BOOK IV. THE PHYSICAL PERIOD

BOOK IV. THE PHYSICAL PERIOD

We have seen how the theory of the solarsystem was slowly developed

by the constant efforts of the human mindto find out what are the

rules of cause and effect by which ourconception of the present

universe and its development seems to bebound. In the primitive ages

a mere record of events in the heavens andon the earth gave the only

hope of detecting those uniform sequencesfrom which to derive rules

or laws of cause and effect upon which torely. Then came the

geometrical age, in which rules were soughtby which to predict the

movements of heavenly bodies. Later, whenthe 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 theseventeenth century, for

the first time in history, it began to berecognised that the laws of

dynamics, exactly as they had beenestablished for our own terrestrial

world, hold good, with the same rigidinvariability, at least as far

as the limits of the solar system.

Throughout this evolution of thought andconjecture there were two

types of astronomers--those who suppliedthe facts, and those who

supplied the interpretation through thelogic 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 theheavens, when Secchi and

Huggins studied the chemistry of the starsby means of the

spectroscope, and when Warren De la Rue setup a photoheliograph at

Kew, we see that a progress in the samedirection as before, in the

evolution of our conception of theuniverse, was being made. Without

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

fact that not only do earthly dynamicsapply to the heavenly bodies,

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

and in the laws of heat, may be extendedwith confidence to the

heavenly bodies. Hence arose the branch ofastronomy called

astronomical physics, a science whichclaims a large portion of the

work of the telescope, spectroscope, andphotography. In this new

development it is more than ever essentialto follow the dictum of

Tycho Brahe--not to make theories until allthe necessary facts are

obtained. The great astronomers of to-daystill hold to Sir Isaac

Newton's declaration, "Hypotheses nonfingo." Each one may have his

suspicions of a theory to guide him in acourse of observation, and

may call it a working hypothesis.  But the cautious astronomer does

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

justified in including in his record thosevague speculations founded

on incomplete data which may be demolishedto-morrow, and which,

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

progress of true science.  Meanwhile the accumulation of facts has

been prodigious, and the revelations of thetelescope and spectroscope

entrancing.

12. THE SUN.

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

telescope to the heavenly bodies, was thatof 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 hasleft us many drawings of

these spots, and he fixed their period ofrotation as a lunar month.

[Illustration: SOLAR SURFACE, AsPhotographed at the Royal

Observatory, Greenwich, showing sun-spotswith umbrae, penumbrae, and

faculae.]

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 neverseen again.  In other cases,

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

bright areas, which are seen all over thesun's surface, but specially

in the neighbourhood of spots, and mostdistinctly near the sun's

edge, were discovered by Galileo. A hightelescopic power resolves

their structure into an appearance likewillow-leaves, or rice-grains,

fairly uniform in size, and more markedthan on other parts of the

sun's surface.

Speculations as to the cause of sun-spotshave never ceased from

Galileo's time to ours. He supposed them tobe clouds. Scheiner[1]

said they were the indications oftumultuous movements occasionally

agitating the ocean of liquid fire of whichhe supposed the sun to be

composed.

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

relative to the penumbra in the transit ofthe spot over the sun's

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

grey shelving sides. This was generallyaccepted, but later

investigations have contradicted itsuniversality. 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

  aremelted 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 beenguessing at it.  The solar

astronomer is still applying all theinstruments of modern research to

find out which of these suppositions, orwhat modification of any of

them, is nearest the truth. Theobstacle--one that is perhaps fatal to

a real theory--lies in the impossibility ofreproducing comparative

experiments in our laboratories or in ouratmosphere.

Sir William Herschel propounded anexplanation of Wilson's observation

which received much notice, but which, outof respect for his memory,

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

Sir John Herschel noticed that the spotsare mostly confined to two

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

zone of equatoreal calms is free fromspots. But it was

R. C. Carrington[4] who, by his continuousobservations at Redhill, in

Surrey, established the remarkable factthat, while the rotation

period in the highest latitudes, 50°, wherespots are seen, is

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

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

to much new information about the averagedistribution of spots at

different epochs.

Schwabe, of Dessau, began in 1826 to studythe solar surface, and,

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

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

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

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

eleven years has been found to be moreexact, all later observations,

besides the earlier ones which have beenhunted up for the purpose, go

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

lately Schuster[6] has given reasons foradmitting a number of

co-existent periods, of which theeleven-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 magneticelements--declination, dip, and

intensity. Australian and Canadianobservations both showed the

decennial period in all three elements.Wolf, of Zurich, and Gauthier,

of Geneva, each independently arrived atthe same conclusion.

It took many years before this coincidencewas accepted as certainly

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

proof for every new theory. But the lastdoubts have long vanished,

and a connection has been further tracedbetween violent outbursts of

solar activity and simultaneous magneticstorms.

The frequency of the Aurora Borealis wasfound by Wolf to follow the

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

magnetism. Wolf also collected oldobservations tracing the

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

Spoerer deduced a law of dependence of theaverage latitude of

sun-spots on the phase of the sun-spotperiod.

All modern total solar eclipse observationsseem to show that the

shape of the luminous corona surroundingthe moon at the moment of

totality has a special distinct characterduring the time of a

sun-spot maximum, and another, totallydifferent, during a sun-spot

minimum.

A suspicion is entertained that the totalquantity of heat received by

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

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

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

from a very long period of observations.

Solar photography has deprived astronomersof the type of Carrington

of the delight in devoting a life's work tocollecting data. It has

now become part of the routine work of anobservatory.

In 1845 Foucault and Fizeau took adaguerreotype photograph of the

sun. In 1850 Bond produced one of the moonof great beauty, Draper

having made some attempts at an evenearlier date. But astronomical

photography really owes its beginning to Dela Rue, who used the

collodion process for the moon in 1853, andconstructed the Kew

photoheliograph in 1857, from which datethese instruments have been

multiplied, and have given us an accuraterecord of the sun's surface.

Gelatine dry plates were first used byHuggins in 1876.

It is noteworthy that from the outset De laRue recognised the value

of stereoscopic vision, which is now knownto be of supreme

accuracy. In 1853 he combined pairs ofphotographs of the moon in the

same phase, but under different conditionsregarding libration,

showing the moon from slightly differentpoints of view.  These in the

stereoscope exhibited all the reliefresulting from binocular vision,

and looked like a solid globe.  In 1860 he used successive photographs

of the total solar eclipsestereoscopically, to prove that the red

prominences belong to the sun, and not tothe moon.  In 1861 he

similarly combined two photographs of asun-spot, the perspective

effect showing the umbra like a floor atthe bottom of a hollow

penumbra; and in one case the faculæ werediscovered to be sailing

over a spot apparently at some considerableheight.  These appearances

may be partly due to a proper motion; but,so far as it went, this was

a beautiful confirmation of Wilson'sdiscovery. Hewlett, however, in

1894, after thirty years of work, showed thatthe spots are not always

depressions, being very subject todisturbance.

The Kew photographs [7] contributed a vastamount of information about

sun-spots, and they showed that the faculægenerally 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, hasalways been a subject of great

interest; and much was done by men withexceptionally keen eyesight,

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

of the changes in appearance of theso-called rice-grains, about 1" in

diameter. The rapid transformations andcirculations of these

rice-grains, if thoroughly studied, mightlead to a much better

knowledge of solar physics. This seemedalmost 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 schemeof photography, which

might almost be called a solarcinematograph. He took photographs of

the sun at intervals of fifteen or thirtyseconds, and then enlarged

selected portions of these two hundredtimes, giving a picture

corresponding to a solar disc of six metresdiameter. In these

enlarged pictures he was able to trace themovements, and changes of

shape and brightness, of individualrice-grains. Some granules become

larger or smaller. Some seem to rise out ofa 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 minuteor less, although each of

the grains may be the size of GreatBritain. Generally they move

together in groups of very variousvelocities, up to forty kilometres

a second. These movements seem to have definite relation to any

sun-spots in the neighbourhood. From theresults already obtained it

seems certain that, if this method ofobservation be continued, it

cannot fail to supply facts of the greatestimportance.

It is quite impossible to do justice hereto 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 ofhuman thought and

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

progress, and an entirely new process ofstudying the photosphere has

been independently perfected by ProfessorHale at Chicago, and

Deslandres at Paris.[8] They have succeededin photographing the sun's

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

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

"Spectroheliograph." Thespectroscope is placed with its slit in the

focus of an equatoreal telescope, pointedto the sun, so that the

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

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

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

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

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

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

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

direction behind the second slit; and assuccessive sections of the

sun's image in the equatoreal enter theapparatus, so are these

sections successively thrown in theirproper place on the photographic

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

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

edge, but all over his surface. The actualmechanical method of

carrying out the observation is not quiteso simple as what is here

described.

By choosing another line of the spectruminstead of calcium K--for

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

showing the appearance of the calciumfloculi, and the other of the

hydrogen floculi, on the same part of thesolar surface; and nothing

is more astonishing than to note the totalwant of resemblance in the

forms shown on the two. This mode ofresearch promises to afford many

new and useful data.

The spectroscope has revealed the factthat, broadly speaking, the sun

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

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

Rowland, of Baltimore, after havingperfected the art of making true

gratings with equidistant lines ruled onmetal for producing spectra,

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

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

spectroscope, and was thus enabled tocompare the spectrum of a spot

with that of the general solar surface. Theobservation proved the

darkness of a spot to be caused byincreased absorption of light, not

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

spectrum. In 1883 Young resolved thiscontinuous obscurity into an

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

way on to the general solar surface.Lockyer also detected

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

produced by a rapid motion in the line ofsight. It has been found

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

predominance of either in a sun-spot. Thevelocity of motion thus

indicated in the line of sight sometimesappears to amount to 320

miles a second. But it must be rememberedthat pressure of a gas has

some effect in displacing the spectrallines.  So we must go on,

collecting data, until a time comes whenthe meaning of all the facts

can be made clear.

_Total Solar Eclipses_.--During total solareclipses the time is so

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

appearance could not always be trusted. Thered prominences of jagged

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

streamers radiating or interlacing, havemuch detail that can hardly

be recorded in a sketch. By the aid ofphotography a number of records

can be taken during the progress oftotality. From a study of these

the extent of the corona is demonstrated inone case to extend to at

least six diameters of the moon, though theeye has traced it

farther. This corona is still one of thewonders of astronomy, and

leads to many questions. What is itsconsistency, if it extends many

million miles from the sun's surface? Howis it that it opposed no

resistance to the motion of comets whichhave almost grazed the sun's

surface? Is this the origin of the zodiacallight?  The character of

the corona in photographic records has beenshown to depend upon the

phase of the sun-spot period. During thesun-spot maximum the corona

seems most developed over thespot-zones--i.e., neither at the

equator nor the poles. The four greatsheaves of light give it a

square appearance, and are made up of raysor plumes, delicate like

the petals of a flower.  During a minimum the nebulous ring seems to

be made of tufts of fine hairs withaigrettes 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, eclipsespectroscopy began with the Indian

eclipse, in which all observers found thatthe red prominences showed

a bright line spectrum, indicating thepresence 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 uneclipsedsun.  Huggins had suggested this

observation in February, 1868, his ideabeing to use prisms of such

great dispersive power that the continuousspectrum reflected by our

atmosphere should be greatly weakened,while a bright line would

suffer no diminution by the highdispersion.  On October 20th

Lockyer,[10] having news of the eclipse,but not of Jansen's

observations the day after, was able to seethese lines. This was a

splendid performance, for it enabled theprominences to be observed,

not only during eclipses, but every day.Moreover, the next year

Huggins was able, by using a wide slit, tosee 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, andhelium; that of the former

including a number of metals.

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

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

the two components of the D line. It didnot correspond with any known

terrestrial element, and the unknownelement was called "helium." It

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

in the mineral cleavite.

The spectrum of the corona is partlycontinuous, indicating light

reflected from the sun's body. But it alsoshows a green line

corresponding with no known terrestrialelement, and the name

"coronium" has been given to thesubstance causing it.

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

by photography and the spectroscope.Speculations and hypotheses in

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

theory evolved to explain all the phenomenaof the storm-swept

metallic atmosphere of the sun.

The proceedings of scientific societiesteem with such facts and

"working hypotheses," and thebest of them have been collected by Miss

Clerke in her _History of Astronomy duringthe Nineteenth Century_. As

to established facts, we learn from thespectroscopic researches (1)

that the continuous spectrum is derivedfrom the _photosphere_ or

solar gaseous material compressed almost toliquid consistency; (2)

that the _reversing layer_ surrounds it andgives rise to black

lines in the spectrum; that the_chromosphere_ surrounds this, is

composed mainly of hydrogen, and is thecause of the red prominences

in eclipses; and that the gaseous _corona_surrounds all of

these, and extends to vast distancesoutside 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 SolarPhysics," by De la Rue, Stewart and Loewy;

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

[8] "The Sun as Photographed on the Kline"; _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 aboutthe moon commence with

Galileo's discovery that her surface hasmountains and valleys, like

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

us, there is periodically a slight twist tolet us see a little round

the eastern or western edge. This wascalled _libration_, and the

explanation was clear when it wasunderstood that in showing always

the same face to us she makes onerevolution a month on her axis

_uniformly_, and that her revolution roundthe earth is not

uniform.

Galileo said that the mountains on the moonshowed greater differences

of level than those on the earth.  Shröter supported this

opinion. W. Herschel opposed it. But Beerand Mädler measured the

heights of lunar mountains by theirshadows, and found four of them

over 20,000 feet above the surroundingplains.

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

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

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

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

of Athens (1878); and, above all, thephotographic atlas by Loewy and

Puiseux.

The details of the moon's surface requirefor their discussion a whole

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

few words must suffice. Mountain rangeslike our Andes or Himalayas

are rare. Instead of that, we see animmense number of circular

cavities, with rugged edges and flatinterior, often with a cone in

the centre, reminding one of instantaneousphotographs of the splash

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

sixty miles across, some more. They aregenerally spoken of as

resembling craters of volcanoes, active orextinct, on the earth. But

some of those who have most fully studiedthe shapes of craters deny

altogether their resemblance to thecircular objects on the moon.

These so-called craters, in many parts, areseen to be closely

grouped, especially in the snow-white partsof the moon. But there are

great smooth dark spaces, like the clearblack ice on a pond, more

free from craters, to which the equallyinappropriate name of seas has

been given. The most conspicuous crater,_Tycho_, is near the south

pole. At full moon there are seen toradiate from Tycho numerous

streaks of light, or "rays,"cutting through all the mountain

formations, and extending over fully halfthe lunar disc, like the

star-shaped cracks made on a sheet of iceby a blow.  Similar cracks

radiate from other large craters. It mustbe mentioned that these

white rays are well seen only in full lightof the sun at full moon,

just as the white snow in the crevasses ofa 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 inthe surface going straight

through mountain and valley.

The moon shares with the sun the advantageof being a good subject for

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

apparent size, and the abundance ofillumination. The consequence is

that the finest details of the moon, asseen in the largest telescope

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

No certain changes have ever been observed;but several suspicions

have been expressed, especially as to thesmall crater _Linné_, in the

_Mare Serenitatis_. It is now generallyagreed that no certainty can

be expected from drawings, and that forreal evidence we must await

the verdict of photography.

No trace of water or of an atmosphere hasbeen found on the moon. It

is possible that the temperature is toolow. In any case, no

displacement of a star by atmosphericrefraction at occultation has

been surely recorded. The moon seems to bedead.

The distance of the moon from the earth isjust now the subject of

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

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

a crater (Mösting A), instead of the moon'sedge, as the point whose

distance is to be measured.

_The Inferior Planets_.--When the telescopewas invented, the phases

of Venus attracted much attention; but thebrightness of this planet,

and her proximity to the sun, as withMercury also, seemed to be a bar

to the discovery of markings by which theaxis and period of rotation

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

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

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

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

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

ascribed to Mercury; but in neither casehas the evidence been

generally accepted. Twenty-four hours isprobably about the period of

rotation for each of these planets.

Several observers have claimed to have seena planet within the orbit

of Mercury, either in transit over thesun's surface or during an

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

have received little attention but for thefact that the motion of

Mercury has irregularities which have notbeen accounted for by known

planets; and Le Verrier[3] has stated thatan intra-Mercurial planet

or ring of asteroids would account for theunexplained part of the

motion of the line of apses of Mercury'sorbit amounting to 38" per

century.

_Mars_.--The first study of the appearanceof Mars by Miraldi led him

to believe that there were changesproceeding 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 hisobservations indicated a

melting of these ice-caps in the Martiansummer.

Schröter attributed the other markings onMars to drifting clouds. But

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

always in the same place, though sometimesblurred by mist in the

local winter. A spot sketched by Huyghensin 1672, one frequently seen

by W. Herschel in 1783, another by Arago in1813, and nearly all the

markings recorded by Beer and Mädler in1830, were seen and drawn by

F. Kaiser in Leyden during seventeen nightsof the opposition of 1862

(_Ast. Nacht._, No. 1,468), whence hededuced the period of rotation

to be 24h. 37m. 22s.,62--or one-tenth of asecond less than the period

deduced by R. A. Proctor from a drawing byHooke in 1666.

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

Venus be about twenty-four hours, as seemsprobable, all the four

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

planets rotate in about ten hours (Uranusand Neptune being still

indeterminate).

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

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

ruddy colour of Mars to its soil ratherthan to its atmosphere.

The observations of that keen-sightedobserver Dawes led to the first

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

interest in the planet by the discovery ofcanals, uniformly about

sixty miles wide, running generally ongreat circles, some of them

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

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

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

to 400 miles distant from its fellow.

The existence of these canals has beendoubted.  Mr. Lowell has now

devoted years to the subject, has drawnthem over and over again, and

has photographed them; and accepts theexplanation that they are

artificial, and that vegetation grows ontheir banks.  Thus is revived

the old controversy between Whewell andBrewster as to the

habitability of the planets. The newarguments are not yet generally

accepted. Lowell believes he has, with thespectroscope, proved the

existence of water on Mars.

One of the most unexpected and interestingof all telescopic

discoveries took place in the opposition of1877, when Mars was

unusually near to the earth. The WashingtonObservatory had acquired

the fine 26-inch refractor, and Asaph Hallsearched for satellites,

concealing the planet's disc to avoid theglare. On August 11th he had

a suspicion of a satellite. This wasconfirmed 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 powerfultelescopes, and only at the

times of opposition. Their diameters areestimated 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 toremarkable results.

Obviously the easterly motion of thesatellite overwhelms the diurnal

rotation of the planet, and Phobos mustappear to the inhabitants, if

they exist, to rise in the west and set inthe east, showing two or

even three full moons in a day, so that,sufficiently well for the

ordinary purposes of life, the hour of theday can be told by its

phases.

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

interesting telescopic visual discoverymade with the large telescopes

of the last half century; photographyhaving been the means of

discovering all the other new satellitesexcept 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 ofJupiter's satellites was followed

by the discovery of his belts. Zucchi andTorricelli seem to have seen

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

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

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

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

observed many spots moving from east towest, whence he concluded that

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

permanent spot during twenty-ninerotations, and fixed the period at

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

than those in higher latitudes (the same asCarrington found for the

sun); and W. Herschel confirmed this in1778-9.

Jupiter's rapid rotation ought, accordingto Newton's theory, to be

accompanied by a great flattening at thepoles. Cassini had noted an

oval form in 1691. This was confirmed by LaHire, Römer, and

Picard. Pound measured the ellipticity =1/(13.25).

W. Herschel supposed the spots to be massesof cloud in the

atmosphere--an opinion still accepted.  Many of them were very

permanent. Cassini's great spot vanishedand reappeared nine times

between 1665 and 1713. It was close to thenorthern margin of the

southern belt. Herschel supposed the beltsto be the body of the

planet, and the lighter parts to be cloudsconfined to certain

latitudes.

In 1665 Cassini observed transits of thefour satellites, and also saw

their shadows on the planet, and worked outa lunar theory for

Jupiter. Mathematical astronomers havetaken great interest in the

perturbations of the satellites, becausetheir relative periods

introduce peculiar effects. Airy, in hisdelightful book,

_Gravitation_, has reduced theseinvestigations to simple

geometrical explanations.

In 1707 and 1713 Miraldi noticed that the fourthsatellite varies much

in brightness. W. Herschel found thisvariation to depend upon its

position in its orbit, and concluded thatin the positions of

feebleness it is always presenting to us aportion of its surface,

which does not well reflect the sun'slight; proving that it always

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

fact had also been established for Saturn'sfifth satellite, and may

be true for all satellites.

In 1826 Struve measured the diameters ofthe four satellites, and

found them to be 2,429, 2,180, 3,561, and3,046 miles.

In modern times much interest has beentaken in watching a rival to

Cassini's famous spot. The "great redspot" was first observed by

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

whitish zone beneath the dark southernequatorial band, shaped like

the new war balloons, 30,000 miles long and7,000 miles across. The

next year it was brick-red. A white spotbeside it completed a

rotation in less time by 5½ minutes thanthe red spot--a difference

of 260 miles an hour. Thus they cametogether again every six weeks,

but the motions did not continueuniform.  The spot was feeble in

1882-4, brightened in 1886, and, after manychanges, is still visible.

Galileo's great discovery of Jupiter's fourmoons 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. Thisproved to be a new satellite

(fifth), nearer to the planet than anyother, and revolving round it

in 11h. 57m. 23s. Between its rising andsetting there must be an

interval of 2½ Jovian days, and two orthree full moons. The sixth

and seventh satellites were found by theexamination of photographic

plates at the Lick Observatory in 1905,since which time they have

been continuously photographed, and theirorbits traced, at Greenwich.

On examining these plates in 1908 Mr.Melotte detected the eighth

satellite, which seems to be revolving in aretrograde orbit three

times as far from its planet as the nextone (seventh), in these two

points agreeing with the outermost ofSaturn's satellites (Phoebe).

_Saturn._--This planet, with its marvellousring, was perhaps the most

wonderful object of those first examined byGalileo's telescope. He

was followed by Dominique Cassini, whodetected bands like Jupiter's

belts. Herschel established the rotation ofthe planet in 1775-94.

From observations during one hundredrotations he found the period to

be 10h. 16m. 0s., 44. Herschel alsomeasured 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 thering was in line with the

earth, and the ring disappeared (December4th, 1612). It was not until

1656 that Huyghens, in his small pamphlet_De Saturni Luna Observatio

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

his Systema Saturnium, he gave his reasonsand translated the cypher:

"The planet is surrounded by a slenderflat ring, everywhere distinct

from its surface, and inclined to theecliptic." This theory explained

all the phases of the ring which hadpuzzled others. This ring was

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

have got accustomed to it. But Huyghens'sdiscovery was received with

amazement.

In 1675 Cassini found the ring to bedouble, the concentric rings

being separated by a black band--a factwhich was placed beyond

dispute by Herschel, who also found thatthe thickness of the ring

subtends an angle less than 0".3.Shröter estimated its thickness at

500 miles.

Many speculations have been advanced toexplain the origin and

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

from Saturn's equator as a liquid ring, andafterwards solidified. He

noticed that the outside would have agreater velocity, and be less

attracted to the planet, than the innerparts, and that equilibrium

would be impossible; so he supposed it tohave solidified into a

number of concentric rings, the exteriorones having the least

velocity.

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

demonstration that the rings must becomposed of meteoritic matter

like gravel. Even so, there must becollisions absorbing the energy of

rotation, and tending to make the ringseventually fall into the

planet. The slower motion of the externalparts has been proved by the

spectroscope in Keeler's hands, 1895.

Saturn has perhaps received more than itsshare of attention owing to

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

J. D. Cassini in 1671, discovered the sixthand eighth satellites

(Titan and Japetus). Cassini lost hissatellite, and in searching for

it found Rhea (the fifth) in 1672, besideshis old friend, whom he

lost again. He added the third and fourthin 1684 (Tethys and

Dione). The first and second (Mimas andEncelades) were added by

Herschel in 1789, and the seventh(Hyperion) simultaneously by Lassel

and Bond in 1848. The ninth (Phoebe) wasfound on photographs, by

Pickering in 1898, with retrograde motion;and he has lately added a

tenth.

The occasional disappearance of Cassini'sJapetus was found on

investigation to be due to the same causesas that of Jupiter's fourth

satellite, and proves that it always turnsthe same face to the

planet.

_Uranus and Neptune_.--The splendiddiscoveries of Uranus and two

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

and Le Verrier in 1846, have been alreadydescribed. Lassel added two

more satellites to Uranus in 1851, andfound Neptune's satellite in

1846. All of the satellites of Uranus haveretrograde motion, and

their orbits are inclined about 80° to theecliptic.

The spectroscope has shown the existence ofan absorbing atmosphere on

Jupiter and Saturn, and there aresuspicions that they partake

something of the character of the sun, and emitsome light besides

reflecting solar light. On both planetssome absorption lines seem to

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

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

agree with a line in the spectrum of somereddish stars.

Uranus and Neptune are difficult to observespectroscopically, but

appear to have peculiar spectra agreeingtogether. Sometimes Uranus

shows Frauenhofer lines, indicatingreflected solar light. But

generally these are not seen, and six broadbands of absorption

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

Jupiter and Saturn. Neptune is a verydifficult object for the

spectroscope.

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

Flagstaff Observatory, succeeded in 1907 inrendering some plates

sensitive far into the red. A reproductionis given of photographed

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

new lines and bands; (2) intensification ofhydrogen 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 greaterincrease in Neptune.

_Asteroids_.--The discovery of these newplanets has been

described. At the beginning of the lastcentury it was an immense

triumph to catch a new one. Sincephotography was called into the

service by Wolf, they have been caughtevery year in shoals. It is

like the difference between sea fishingwith the line and using a

steam trawler. In the 1908 almanacs nearlyseven hundred asteroids are

included. The computation of theirperturbations and ephemerides by

Euler's and Lagrange's method of variableelements became so laborious

that Encke devised a special process forthese, which can be applied

to many other disturbed orbits. [8]

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

asteroid, the stars are photographed aspoints because the telescope

is made to follow their motion; but theasteroids, by their proper

motion, appear as short lines.

The discovery of Eros and the photographicattack upon its path have

been described in their relation to findingthe sun's distance.

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

and period equal to that of Jupiter. Tothree 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 cometof 1682 was a member of

the solar system, these wonderful objectshave had a new interest for

astronomers; and a comparison of orbits hasoften identified the

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

the difference from a parabola wasimperceptible in the small portion

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

of 1556, of whose identity with the cometof 1264 there could be

little doubt.  Hind wanted to compute the orbit more exactlythan

Halley had done. He knew that observationshad been made, but they

were lost. Having expressed his desire fora search, all the

observations of Fabricius and of Heller,and also a map of the comet's

path among the stars, were eventuallyunearthed in the most unlikely

manner, after being lost nearly threehundred years.  Hind and others

were certain that this comet would returnbetween 1844 and 1848, but

it never appeared.

When the spectroscope was first applied tofinding the composition of

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

are made of. The first opportunity came in1864, when Donati observed

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

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

compared the spectrum of Winnecke's cometwith that of a Geissler tube

containing olefiant gas, and found exactagreement. Nearly all comets

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

band spectra differing from the normaltype. Also a certain kind of

continuous spectrum, as well as reflectedsolar light showing

Frauenhofer lines, have been seen.

[Illustration: COPY OF THE DRAWING MADE BYPAUL FABRICIUS.  To define

the path of comet 1556. After being lostfor 300 years, this drawing

was recovered by the prolonged efforts ofMr. Hind and Professor

Littrow in 1856.]

When Wells's comet, in 1882, approachedvery close indeed to the sun,

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

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

referred to books on descriptive astronomy,or to monographs on

comets.[2] Nor can the very uncertainspeculations about the structure

of comets' tails be given here. A newexplanation has been proposed

almost every time that a great discoveryhas been made in the theory

of light, heat, chemistry, or electricity.

Halley's comet remained the only one ofwhich a prediction of the

return had been confirmed, until the orbitof the small, ill-defined

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

period of 3⅓years. It was predicted to return in 1822, and was

recognised by him as identical with manyprevious comets. This comet,

called after Encke, has showed in each ofits returns an inexplicable

reduction of mean distance, which led tothe assertion of a resisting

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

Since that date fourteen comets have beenfound with elliptic orbits,

whose aphelion distances are all about thesame as Jupiter's mean

distance; and six have an aphelion distanceabout ten per cent,

greater than Neptune's mean distance. Othercomets are similarly

associated with the planets Saturn andUranus.

The physical transformations of comets areamong the most wonderful of

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

the greatest interest attaches to thereduction of radius vector of

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

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

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

travel in exactly the same orbit under thesun's gravitation, and that

their mass is not sufficient to retain theparts together very

forcibly; also that the inevitablecollision of particles, or else

fluid friction, is absorbing energy, and soreducing the comet's

velocity.

In 1770 Lexell discovered a comet which, aswas afterwards proved by

investigations of Lexell, Burchardt, andLaplace, had in 1767 been

deflected by Jupiter out of an orbit inwhich it was invisible from

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

seen. In 1779 it again approached Jupitercloser than some of his

satellites, and was sent off in anotherorbit, never to be again

recognised.

But our interest in cometary orbits hasbeen added to by the discovery

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

into discrete parts like Biela's, must intime 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'sorbit, a meteor shower is

the result.

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

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

in the constellation Leo. A similar displayhad been witnessed in

Mexico by Humboldt and Bonpland on November12th, 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 wasindicated. 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 therate of one day in seventy

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

to calculate what the amount would be onall the five suppositions

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

that none gave one day in seventy yearsexcept the 33¼-year period,

which did so exactly. H. A. Newtonpredicted a return of the shower on

the night of November 13th-14th, 1866. Heis now dead; but many of us

are alive to recall the wonder andenthusiasm with which we saw this

prediction being fulfilled by the grandestdisplay of meteors ever

seen by anyone now alive.

The _progression_ of the nodes proved thepath of the meteor

stream to be retrograde. The _radiant_ hadalmost the exact

longitude of the point towards which theearth 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, bycomparing this with the earth's

velocity, the latitude of the radiantenabled the inclination to be

determined, while the longitude of theearth that night was the

longitude of the node. In such a waySchiaparelli was able to find

first the elements of the orbit of theAugust meteor shower

(Perseids), and to show its identity withthe orbit of Tuttle's comet

1862.iii. Then, in January 1867, Le Verriergave the elements of the

November meteor shower (Leonids); andPeters, of Altona, identified

these with Oppolzer's elements for Tempel'scomet 1866--Schiaparelli

having independently attained both of theseresults. Subsequently

Weiss, of Vienna, identified the meteorshower of April 20th (Lyrids)

with comet 1861. Finally, thatindefatigable worker on meteors,

A. S. Herschel, added to the number, and in1878 gave a list of

seventy-six coincidences between cometaryand meteoric orbits.

Cometary astronomy is now largely indebtedto photography, not merely

for accurate delineations of shape, butactually for the discovery of

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

comets at distances from their perihelia sogreat as to prevent their

visual observation. Thus has Wolf, ofHeidelburg, found upon old

plates the position of comet 1905.v., as astar of the 15.5 magnitude,

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

the importance of finding out thedivergence of a cometary orbit from

a parabola, its period, and its apheliondistance, this increase of

range attains the very highest value.

The present Astronomer Royal, appreciatingthis possibility, has been

searching by photography for Halley's cometsince November, 1907,

although its perihelion passage will nottake place until April, 1910.

FOOTNOTES:

[1] In 1874, when the writer was crossingthe Pacific Ocean in

H.M.S. "Scout," Coggia's cometunexpectedly appeared, and (while

Colonel Tupman got its positions with thesextant) he tried to use the

prism out of a portable direct-visionspectroscope, without success

until it was put in front of theobject-glass of a binocular, when, to

his great joy, the three band images wereclearly 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_, byPingré, Paris, 1783; _Donati's

Comet_, by Bond.

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

support, and later ones, especially thoseby Backlund (also of

St. Petersburg), seem to discredit, theidea of a resisting medium.

15. THE FIXED STARS AND NEBUL.

Passing now from our solar system, whichappears to be subject to the

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

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

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

earnestly directed since telescopes havebeen so much enlarged.

Photography also has enabled a vast amountof work to be covered in a

comparatively short period, and thespectroscope has given them the

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

detecting any motion in the line of sightfrom less than a mile a

second and upwards in any star, howeverdistant, provided it be bright

enough.

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

Lemuel F. Abbott; National PortraitGallery, Room XX.]

In the field of telescopic discovery beyondour solar system there is

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

to whom we owe the greatest discovery indynamical astronomy among the

stars--viz., that the law of gravitationextends to the most distant

stars, and that many of them describeelliptic orbits about each

other. W. Herschel was born at Hanover in1738, came to England in

1758 as a trained musician, and died in1822. He studied science when

he could, and hired a telescope, until helearnt to make his own

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

years. He discovered 2,500 nebulæ and 806double stars, counted the

stars in 3,400 guage-fields, and comparedthe principal stars

photometrically.

Some of the things for which he is bestknown were results of those

accidents that happen only to theindefatigable enthusiast. Such was

the discovery of Uranus, which led to fundsbeing provided for

constructing his 40-feet telescope, afterwhich, in 1786, he settled

at Slough. In the same way, while trying todetect the annual parallax

of the stars, he failed in that quest, butdiscovered binary systems

of stars revolving in ellipses round eachother; just as Bradley's

attack on stellar parallax failed, but ledto the discovery of

aberration, nutation, and the true velocityof light.

_Parallax_.--The absence of stellarparallax was the great

objection to any theory of the earth'smotion prior to Kepler's

time. It is true that Kepler's theoryitself could have been

geometrically expressed equally well withthe earth or any other point

fixed. But in Kepler's case the obviouslyimplied physical theory of

the planetary motions, even before Newtonexplained the simplicity of

conception involved, made astronomers quiteready to waive the claim

for a rigid proof of the earth's motion bymeasurement of an annual

parallax of stars, which they had insistedon in respect of

Copernicus's revival of the idea of theearth's orbital motion.

Still, the desire to measure this parallaxwas only intensified by the

practical certainty of its existence, andby repeated failures. The

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

early in the nineteenth century, alsofailed. The first successes,

afterwards confirmed, were by Bessel andHenderson.  Both used stars

whose proper motion had been found to belarge, as this argued

proximity. Henderson, at the Cape of GoodHope, observed α

Centauri, whose annual proper motion hefound to amount to 3".6, in

1832-3; and a few years later deduced itsparallax 1".16.  His

successor at the Cape, Maclear, reducedthis to 0".92.

In 1835 Struve assigned a doubtful parallaxof 0".261 to Vega (α

Lyræ). But Bessel's observations, between1837 and 1840, of 61 Cygni,

a star with the large proper motion of over5", established its annual

parallax to be 0".3483; and this wasconfirmed 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 fixedstar; and its light takes 4⅓

years to reach us. The light year is takenas the unit of measurement

in the starry heavens, as the earth's meandistance is "the

astronomical unit" for the solarsystem.[3] The proper motions and

parallaxes combined tell us the velocity ofthe motion of these stars

across the line of sight: α Centauri 14.4miles a second=4.2

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

astronomical units a year. These successesled to renewed zeal, and

now the distances of many stars are knownmore or less accurately.

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

nearest, have not shown a parallaxamounting to a twentieth of a

second of arc. Among these are Canopus, αOrionis, α Cygni, β

Centauri, and γ Cassiopeia. Oudemans haspublished 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 TychoBrahe fixed its position. In

1792 Piazzi noted the motion of 61 Cygni asgiven above. For a long

time the greatest observed proper motionwas that of a small star 1830

Groombridge, nearly 7" a year; butothers have since been found

reaching as much as 10".

Now the spectroscope enables the motion ofstars to be detected at a

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

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

motion and parallax must also be known.

When Huggins first applied the Dopplerprinciple to measure velocities

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

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

exposures of photographic plates.

It has often been noticed that stars whichseem to belong to a group

of nearly uniform magnitude have the same propermotion. The

spectroscope has shown that these have alsooften 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 wastoo faint for a

spectroscopic measurement, but all the othershave been shown to be

approaching us at a rate of twelve totwenty miles a second. The same

has been proved for proper motion, and lineof sight motion, in the

case of Pleiades and other groups.

Maskelyne measured many proper motions ofstars, from which W.

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

the most part due to a motion of the solarsystem 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 thatthe point first indicated

by Herschel, from totally insufficientdata, agrees well with modern

estimates.

Comparing the proper motions and parallaxesto 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 befifteen miles a second, or

five astronomical units a year.

The work of Herschel in this matter hasbeen checked by comparing

spectroscopic velocities in the line ofsight which, so far as the

sun's motion is concerned, would give amaximum rate of approach for

stars near Hercules, a maximum rate ofrecession for stars in the

opposite part of the heavens, and no effectfor stars half-way

between. In this way the spectroscope hasconfirmed generally

Herschel's view of the direction, and makesthe velocity eleven miles

a second, or nearly four astronomical unitsa year.

The average proper motion of a firstmagnitude star has been found to

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

bright stars are nearer than all smallstars, or that they show

greater proper motion for that reason, isfound to be far from the

truth. Many statistical studies have beenmade in this connection, and

interesting results may be expected fromthis treatment in the hands

of Kapteyn of Groningen, and others.[9]

On analysis of the directions of propermotions of stars in all parts

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

solar motion towards Hercules, two generaldrifts of stars in nearly

opposite directions, which can be detectedin any part of the

heavens. This result has been confirmedfrom independent data by

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

Photography promises to assist in themeasurement of parallax and

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

extended the applications of stereoscopicvision to astronomy--a

subject which De la Rue took up in theearly days of photography.  He

has made a stereo-comparator of greatbeauty and convenience for

comparing stereoscopically two starphotographs taken at different

dates. Wolf of Heidelberg has used this formany purposes. His

investigations depending on the solarmotion in space are remarkable.

He photographs stars in a direction atright angles to the line of the

sun's motion. He has taken photographs ofthe same region fourteen

years apart, the two positions of hiscamera 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 seemto be much nearer. Many of

the stars are thus seen to be suspended inspace at different

distances corresponding exactly to theirreal distances from our solar

system, except when their proper motioninterferes. The effect is most

striking; the accuracy of measurementexceeds that of any other method

of measuring such displacements, and itseems that with a long

interval of time the advantage of themethod increases.

_Double Stars._--The large class of doublestars has always been much

studied by amateurs, partly for theirbeauty and colour, and partly as

a test for telescopic definition. Among themany unexplained stellar

problems there is one noticed in doublestars that is thought by some

to be likely to throw light on stellarevolution. It is this: There

are many instances where one star of thepair is comparatively faint,

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

the general colour of the faint companionis invariably to be classed

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

the principal star.

_Binary Stars._--Sir William Herschel beganhis observations of double

stars in the hope of discovering an annualparallax of the stars. In

this he was following a suggestion ofGalileo's. The presumption is

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

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

noting the distance between the pair atdifferent times of the year, a

delicate test of parallax is provided,unaffected by major

instrumental errors.

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

character to indicate parallax. Followingthis by further observation,

he found that the motions were not uniformnor rectilinear, and by a

clear analysis of the movements he establishedthe remarkable and

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

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

approximate period of revolution of some ofthese: Castor, 342 years;

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

Twenty years later Sir John Herschel andSir James South, after

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

one pair of Coronæ having in the intervalcompleted more than a whole

revolution.

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

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

whole universe. His observations wereconfirmed by F.G.W. Struve (born

1793, died 1864), who carried on the workat Dorpat. But it was first

to Savary,[13] and later to Encke and SirJohn Herschel, that we owe

the computation of the elliptic elements ofthese stars; also the

resulting identification of their law offorce with Newton's force of

gravitation applied to the solar system,and the force that makes an

apple fall to the ground. As Grant wellsays in his _History_:

"This may be justly asserted to be oneof the most sublime truths

which astronomical science has hithertodisclosed to the researches of

the human mind."

Latterly the best work on double stars hasbeen 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 orbitsare about the size of the

orbit of Uranus, these being probably amongthe smallest stellar

orbits.

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

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

labour and originality involved in thecalculation) that entrances the

imagination became possible, and wasrealised by Bessel--the discovery

of an unknown body by its gravitationaldisturbance on one that was

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

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

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

each case to the attraction of an invisiblecompanion, the period of

revolution of Sirius being about half acentury. Later he said: "I

adhere to the conviction that Procyon andSirius form real binary

systems, consisting of a visible and aninvisible star.  There is no

reason to suppose luminosity an essentialquality of cosmical

bodies. The visibility of countless starsis no argument against the

invisibility of countless others."This grand conception led Peters to

compute more accurately the orbit, and toassign the place of the

invisible companion of Sirius. In 1862Alvan G. Clark was testing a

new 18-inch object-glass (now at Chicago)upon Sirius, and, knowing

nothing of these predictions, actuallyfound the companion in the very

place assigned to it. In 1896 the companionof Procyon was discovered

by Professor Schaeberle at the LickObservatory.

Now, by the refined parallax determinationsof Gill at the Cape, we

know that of Sirius to be 0".38. Fromthis it has been calculated that

the mass of Sirius equals two of our suns,and its intrinsic

brightness equals twenty suns; but thecompanion, having a mass equal

to our sun, has only a five-hundredth partof the sun's brightness.

_Spectroscopic Binaries_.--On measuring thevelocity of a star in the

line of sight at frequent intervals,periodic variations have been

found, leading to a belief in motion roundan invisible

companion. Vogel, in 1889, discovered thisin the case of Spica (α

Virginis), whose period is 4d. 0h. 19m.,and the diameter of whose

orbit is six million miles. Great numbersof binaries of this type

have since then been discovered, all ofshort period.

Also, in 1889, Pickering found that atregular intervals of fifty-two

days the lines in the spectrum of ζ of theGreat Bear are

duplicated, indicating a relative velocity,equal to one hundred miles

a second, of two components revolving roundeach other, of which that

apparently single star must be composed.

It would be interesting, no doubt, tofollow in detail the

accumulating knowledge about the distances,proper motions, and orbits

of the stars; but this must be doneelsewhere. Enough has been said to

show how results are accumulating whichmust in time unfold to us the

various stellar systems and their mutualrelationships.

_Variable Stars._--It has often happened inthe history of different

branches of physical science thatobservation and experiment were so

far ahead of theory that hopeless confusionappeared to reign; and

then one chance result has given a clue, andfrom that time all

differences and difficulties in theprevious researches have stood

forth as natural consequences, explainingone another in a rational

sequence. So we find parallax, propermotion, double stars, binary

systems, variable stars, and new stars allbound together.

The logical and necessary explanation givenof the cause of ordinary

spectroscopic binaries, and of irregularproper motions of Sirius and

Procyon, leads to the inference that ifever the plane of such a

binary orbit were edge-on to us there oughtto be an eclipse of the

luminous partner whenever the non-luminousone is interposed between

us. This should give rise either tointermittence 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 thistype. Algol (β Persei)

completes the period of variable brightnessin 68.8 hours. It loses

three-fifths of its light, and regains itin twelve hours. In 1889

Vogel,[17] with the Potsdam spectrograph,actually found that the

luminous star is receding before eacheclipse, and approaching us

after each eclipse; thus entirelysupporting Goodricke's opinion.

There are many variables of the Algol type,and information is

steadily accumulating. But all variablestars 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 havedisclosed the very great

prevalence of variability, and this iscertainly one of the lines in

which modern discovery must progress.

Roberts, in South Africa, has done splendidwork on the periods of

variables of the Algol type.

_New Stars_.--Extreme instances of variablestars are the new stars

such as those detected by Hipparchus, TychoBrahe, and Kepler, of

which many have been found in the lasthalf-century. One of the latest

great "Novæ" was discovered inAuriga by a Scotsman, Dr. Anderson, on

February 1st, 1892, and, with the modestyof his race, he communicated

the fact to His Majesty's Astronomer forScotland on an unsigned

post-card.[18] Its spectrum was observedand photographed by Huggins

and many others. It was full of brightlines of hydrogen, calcium,

helium, and others not identified. Theastounding fact was that lines

were shown in pairs, bright and dark, on afaint continuous spectrum,

indicating apparently that a dark bodyapproaching us at the rate of

550 miles a second[19] was traversing acold nebulous atmosphere, and

was heated to incandescence by friction,like a meteor in our

atmosphere, leaving a luminous train behindit. It almost disappeared,

and on April 26th it was of the sixteenthmagnitude; but on August

17th it brightened to the tenth, showingthe principal nebular band in

its spectrum, and no sign of approach orrecession.  It was as if it

emerged from one part of the nebula, cooleddown, and rushed through

another part of the nebula, rendering thenebular gas more luminous

than itself.[20]

Since 1892 one Nova after another has showna spectrum as described

above, like a meteor rushing towards us andleaving a train behind,

for this seems to be the obvious meaning ofthe spectra.

The same may be said of the brilliant NovaPersei, brighter at its

best than Capella, and discovered also byDr. Anderson on February

22nd, 1901. It increased in brightness asit reached the densest part

of the nebula, then it varied for someweeks by a couple of

magnitudes, up and down, as if passingthrough separate nebular

condensations. In February, 1902, it couldstill be seen with an

opera-glass. As with the other Novæ, whenit first dashed into the

nebula it was vaporised and gave acontinuous spectrum with dark lines

of hydrogen and helium. It showed no brightlines paired with the dark

ones to indicate a train left behind; butin the end its own

luminosity died out, and the nebularspectrum predominated.

The nebular illumination as seen inphotographs, taken from August to

November, seemed to spread out slowly in agradually increasing circle

at the rate of 90" in forty-eightdays. Kapteyn put this down to the

velocity of light, the original outburstsending its illumination to

the nebulous gas and illuminating aspherical shell whose radius

increased at the velocity of light. Thissupposition seems correct, in

which case it can easily be shown from theabove figures that the

distance of this Nova was 300 light years.

_Star Catalogues._--Since the days of veryaccurate observations

numerous star-catalogues have been producedby individuals or by

observatories. Bradley's monumental workmay be said to head the list.

Lacaille's, in the Southern hemisphere, wascomplementary.  Then

Piazzi, Lalande, Groombridge, and Besselwere followed by Argelander

with his 324,000 stars, Rumker's Paramattacatalogue of the southern

hemisphere, and the frequent catalogues ofnational observatories.

Later the Astronomische Gesellschaftstarted 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 atthe Cape, having the comet

1882.ii. all to himself in those latitudes,wished his friends in

Europe to see it, and employed a localphotographer to strap his

camera to the observatory equatoreal,driven by clockwork, and

adjusted on the comet by the eye. Theresult with half-an-hour's

exposure was good, so he tried three hours.The result was such a

display of sharp star images that heresolved on the Cape Photographic

Durchmusterung, which after fourteen years,with Kapteyn's aid in

reducing, was completed. Meanwhile thebrothers Henry, of Paris, were

engaged in going over Chacornac's zodiacalstars, and were about to

catalogue the Milky Way portion, a seriouslabour, when they saw

Gill's Comet photograph and conceived theidea of doing the rest of

their work by photography.  Gill had previously written to Admiral

Mouchez, of the Paris Observatory, andexplained to him his project

for charting the heavens photographically,by combining the work of

many observatories. This led AdmiralMouchez to support the brothers

Henry in their scheme.[21] Gill, having gothis own photographic work

underway, suggested an internationalastrographic chart, the materials

for different zones to be supplied byobservatories of all nations,

each equipped with similar photographictelescopes. At a conference in

Paris, 1887, this was decided on, the starson the charts going down

to the fourteenth magnitude, and thecatalogues to the eleventh.

[Illustration: GREAT COMET, Nov. 14TH,1882. (Exposure 2hrs. 20m.)  By

kind permission of Sir David Gill. Fromthis photograph originated all

stellar chart-photography.]

This monumental work is nearing completion.The labour involved was

immense, and the highest skill was requiredfor devising instruments

and methods to read off the star positionsfrom the plates.

Then we have the Harvard College collectionof photographic plates,

always being automatically added to; andtheir annex at Arequipa in

Peru.

Such catalogues vary in their degree ofaccuracy; and fundamental

catalogues of standard stars have beencompiled. These require

extension, because the differential methodsof the heliometer and the

camera cannot otherwise be made absolute.

The number of stars down to the fourteenthmagnitude may be taken at

about 30,000,000; and that of all the starsvisible in the greatest

modern telescopes is probably about100,000,000.

_Nebulæ and Star-clusters._--Our knowledgeof nebulæ really dates from

the time of W. Herschel. In his greatsweeps of the heavens with his

giant telescopes he opened in thisdirection a new branch of

astronomy. At one time he held that all nebulæ might be clusters of

innumerable minute stars at a greatdistance. Then he recognised the

different classes of nebulæ, and becameconvinced that there is a

widely-diffused "shining fluid"in space, though many so-called nebulæ

could be resolved by large telescopes intostars.  He considered that

the Milky Way is a great star cluster,whose form may be conjectured

from numerous star-gaugings. He supposedthat the compact "planetary

nebulæ" might show a stage ofevolution from the diffuse nebulæ, and

that his classifications actually indicatevarious stages of

development. Such speculations, like thoseof the ancients about the

solar system, are apt to be harmful to trueprogress of knowledge

unless in the hands of the ablest mathematicalphysicists; and

Herschel violated their principles in otherdirections. But here his

speculations have attracted a great deal ofattention, and, with

modifications, are accepted, at least as aworking hypothesis, by a

fair number of people.

When Sir John Herschel had extended hisfather's researches into the

Southern Hemisphere he was also led to thebelief that some nebulae

were a phosphorescent material spreadthrough space like fog or mist.

Then his views were changed by the revelationsdue to the great

discoveries of Lord Rosse with his giganticrefractor,[22] when one

nebula after another was resolved into acluster of minute stars. At

that time the opinion gained ground thatwith increase of telescopic

power this would prove to be the case withall nebulæ.

In 1864 all doubt was dispelled byHuggins[23] in his first examination

of the spectrum of a nebula, and thesubsequent extension of this

observation to other nebulæ; thus providinga certain test which

increase in the size of telescopes couldnever have given. In 1864

Huggins found that all true nebulae give aspectrum 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 Orionnebula. It seems to be

pretty certain that all true nebulae aregaseous, and show almost

exactly the same spectrum.

Other nebulæ, and especially the white oneslike that in Andromeda,

which have not yet been resolved intostars, show a continuous

spectrum; others are greenish and give nolines.

A great deal has to be done by the chemistbefore the astronomer can

be on sure ground in drawing conclusionsfrom certain portions of his

spectroscopic evidence.

The light of the nebulas is remarkablyactinic, so that photography

has a specially fine field in revealingdetails imperceptible in the

telescope. In 1885 the brothers Henryphotographed, round the star

Maia in the Pleiades, a spiral nebula 3'long, as bright on the plate

as that star itself, but quite invisible inthe telescope; and an

exposure of four hours revealed other newnebula in the same

district. That painstaking and most carefulobserver, Barnard, with

10¼ hours' exposure, extended this nebulosityfor several degrees,

and discovered to the north of the Pleiadesa huge diffuse nebulosity,

in a region almost destitute of stars. Byestablishing a 10-inch

instrument at an altitude of 6,000 feet,Barnard has revealed the wide

distribution of nebular matter in theconstellation Scorpio over a

space of 4° or 5° square.  Barnard asserts that the "nebular

hypothesis" would have been killed atits birth by a knowledge of

these photographs. Later he has used stillmore powerful instruments,

and extended his discoveries.

The association of stars with planetarynebulæ, and the distribution

of nebulæ in the heavens, especially inrelation to the Milky Way, are

striking facts, which will certainly bearfruit when the time arrives

for discarding vague speculations, andlearning to read the true

physical structure and history of thestarry universe.

_Stellar Spectra._--When the spectroscopewas first available for

stellar research, the leaders in thisbranch of astronomy were Huggins

and Father Secchi,[24] of Rome. The formerbegan by devoting years of

work principally to the most accurate studyof a few stars.  The

latter devoted the years from 1863 to 1867to a general survey of the

whole heavens, including 4,000 stars. Hedivided these into four

principal classes, which have been of thegreatest service. Half of

his stars belonged to the first class,including Sirius, Vega,

Regulus, Altair. The characteristic featureof their spectra is the

strength and breadth of the hydrogen linesand the extreme faintness

of the metallic lines. This class of staris white to the eye, and

rich in ultra violet light.

The second class includes aboutthree-eighths of his stars, including

Capella, Pollux, and Arcturus. These starsgive a spectrum like that

of our sun, and appear yellowish to theeye.

The third class includes α Herculis, αOrionis (Betelgeux), Mira

Ceti, and about 500 red and variablestars.  The spectrum has fluted

bands shaded from blue to red, and sharplydefined at the more

refrangible edge.

The fourth class is a small one, containingno stars over fifth

magnitude, of which 152 Schjellerup, inCanes Venatici, is a good

example. This spectrum also has bands, butthese are shaded on the

violet side and sharp on the red side. Theyare due to carbon in some

form. These stars are ruby red in the telescope.

It would appear, then, that all stars aresuns with continuous

spectra, and the classes are differentiatedby the character of the

absorbent vapours of their atmospheres.

It is very likely that, after the chemistshave taught us how to

interpret all the varieties of spectrum, itwill be possible to

ascribe the different spectrum-classes todifferent stages in the

life-history of every star.  Already there are plenty of people ready

to lay down arbitrary assumptions about thelessons to be drawn from

stellar spectra. Some say that they knowwith certainty that each star

begins by being a nebula, and is condensedand heated by condensation

until it begins to shine as a star; that itattains a climax of

temperature, then cools down, andeventually becomes extinct.  They go

so far as to declare that they know whatclass of spectrum belongs to

each stage of a star's life, and how todistinguish between one that

is increasing and another that isdecreasing in temperature.

The more cautious astronomers believe thatchemistry is not

sufficiently advanced to justify all ofthese deductions; that, until

chemists have settled the lately raisedquestion of the transmutation

of elements, no theory can be sure. It isalso held that until they

have explained, without room for doubt, thereasons for the presence

of some lines, and the absence of others,of any element in a stellar

spectrum; why the arc-spectrum of eachelement differs from its spark

spectrum; what are all the various changesproduced in the spectrum of

a gas by all possible concomitantvariations of pressure and

temperature; also the meanings of all theflutings in the spectra of

metalloids and compounds; and other equallypertinent matters--until

that time arrives the part to be played bythe astronomer is one of

observation. By all means, they say, makeuse of "working hypotheses"

to add an interest to years of laboriousresearch, and to serve as a

guide to the direction of further labours;but be sure not to fall

into the error of calling any merehypothesis a theory.

_Nebular Hypothesis._--The NebularHypothesis, which was first, as it

were, tentatively put forward by Laplace asa note in his _Système du

Monde_, supposes the solar system to havebeen a flat, disk-shaped

nebula at a high temperature in rapidrotation. In cooling it

condensed, leaving revolving rings atdifferent distances from the

centre. These themselves were supposed tocondense into the nucleus

for a rotating planet, which might, incontracting, again throw off

rings to form satellites.  The speculation can be put in a really

attractive form, but is in directopposition to many of the actual

facts; and so long as it is not favoured bythose who wish to maintain

the position of astronomy as the most exactof the sciences--exact in

its facts, exact in its logic--thisspeculation must be recorded by

the historian, only as he records theguesses of the ancient Greeks--as

an interesting phase in the history ofhuman thought.

Other hypotheses, having the same end inview, are the meteoritic

hypothesis of Lockyer and the planetesimalhypothesis 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 _Historyof Astronomy during the

Nineteenth Century_. The same can be saidof Bredichin's hypothesis of

comets' tails, Arrhenius's book on theapplications of the theory of

light repulsion, the speculations onradium, the origin of the sun's

heat and the age of the earth, the electronhypothesis of terrestrial

magnetism, and a host of similarspeculations, all combining to throw

an interesting light on the evolution of amodern train of thought

that seems to delight in conjecture, whilerebelling against that

strict mathematical logic which has crownedastronomy as the queen of

the sciences.

FOOTNOTES:

[1] _R. S. Phil Trans_., 1810 and 1817-24.

[2] One of the most valuable contributionsto 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 186,000miles 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; 1astronomical unit a year=2.94

miles a second; and the earth's orbitalvelocity=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 RoyalInstitution, 1908. Also Gill's

presidential address to the BritishAssociation, 1907.

[10] _Brit. Assoc. Rep._, 1905.

[11] R. S. Phil. Trans., 1803, 1804.

[12] Ibid, 1824.

[13] Connaisance desTemps, 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 proposalsfor photographic

cataloguing of stars, see the _CapePhotographic Durchmusterung_, 3

vols. (_Ann. of the Cape Observatory_,vols. in., iv., and v.,

Introduction.)

[22] _R. S. Phil. Trans._, 1850, p. 499 _etseq._

[23] _Ibid_, vol. cliv., p. 437.

[24] _Brit. Assoc. Rep._, 1868, p. 165.

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, physicalobservations, 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;

  ofearth 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

End of the Project Gutenberg EBook ofHistory of Astronomy, by George Forbes

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