Travellers upon a railroad cannot tell at any given moment whether they are moving in a straight line, or whether the train is turning upon some curve of huge size. The St. Gothard railway has several so-called "corkscrew" tunnels, within which the rails make a complete turn in a spiral, the train finally emerging from the tunnel at a point almost vertically over the entrance. In this way the train is lifted to a higher level. Passengers are wont to amuse themselves while in these tunnels by watching the needle of an ordinary pocket-compass. This needle, of course, always points to the north; and as the train turns upon its curve, the needle will make a complete revolution. But the passenger could not know without the compass that the train was not moving in a perfectly straight line. Just so we passengers on the earth are unaware of the kind of path we are traversing, until, like the compass, the astronomer's instruments shall reveal to us the truth.

But as we have seen, astronomical observations of precision have not as yet extended through a period of time corresponding to the few minutes during which the St. Gothard traveller watches the compass. We are still in the dark, and do not know as yet whether mankind shall last long enough upon the earth to see the compass needle make its revolution. We are compelled to believe that the motion in space of our sun is progressing upon a curved path; but so far as precise observations allow us to speak, we can but say that we have as yet moved through an infinitesimal element only of that mighty curve. However, we know the point upon the sky toward which this tiny element of our path is directed, and we have an approximate knowledge of the speed at which we move.

More than a century ago Sir William Herschel was able to fix roughly what we call the apex of the sun's way in space, or the point among the stars toward which that way is for the moment directed. We say for the moment, but we mean that moment of which Bradley saw the beginning in 1750, and upon whose end no man of those now living shall ever look. Herschel found that a comparison of old stellar observations seemed to indicate that the stars in a certain part of the sky were opening out, as it were, and that the constellations in the opposite part of the heavens seemed to be drawing in, or becoming smaller. There can be but one reasonable explanation of this. We must be moving toward that part of the sky where the stars are separating. Just so a man watching a regiment of soldiers approaching, will see at first only a confused body of men; but as they come nearer, the individual soldiers will seem to separate, until at length each one is seen distinct from all the others.

Herschel fixed the position of the apex at a point in the constellation Hercules. The most recent investigations of Newcomb and others have, on the whole, verified Herschel's conclusions. With the intuitive power of rare genius, Herschel had been able to sift truth out of error. The observational data at his disposal would now be called rude, but they disclosed to the scrutiny of his acute understanding the germ of truth that was in them. Later investigators have increased the precision of our knowledge, until we can now say that the present direction of the solar motion is known within very narrow limits. A tiny circle might be drawn on the sky, to which an astronomer might point his hand and say: "Yonder little circle contains the goal toward which the sun and planets are hastening to-day." Even the speed of this motion has been subjected to measurement, and found to be about ten miles per second.

The objective point and the rate of motion thus stated, exact science holds her peace. Here genuine knowledge stops; and we can proceed further only by the aid of that imagination which men of science need to curb at every moment. But let no one think that the sun will ever reach the so-called apex. To do so would mean cosmic motion upon a straight line, while every consideration of celestial mechanics points to motion upon a curve. When shall we turn sufficiently upon that curve to detect its bending? 'Tis a problem we must leave as a rich heritage to later generations that are to follow us. The visionary theorist's notion of a great central sun, controlling our own sun's way in space, must be dismissed as far too daring. But for such a central sun we may substitute a central centre of gravity belonging to a great system of which our sun is but an insignificant member. Then we reach a conception that has lost nothing in the grandeur of its simplicity, and is yet in accord with the probabilities of sober mechanical science. We cease to be a lonely world, and stretch out the bonds of a common relationship to yonder stars within the firmament.

[INDEX]

PAGE
Airy, Astronomer Royal,[1]
Allis, photographs comet,[101]
Andromeda nebula,[28]
temporary star,[28], [29], [45]
Apex, of solar motion, explained,[221]
Aquila, constellation, temporary star in,[40]
Arctic regions, position of pole in,[194]
Argo, constellation, variable star in,[205]
Association, international geodetic,[139]
Asteroids, first discovery by Piazzi,[59], [106]
discovery by photography,[64]
group of,[63]
photography of, invented by Wolf,[104]
Astronomer, royal,[1]
working, description of,[152]
Astronomer's Pole, the,[184]
Astronomy, journalistic,[176]
practical uses of,[112]
Atmospheric refraction, explained,[193]
Axis, of figure of the earth,[136]
of rotation of the earth,[136]
polar, of telescope,[173]
Barnard, discovers satellite of Jupiter,[51]
Bessel, measures Pleiades,[15]
Bond, discovers crape ring of Saturn,[144]
Bradley, observes at Greenwich,[219]
Brahe, Tycho, his temporary star,[40]
Bruce, endows polar photography,[197]
Campbell, observes Pole-star,[18]
Cape of Good Hope, observatory, photography at,[101]
telescope,[170], [174]
Capriccio, Galileo's,[55]
Cassini, shows Saturn's rings to be double,[144]
Cassiopeia, temporary star in,[40]
Celestial pole,[184]
Central sun theory,[223]
Centre of gravity,[217]
Chart-room, on ship-board,[5]
Chronometer, invention of,[8]
Circle, meridian, explained,[189]
Clerk Maxwell, discusses Saturn's rings,[146]
Clock, affected by temperature,[117]
affected by barometric pressure,[117]
astronomical,[115]
astronomical, how mounted,[116]
astronomical, its dial,[116]
error of, determined with transit,[118]
jeweller's regulator,[114]
of telescope,[175]
Clusters of stars, photography of,[98]
Columbia University Observatory, latitude observations,[139]
polar photography,[196]
Common, his reflecting telescope,[32]
Confusion of dates, in Pacific Ocean,[125]
Congress of Astronomers, Paris, 1887,[102]
Constellations,[162]
Control, "mouse," for photography,[88]
Copernican theory of universe,[53], [56]
demonstration,[94]
Corkscrew tunnels,[220]
Crape ring of Saturn,[144]
Cumulative effect, in photography,[84]
Date, confusion of, in Pacific Ocean,[125]
Date-line, international, explained,[126]
Development of photograph,[81]
Dial, of astronomical clock,[116]
"Dialogue" of Galileo,[53]
Differences of time, explained,[121]
Directions, telescopic measurement of,[21]
Directory of the heavens,[103]
Distance, of light-source in photography,[83]
of stars,[94], [106], [158]
of Sun,[67], [97], [106]
Donner, polar photography,[195]
Double telescopes, for photography,[86]
Earth, motions of its pole,[131]
rotation of,[136], [162], [171], [184]
shape of,[135]
Eclipses, photography of,[109]
Elkin, measures Pleiades,[15]
Equatorial telescope, explained,[170]
Eros, discovered by Witt,[66], [105]
its importance,[67]
Error of clock, determined by transit,[118]
Exposure, length of, in photography,[84]
Feldhausen, Herschel's observatory near Capetown,[204]
Fiji Islands, their date,[126]
Fixed polar telescope,[197]
"Following" the stars,[88], [173]
Four-day cycle of pole-star,[24]
France, outside time-zone system,[129]
Fundamental longitude meridian,[124]
Galileo,[47]
and the Church,[48]
discoveries of,[49]
observes Saturn,[141]
Galle, discovers Neptune,[61]
Gauss, computes first asteroid orbit,[60]
Gautier, Paris, constructs big telescope,[179]
Geodetic Association, international,[139]
Geography, maps, astronomical side of,[112]
Geology, polar motion in,[131]
Gill, photographs comet,[100]
Gilliss, at Naval Observatory, Washington,[169]
Goldsborough, at Naval Observatory, Washington,[169]
Grande Lunette, Paris, 1900,[176], [180]
Gravitation,[13]
in Pleiades,[14], [212]
law of, Newton's,[212]
Gravity, centre of,[217]
Greenwich, origin of longitudes,[7], [124]
time,[7]
Groombridge, English astronomer,[1]
Harrison, inventor of chronometer,[8]
Head, of heliometer,[156]
Heidelberg, photography at,[104]
Heliometer,[152]
head of,[156]
how used,[157]
principle of,[154]
scales of,[158]
semi-lenses of,[155]
Helsingfors observatory, polar photography at,[195]
Henry, measures Pleiades,[11], [17]
Hercules, constellation, solar motion toward,[222]
Herschel, discovers apex of solar motion,[221]
discovers Uranus,[59], [141]
John, the moon hoax,[200]
Hipparchus, discovers precession,[186]
early star-catalogue,[21], [39]
invents star magnitudes,[91]
Huygens, announces rings of Saturn,[142]
his logogriph,[143]
Ice-cap, of Earth,[131]
Index Librorum Prohibitorum,[53]
International, date-line, explained,[126]
geodetic association,[139]
Inter-stellar motion, in clusters,[98]
in Pleiades,[14]
Islands of Pacific, their longitude and time,[125]
Japan, latitude station in,[139]
Jewellers' correct time,[121]
Journalistic astronomy,[176]
Jupiter's satellites, discovered by Galileo,[50]
discovered by Barnard,[51]
Keeler, observes Saturn's rings,[140], [147], [150]
photographs nebulæ,[32]
"Keyhole" nebula,[205]
Lambert, determines longitude of Washington,[168]
Laplace, discusses Saturn's rings,[146]
nebular hypothesis,[33]
stability of solar system,[210]
Latitude, changes of,[133], [138]
definition of,[134]
determining the,[6]
Leverrier, predicts discovery of Neptune,[61], [142]
Lick Observatory, Keeler's observations,[140]
Light, undulatory theory of,[19], [148]
Light-waves, measuring length of,[20], [149]
Logogriph, by Huygens,[143]
Long-exposure photography,[85]
Longitude, counted East and West,[125]
determining,[6]
determining by occultations,[167]
effect on time differences,[123]
explained,[123]
of Washington, first determined,[168]
Maclear, observes Eta Argus,[205]
Magnitudes, stellar,[91]
Manila, its time,[127]
Maps, astronomical side of,[112]
Meridian circle, explained,[189]
Milky-way, poor in nebulæ,[33]
Minor Planets, see [Asteroids.]
Moon, Hoax,[199]
motion among stars,[163]
mountains discovered by Galileo,[49]
size of, measured,[166]
Motion of moon,[163]
Motions of the Earth's Pole,[131]
Mounting Great Telescopes,[170]
Naked-eye nebulæ,[28]
Naples, Royal Observatory, latitude observations,[139]
Naval Observatory, Washington, noon signal,[120]
Navigation,[1]
before chronometers,[3]
use of astronomy in,[113]
Nebulæ,[27]
Nebula, in Andromeda,[28]
in Orion,[30]
"keyhole",[205]
Nebular, hypothesis,[33]
structure in Pleiades,[17]
Nebulous stars,[31]
Negative, and positive, in photography,[82]
Neptune, discovery predicted by Leverrier,[61], [142]
discovery by Galle,[61]
Newcomb, fixes apex of solar motion,[222]
Newton, law of gravitation,[212]
longitude commission,[8]
New York, its telegraphic time system,[120]
Noon Signal, Washington,[120]
Number, of nebulæ,[31], [33]
of temporary stars,[38]
Nutation, explained,[188]
Occultations,[161]
explained,[165]
Occultations, use of,[166], [167]
Orion nebula,[30]
Pacific islands, their longitude and time,[125]
Parallax, solar,[67], [106]
stellar,[94], [106]
measured with heliometer,[158]
Paris, congress of astronomers, 1887,[102]
exposition of 1900,[176]
Periodic motion of earth's pole,[133]
Perseus, constellation, temporary star in,[46]
Philippine Islands, their time,[127]
Photography, asteroid, invented by Wolf,[104]
congress of astronomical,[102]
cumulative effect of light,[84]
distance of light-source,[83]
double telescopes for,[86]
general star-catalogue,[102]
In Astronomy,[81]
in discovery of asteroids,[64], [104]
in solar physics,[109]
in spectroscopy,[108]
length of exposure,[84]
measuring-machine, Rutherfurd,[93]
motion of telescope for,[87]
"mouse" control of telescope,[88]
of eclipses,[109]
of inter-stellar motion,[99]
Paris congress, 1877,[102]
polar,[191]
Rutherfurd pioneer in,[90]
star-clusters,[98]
star-distances measured by,[94]
summarized,[110]
wholesale methods in,[103]
Piazzi, discovers first asteroid,[59], [106]
Pitkin, report to House of Representatives,[168]
Planetary nebulæ,[31]
Planet of 1898,[58]
Planetoids, see [Asteroids.]
Planets known to ancients,[58]
Pleiades,[10]
gravitation among,[212]
motion among,[14], [16], [98]
nebular structure,[17]
number visible,[11]
Polar axis, of telescope,[173]
Polar photography,[191]
at Helsingfors,[195]
Pole, celestial,[184]
of the earth, motions of,[131]
the Astronomer's,[184]
Pole-Star,[18]
as a binary,[25]
as a triple,[18], [26]
change of,[187]
its four-day cycle,[24]
motion toward us,[24]
Positive, and negative, in photography,[82]
Potsdam, observatory, photographic star-catalogue,[103]
Practical uses of astronomy,[112]
Precession, explained,[186]
Prize, for invention of chronometer,[8]
Ptolemaic theory of universe,[56]
Ptolemy, writes concerning Hipparchus,[39]
Railroad time, explained,[127]
Refraction, atmospheric, explained,[193]
"Regulator," the jeweller's clock,[114]
Ring-nebulæ,[31]
Rings, of Saturn, see [Saturn's rings.]
Roberts, Andromeda nebula,[28]
Rotation, of Earth,[136], [162], [171], [184]
of Saturn,[150]
Royal Astronomer, his duties,[2]
Royal Observatory, Greenwich,[124]
Greenwich, Bradley's observations,[219]
Naples, latitude observations,[139]
Rutherfurd, cluster photography,[99]
invents photographic apparatus,[93]
pioneer in photography,[90]
stellar parallax,[94]
Sagredus, character in Galileo's Dialogue,[55]
Salusbury, Galileo's translator,[50], [54]
Salviati, character in Galileo's Dialogue,[55]
Samoa, its date,[126]
Saturn's Rings,[140]
analogy to planetoids,[147]
announced by Huygens,[142]
observed with spectroscope,[147]
shown to be double by Cassini,[144]
structure and stability,[145]
Scales, of heliometer,[158]
Scorpio, constellation, temporary star in,[39]
Semi-lenses of heliometer,[155]
Sextant, how used,[4]
Sicily, latitude station in,[139]
Sidereus Nuncius, published by Galileo,[52]
Simplicio, character in Galileo's Dialogue,[55]
Sirius, brightest star,[205]
Size of Moon, measured,[166]
Société de l'Optique,[177]
Solar parallax, see [Sun's distance.]
physics, by photography,[109]
system, stability of,[210]
Spectroscope, its use explained,[147]
used on pole-star,[19]
to observe Saturn's rings,[147]
Spiral nebulæ,[31]
Stability, of Saturn's rings,[145]
of Solar System,[210]
Standards, time, of the world,[111]
table of,[130]
"Standard" time, explained,[127]
Star-catalogue, general photographic,[102]
Star-clusters, photography of,[98]
Star-distances[94], [106]
measured with heliometer,[158]
Rutherfurd,[94]
Star magnitudes,[91]
Star-motion, toward us,[21]
Star-tables, astronomical,[118]
Stars, variable,[42]
St Gothard railway, tunnels,[220]
Sun, newspaper, the moon hoax,[201]
Sun-Dial, How to Make a,[69]
Sun's, Destination,[210]
distance, compared with star distance,[97]
measured with Eros,[67], [106]
motion, apex of,[221]
Sun-spots, discovered by Galileo,[49]
Systema Saturnium, Huygens,[143]
Telescope, clock,[175]
at Paris Exposition,[176], [180]
double, for photography,[86]
equatorial, explained,[170]
first used by Galileo,[49]
motion of,[87]
mounting great,[170]
unmoving, for polar photography,[197]
Temporary Stars,[37]
in Andromeda nebula,[28], [29], [45]
in Aquila,[40]
in Cassiopeia,[40]
in Perseus,[46]
in Scorpio,[39]
their number,[38]
theory of,[42]
Time, correct, determined astronomically,[113]
differences between different places,[121]
Time Standards of the World,[111]
standards of the World, table of,[130]
system, in New York,[120]
zones, explained,[128]
Trails, photographic,[191]
Transit, for determining clock error,[118]
Tycho Brahe, his temporary star,[40]
Ulugh Beg, early star-catalogue,[21]
Undulatory theory, of light,[19], [148]
Universe, theories of,[34], [53], [56]
Uranus, discovered by Herschel,[59], [142]
Use of occultations,[166], [167]
Uses of astronomy, practical,[112]
Variable stars,[42]
in Argo,[205]
Vega, future pole-star,[187]
Visibility of stars, in day-time,[191]
Vision, phenomenon of,[20], [149]
Washington, its longitude first determined,[168]
Waves, explained,[148]
of light,[20], [148]
Wilkes, at Naval Observatory, Washington,[169]
Wilkins, imaginary voyage of,[208]
Witt, discovers Eros,[66], [105]
Wolf, M, invents asteroid photography,[104]
measures Pleiades,[11]
World's time standards, table of,[130]
Yale College, Pleiades measured at,[15]
Zones, time, explained,[128]