as I have already

intimated, display astonishing colors, particularly shades of red and

green, as they flit from place to place in the sky. The discovery that

the magnetic needle is affected by the Aurora, quivering and darting

about in a state of extraordinary excitement when the lights are

playing in the sky, only added to the mystery of the phenomenon until

its electro-magnetic nature had been established. This became evident

as soon as it was known that the focus of the displays was the magnetic

pole; and when the far South was visited the Aurora Australis was

found, having its center at the South Magnetic Pole. Then, if not

before, it was clear that the earth was a great globular magnet, having

its poles of opposite magnetism, and that the auroral lights, whatever

their precise cause might be, were manifestations of the magnetic

activity of our planet. After the invention of magnetic telegraphy it

was found that whenever a great Aurora occurred the telegraph lines

were interrupted in their operation, and the ocean cables ceased to

work. Such a phenomenon is called a “magnetic storm.”

The interest excited by the Aurora in scientific circles was greatly

stimulated when, in the last half of the nineteenth century, it was

discovered that it is a phenomenon intimately associated with

disturbances on the sun. The ancient “Zurich Chronicles,” extending

from the year 1000 to the year 1800, in which both sun-spots visible to

the naked eye and great displays of the auroral lights were recorded,

first set Rudolf Wolf on the track of this discovery. The first notable

proof of the suspected connection was furnished with dramatic emphasis

by an occurrence which happened on September 1, 1859. Near noon on that

day two intensely brilliant points suddenly broke out in a group of

sun-spots which were under observation by Mr R. C. Carrington at his

observatory at Redhill, England. The points remained visible for not

more than five minutes, during which interval they moved _thirty-five

thousand miles_ across the solar disk. Mr R. Hodgson happened to see

the same phenomenon at his observatory at Highgate, and thus all

possibility of deception was removed. But neither of the startled

observers could have anticipated what was to follow, and, indeed, it

was an occurrence which has never been precisely duplicated. I quote

the eloquent account given by Miss Clerke in her _History of Astronomy

During the Nineteenth Century._

This unique phenomenon seemed as if specially designed to accentuate

the inference of a sympathetic relation between the earth and the sun.

From August 28 to September 4, 1859, a magnetic storm of unparalleled

intensity, extent, and duration was in progress over the entire globe.

Telegraphic communication was everywhere interrupted—except, indeed,

that it was in some cases found practicable to work the lines _without

batteries_ by the agency of the earth-currents alone; sparks issued

from the wires; gorgeous auroras draped the skies in solemn crimson

over both hemispheres, and even in the tropics; the magnetic needle

lost all trace of continuity in its movements and darted to and fro as

if stricken with inexplicable panic. The coincidence was even closer.

_At the very instant_ of the solar outburst witnessed by Carrington and

Hodgson the photographic apparatus at Kew registered a marked

disturbance of all the three magnetic elements; while shortly after the

ensuing midnight the electric agitation culminated, thrilling the whole

earth with subtle vibrations, and lighting up the atmosphere from pole

to pole with coruscating splendors which perhaps dimly recall the times

when our ancient planet itself shone as a star.

If this amazing occurrence stood alone, and as I have already said it

has never been exactly duplicated, doubt might be felt concerning some

of the inferences drawn from it; but in varying forms it has been

repeated many times, so that now hardly anyone questions the reality of

the assumed connection between solar outbursts and magnetic storms

accompanied by auroral displays on the earth. It is true that the late

Lord Kelvin raised difficulties in the way of the hypothesis of a

direct magnetic action of the sun upon the earth, because it seemed to

him that an inadmissible quantity of energy was demanded to account for

such action. But no calculation like that which he made is final, since

all calculations depend upon the validity of the data; and no authority

is unshakable in science, because no man can possess omniscience. It

was Lord Kelvin who, but a few years before the thing was actually

accomplished, declared that aerial navigation was an impracticable

dream, and demonstrated its impracticability by calculation. However

the connection may be brought about, it is as certain as evidence can

make it that solar outbursts are coincident with terrestial magnetic

disturbances, and coincident in such a way as to make the inference of

a causal connection irresistible. The sun is only a little more than a

hundred times its own diameter away from the earth. Why, then, with the

subtle connection between them afforded by the ether which conveys to

us the blinding solar light and the life-sustaining solar heat, should

it be so difficult to believe that the sun’s enormous electric energies

find a way to us also? No doubt the impulse coming from the sun acts

upon the earth after the manner of a touch upon a trigger, releasing

energies which are already stored up in our planet.

But besides the evidence afforded by such occurrences as have been

related of an intimate connection between solar outbreaks and

terrestial magnetic flurries, attended by magnificent auroral displays,

there is another line of proof pointing in the same direction. Thus, it

is known that the sun-spot period, as remarked in a preceding chapter,

coincides in a most remarkable manner with the periodic fluctuations in

the magnetic state of the earth. This coincidence runs into the most

astonishing details. For instance, when the sun-spot period shortens,

the auroral period shortens to precisely the same extent; as the short

sun-spot periods usually bring the most intense outbreaks of solar

activity, so the corresponding short auroral periods are attended by

the most violent magnetic storms; a secular period of about two hundred

and twenty-two years affecting sun-spots is said to have its auroral

duplicate; a shorter period of fifty-five and a half years, which some

observers believe that they have discovered appears also to be common

to the two phenomena; and yet another “superposed” period of about

thirty-five years, which some investigators aver exists, affects

sun-spots and aurora alike. In short, the coincidences are so numerous

and significant that one would have to throw the doctrine of

probability to the winds in order to be able to reject the conclusion

to which they so plainly lead.

[Illustration: Auroral curtain seen in Scandinavia]

But still the question recurs: How is the influence transmitted? Here

Arrhenius comes once more with his hypothesis of negative corpuscles,

or ions, driven away from the sun by light-pressure—a hypothesis which

seems to explain so many things—and offers it also as an explanation of

the way in which the sun creates the Aurora. He would give the Aurora

the same lineage with the Zodiacal Light. To understand the application

of this theory we must first recall the fact that the earth is a great

magnet having its two opposite poles of magnetism, one near the Arctic

and the other near the Antarctic Circle. Like all magnets, the earth is

surrounded with “lines of force,” which, after the manner of the curved

rays we saw in the photograph of a solar eclipse, start from a pole,

rising at first nearly vertically, then bend gradually over, passing

high above the equator, and finally descending in converging sheaves to

the opposite pole. Now the axis of the earth is so placed in space that

it lies at nearly a right angle to the direction of the sun, and as the

streams of negatively charged particles come pouring on from the sun

(see the last preceding chapter), they arrive in the greatest numbers

over the earth’s equatorial regions. There they encounter the lines of

magnetic force at the place where the latter have their greatest

elevation above the earth, and where their direction is horizontal to

the earth’s surface. Obeying a law which has been demonstrated in the

laboratory, the particles then follow the lines of force toward the

poles. While they are above the equatorial regions they do not become

luminescent, because at the great elevation that they there occupy

there is virtually no atmosphere; but as they pass on toward the north

and the south they begin to descend with the lines of force, curving

down to meet at the poles; and, encountering a part of the atmosphere

comparable in density with what remains in an exhausted Crookes tube,

they produce a glow of cathode rays. This glow is conceived to

represent the Aurora, which may consequently be likened to a gigantic

exhibition of vacuum-tube lights. Anybody who recalls his student days

in the college laboratory and who has witnessed a display of Northern

Lights will at once recognize the resemblance between them in colors,

forms, and behavior. This resemblance had often been noted before

Arrhenius elaborated his hypothesis.

Without intending to treat his interesting theory as more than a

possibly correct explanation of the phenomena of the Aurora, we may

call attention to some apparently confirmatory facts. One of the most

striking of these relates to a seasonal variation in the average number

of auroræ. It has been observed that there are more in March and

September than at any other time of the year, and fewer in June and

December; moreover (and this is a delicate test as applied to the

theory), they are slightly rarer in June than in December. Now all

these facts seem to find a ready explanation in the hypothesis of

Arrhenius, thus: (1) The particles issuing from the sun are supposed to

come principally from the regions whose excitement is indicated by the

presence of sun-spots (which accords with Hale’s observation that

sun-spots are columns of ionized vapors), and these regions have a

definite location on either side of the solar equator, seldom

approaching it nearer than within 5° or 10° north or south, and never

extending much beyond 35° toward either pole; (2) The equator of the

sun is inclined about 7° to the plane of the earth’s orbit, from which

it results that twice in a year—_viz.,_ in June and December—the earth

is directly over the solar equator, and twice a year—_viz.,_ in March

and September—when it is farthest north or south of the solar equator,

it is over the inner edge of the sun-spot belts. Since the corpuscles

must be supposed to be propelled radially from the sun, few will reach

the earth when the latter is over the solar equator in June and

December, but when it is over, or nearly over, the spot belts, in March

and September, it will be in the line of fire of the more active parts

of the solar surface, and relatively rich streams of particles will

reach it. This, as will be seen from what has been said above, is in

strict accord with the observed variations in the frequency of auroræ.

Even the fact that somewhat fewer auroræ are seen in June than in

December also finds its explanation in the known fact that the earth is

about three million miles nearer the sun in the winter than in the

summer, and the number of particles reaching it will vary, like the

intensity of light, inversely as the square of the distance. These

coincidences are certainly very striking, and they have a cumulative

force. If we accept the theory, it would appear that we ought to

congratulate ourselves that the inclination of the sun’s equator is so

slight, for as things stand the earth is never directly over the most

active regions of the sun-spots, and consequently never suffers from

the maximum bombardment of charged particles of which the sun is

capable. Incessant auroral displays, with their undulating draperies,

flitting colors, and marching columns might not be objectionable from

the point of view of picturesqueness, but one magnetic storm of extreme

intensity following closely upon the heels of another, for months on

end, crazing the magnetic needle and continually putting the telegraph

and cable lines out of commission, to say nothing of their effect upon

“wireless telegraphy”, would hardly add to the charms of terrestrial

existence.

[Illustration: Auroral arches seen in Scandinavia]

One or two other curious points in connection with Arrhenius’

hypothesis may be mentioned. First, the number of auroræ, according to

his explanation, ought to be greatest in the daytime, when the face of

the earth on the sunward side is directly exposed to the atomic

bombardment. Of course visual observation can give us no information

about this, since the light of the Aurora is never sufficiently intense

to be visible in the presence of daylight, but the records of the

magnetic observatories can be, and have been, appealed to for

information, and they indicate that the facts actually accord with the

theory. Behind the veil of sunlight in the middle of the afternoon,

there is good reason to believe, auroral exhibitions often take place

which would eclipse in magnificence those seen at night if we could

behold them. Observation shows, too, that auroræ are more frequent

before than after midnight, which is just what we should expect if they

originate in the way that Arrhenius supposes. Second, the theory offers

an explanation of the alleged fact that the formation of clouds in the

upper air is more frequent in years when auroræ are most abundant,

because clouds are the result of the condensation of moisture upon

floating particles in the atmosphere (in an absolutely dustless

atmosphere there would be no clouds), and it has been proved that

negative ions like those supposed to come from the sun play a master

part in the phenomena of cloud formation.

Yet another singular fact, almost mystical in its suggestions, may be

mentioned. It seems that the dance of the auroral lights occurs most

frequently during the absence of the moon from the hemisphere in which

they appear, and that they flee, in greater part, to the opposite

hemisphere when the moon’s revolution in an orbit considerably inclined

to the earth’s equator brings her into that where they have been

performing. Arrhenius himself discovered this curious relation of

auroral frequency to the position of the moon north or south of the

equator, and he explains it in this way. The moon, like the earth, is

exposed to the influx of the ions from the sun; but having no

atmosphere, or almost none, to interfere with them, they descend

directly upon her surface and charge her with an electric negative

potential to a very high degree. In consequence of this she affects the

electric state of the upper parts of the earth’s atmosphere where they

lie most directly beneath her, and thus prevents, to a large extent,

the negative discharges to which the appearance of the Aurora is due.

And so “the extravagant and erring spirit” of the Aurora avoids the

moon as Hamlet’s ghost fled at the voice of the cock announcing the

awakening of the god of day.

There are even other apparent confirmations of the hypothesis, but we

need not go into them. We shall, however, find one more application of

it in the next chapter, for it appears to be a kind of cure-all for

astronomical troubles; at any rate it offers a conceivable solution of

the question, How does the sun manage to transmit its electric

influence to the earth? And this solution is so grandiose in

conception, and so novel in the mental pictures that it offers, that

its acceptance would not in the least detract from the impression that

the Aurora makes upon the imagination.

X

Strange Adventures of Comets

The fears and legends of ancient times before Science was born, and the

superstitions of the Dark Ages, sedulously cultivated for theological

purposes by monks and priests, have so colored our ideas of the

influence that comets have had upon the human mind that many readers

may be surprised to learn that it was the apparition of a wonderful

comet, that of 1843, which led to the foundation of our greatest

astronomical institution, the Harvard College Observatory. No doubt the

comet superstition existed half a century ago, as, indeed, it exists

yet today, but in this case the marvelous spectacle in the sky proved

less effective in inspiring terror than in awakening a desire for

knowledge. Even in the sixteenth century the views that enlightened

minds took of comets tended powerfully to inspire popular confidence in

science, and Halley’s prediction, after seeing and studying the motion

of the comet which appeared in 1682, that it would prove to be a

regular member of the sun’s family and would be seen returning after a

period of about seventy-six years, together with the fulfillment of

that prediction, produced a revulsion from the superstitious notions

which had so long prevailed.

[Illustration: Swift’s comet. Taken at Arequipa, March 30 1892]

Then the facts were made plain that comets are subject to the law of

gravitation equally with the planets; that there are many which

regularly return to the neighborhood of the sun (perihelion); and that

these travel in orbits differing from those of the planets only in

their greater eccentricity, although they have the peculiarity that

they do not, like the planets, all go round the sun in the same

direction, and do not keep within the general plane of the planetary

system, but traverse it sometimes from above and sometimes from below.

Other comets, including most of the “great” ones, appear to travel in

parabolic or, in a few cases, hyperbolic orbits, which, not being

closed curves, never bring them back again. But it is not certain that

these orbits may not be extremely eccentric ellipses, and that after

the lapse of hundreds, or thousands, of years the comets that follow

them may not reappear. The question is an interesting one, because if

all orbits are really ellipses, then all comets must be permanent

members of the solar system, while in the contrary case many of them

are simply visitors, seen once and never to be seen again. The

hypothesis that comets are originally interlopers might seem to derive

some support from the fact that the certainly periodic ones are

associated, in groups, with the great outer planets, whose attraction

appears to have served as a trap for them by turning them into

elliptical orbits and thus making them prisoners in the solar system.

Jupiter, owing to his great mass and his commanding situation in the

system, is the chief “comet-catcher;” but he catches them not for

himself, but for the sun. Yet if comets do come originally from without

the borders of the planetary system, it does not, by any means, follow

that they were wanderers at large in space before they yielded to the

overmastering attraction of the sun. Investigation of the known

cometary orbits, combined with theoretical considerations, has led some

astronomers to the conclusion that as the sun travels onward through

space he “picks up _en route_” cometary masses which, without belonging

strictly to his empire, are borne along in the same vast “cosmical

current” that carries the solar system.

But while no intelligent person any longer thinks that the appearance

of a great comet is a token from the heavenly powers of the approaching

death of a mighty ruler, or the outbreak of a devastating war, or the

infliction of a terrible plague upon wicked mankind, science itself has

discovered mysteries about comets which are not less fascinating

because they are more intellectual than the irrational fancies that

they have displaced. To bring the subject properly before the mind, let

us see what the principal phenomena connected with a comet are.

At the present day comets are ordinarily “picked up” with the telescope

or the photographic plate before any one except their discoverer is

aware of their existence, and usually they remain so insignificant in

appearance that only astronomers ever see them. Yet so great is the

prestige of the word “comet” that the discovery of one of these

inconspicuous wanderers, and its subsequent movements, become items of

the day’s news which everybody reads with the feeling, perhaps, that at

least he knows what is going on in the universe even if he doesn’t

understand it. But a truly great comet presents quite a different

proposition. It, too, is apt to be detected coming out of the depths of

space before the world at large can get a glimpse of it, but as it

approaches the sun its aspect undergoes a marvelous change. Agitated

apparently by solar influence, it throws out a long streaming tail of

nebulous light, directed away from the sun and looking as if blown out

like a pennon by a powerful wind. Whatever may be the position of the

comet with regard to the sun, as it circles round him it continually

keeps its tail on the off side. This, as we shall soon see, is a fact

of capital importance in relation to the probable nature of comets’

tails. Almost at the same time that the formation of the tail is

observed a remarkable change takes place in the comet’s head, which, by

the way, is invariably and not merely occasionally its most important

part. On approaching the sun the head usually contracts. Coincidently

with this contraction a nucleus generally makes its appearance. This is

a bright, star-like point in the head, and it probably represents the

totality of solid matter that the comet possesses. But it is regarded

as extremely unlikely that even the nucleus consists of a uniformly

solid mass. If it were such, comets would be far more formidable

visitors when they pass near the planets than they have been found to

be. The diameter of the nucleus may vary from a few hundred up to

several thousand miles; the heads, on the average, are from twenty-five

thousand to one hundred thousand miles in diameter, although a few have

greatly exceeded these dimensions; that of the comet of 1811, one of

the most stupendous ever seen, was a million and a quarter miles in

diameter! As to the tails, not withstanding their enormous length—some

have been more than a hundred million miles long—there is reason to

believe that they are of extreme tenuity, “as rare as vacuum.” The

smallest stars have been seen shining through their most brilliant

portions with undiminished luster.

After the nucleus has been formed it begins to throw out bright jets

directed toward the sun. A stream, and sometimes several streams, of

light also project sunward from the nucleus, occasionally appearing

like a stunted tail directed oppositely to the real tail. Symmetrical

envelopes which, seen in section, appear as half circles or parabolas,

rise sunward from the nucleus, forming a concentric series. The ends of

these stream backward into the tail, to which they seem to supply

material. Ordinarily the formation of these ejections and envelopes is

attended by intense agitation of the nucleus, which twists and turns,

swinging and gyrating with an appearance of the greatest violence.

Sometimes the nucleus is seen to break up into several parts. The

entire heads of some comets have been split asunder in passing close

around the sun; The comet of 1882 retreated into space after its

perihelion passage with _five heads_ instead of the one that it had

originally, and each of these heads had its own tail!

The possession of the spectroscope has enabled astronomers during later

years to study the chemical composition of comets by analyzing their

light. At first the only substances thus discovered in them were

hydro-carbon compounds, due evidently to the gaseous envelopes in which

some combination of hydrogen with carbon existed. Behind this gaseous

spectrum was found a faint continuous spectrum ascribed to the nucleus,

which apparently both reflects the sunlight and gives forth the light

of a glowing solid or liquid. Subsequently sodium and iron lines were

found in cometary spectra. The presence of iron would seem to indicate

that some of these bodies may be much more massive than observations on

their attractive effects have indicated. In some recent comets, such as

Morehouse’s, in 1908, several lines have been found, the origin of

which is unknown.

Without going back of the nineteenth century we may find records of

some of the most extraordinary comets that man has ever looked upon. In

1811, still spoken of as “the year of the comet,” because of the

wonderful vintage ascribed to the skyey visitor, a comet shaped like a

gigantic sword amazed the whole world, and, as it remained visible for

seventeen months, was regarded by superstitious persons as a symbol of

the fearful happenings of Napoleon’s Russian campaign. This comet, the

extraordinary size of whose head, greatly exceeding that of the sun

itself, has already been mentioned, was also remarkable for exhibiting

so great a brilliancy without approaching even to the earth’s distance

from the sun. But there was once a comet (and only once—in the year

1729) which never got nearer to the sun than four times the distance of

the earth and yet appeared as a formidable object in the sky. As

Professor Young has remarked, “it must have been an enormous comet to

be visible from such a distance.” And we are to remember that there

were no great telescopes in the year 1729. That comet affects the

imagination like a phantom of space peering into the solar system,

displaying its enormous train afar off (which, if it had approached as

near as other comets, would probably have become _the_ celestial wonder

of all human memory), and then turning away and vanishing in the depths

of immensity.