BOOK IV. THE PHYSICAL PERIOD

We have seen how the theory of the solar system was slowly developed by

the constant efforts of the human mind to find out what are the rules

of cause and effect by which our conception of the present universe and

its development seems to be bound. In the primitive ages a mere record

of events in the heavens and on the earth gave the only hope of

detecting those uniform sequences from which to derive rules or laws of

cause and effect upon which to rely. Then came the geometrical age, in

which rules were sought by which to predict the movements of heavenly

bodies. Later, when the relation of the sun to the courses of the

planets was established, the sun came to be looked upon as a cause; and

finally, early in the seventeenth century, for the first time in

history, it began to be recognised that the laws of dynamics, exactly

as they had been established for our own terrestrial world, hold good,

with the same rigid invariability, at least as far as the limits of the

solar system.

Throughout this evolution of thought and conjecture there were two

types of astronomers—those who supplied the facts, and those who

supplied the interpretation through the logic of mathematics. So

Ptolemy was dependent upon Hipparchus, Kepler on Tycho Brahe, and

Newton in much of his work upon Flamsteed.

When Galileo directed his telescope to the heavens, when Secchi and

Huggins studied the chemistry of the stars by means of the

spectroscope, and when Warren De la Rue set up a photoheliograph at

Kew, we see that a progress in the same direction as before, in the

evolution of our conception of the universe, was being made. Without

definite expression at any particular date, it came to be an accepted

fact that not only do earthly dynamics apply to the heavenly bodies,

but that the laws we find established here, in geology, in chemistry,

and in the laws of heat, may be extended with confidence to the

heavenly bodies. Hence arose the branch of astronomy called

astronomical physics, a science which claims a large portion of the

work of the telescope, spectroscope, and photography. In this new

development it is more than ever essential to follow the dictum of

Tycho Brahe—not to make theories until all the necessary facts are

obtained. The great astronomers of to-day still hold to Sir Isaac

Newton’s declaration, “Hypotheses non fingo.” Each one may have his

suspicions of a theory to guide him in a course of observation, and may

call it a working hypothesis. But the cautious astronomer does not

proclaim these to the world; and the historian is certainly not

justified in including in his record those vague speculations founded

on incomplete data which may be demolished to-morrow, and which,

however attractive they may be, often do more harm than good to the

progress of true science. Meanwhile the accumulation of facts has been

prodigious, and the revelations of the telescope and spectroscope

entrancing.

12. THE SUN.

One of Galileo’s most striking discoveries, when he pointed his

telescope to the heavenly bodies, was that of the irregularly shaped

spots on the sun, with the dark central _umbra_ and the less dark, but

more extensive, _penumbra_ surrounding it, sometimes with several

umbrae in one penumbra. He has left us many drawings of these spots,

and he fixed their period of rotation as a lunar month.

It is not certain whether Galileo, Fabricius, or Schemer was the first

to see the spots. They all did good work. The spots were found to be

ever varying in size and shape. Sometimes, when a spot disappears at

the western limb of the sun, it is never seen again. In other cases,

after a fortnight, it reappears at the eastern limb. The faculae, or

bright areas, which are seen all over the sun’s surface, but specially

in the neighbourhood of spots, and most distinctly near the sun’s edge,

were discovered by Galileo. A high telescopic power resolves their

structure into an appearance like willow-leaves, or rice-grains, fairly

uniform in size, and more marked than on other parts of the sun’s

surface.

[Illustration: SOLAR SURFACE.

As Photographed at the Royal Observatory, Greenwich, showing sun-spots

with umbræ, penumbræ, and faculæ.]

Speculations as to the cause of sun-spots have never ceased from

Galileo’s time to ours. He supposed them to be clouds. Scheiner[1] said

they were the indications of tumultuous movements occasionally

agitating the ocean of liquid fire of which he supposed the sun to be

composed.

A. Wilson, of Glasgow, in 1769,[2] noticed a movement of the umbra

relative to the penumbra in the transit of the spot over the sun’s

surface; exactly as if the spot were a hollow, with a black base and

grey shelving sides. This was generally accepted, but later

investigations have contradicted its universality. Regarding the cause

of these hollows, Wilson said:—

Whether their first production and subsequent numberless changes depend

upon the eructation of elastic vapours from below, or upon eddies or

whirlpools commencing at the surface, or upon the dissolving of the

luminous matter in the solar atmosphere, as clouds are melted and again

given out by our air; or, if the reader pleases, upon the annihilation

and reproduction of parts of this resplendent covering, is left for

theory to guess at.[3]

Ever since that date theory has been guessing at it. The solar

astronomer is still applying all the instruments of modern research to

find out which of these suppositions, or what modification of any of

them, is nearest the truth. The obstacle—one that is perhaps fatal to a

real theory—lies in the impossibility of reproducing comparative

experiments in our laboratories or in our atmosphere.

Sir William Herschel propounded an explanation of Wilson’s observation

which received much notice, but which, out of respect for his memory,

is not now described, as it violated the elementary laws of heat.

Sir John Herschel noticed that the spots are mostly confined to two

zones extending to about 35° on each side of the equator, and that a

zone of equatoreal calms is free from spots. But it was R. C.

Carrington[4] who, by his continuous observations at Redhill, in

Surrey, established the remarkable fact that, while the rotation period

in the highest latitudes, 50°, where spots are seen, is

twenty-seven-and-a-half days, near the equator the period is only

twenty-five days. His splendid volume of observations of the sun led to

much new information about the average distribution of spots at

different epochs.

Schwabe, of Dessau, began in 1826 to study the solar surface, and,

after many years of work, arrived at a law of frequency which has been

more fruitful of results than any discovery in solar physics.[5] In

1843 he announced a decennial period of maxima and minima of sun-spot

displays. In 1851 it was generally accepted, and, although a period of

eleven years has been found to be more exact, all later observations,

besides the earlier ones which have been hunted up for the purpose, go

to establish a true periodicity in the number of sun-spots. But quite

lately Schuster[6] has given reasons for admitting a number of

co-existent periods, of which the eleven-year period was predominant in

the nineteenth century.

In 1851 Lament, a Scotchman at Munich, found a decennial period in the

daily range of magnetic declination. In 1852 Sir Edward Sabine

announced a similar period in the number of “magnetic storms” affecting

all of the three magnetic elements—declination, dip, and intensity.

Australian and Canadian observations both showed the decennial period

in all three elements. Wolf, of Zurich, and Gauthier, of Geneva, each

independently arrived at the same conclusion.

It took many years before this coincidence was accepted as certainly

more than an accident by the old-fashioned astronomers, who want rigid

proof for every new theory. But the last doubts have long vanished, and

a connection has been further traced between violent outbursts of solar

activity and simultaneous magnetic storms.

The frequency of the Aurora Borealis was found by Wolf to follow the

same period. In fact, it is closely allied in its cause to terrestrial

magnetism. Wolf also collected old observations tracing the periodicity

of sun-spots back to about 1700 A.D.

Spoerer deduced a law of dependence of the average latitude of

sun-spots on the phase of the sun-spot period.

All modern total solar eclipse observations seem to show that the shape

of the luminous corona surrounding the moon at the moment of totality

has a special distinct character during the time of a sun-spot maximum,

and another, totally different, during a sun-spot minimum.

A suspicion is entertained that the total quantity of heat received by

the earth from the sun is subject to the same period. This would have

far-reaching effects on storms, harvests, vintages, floods, and

droughts; but it is not safe to draw conclusions of this kind except

from a very long period of observations.

Solar photography has deprived astronomers of the type of Carrington of

the delight in devoting a life’s work to collecting data. It has now

become part of the routine work of an observatory.

In 1845 Foucault and Fizeau took a daguerreotype photograph of the sun.

In 1850 Bond produced one of the moon of great beauty, Draper having

made some attempts at an even earlier date. But astronomical

photography really owes its beginning to De la Rue, who used the

collodion process for the moon in 1853, and constructed the Kew

photoheliograph in 1857, from which date these instruments have been

multiplied, and have given us an accurate record of the sun’s surface.

Gelatine dry plates were first used by Huggins in 1876.

It is noteworthy that from the outset De la Rue recognised the value of

stereoscopic vision, which is now known to be of supreme accuracy. In

1853 he combined pairs of photographs of the moon in the same phase,

but under different conditions regarding libration, showing the moon

from slightly different points of view. These in the stereoscope

exhibited all the relief resulting from binocular vision, and looked

like a solid globe. In 1860 he used successive photographs of the total

solar eclipse stereoscopically, to prove that the red prominences

belong to the sun, and not to the moon. In 1861 he similarly combined

two photographs of a sun-spot, the perspective effect showing the umbra

like a floor at the bottom of a hollow penumbra; and in one case the

faculæ were discovered to be sailing over a spot apparently at some

considerable height. These appearances may be partly due to a proper

motion; but, so far as it went, this was a beautiful confirmation of

Wilson’s discovery. Hewlett, however, in 1894, after thirty years of

work, showed that the spots are not always depressions, being very

subject to disturbance.

The Kew photographs[7] contributed a vast amount of information about

sun-spots, and they showed that the faculæ generally follow the spots

in their rotation round the sun.

The constitution of the sun’s photosphere, the layer which is the

principal light-source on the sun, has always been a subject of great

interest; and much was done by men with exceptionally keen eyesight,

like Mr. Dawes. But it was a difficult subject, owing to the rapidity

of the changes in appearance of the so-called rice-grains, about 1” in

diameter. The rapid transformations and circulations of these

rice-grains, if thoroughly studied, might lead to a much better

knowledge of solar physics. This seemed almost hopeless, as it was

found impossible to identify any “rice-grain” in the turmoil after a

few minutes. But M. Hansky, of Pulkowa (whose recent death is

deplored), introduced successfully a scheme of photography, which might

almost be called a solar cinematograph. He took photographs of the sun

at intervals of fifteen or thirty seconds, and then enlarged selected

portions of these two hundred times, giving a picture corresponding to

a solar disc of six metres diameter. In these enlarged pictures he was

able to trace the movements, and changes of shape and brightness, of

individual rice-grains. Some granules become larger or smaller. Some

seem to rise out of a mist, as it were, and to become clearer. Others

grow feebler. Some are split in two. Some are rotated through a right

angle in a minute or less, although each of the grains may be the size

of Great Britain. Generally they move together in groups of very

various velocities, up to forty kilometres a second. These movements

seem to have definite relation to any sun-spots in the neighbourhood.

From the results already obtained it seems certain that, if this method

of observation be continued, it cannot fail to supply facts of the

greatest importance.

It is quite impossible to do justice here to the work of all those who

are engaged on astronomical physics. The utmost that can be attempted

is to give a fair idea of the directions of human thought and

endeavour. During the last half-century America has made splendid

progress, and an entirely new process of studying the photosphere has

been independently perfected by Professor Hale at Chicago, and

Deslandres at Paris.[8] They have succeeded in photographing the sun’s

surface in monochromatic light, such as the light given off as one of

the bright lines of hydrogen or of calcium, by means of the

“Spectroheliograph.” The spectroscope is placed with its slit in the

focus of an equatoreal telescope, pointed to the sun, so that the

circular image of the sun falls on the slit. At the other end of the

spectroscope is the photographic plate. Just in front of this plate

there is another slit parallel to the first, in the position where the

image of the first slit formed by the K line of calcium falls. Thus is

obtained a photograph of the section of the sun, made by the first

slit, only in K light. As the image of the sun passes over the first

slit the photographic plate is moved at the same rate and in the same

direction behind the second slit; and as successive sections of the

sun’s image in the equatoreal enter the apparatus, so are these

sections successively thrown in their proper place on the photographic

plate, always in K light. By using a high dispersion the faculæ which

give off K light can be correctly photographed, not only at the sun’s

edge, but all over his surface. The actual mechanical method of

carrying out the observation is not quite so simple as what is here

described.

By choosing another line of the spectrum instead of calcium K—for

example, the hydrogen line H(3)—we obtain two photographs, one showing

the appearance of the calcium floculi, and the other of the hydrogen

floculi, on the same part of the solar surface; and nothing is more

astonishing than to note the total want of resemblance in the forms

shown on the two. This mode of research promises to afford many new and

useful data.

The spectroscope has revealed the fact that, broadly speaking, the sun

is composed of the same materials as the earth. Ångstrom was the first

to map out all of the lines to be found in the solar spectrum. But

Rowland, of Baltimore, after having perfected the art of making true

gratings with equidistant lines ruled on metal for producing spectra,

then proceeded to make a map of the solar spectrum on a large scale.

In 1866 Lockyer[9] threw an image of the sun upon the slit of a

spectroscope, and was thus enabled to compare the spectrum of a spot

with that of the general solar surface. The observation proved the

darkness of a spot to be caused by increased absorption of light, not

only in the dark lines, which are widened, but over the entire

spectrum. In 1883 Young resolved this continuous obscurity into an

infinite number of fine lines, which have all been traced in a shadowy

way on to the general solar surface. Lockyer also detected

displacements of the spectrum lines in the spots, such as would be

produced by a rapid motion in the line of sight. It has been found that

both uprushes and downrushes occur, but there is no marked predominance

of either in a sun-spot. The velocity of motion thus indicated in the

line of sight sometimes appears to amount to 320 miles a second. But it

must be remembered that pressure of a gas has some effect in displacing

the spectral lines. So we must go on, collecting data, until a time

comes when the meaning of all the facts can be made clear.

_Total Solar Eclipses_.—During total solar eclipses the time is so

short, and the circumstances so impressive, that drawings of the

appearance could not always be trusted. The red prominences of jagged

form that are seen round the moon’s edge, and the corona with its

streamers radiating or interlacing, have much detail that can hardly be

recorded in a sketch. By the aid of photography a number of records can

be taken during the progress of totality. From a study of these the

extent of the corona is demonstrated in one case to extend to at least

six diameters of the moon, though the eye has traced it farther. This

corona is still one of the wonders of astronomy, and leads to many

questions. What is its consistency, if it extends many million miles

from the sun’s surface? How is it that it opposed no resistance to the

motion of comets which have almost grazed the sun’s surface? Is this

the origin of the zodiacal light? The character of the corona in

photographic records has been shown to depend upon the phase of the

sun-spot period. During the sun-spot maximum the corona seems most

developed over the spot-zones—i.e., neither at the equator nor the

poles. The four great sheaves of light give it a square appearance, and

are made up of rays or plumes, delicate like the petals of a flower.

During a minimum the nebulous ring seems to be made of tufts of fine

hairs with aigrettes or radiations from both poles, and streamers from

the equator.

[Illustration: SOLAR ECLIPSE, 1882. From drawing by W. H. Wesley,

Secretary R.A.S.; showing the prominences, the corona, and an unknown

comet.]

On September 19th, 1868, eclipse spectroscopy began with the Indian

eclipse, in which all observers found that the red prominences showed a

bright line spectrum, indicating the presence of hydrogen and other

gases. So bright was it that Jansen exclaimed: “_Je verrai ces

lignes-là en dehors des éclipses_.” And the next day he observed the

lines at the edge of the uneclipsed sun. Huggins had suggested this

observation in February, 1868, his idea being to use prisms of such

great dispersive power that the continuous spectrum reflected by our

atmosphere should be greatly weakened, while a bright line would suffer

no diminution by the high dispersion. On October 20th Lockyer,[10]

having news of the eclipse, but not of Jansen’s observations the day

after, was able to see these lines. This was a splendid performance,

for it enabled the prominences to be observed, not only during

eclipses, but every day. Moreover, the next year Huggins was able, by

using a wide slit, to see the whole of a prominence and note its shape.

Prominences are classified, according to their form, into “flame” and

“cloud” prominences, the spectrum of the latter showing calcium,

hydrogen, and helium; that of the former including a number of metals.

The D line of sodium is a double line, and in the same eclipse (1868)

an orange line was noticed which was afterwards found to lie close to

the two components of the D line. It did not correspond with any known

terrestrial element, and the unknown element was called “helium.” It

was not until 1895 that Sir William Ramsay found this element as a gas

in the mineral cleavite.

The spectrum of the corona is partly continuous, indicating light

reflected from the sun’s body. But it also shows a green line

corresponding with no known terrestrial element, and the name

“coronium” has been given to the substance causing it.

A vast number of facts have been added to our knowledge about the sun

by photography and the spectroscope. Speculations and hypotheses in

plenty have been offered, but it may be long before we have a complete

theory evolved to explain all the phenomena of the storm-swept metallic

atmosphere of the sun.

The proceedings of scientific societies teem with such facts and

“working hypotheses,” and the best of them have been collected by Miss

Clerke in her _History of Astronomy during the Nineteenth Century_. As

to established facts, we learn from the spectroscopic researches (1)

that the continuous spectrum is derived from the _photosphere_ or solar

gaseous material compressed almost to liquid consistency; (2) that the

_reversing layer_ surrounds it and gives rise to black lines in the

spectrum; that the _chromosphere_ surrounds this, is composed mainly of

hydrogen, and is the cause of the red prominences in eclipses; and that

the gaseous _corona_ surrounds all of these, and extends to vast

distances outside the sun’s visible surface.

FOOTNOTES:

[1] _Rosa Ursina_, by C. Scheiner, _fol_.; Bracciani, 1630.

[2] _R. S. Phil. Trans_., 1774.

[3] _Ibid_, 1783.

[4] _Observations on the Spots on the Sun, etc.,_ 4°; London and

Edinburgh, 1863.

[5] _Periodicität der Sonnenflecken. Astron. Nach. XXI._, 1844, P.

234.

[6] _R.S. Phil. Trans._ (ser. A), 1906, p. 69-100.

[7] “Researches on Solar Physics,” by De la Rue, Stewart and Loewy;

_R. S. Phil. Trans_., 1869, 1870.

[8] “The Sun as Photographed on the K line”; _Knowledge_, London,

1903, p. 229.

[9] _R. S. Proc._, xv., 1867, p. 256.

[10] _Acad. des Sc._, Paris; _C. R._, lxvii., 1868, p. 121.

13. THE MOON AND PLANETS.

_The Moon_.—Telescopic discoveries about the moon commence with

Galileo’s discovery that her surface has mountains and valleys, like

the earth. He also found that, while she always turns the same face to

us, there is periodically a slight twist to let us see a little round

the eastern or western edge. This was called _libration_, and the

explanation was clear when it was understood that in showing always the

same face to us she makes one revolution a month on her axis

_uniformly_, and that her revolution round the earth is not uniform.

Galileo said that the mountains on the moon showed greater differences

of level than those on the earth. Shröter supported this opinion. W.

Herschel opposed it. But Beer and Mädler measured the heights of lunar

mountains by their shadows, and found four of them over 20,000 feet

above the surrounding plains.

Langrenus[1] was the first to do serious work on selenography, and

named the lunar features after eminent men. Riccioli also made lunar

charts. In 1692 Cassini made a chart of the full moon. Since then we

have the charts of Schröter, Beer and Mädler (1837), and of Schmidt, of

Athens (1878); and, above all, the photographic atlas by Loewy and

Puiseux.

The details of the moon’s surface require for their discussion a whole

book, like that of Neison or the one by Nasmyth and Carpenter. Here a

few words must suffice. Mountain ranges like our Andes or Himalayas are

rare. Instead of that, we see an immense number of circular cavities,

with rugged edges and flat interior, often with a cone in the centre,

reminding one of instantaneous photographs of the splash of a drop of

water falling into a pool. Many of these are fifty or sixty miles

across, some more. They are generally spoken of as resembling craters

of volcanoes, active or extinct, on the earth. But some of those who

have most fully studied the shapes of craters deny altogether their

resemblance to the circular objects on the moon. These so-called

craters, in many parts, are seen to be closely grouped, especially in

the snow-white parts of the moon. But there are great smooth dark

spaces, like the clear black ice on a pond, more free from craters, to

which the equally inappropriate name of seas has been given. The most

conspicuous crater, _Tycho_, is near the south pole. At full moon there

are seen to radiate from Tycho numerous streaks of light, or “rays,”

cutting through all the mountain formations, and extending over fully

half the lunar disc, like the star-shaped cracks made on a sheet of ice

by a blow. Similar cracks radiate from other large craters. It must be

mentioned that these white rays are well seen only in full light of the

sun at full moon, just as the white snow in the crevasses of a glacier

is seen bright from a distance only when the sun is high, and

disappears at sunset. Then there are deep, narrow, crooked “rills”

which may have been water-courses; also “clefts” about half a mile

wide, and often hundreds of miles long, like deep cracks in the surface

going straight through mountain and valley.

The moon shares with the sun the advantage of being a good subject for

photography, though the planets are not. This is owing to her larger

apparent size, and the abundance of illumination. The consequence is

that the finest details of the moon, as seen in the largest telescope

in the world, may be reproduced at a cost within the reach of all.

No certain changes have ever been observed; but several suspicions have

been expressed, especially as to the small crater _Linné_, in the _Mare

Serenitatis_. It is now generally agreed that no certainty can be

expected from drawings, and that for real evidence we must await the

verdict of photography.

No trace of water or of an atmosphere has been found on the moon. It is

possible that the temperature is too low. In any case, no displacement

of a star by atmospheric refraction at occultation has been surely

recorded. The moon seems to be dead.

The distance of the moon from the earth is just now the subject of

re-measurement. The base line is from Greenwich to Cape of Good Hope,

and the new feature introduced is the selection of a definite point on

a crater (Mösting A), instead of the moon’s edge, as the point whose

distance is to be measured.

_The Inferior Planets_.—When the telescope was invented, the phases of

Venus attracted much attention; but the brightness of this planet, and

her proximity to the sun, as with Mercury also, seemed to be a bar to

the discovery of markings by which the axis and period of rotation

could be fixed. Cassini gave the rotation as twenty-three hours, by

observing a bright spot on her surface. Shröter made it 23h. 21m. 19s.

This value was supported by others. In 1890 Schiaparelli[2] announced

that Venus rotates, like our moon, once in one of her revolutions, and

always directs the same face to the sun. This property has also been

ascribed to Mercury; but in neither case has the evidence been

generally accepted. Twenty-four hours is probably about the period of

rotation for each of these planets.

Several observers have claimed to have seen a planet within the orbit

of Mercury, either in transit over the sun’s surface or during an

eclipse. It has even been named _Vulcan_. These announcements would

have received little attention but for the fact that the motion of

Mercury has irregularities which have not been accounted for by known

planets; and Le Verrier[3] has stated that an intra-Mercurial planet or

ring of asteroids would account for the unexplained part of the motion

of the line of apses of Mercury’s orbit amounting to 38” per century.

_Mars_.—The first study of the appearance of Mars by Miraldi led him to

believe that there were changes proceeding in the two white caps which

are seen at the planet’s poles. W. Herschel attributed these caps to

ice and snow, and the dates of his observations indicated a melting of

these ice-caps in the Martian summer.

Schroter attributed the other markings on Mars to drifting clouds. But

Beer and Mädler, in 1830-39, identified the same dark spots as being

always in the same place, though sometimes blurred by mist in the local

winter. A spot sketched by Huyghens in 1672, one frequently seen by W.

Herschel in 1783, another by Arago in 1813, and nearly all the markings

recorded by Beer and Mädler in 1830, were seen and drawn by F. Kaiser

in Leyden during seventeen nights of the opposition of 1862 (_Ast.

Nacht._, No. 1,468), whence he deduced the period of rotation to be

24h. 37m. 22s.,62—or one-tenth of a second less than the period deduced

by R. A. Proctor from a drawing by Hooke in 1666.

It must be noted that, if the periods of rotation both of Mercury and

Venus be about twenty-four hours, as seems probable, all the four

planets nearest to the sun rotate in the same period, while the great

planets rotate in about ten hours (Uranus and Neptune being still

indeterminate).

The general surface of Mars is a deep yellow; but there are dark grey

or greenish patches. Sir John Herschel was the first to attribute the

ruddy colour of Mars to its soil rather than to its atmosphere.

The observations of that keen-sighted observer Dawes led to the first

good map of Mars, in 1869. In the 1877 opposition Schiaparelli revived

interest in the planet by the discovery of canals, uniformly about

sixty miles wide, running generally on great circles, some of them

being three or four thousand miles long. During the opposition of

1881-2 the same observer re-observed the canals, and in twenty of them

he found the canals duplicated,[4] the second canal being always 200 to

400 miles distant from its fellow.

The existence of these canals has been doubted. Mr. Lowell has now

devoted years to the subject, has drawn them over and over again, and

has photographed them; and accepts the explanation that they are

artificial, and that vegetation grows on their banks. Thus is revived

the old controversy between Whewell and Brewster as to the habitability

of the planets. The new arguments are not yet generally accepted.

Lowell believes he has, with the spectroscope, proved the existence of

water on Mars.

One of the most unexpected and interesting of all telescopic

discoveries took place in the opposition of 1877, when Mars was

unusually near to the earth. The Washington Observatory had acquired

the fine 26-inch refractor, and Asaph Hall searched for satellites,

concealing the planet’s disc to avoid the glare. On August 11th he had

a suspicion of a satellite. This was confirmed on the 16th, and on the

following night a second one was added. They are exceedingly faint, and

can be seen only by the most powerful telescopes, and only at the times

of opposition. Their diameters are estimated at six or seven miles. It

was soon found that the first, Deimos, completes its orbit in 30h. 18m.

But the other, Phobos, at first was a puzzle, owing to its incredible

velocity being unsuspected. Later it was found that the period of

revolution was only 7h. 39m. 22s. Since the Martian day is twenty-four

and a half hours, this leads to remarkable results. Obviously the

easterly motion of the satellite overwhelms the diurnal rotation of the

planet, and Phobos must appear to the inhabitants, if they exist, to

rise in the west and set in the east, showing two or even three full

moons in a day, so that, sufficiently well for the ordinary purposes of

life, the hour of the day can be told by its phases.

The discovery of these two satellites is, perhaps, the most interesting

telescopic visual discovery made with the large telescopes of the last

half century; photography having been the means of discovering all the

other new satellites except Jupiter’s fifth (in order of discovery).

[Illustration: JUPITER. From a drawing by E. M. Antoniadi, showing

transit of a satellite’s shadow, the belts, and the “great red spot”

(_Monthly Notices_, R. A. S., vol. lix., pl. x.).]

_Jupiter._—Galileo’s discovery of Jupiter’s satellites was followed by

the discovery of his belts. Zucchi and Torricelli seem to have seen

them. Fontana, in 1633, reported three belts. In 1648 Grimaldi saw but

two, and noticed that they lay parallel to the ecliptic. Dusky spots

were also noticed as transient. Hooke[5] measured the motion of one in

1664. In 1665 Cassini, with a fine telescope, 35-feet focal length,

observed many spots moving from east to west, whence he concluded that

Jupiter rotates on an axis like the earth. He watched an unusually

permanent spot during twenty-nine rotations, and fixed the period at

9h. 56m. Later he inferred that spots near the equator rotate quicker

than those in higher latitudes (the same as Carrington found for the

sun); and W. Herschel confirmed this in 1778-9.

Jupiter’s rapid rotation ought, according to Newton’s theory, to be

accompanied by a great flattening at the poles. Cassini had noted an

oval form in 1691. This was confirmed by La Hire, Römer, and Picard.

Pound measured the ellipticity = 1/(13.25).

W. Herschel supposed the spots to be masses of cloud in the

atmosphere—an opinion still accepted. Many of them were very permanent.

Cassini’s great spot vanished and reappeared nine times between 1665

and 1713. It was close to the northern margin of the southern belt.

Herschel supposed the belts to be the body of the planet, and the

lighter parts to be clouds confined to certain latitudes.

In 1665 Cassini observed transits of the four satellites, and also saw

their shadows on the planet, and worked out a lunar theory for Jupiter.

Mathematical astronomers have taken great interest in the perturbations

of the satellites, because their relative periods introduce peculiar

effects. Airy, in his delightful book, _Gravitation_, has reduced these

investigations to simple geometrical explanations.

In 1707 and 1713 Miraldi noticed that the fourth satellite varies much

in brightness. W. Herschel found this variation to depend upon its

position in its orbit, and concluded that in the positions of

feebleness it is always presenting to us a portion of its surface,

which does not well reflect the sun’s light; proving that it always

turns the same face to Jupiter, as is the case with our moon. This fact

had also been established for Saturn’s fifth satellite, and may be true

for all satellites.

In 1826 Struve measured the diameters of the four satellites, and found

them to be 2,429, 2,180, 3,561, and 3,046 miles.

In modern times much interest has been taken in watching a rival to

Cassini’s famous spot. The “great red spot” was first observed by

Niesten, Pritchett, and Tempel, in 1878, as a rosy cloud attached to a

whitish zone beneath the dark southern equatorial band, shaped like the

new war balloons, 30,000 miles long and 7,000 miles across. The next

year it was brick-red. A white spot beside it completed a rotation in

less time by 5½ minutes than the red spot—a difference of 260 miles an

hour. Thus they came together again every six weeks, but the motions

did not continue uniform. The spot was feeble in 1882-4, brightened in

1886, and, after many changes, is still visible.

Galileo’s great discovery of Jupiter’s four moons was the last word in

this connection until September 9th, 1892, when Barnard, using the

36-inch refractor of the Lick Observatory, detected a tiny spot of

light closely following the planet. This proved to be a new satellite

(fifth), nearer to the planet than any other, and revolving round it in

11h. 57m. 23s. Between its rising and setting there must be an interval

of 2½ Jovian days, and two or three full moons. The sixth and seventh

satellites were found by the examination of photographic plates at the

Lick Observatory in 1905, since which time they have been continuously

photographed, and their orbits traced, at Greenwich. On examining these

plates in 1908 Mr. Melotte detected the eighth satellite, which seems

to be revolving in a retrograde orbit three times as far from its

planet as the next one (seventh), in these two points agreeing with the

outermost of Saturn’s satellites (Phoebe).

_Saturn._—This planet, with its marvellous ring, was perhaps the most

wonderful object of those first examined by Galileo’s telescope. He was

followed by Dominique Cassini, who detected bands like Jupiter’s belts.

Herschel established the rotation of the planet in 1775-94. From

observations during one hundred rotations he found the period to be

10h. 16m. 0s., 44. Herschel also measured the ratio of the polar to the

equatoreal diameter as 10:11.

The ring was a complete puzzle to Galileo, most of all when the planet

reached a position where the plane of the ring was in line with the

earth, and the ring disappeared (December 4th, 1612). It was not until

1656 that Huyghens, in his small pamphlet _De Saturni Luna Observatio

Nova_, was able to suggest in a cypher the ring form; and in 1659, in

his Systema Saturnium, he gave his reasons and translated the cypher:

“The planet is surrounded by a slender flat ring, everywhere distinct

from its surface, and inclined to the ecliptic.” This theory explained

all the phases of the ring which had puzzled others. This ring was

then, and has remained ever since, a unique structure. We in this age

have got accustomed to it. But Huyghens’s discovery was received with

amazement.

In 1675 Cassini found the ring to be double, the concentric rings being

separated by a black band—a fact which was placed beyond dispute by

Herschel, who also found that the thickness of the ring subtends an

angle less than 0".3. Shröter estimated its thickness at 500 miles.

Many speculations have been advanced to explain the origin and

constitution of the ring. De Sejour said[6] that it was thrown off from

Saturn’s equator as a liquid ring, and afterwards solidified. He

noticed that the outside would have a greater velocity, and be less

attracted to the planet, than the inner parts, and that equilibrium

would be impossible; so he supposed it to have solidified into a number

of concentric rings, the exterior ones having the least velocity.

Clerk Maxwell, in the Adams prize essay, gave a physico-mathematical

demonstration that the rings must be composed of meteoritic matter like

gravel. Even so, there must be collisions absorbing the energy of

rotation, and tending to make the rings eventually fall into the

planet. The slower motion of the external parts has been proved by the

spectroscope in Keeler’s hands, 1895.

Saturn has perhaps received more than its share of attention owing to

these rings. This led to other discoveries. Huyghens in 1655, and J. D.

Cassini in 1671, discovered the sixth and eighth satellites (Titan and

Japetus). Cassini lost his satellite, and in searching for it found

Rhea (the fifth) in 1672, besides his old friend, whom he lost again.

He added the third and fourth in 1684 (Tethys and Dione). The first and

second (Mimas and Encelades) were added by Herschel in 1789, and the

seventh (Hyperion) simultaneously by Lassel and Bond in 1848. The ninth

(Phoebe) was found on photographs, by Pickering in 1898, with

retrograde motion; and he has lately added a tenth.

The occasional disappearance of Cassini’s Japetus was found on

investigation to be due to the same causes as that of Jupiter’s fourth

satellite, and proves that it always turns the same face to the planet.

_Uranus and Neptune_.—The splendid discoveries of Uranus and two

satellites by Sir William Herschel in 1787, and of Neptune by Adams and

Le Verrier in 1846, have been already described. Lassel added two more

satellites to Uranus in 1851, and found Neptune’s satellite in 1846.

All of the satellites of Uranus have retrograde motion, and their

orbits are inclined about 80° to the ecliptic.

The spectroscope has shown the existence of an absorbing atmosphere on

Jupiter and Saturn, and there are suspicions that they partake

something of the character of the sun, and emit some light besides

reflecting solar light. On both planets some absorption lines seem to

agree with the aqueous vapour lines of our own atmosphere; while one,

which is a strong band in the red common to both planets, seems to

agree with a line in the spectrum of some reddish stars.

Uranus and Neptune are difficult to observe spectroscopically, but

appear to have peculiar spectra agreeing together. Sometimes Uranus

shows Frauenhofer lines, indicating reflected solar light. But

generally these are not seen, and six broad bands of absorption appear.

One is the F. of hydrogen; another is the red-star line of Jupiter and

Saturn. Neptune is a very difficult object for the spectroscope.

Quite lately[7] P. Lowell has announced that V. M. Slipher, at

Flagstaff Observatory, succeeded in 1907 in rendering some plates

sensitive far into the red. A reproduction is given of photographed

spectra of the four outermost planets, showing (1) a great number of

new lines and bands; (2) intensification of hydrogen F. and C. lines;

(3) a steady increase of effects (1) and (2) as we pass from Jupiter

and Saturn to Uranus, and a still greater increase in Neptune.

_Asteroids_.—The discovery of these new planets has been described. At

the beginning of the last century it was an immense triumph to catch a

new one. Since photography was called into the service by Wolf, they

have been caught every year in shoals. It is like the difference

between sea fishing with the line and using a steam trawler. In the

1908 almanacs nearly seven hundred asteroids are included. The

computation of their perturbations and ephemerides by Euler’s and

Lagrange’s method of variable elements became so laborious that Encke

devised a special process for these, which can be applied to many other

disturbed orbits.[8]

When a photograph is taken of a region of the heavens including an

asteroid, the stars are photographed as points because the telescope is

made to follow their motion; but the asteroids, by their proper motion,

appear as short lines.

The discovery of Eros and the photographic attack upon its path have

been described in their relation to finding the sun’s distance.

A group of four asteroids has lately been found, with a mean distance

and period equal to that of Jupiter. To three of these masculine names

have been given—Hector, Patroclus, Achilles; the other has not yet been

named.

FOOTNOTES:

[1] Langrenus (van Langren), F. Selenographia sive lumina austriae

philippica; Bruxelles, 1645.

[2] _Astr. Nach._, 2,944.

[3] _Acad. des Sc._, Paris; _C.R._, lxxxiii., 1876.

[4] _Mem. Spettr. Ital._, xi., p. 28.

[5] _R. S. Phil. Trans_., No. 1.

[6] Grant’s _Hist. Ph. Ast_., p. 267.

[7] _Nature_, November 12th, 1908.

[8] _Ast. Nach_., Nos. 791, 792, 814, translated by G. B. Airy. _Naut.

Alm_., Appendix, 1856.

14. COMETS AND METEORS.

Ever since Halley discovered that the comet of 1682 was a member of the

solar system, these wonderful objects have had a new interest for

astronomers; and a comparison of orbits has often identified the return

of a comet, and led to the detection of an elliptic orbit where the

difference from a parabola was imperceptible in the small portion of

the orbit visible to us. A remarkable case in point was the comet of

1556, of whose identity with the comet of 1264 there could be little

doubt. Hind wanted to compute the orbit more exactly than Halley had

done. He knew that observations had been made, but they were lost.

Having expressed his desire for a search, all the observations of

Fabricius and of Heller, and also a map of the comet’s path among the

stars, were eventually unearthed in the most unlikely manner, after

being lost nearly three hundred years. Hind and others were certain

that this comet would return between 1844 and 1848, but it never

appeared.

When the spectroscope was first applied to finding the composition of

the heavenly bodies, there was a great desire to find out what comets

are made of. The first opportunity came in 1864, when Donati observed

the spectrum of a comet, and saw three bright bands, thus proving that

it was a gas and at least partly self-luminous. In 1868 Huggins

compared the spectrum of Winnecke’s comet with that of a Geissler tube

containing olefiant gas, and found exact agreement. Nearly all comets

have shown the same spectrum.[1] A very few comets have given bright

band spectra differing from the normal type. Also a certain kind of

continuous spectrum, as well as reflected solar light showing

Frauenhofer lines, have been seen.

[Illustration: COPY OF THE DRAWING MADE BY PAUL FABRICIUS.

To define the path of comet 1556. After being lost for 300 years, this

drawing was recovered by the prolonged efforts of Mr. Hind and

Professor Littrow in 1856.]

When Wells’s comet, in 1882, approached very close indeed to the sun,

the spectrum changed to a mono-chromatic yellow colour, due to sodium.

For a full account of the wonders of the cometary world the reader is

referred to books on descriptive astronomy, or to monographs on

comets.[2] Nor can the very uncertain speculations about the structure

of comets’ tails be given here. A new explanation has been proposed

almost every time that a great discovery has been made in the theory of

light, heat, chemistry, or electricity.

Halley’s comet remained the only one of which a prediction of the

return had been confirmed, until the orbit of the small, ill-defined

comet found by Pons in 1819 was computed by Encke, and found to have a

period of 3 1/3 years. It was predicted to return in 1822, and was

recognised by him as identical with many previous comets. This comet,

called after Encke, has showed in each of its returns an inexplicable

reduction of mean distance, which led to the assertion of a resisting

medium in space until a better explanation could be found.[3]

Since that date fourteen comets have been found with elliptic orbits,

whose aphelion distances are all about the same as Jupiter’s mean

distance; and six have an aphelion distance about ten per cent, greater

than Neptune’s mean distance. Other comets are similarly associated

with the planets Saturn and Uranus.

The physical transformations of comets are among the most wonderful of

unexplained phenomena in the heavens. But, for physical astronomers,

the greatest interest attaches to the reduction of radius vector of

Encke’s comet, the splitting of Biela’s comet into two comets in 1846,

and the somewhat similar behaviour of other comets. It must be noted,

however, that comets have a sensible size, that all their parts cannot

travel in exactly the same orbit under the sun’s gravitation, and that

their mass is not sufficient to retain the parts together very

forcibly; also that the inevitable collision of particles, or else

fluid friction, is absorbing energy, and so reducing the comet’s

velocity.

In 1770 Lexell discovered a comet which, as was afterwards proved by

investigations of Lexell, Burchardt, and Laplace, had in 1767 been

deflected by Jupiter out of an orbit in which it was invisible from the

earth into an orbit with a period of 5½ years, enabling it to be seen.

In 1779 it again approached Jupiter closer than some of his satellites,

and was sent off in another orbit, never to be again recognised.

But our interest in cometary orbits has been added to by the discovery

that, owing to the causes just cited, a comet, if it does not separate

into discrete parts like Biela’s, must in time have its parts spread

out so as to cover a sensible part of the orbit, and that, when the

earth passes through such part of a comet’s orbit, a meteor shower is

the result.

A magnificent meteor shower was seen in America on November 12th-13th,

1833, when the paths of the meteors all seemed to radiate from a point

in the constellation Leo. A similar display had been witnessed in

Mexico by Humboldt and Bonpland on November 12th, 1799. H. A. Newton

traced such records back to October 13th, A.D. 902. The orbital motion

of a cloud or stream of small particles was indicated. The period

favoured by H. A. Newton was 354½ days; another suggestion was 375½

days, and another 33¼ years. He noticed that the advance of the date of

the shower between 902 and 1833, at the rate of one day in seventy

years, meant a progression of the node of the orbit. Adams undertook to

calculate what the amount would be on all the five suppositions that

had been made about the period. After a laborious work, he found that

none gave one day in seventy years except the 33¼-year period, which

did so exactly. H. A. Newton predicted a return of the shower on the

night of November 13th-14th, 1866. He is now dead; but many of us are

alive to recall the wonder and enthusiasm with which we saw this

prediction being fulfilled by the grandest display of meteors ever seen

by anyone now alive.

The _progression_ of the nodes proved the path of the meteor stream to

be retrograde. The _radiant_ had almost the exact longitude of the

point towards which the earth was moving. This proved that the meteor

cluster was at perihelion. The period being known, the eccentricity of

the orbit was obtainable, also the orbital velocity of the meteors in

perihelion; and, by comparing this with the earth’s velocity, the

latitude of the radiant enabled the inclination to be determined, while

the longitude of the earth that night was the longitude of the node. In

such a way Schiaparelli was able to find first the elements of the

orbit of the August meteor shower (Perseids), and to show its identity

with the orbit of Tuttle’s comet 1862.iii. Then, in January 1867, Le

Verrier gave the elements of the November meteor shower (Leonids); and

Peters, of Altona, identified these with Oppolzer’s elements for

Tempel’s comet 1866—Schiaparelli having independently attained both of

these results. Subsequently Weiss, of Vienna, identified the meteor

shower of April 20th (Lyrids) with comet 1861. Finally, that

indefatigable worker on meteors, A. S. Herschel, added to the number,

and in 1878 gave a list of seventy-six coincidences between cometary

and meteoric orbits.

Cometary astronomy is now largely indebted to photography, not merely

for accurate delineations of shape, but actually for the discovery of

most of them. The art has also been applied to the observation of

comets at distances from their perihelia so great as to prevent their

visual observation. Thus has Wolf, of Heidelburg, found upon old plates

the position of comet 1905.v., as a star of the 15.5 magnitude, 783

days before the date of its discovery. From the point of view of the

importance of finding out the divergence of a cometary orbit from a

parabola, its period, and its aphelion distance, this increase of range

attains the very highest value.

The present Astronomer Royal, appreciating this possibility, has been

searching by photography for Halley’s comet since November, 1907,

although its perihelion passage will not take place until April, 1910.

FOOTNOTES:

[1] In 1874, when the writer was crossing the Pacific Ocean in H.M.S.

“Scout,” Coggia’s comet unexpectedly appeared, and (while Colonel

Tupman got its positions with the sextant) he tried to use the prism

out of a portable direct-vision spectroscope, without success until it

was put in front of the object-glass of a binocular, when, to his

great joy, the three band images were clearly seen.

[2] Such as _The World of Comets_, by A. Guillemin; _History of

Comets_, by G. R. Hind, London, 1859; _Theatrum Cometicum_, by S. de

Lubienietz, 1667; _Cometographie_, by Pingré, Paris, 1783; _Donati’s

Comet_, by Bond.

[3] The investigations by Von Asten (of St. Petersburg) seem to

support, and later ones, especially those by Backlund (also of St.

Petersburg), seem to discredit, the idea of a resisting medium.

15. THE FIXED STARS AND NEBULÆ.

Passing now from our solar system, which appears to be subject to the

action of the same forces as those we experience on our globe, there

remains an innumerable host of fixed stars, nebulas, and nebulous

clusters of stars. To these the attention of astronomers has been more

earnestly directed since telescopes have been so much enlarged.

Photography also has enabled a vast amount of work to be covered in a

comparatively short period, and the spectroscope has given them the

means, not only of studying the chemistry of the heavens, but also of

detecting any motion in the line of sight from less than a mile a

second and upwards in any star, however distant, provided it be bright

enough.

[Illustration: SIR WILLIAM HERSCHEL, F.R.S.—1738-1822.

Painted by Lemuel F. Abbott; National Portrait Gallery, Room XX.]

In the field of telescopic discovery beyond our solar system there is

no one who has enlarged our knowledge so much as Sir William Herschel,

to whom we owe the greatest discovery in dynamical astronomy among the

stars—viz., that the law of gravitation extends to the most distant

stars, and that many of them describe elliptic orbits about each other.

W. Herschel was born at Hanover in 1738, came to England in 1758 as a

trained musician, and died in 1822. He studied science when he could,

and hired a telescope, until he learnt to make his own specula and

telescopes. He made 430 parabolic specula in twenty-one years. He

discovered 2,500 nebulæ and 806 double stars, counted the stars in

3,400 guage-fields, and compared the principal stars photometrically.

Some of the things for which he is best known were results of those

accidents that happen only to the indefatigable enthusiast. Such was

the discovery of Uranus, which led to funds being provided for

constructing his 40-feet telescope, after which, in 1786, he settled at

Slough. In the same way, while trying to detect the annual parallax of

the stars, he failed in that quest, but discovered binary systems of

stars revolving in ellipses round each other; just as Bradley’s attack

on stellar parallax failed, but led to the discovery of aberration,

nutation, and the true velocity of light.

_Parallax_.—The absence of stellar parallax was the great objection to

any theory of the earth’s motion prior to Kepler’s time. It is true

that Kepler’s theory itself could have been geometrically expressed

equally well with the earth or any other point fixed. But in Kepler’s

case the obviously implied physical theory of the planetary motions,

even before Newton explained the simplicity of conception involved,

made astronomers quite ready to waive the claim for a rigid proof of

the earth’s motion by measurement of an annual parallax of stars, which

they had insisted on in respect of Copernicus’s revival of the idea of

the earth’s orbital motion.

Still, the desire to measure this parallax was only intensified by the

practical certainty of its existence, and by repeated failures. The

attempts of Bradley failed. The attempts of Piazzi and Brinkley,[1]

early in the nineteenth century, also failed. The first successes,

afterwards confirmed, were by Bessel and Henderson. Both used stars

whose proper motion had been found to be large, as this argued

proximity. Henderson, at the Cape of Good Hope, observed α Centauri,

whose annual proper motion he found to amount to 3".6, in 1832-3; and a

few years later deduced its parallax 1".16. His successor at the Cape,

Maclear, reduced this to 0".92.

In 1835 Struve assigned a doubtful parallax of 0".261 to Vega (α Lyræ).

But Bessel’s observations, between 1837 and 1840, of 61 Cygni, a star

with the large proper motion of over 5”, established its annual

parallax to be 0".3483; and this was confirmed by Peters, who found the

value 0".349.

Later determinations for α2 Centauri, by Gill,[2] make its parallax

0".75—This is the nearest known fixed star; and its light takes 4 1/3

years to reach us. The lightyear is taken as the unit of measurement in

the starry heavens, as the earth’s mean distance is “the astronomical

unit” for the solar system.[3] The proper motions and parallaxes

combined tell us the velocity of the motion of these stars across the

line of sight: α Centauri 14.4 miles a second=4.2 astronomical units a

year; 61 Cygni 37.9 miles a second=11.2 astronomical units a year.

These successes led to renewed zeal, and now the distances of many

stars are known more or less accurately.

Several of the brightest stars, which might be expected to be the

nearest, have not shown a parallax amounting to a twentieth of a second

of arc. Among these are Canopus, α Orionis, α Cygni, β Centauri, and γ

Cassiopeia. Oudemans has published a list of parallaxes observed.[4]

_Proper Motion._—In 1718 Halley[5] detected the proper motions of

Arcturus and Sirius. In 1738 J. Cassinis[6] showed that the former had

moved five minutes of arc since Tycho Brahe fixed its position. In 1792

Piazzi noted the motion of 61 Cygni as given above. For a long time the

greatest observed proper motion was that of a small star 1830

Groombridge, nearly 7” a year; but others have since been found

reaching as much as 10”.

Now the spectroscope enables the motion of stars to be detected at a

single observation, but only that part of the motion that is in the

line of sight. For a complete knowledge of a star’s motion the proper

motion and parallax must also be known.

When Huggins first applied the Doppler principle to measure velocities

in the line of sight,[7] the faintness of star spectra diminished the

accuracy; but Vögel, in 1888, overcame this to a great extent by long

exposures of photographic plates.

It has often been noticed that stars which seem to belong to a group of

nearly uniform magnitude have the same proper motion. The spectroscope

has shown that these have also often the same velocity in the line of

sight. Thus in the Great Bear, β, γ, δ, ε, ζ, all agree as to angular

proper motion. δ was too faint for a spectroscopic measurement, but all

the others have been shown to be approaching us at a rate of twelve to

twenty miles a second. The same has been proved for proper motion, and

line of sight motion, in the case of Pleiades and other groups.

Maskelyne measured many proper motions of stars, from which W.

Herschel[8] came to the conclusion that these apparent motions are for

the most part due to a motion of the solar system in space towards a

point in the constellation Hercules, R.A. 257°; N. Decl. 25°. This

grand discovery has been amply confirmed, and, though opinions differ

as to the exact direction, it happens that the point first indicated by

Herschel, from totally insufficient data, agrees well with modern

estimates.

Comparing the proper motions and parallaxes to get the actual velocity

of each star relative to our system, C.L. Struve found the probable

velocity of the solar system in space to be fifteen miles a second, or

five astronomical units a year.

The work of Herschel in this matter has been checked by comparing

spectroscopic velocities in the line of sight which, so far as the

sun’s motion is concerned, would give a maximum rate of approach for

stars near Hercules, a maximum rate of recession for stars in the

opposite part of the heavens, and no effect for stars half-way between.

In this way the spectroscope has confirmed generally Herschel’s view of

the direction, and makes the velocity eleven miles a second, or nearly

four astronomical units a year.

The average proper motion of a first magnitude star has been found to

be 0".25 annually, and of a sixth magnitude star 0".04. But that all

bright stars are nearer than all small stars, or that they show greater

proper motion for that reason, is found to be far from the truth. Many

statistical studies have been made in this connection, and interesting

results may be expected from this treatment in the hands of Kapteyn of

Groningen, and others.[9]

On analysis of the directions of proper motions of stars in all parts

of the heavens, Kapteyn has shown[10] that these indicate, besides the

solar motion towards Hercules, two general drifts of stars in nearly

opposite directions, which can be detected in any part of the heavens.

This result has been confirmed from independent data by Eddington

(_R.A.S., M.N._) and Dyson (_R.S.E. Proc._).

Photography promises to assist in the measurement of parallax and

proper motions. Herr Pulfrich, of the firm of Carl Zeiss, has vastly

extended the applications of stereoscopic vision to astronomy—a subject

which De la Rue took up in the early days of photography. He has made a

stereo-comparator of great beauty and convenience for comparing

stereoscopically two star photographs taken at different dates. Wolf of

Heidelberg has used this for many purposes. His investigations

depending on the solar motion in space are remarkable. He photographs

stars in a direction at right angles to the line of the sun’s motion.

He has taken photographs of the same region fourteen years apart, the

two positions of his camera being at the two ends of a base-line over

5,000,000,000 miles apart, or fifty-six astronomical units. On

examining these stereoscopically, some of the stars rise out of the

general plane of the stars, and seem to be much nearer. Many of the

stars are thus seen to be suspended in space at different distances

corresponding exactly to their real distances from our solar system,

except when their proper motion interferes. The effect is most

striking; the accuracy of measurement exceeds that of any other method

of measuring such displacements, and it seems that with a long interval

of time the advantage of the method increases.

_Double Stars._—The large class of double stars has always been much

studied by amateurs, partly for their beauty and colour, and partly as

a test for telescopic definition. Among the many unexplained stellar

problems there is one noticed in double stars that is thought by some

to be likely to throw light on stellar evolution. It is this: There are

many instances where one star of the pair is comparatively faint, and

the two stars are contrasted in colour; and in every single case the

general colour of the faint companion is invariably to be classed with

colours more near to the blue end of the spectrum than that of the

principal star.

_Binary Stars._—Sir William Herschel began his observations of double

stars in the hope of discovering an annual parallax of the stars. In

this he was following a suggestion of Galileo’s. The presumption is

that, if there be no physical connection between the stars of a pair,

the largest is the nearest, and has the greatest parallax. So, by

noting the distance between the pair at different times of the year, a

delicate test of parallax is provided, unaffected by major instrumental

errors.

Herschel did, indeed, discover changes of distance, but not of the

character to indicate parallax. Following this by further observation,

he found that the motions were not uniform nor rectilinear, and by a

clear analysis of the movements he established the remarkable and

wholly unexpected fact that in all these cases the motion is due to a

revolution about their common centre of gravity.[11] He gave the

approximate period of revolution of some of these: Castor, 342 years; δ

Serpentis, 375 years; γ Leonis, 1,200 years; ε Bootis, 1,681 years.

Twenty years later Sir John Herschel and Sir James South, after

re-examination of these stars, confirmed[12] and extended the results,

one pair of Coronæ having in the interval completed more than a whole

revolution.

It is, then, to Sir William Herschel that we owe the extension of the

law of gravitation, beyond the limits of the solar system, to the whole

universe. His observations were confirmed by F.G.W. Struve (born 1793,

died 1864), who carried on the work at Dorpat. But it was first to

Savary,[13] and later to Encke and Sir John Herschel, that we owe the

computation of the elliptic elements of these stars; also the resulting

identification of their law of force with Newton’s force of gravitation

applied to the solar system, and the force that makes an apple fall to

the ground. As Grant well says in his _History_: “This may be justly

asserted to be one of the most sublime truths which astronomical

science has hitherto disclosed to the researches of the human mind.”

Latterly the best work on double stars has been done by S. W.

Burnham,[14] at the Lick Observatory. The shortest period he found was

eleven years (κ Pegasi). In the case of some of these binaries the

parallax has been measured, from which it appears that in four of the

surest cases the orbits are about the size of the orbit of Uranus,

these being probably among the smallest stellar orbits.

The law of gravitation having been proved to extend to the stars, a

discovery (like that of Neptune in its origin, though unlike it in the

labour and originality involved in the calculation) that entrances the

imagination became possible, and was realised by Bessel—the discovery

of an unknown body by its gravitational disturbance on one that was

visible. In 1834 and 1840 he began to suspect a want of uniformity in

the proper motion of Sirius and Procyon respectively. In 1844, in a

letter to Sir John Herschel,[15] he attributed these irregularities in

each case to the attraction of an invisible companion, the period of

revolution of Sirius being about half a century. Later he said: “I

adhere to the conviction that Procyon and Sirius form real binary

systems, consisting of a visible and an invisible star. There is no

reason to suppose luminosity an essential quality of cosmical bodies.

The visibility of countless stars is no argument against the

invisibility of countless others.” This grand conception led Peters to

compute more accurately the orbit, and to assign the place of the

invisible companion of Sirius. In 1862 Alvan G. Clark was testing a new

18-inch object-glass (now at Chicago) upon Sirius, and, knowing nothing

of these predictions, actually found the companion in the very place

assigned to it. In 1896 the companion of Procyon was discovered by

Professor Schaeberle at the Lick Observatory.

Now, by the refined parallax determinations of Gill at the Cape, we

know that of Sirius to be 0".38. From this it has been calculated that

the mass of Sirius equals two of our suns, and its intrinsic brightness

equals twenty suns; but the companion, having a mass equal to our sun,

has only a five-hundredth part of the sun’s brightness.

_Spectroscopic Binaries_.—On measuring the velocity of a star in the

line of sight at frequent intervals, periodic variations have been

found, leading to a belief in motion round an invisible companion.

Vogel, in 1889, discovered this in the case of Spica (α Virginis),

whose period is 4d. 0h. 19m., and the diameter of whose orbit is six

million miles. Great numbers of binaries of this type have since then

been discovered, all of short period.

Also, in 1889, Pickering found that at regular intervals of fifty-two

days the lines in the spectrum of ζ of the Great Bear are duplicated,

indicating a relative velocity, equal to one hundred miles a second, of

two components revolving round each other, of which that apparently

single star must be composed.

It would be interesting, no doubt, to follow in detail the accumulating

knowledge about the distances, proper motions, and orbits of the stars;

but this must be done elsewhere. Enough has been said to show how

results are accumulating which must in time unfold to us the various

stellar systems and their mutual relationships.

_Variable Stars._—It has often happened in the history of different

branches of physical science that observation and experiment were so

far ahead of theory that hopeless confusion appeared to reign; and then

one chance result has given a clue, and from that time all differences

and difficulties in the previous researches have stood forth as natural

consequences, explaining one another in a rational sequence. So we find

parallax, proper motion, double stars, binary systems, variable stars,

and new stars all bound together.

The logical and necessary explanation given of the cause of ordinary

spectroscopic binaries, and of irregular proper motions of Sirius and

Procyon, leads to the inference that if ever the plane of such a binary

orbit were edge-on to us there ought to be an eclipse of the luminous

partner whenever the non-luminous one is interposed between us. This

should give rise either to intermittence in the star’s light or else to

variability. It was by supposing the existence of a dark companion to

Algol that its discoverer, Goodricke of York,[16] in 1783, explained

variable stars of this type. Algol (β Persei) completes the period of

variable brightness in 68.8 hours. It loses three-fifths of its light,

and regains it in twelve hours. In 1889 Vogel,[17] with the Potsdam

spectrograph, actually found that the luminous star is receding before

each eclipse, and approaching us after each eclipse; thus entirely

supporting Goodricke’s opinion. There are many variables of the Algol

type, and information is steadily accumulating. But all variable stars

do not suffer the sudden variations of Algol. There are many types, and

the explanations of others have not proved so easy.

The Harvard College photographs have disclosed the very great

prevalence of variability, and this is certainly one of the lines in

which modern discovery must progress.

Roberts, in South Africa, has done splendid work on the periods of

variables of the Algol type.

_New Stars_.—Extreme instances of variable stars are the new stars such

as those detected by Hipparchus, Tycho Brahe, and Kepler, of which many

have been found in the last half-century. One of the latest great

“Novæ” was discovered in Auriga by a Scotsman, Dr. Anderson, on

February 1st, 1892, and, with the modesty of his race, he communicated

the fact to His Majesty’s Astronomer for Scotland on an unsigned

post-card.[18] Its spectrum was observed and photographed by Huggins

and many others. It was full of bright lines of hydrogen, calcium,

helium, and others not identified. The astounding fact was that lines

were shown in pairs, bright and dark, on a faint continuous spectrum,

indicating apparently that a dark body approaching us at the rate of

550 miles a second[19] was traversing a cold nebulous atmosphere, and

was heated to incandescence by friction, like a meteor in our

atmosphere, leaving a luminous train behind it. It almost disappeared,

and on April 26th it was of the sixteenth magnitude; but on August 17th

it brightened to the tenth, showing the principal nebular band in its

spectrum, and no sign of approach or recession. It was as if it emerged

from one part of the nebula, cooled down, and rushed through another

part of the nebula, rendering the nebular gas more luminous than

itself.[20]

Since 1892 one Nova after another has shown a spectrum as described

above, like a meteor rushing towards us and leaving a train behind, for

this seems to be the obvious meaning of the spectra.

The same may be said of the brilliant Nova Persei, brighter at its best

than Capella, and discovered also by Dr. Anderson on February 22nd,

1901. It increased in brightness as it reached the densest part of the

nebula, then it varied for some weeks by a couple of magnitudes, up and

down, as if passing through separate nebular condensations. In

February, 1902, it could still be seen with an opera-glass. As with the

other Novæ, when it first dashed into the nebula it was vaporised and

gave a continuous spectrum with dark lines of hydrogen and helium. It

showed no bright lines paired with the dark ones to indicate a train

left behind; but in the end its own luminosity died out, and the

nebular spectrum predominated.

The nebular illumination as seen in photographs, taken from August to

November, seemed to spread out slowly in a gradually increasing circle

at the rate of 90” in forty-eight days. Kapteyn put this down to the

velocity of light, the original outburst sending its illumination to

the nebulous gas and illuminating a spherical shell whose radius

increased at the velocity of light. This supposition seems correct, in

which case it can easily be shown from the above figures that the

distance of this Nova was 300 light years.

_Star Catalogues._—Since the days of very accurate observations

numerous star-catalogues have been produced by individuals or by

observatories. Bradley’s monumental work may be said to head the list.

Lacaille’s, in the Southern hemisphere, was complementary. Then Piazzi,

Lalande, Groombridge, and Bessel were followed by Argelander with his

324,000 stars, Rumker’s Paramatta catalogue of the southern hemisphere,

and the frequent catalogues of national observatories. Later the

Astronomische Gesellschaft started their great catalogue, the combined

work of many observatories. Other southern ones were Gould’s at Cordova

and Stone’s at the Cape.

After this we have a new departure. Gill at the Cape, having the comet

1882.ii. all to himself in those latitudes, wished his friends in

Europe to see it, and employed a local photographer to strap his camera

to the observatory equatoreal, driven by clockwork, and adjusted on the

comet by the eye. The result with half-an-hour’s exposure was good, so

he tried three hours. The result was such a display of sharp star

images that he resolved on the Cape Photographic Durchmusterung, which

after fourteen years, with Kapteyn’s aid in reducing, was completed.

Meanwhile the brothers Henry, of Paris, were engaged in going over

Chacornac’s zodiacal stars, and were about to catalogue the Milky Way

portion, a serious labour, when they saw Gill’s Comet photograph and

conceived the idea of doing the rest of their work by photography. Gill

had previously written to Admiral Mouchez, of the Paris Observatory,

and explained to him his project for charting the heavens

photographically, by combining the work of many observatories. This led

Admiral Mouchez to support the brothers Henry in their scheme.[21]

Gill, having got his own photographic work underway, suggested an

international astrographic chart, the materials for different zones to

be supplied by observatories of all nations, each equipped with similar

photographic telescopes. At a conference in Paris, 1887, this was

decided on, the stars on the charts going down to the fourteenth

magnitude, and the catalogues to the eleventh.

[Illustration: GREAT COMET, NOV. 14TH, 1882.

(Exposure 2hrs. 20m.) By kind permission of Sir David Gill. From this

photograph originated all stellar chart-photography.]

This monumental work is nearing completion. The labour involved was

immense, and the highest skill was required for devising instruments

and methods to read off the star positions from the plates.

Then we have the Harvard College collection of photographic plates,

always being automatically added to; and their annex at Arequipa in

Peru.

Such catalogues vary in their degree of accuracy; and fundamental

catalogues of standard stars have been compiled. These require

extension, because the differential methods of the heliometer and the

camera cannot otherwise be made absolute.

The number of stars down to the fourteenth magnitude may be taken at

about 30,000,000; and that of all the stars visible in the greatest

modern telescopes is probably about 100,000,000.

_Nebulæ and Star-clusters._—Our knowledge of nebulæ really dates from

the time of W. Herschel. In his great sweeps of the heavens with his

giant telescopes he opened in this direction a new branch of astronomy.

At one time he held that all nebulæ might be clusters of innumerable

minute stars at a great distance. Then he recognised the different

classes of nebulæ, and became convinced that there is a widely-diffused

“shining fluid” in space, though many so-called nebulæ could be

resolved by large telescopes into stars. He considered that the Milky

Way is a great star cluster, whose form may be conjectured from

numerous star-gaugings. He supposed that the compact “planetary nebulæ”

might show a stage of evolution from the diffuse nebulæ, and that his

classifications actually indicate various stages of development. Such

speculations, like those of the ancients about the solar system, are

apt to be harmful to true progress of knowledge unless in the hands of

the ablest mathematical physicists; and Herschel violated their

principles in other directions. But here his speculations have

attracted a great deal of attention, and, with modifications, are

accepted, at least as a working hypothesis, by a fair number of people.

When Sir John Herschel had extended his father’s researches into the

Southern Hemisphere he was also led to the belief that some nebulae

were a phosphorescent material spread through space like fog or mist.

Then his views were changed by the revelations due to the great

discoveries of Lord Rosse with his gigantic refractor,[22] when one

nebula after another was resolved into a cluster of minute stars. At

that time the opinion gained ground that with increase of telescopic

power this would prove to be the case with all nebulæ.

In 1864 all doubt was dispelled by Huggins[23] in his first examination

of the spectrum of a nebula, and the subsequent extension of this

observation to other nebulæ; thus providing a certain test which

increase in the size of telescopes could never have given. In 1864

Huggins found that all true nebulae give a spectrum of bright lines.

Three are due to hydrogen; two (discovered by Copeland) are helium

lines; others are unknown. Fifty-five lines have been photographed in

the spectrum of the Orion nebula. It seems to be pretty certain that

all true nebulae are gaseous, and show almost exactly the same

spectrum.

Other nebulæ, and especially the white ones like that in Andromeda,

which have not yet been resolved into stars, show a continuous

spectrum; others are greenish and give no lines.

A great deal has to be done by the chemist before the astronomer can be

on sure ground in drawing conclusions from certain portions of his

spectroscopic evidence.

The light of the nebulas is remarkably actinic, so that photography has

a specially fine field in revealing details imperceptible in the

telescope. In 1885 the brothers Henry photographed, round the star Maia

in the Pleiades, a spiral nebula 3’ long, as bright on the plate as

that star itself, but quite invisible in the telescope; and an exposure

of four hours revealed other new nebula in the same district. That

painstaking and most careful observer, Barnard, with 10¼ hours’

exposure, extended this nebulosity for several degrees, and discovered

to the north of the Pleiades a huge diffuse nebulosity, in a region

almost destitute of stars. By establishing a 10-inch instrument at an

altitude of 6,000 feet, Barnard has revealed the wide distribution of

nebular matter in the constellation Scorpio over a space of 4° or 5°

square. Barnard asserts that the “nebular hypothesis” would have been

killed at its birth by a knowledge of these photographs. Later he has

used still more powerful instruments, and extended his discoveries.

The association of stars with planetary nebulæ, and the distribution of

nebulæ in the heavens, especially in relation to the Milky Way, are

striking facts, which will certainly bear fruit when the time arrives

for discarding vague speculations, and learning to read the true

physical structure and history of the starry universe.

_Stellar Spectra._—When the spectroscope was first available for

stellar research, the leaders in this branch of astronomy were Huggins

and Father Secchi,[24] of Rome. The former began by devoting years of

work principally to the most accurate study of a few stars. The latter

devoted the years from 1863 to 1867 to a general survey of the whole

heavens, including 4,000 stars. He divided these into four principal

classes, which have been of the greatest service. Half of his stars

belonged to the first class, including Sirius, Vega, Regulus, Altair.

The characteristic feature of their spectra is the strength and breadth

of the hydrogen lines and the extreme faintness of the metallic lines.

This class of star is white to the eye, and rich in ultra violet light.

The second class includes about three-eighths of his stars, including

Capella, Pollux, and Arcturus. These stars give a spectrum like that of

our sun, and appear yellowish to the eye.

The third class includes α Herculis, α Orionis (Betelgeux), Mira Ceti,

and about 500 red and variable stars. The spectrum has fluted bands

shaded from blue to red, and sharply defined at the more refrangible

edge.

The fourth class is a small one, containing no stars over fifth

magnitude, of which 152 Schjellerup, in Canes Venatici, is a good

example. This spectrum also has bands, but these are shaded on the

violet side and sharp on the red side. They are due to carbon in some

form. These stars are ruby red in the telescope.

It would appear, then, that all stars are suns with continuous spectra,

and the classes are differentiated by the character of the absorbent

vapours of their atmospheres.

It is very likely that, after the chemists have taught us how to

interpret all the varieties of spectrum, it will be possible to ascribe

the different spectrum-classes to different stages in the life-history

of every star. Already there are plenty of people ready to lay down

arbitrary assumptions about the lessons to be drawn from stellar

spectra. Some say that they know with certainty that each star begins

by being a nebula, and is condensed and heated by condensation until it

begins to shine as a star; that it attains a climax of temperature,

then cools down, and eventually becomes extinct. They go so far as to

declare that they know what class of spectrum belongs to each stage of

a star’s life, and how to distinguish between one that is increasing

and another that is decreasing in temperature.

The more cautious astronomers believe that chemistry is not

sufficiently advanced to justify all of these deductions; that, until

chemists have settled the lately raised question of the transmutation

of elements, no theory can be sure. It is also held that until they

have explained, without room for doubt, the reasons for the presence of

some lines, and the absence of others, of any element in a stellar

spectrum; why the arc-spectrum of each element differs from its spark

spectrum; what are all the various changes produced in the spectrum of

a gas by all possible concomitant variations of pressure and

temperature; also the meanings of all the flutings in the spectra of

metalloids and compounds; and other equally pertinent matters—until

that time arrives the part to be played by the astronomer is one of

observation. By all means, they say, make use of “working hypotheses”

to add an interest to years of laborious research, and to serve as a

guide to the direction of further labours; but be sure not to fall into

the error of calling any mere hypothesis a theory.

_Nebular Hypothesis._—The Nebular Hypothesis, which was first, as it

were, tentatively put forward by Laplace as a note in his _Système du

Monde_, supposes the solar system to have been a flat, disk-shaped

nebula at a high temperature in rapid rotation. In cooling it

condensed, leaving revolving rings at different distances from the

centre. These themselves were supposed to condense into the nucleus for

a rotating planet, which might, in contracting, again throw off rings

to form satellites. The speculation can be put in a really attractive

form, but is in direct opposition to many of the actual facts; and so

long as it is not favoured by those who wish to maintain the position

of astronomy as the most exact of the sciences—exact in its facts,

exact in its logic—this speculation must be recorded by the historian,

only as he records the guesses of the ancient Greeks--as an interesting

phase in the history of human thought.

Other hypotheses, having the same end in view, are the meteoritic

hypothesis of Lockyer and the planetesimal hypothesis that has been

largely developed in the United States. These can best be read in the

original papers to various journals, references to which may be found

in the footnotes of Miss Clerke’s _History of Astronomy during the

Nineteenth Century_. The same can be said of Bredichin’s hypothesis of

comets’ tails, Arrhenius’s book on the applications of the theory of

light repulsion, the speculations on radium, the origin of the sun’s

heat and the age of the earth, the electron hypothesis of terrestrial

magnetism, and a host of similar speculations, all combining to throw

an interesting light on the evolution of a modern train of thought that

seems to delight in conjecture, while rebelling against that strict

mathematical logic which has crowned astronomy as the queen of the

sciences.

FOOTNOTES:

[1] _R. S. Phil Trans_., 1810 and 1817-24.

[2] One of the most valuable contributions to our knowledge of stellar

parallaxes is the result of Gill’s work (_Cape Results_, vol. iii.,

part ii., 1900).

[3] Taking the velocity of light at 186,000 miles a second, and the

earth’s mean distance at 93,000,000 miles, 1

light-year=5,865,696,000,000 miles or 63,072 astronomical units; 1

astronomical unit a year=2.94 miles a second; and the earth’s orbital

velocity=18.5 miles a second.

[4] Ast. Nacht., 1889.

[5] R. S. Phil. Trans., 1718.

[6] Mem. Acad. des Sciences, 1738, p. 337.

[7] R. S Phil. Trans., 1868.

[8] _R.S. Phil Trans._, 1783.

[9] See Kapteyn’s address to the Royal Institution, 1908. Also Gill’s

presidential address to the British Association, 1907.

[10] _Brit. Assoc. Rep._, 1905.

[11] R. S. Phil. Trans., 1803, 1804.

[12] Ibid, 1824.

[13] Connaisance des Temps, 1830.

[14] _R. A. S. Mem._, vol. xlvii., p. 178; _Ast. Nach._, No. 3,142;

Catalogue published by Lick Observatory, 1901.

[15] _R. A. S., M. N._, vol. vi.

[16] _R. S. Phil. Trans._, vol. lxxiii., p. 484.

[17] _Astr. Nach._, No. 2,947.

[18] _R. S. E. Trans_., vol. xxvii. In 1901 Dr. Anderson discovered

Nova Persei.

[19] _Astr. Nach_., No. 3,079.

[20] For a different explanation see Sir W. Huggins’s lecture, Royal

Institution, May 13th, 1892.

[21] For the early history of the proposals for photographic

cataloguing of stars, see the _Cape Photographic Durchmusterung_, 3

vols. (_Ann. of the Cape Observatory_, vols. in., iv., and v.,

Introduction.)

[22] _R. S. Phil. Trans._, 1850, p. 499 _et seq._

[23] _Ibid_, vol. cliv., p. 437.

[24] _Brit. Assoc. Rep._, 1868, p. 165.

ILLUSTRATIONS

SIR ISAAC NEWTON

(From the bust by Roubiliac In Trinity College, Cambridge.)

CHALDÆAN BAKED BRICK OR TABLET

Obverse and reverse sides, containing record of solar eclipse, 1062

B.C., used lately by Cowell for rendering the lunar theory more

accurate than was possible by finest modern observations. (British

Museum collection, No. 35908.)

“QUADRANS MURALIS SIVE TICHONICUS.”

With portrait of Tycho Brahe, instruments, etc., painted on the wall;

showing assistants using the sight, watching the clock, and recording.

(From the author’s copy of the _Astronomiæ Instauratæ Mechanica_.)

PORTRAIT OF JOHANNES KEPLER.

By F. Wanderer, from Reitlinger’s “Johannes Kepler” (Original in

Strassburg).

DEATH-MASK OF SIR ISAAC NEWTON.

Photographed specially for this work from the original, by kind

permission of the Royal Society, London.

ANCIENT CHINESE INSTRUMENTS,

Including quadrant, celestial globe, and two armillae, in the

Observatory at Peking. Photographed in Peking by the author in 1875,

and stolen by the Germans when the Embassies were relieved by the

allies in 1900.

SOLAR SURFACE.

As Photographed at the Royal Observatory, Greenwich, showing sun spots

with umbræ, penumbræ, and faculæ.

SOLAR ECLIPSE, 1882.

From drawing by W. H. Wesley, Secretary R.A.S.; showing the

prominences, the corona, and an unknown comet.

JUPITER.

From a drawing by E. M. Antoniadi, showing transit of a satellite’s

shadow, the belts, and the “great red spot” (_Monthly Notices_, R. A.

S., vol. lix., pl. x.).

COPY OF THE DRAWING MADE BY PAUL FABRICIUS.

To define the path of comet 1556. After being lost for 300 years, this

drawing was recovered by the prolonged efforts of Mr. Hind and

Professor Littrow in 1856.

SIR WILLIAM HERSCHEL, F.R.S.—1738-1822.

Painted by Lemuel F. Abbott; National Portrait Gallery, Room XX.

GREAT COMET, NOV. 14TH, 1882. (Exposure 2hrs. 20m.)

By kind permission of Sir David Gill. From this photograph originated

all stellar chart-photography.

INDEX

Abul Wefa, 24

Acceleration of moon’s mean motion, 60

Achromatic lens invented, 88

Adams, J. C., 61, 65, 68, 69, 70, 87, 118, 124

Airy, G. B., 13, 30, 37, 65, 69, 70, 80, 81, 114, 119

Albetegnius, 24

Alphonso, 24

Altazimuth, 81

Anaxagoras, 14, 16

Anaximander, 14

Anaximenes, 14

Anderson, T. D., 137, 138

Ångstrom, A. J., 102

Antoniadi, 113

Apian, P., 63

Apollonius, 22, 23

Arago, 111

Argelander, F. W. A., 139

Aristarchus, 18, 29

Aristillus, 17, 19

Aristotle, 16, 30, 47

Arrhenius, 146

Arzachel, 24

Asshurbanapal, 12

Asteroids, discovery of, 67, 119

Astrology, ancient and modern, 1-7, 38

Backlund, 122

Bacon, R., 86

Bailly, 8, 65

Barnard, E. E., 115, 143

Beer and Mädler, 107, 110, 111

Behaim, 74

Bessel, F.W., 65, 79, 128, 134, 139

Biela, 123

Binet, 65

Biot, 10

Bird, 79, 80

Bliss, 80

Bode, 66, 69

Bond, G. P., 99, 117, 122

Bouvard, A., 65, 68

Bradley, J., 79, 80, 81, 87, 127, 128, 139

Bredechin, 146

Bremiker, 71

Brewster, D., 52, 91, 112

Brinkley, 128

Bruno, G., 49

Burchardt, 65, 123

Burnham, S. W., 134

Callippus, 15, 16, 31

Carrington, R. C., 97, 99, 114

Cassini, G. D., 107, 114, 115, 116, 117, 118

Cassini, J., 109, 129

Chacornac, 139

Chaldæan astronomy, 11-13

Challis, J., 69, 70, 71, 72

Chance, 88

Charles, II., 50, 81

Chinese astronomy, 8-11

Christie, W. M. H. (Ast. Roy.), 64, 82, 125

Chueni, 9

Clairaut, A. C., 56, 63, 65

Clark, A. G., 89, 135

Clerke, Miss, 106, 146

Comets, 120

Common, A. A., 88

Cooke, 89

Copeland, R., 142

Copernicus, N., 14, 24-31, 37, 38, 41, 42, 49, 128

Cornu, 85

Cowell, P. H., 3, 5, 64, 83

Crawford, Earl of, 84

Cromellin, A. C., 5, 64

D’Alembert, 65

Damoiseau, 65

D’Arrest, H. L., 34

Dawes, W. R., 100, 111

Delambre, J. B. J., 8, 27, 51, 65, 68

De la Rue, W., 2, 94, 99, 100, 131

Delaunay, 65

Democritus, 16

Descartes, 51

De Sejour, 117

Deslandres, II., 101

Desvignolles, 9

De Zach, 67

Digges, L., 86

Dollond, J., 87, 90

Dominis, A. di., 86

Donati, 120

Doppler, 92, 129

Draper, 99

Dreyer, J. L. E., 29,77

Dunthorne, 60

Dyson, 131

Eclipses, total solar, 103

Ecphantes, 16

Eddington, 131

Ellipse, 41

Empedocles, 16

Encke, J. F., 119, 122, 123, 133

Epicycles, 22

Eratosthenes, 18

Euclid, 17

Eudoxus, 15, 31

Euler, L., 60, 61, 62, 65, 88, 119

Fabricius, D.,95, 120, 121

Feil and Mantois, 88

Fizeau, H. L., 85, 92, 99

Flamsteed, J., 50, 58, 68, 78, 79, 93

Fohi, 8

Forbes, J. D., 52, 91

Foucault, L., 85, 99

Frauenhofer, J., 88, 90, 91

Galilei, G., 38, 46-49, 77, 93, 94, 95, 96, 107, 113, 115, 116, 133

Galle, 71, 72

Gascoigne, W., 45, 77

Gauss, C. F., 65, 67

Gauthier, 98

Gautier, 89

Gilbert, 44

Gill, D., 84, 85, 128, 135, 139, 140

Goodricke, J., 136

Gould, B. A., 139

Grant, R., 27, 47, 51, 86, 134

Graham, 79

Greek astronomy, 8-11

Gregory, J. and D., 87

Grimaldi, 113

Groombridge, S., 139

Grubb, 88, 89

Guillemin, 122

Guinand, 88

Hale, G. E., 101

Hall, A., 112

Hall, C. M., 88

Halley, E., 19, 51, 58, 60, 61, 62, 63, 64, 79, 120, 122, 125, 129

Halley’s comet, 62-64

Halm, 85

Hansen, P. A., 3, 65

Hansky, A. P., 100

Harding, C. L., 67

Heliometer, 83

Heller, 120

Helmholtz, H. L. F., 35

Henderson, T., 128

Henry, P. and P., 139, 140, 143

Heraclides, 16

Heraclitus, 14

Herodotus, 13

Herschel, W., 65, 68, 97, 107, 110, 114, 115, 116, 117, 118, 126, 127,

130, 131, 132, 141, 142

Herschel, J., 97, 111, 133, 134, 142

Herschel, A. S., 125

Hevelius, J., 178

Hind, J. R., 5, 64, 120, 121, 122

Hipparchus, 3, 18, 19, 20, 22, 23, 24, 26, 36, 55, 60, 74, 93, 137

Hooke, R., 51, 111, 114

Horrocks, J., 50, 56

Howlett, 100

Huggins, W., 92, 93, 99, 106, 120, 129, 137, 138, 142, 144

Humboldt and Bonpland, 124

Huyghens, C., 47, 77, 87, 110, 116, 117

Ivory, 65

Jansen, P. J. C., 105, 106

Jansen, Z., 86

Kaiser, F., 111

Kapteyn, J. C., 131, 138, 139

Keeler, 117

Kepler, J., 17, 23, 26, 29, 30, 36, 37, 38-46, 48, 49, 50, 52, 53, 63,

66, 77, 87, 93, 127, 137

Kepler’s laws, 42

Kirchoff, G.R., 91

Kirsch, 9

Knobel, E.B., 12, 13

Ko-Show-King, 76

Lacaile, N.L., 139

Lagrange, J.L., 61, 62, 65, 119

La Hire, 114

Lalande, J.J.L., 60, 63, 65, 66, 72, 139

Lamont, J., 98

Langrenus, 107

Laplace, P.S. de, 50, 58, 61, 62, 65,66, 123, 146

Lassel, 72, 88, 117, 118

Law of universal gravitation, 53

Legendre, 65

Leonardo da Vinci, 46

Lewis, G.C., 17

Le Verrier, U.J.J., 65, 68, 70, 71,72, 110, 118, 125

Lexell, 66, 123

Light year, 128

Lipperhey, H., 86

Littrow, 121

Lockyer, J.N., 103, 105, 146

Logarithms invented, 50

Loewy, 2, 100

Long inequality of Jupiter and Saturn, 50, 62

Lowell, P., 111, 112, 118

Lubienietz, S. de, 122

Luther, M., 38

Lunar theory, 37, 50, 56, 64

Maclaurin, 65

Maclear, T., 128

Malvasia, 77

Martin, 9

Maxwell, J. Clerk, 117

Maskelyne, N., 80, 130

McLean, F., 89

Medici, Cosmo di, 48

Melancthon, 38

Melotte, 83, 116

Meteors, 123

Meton, 15

Meyer, 57, 65

Michaelson, 85

Miraldi, 110, 114

Molyneux, 87

Moon, physical observations, 107

Mouchez, 139

Moyriac de Mailla, 8

Napier, Lord, 50

Nasmyth and Carpenter, 108

Nebulae, 141, 146

Neison, E., 108

Neptune, discovery of, 68-72

Newall, 89

Newcomb, 85

Newton, H.A., 124

Newton, I., 5, 19, 43, 49, 51-60, 62, 64, 68, 77, 79, 87, 90, 93, 94,

114, 127, 133

Nicetas, 16, 25

Niesten, 115

Nunez, P., 35

Olbers, H.W.M., 67

Omar, 11, 24

Oppolzer, 13, 125

Oudemans, 129

Palitsch, G., 64

Parallax, solar, 85, 86

Parmenides, 14

Paul III., 30

Paul V., 48

Pemberton, 51

Peters, C.A.F., 125, 128, 135

Photography, 99

Piazzi, G., 67, 128, 129, 139

Picard, 54, 77, 114

Pickering, E.C., 118, 135

Pingré, 13, 122

Plana, 65

Planets and satellites, physical observations, 109-119

Plato, 17, 23, 26, 40

Poisson, 65

Pond, J., 80

Pons, 122

Porta, B., 86

Pound, 87, 114

Pontecoulant, 64

Precession of the equinoxes, 19-21, 55, 57

Proctor, R.A., 111

Pritchett, 115

Ptolemy, 11, 13, 21, 22, 23, 24, 93

Puiseux and Loewy, 108

Pulfrich, 131

Purbach, G., 24

Pythagoras, 14, 17, 25, 29

Ramsay, W., 106

Ransome and May, 81

Reflecting telescopes invented, 87

Regiomontanus (Müller), 24

Respighi, 82

Retrograde motion of planets, 22

Riccioli, 107

Roberts, 137

Römer, O.,78, 114

Rosse, Earl of, 88, 142

Rowland, H. A., 92, 102

Rudolph H.,37, 39

Rumker, C., 139

Sabine, E., 98

Savary, 133

Schaeberle, J. M., 135

Schiaparelli, G. V., 110, 111, 124, 125

Scheiner, C., 87, 95, 96

Schmidt, 108

Schott, 88

Schröter, J. H., 107, 110, 111, 124, 125

Schuster, 98

Schwabe, G. H., 97

Secchi, A., 93, 144

Short, 87

Simms, J., 81

Slipher, V. M., 119

Socrates, 17

Solon, 15

Souciet, 8

South, J., 133

Spectroscope, 89-92

Spectroheliograph, 101

Spoerer, G. F. W., 98

Spots on the sun, 84;

periodicity of, 97

Stars, Parallax, 127;

proper motion, 129;

double, 132;

binaries, 132, 135;

new, 19, 36, 137;

catalogues of, 19, 36, 139;

spectra of, 143

Stewart, B., 2, 100

Stokes, G. G., 91

Stone, E. J., 139

Struve, C. L., 130

Struve, F. G. W,, 88, 115, 128, 133

Telescopes invented, 47, 86;

large, 88

Temple, 115, 125

Thales, 13, 16

Theon, 60

Transit circle of Römer, 78

Timocharis, 17, 19

Titius, 66

Torricelli, 113

Troughton, E., 80

Tupman, G. L., 120

Tuttle, 125

Tycho Brahe, 23, 25, 30, 33-38, 39, 40, 44, 50, 75, 77, 93, 94, 129,

137

Ulugh Begh, 24

Uranus, discovery of, 65

Velocity of light, 86, 128;

of earth in orbit, 128

Verbiest, 75

Vogel, H. C., 92, 129, 135, 136

Von Asten, 122

Walmsley, 65

Walterus, B., 24, 74

Weiss, E., 125

Wells, 122

Wesley, 104

Whewell, 112

Williams, 10

Wilson, A., 96, 100

Winnecke, 120

Witte, 86

Wollaston, 90

Wolf, M., 119, 125, 132

Wolf, R., 98

Wren, C., 51

Wyllie, A., 77

Yao, 9

Young, C. A., 103

Yu-Chi, 8

Zenith telescopes, 79, 82

Zöllner, 92

Zucchi, 113