Part 11

Having thus defined the centre of gravity in its relation to the constituent parts of any cosmic system, we can pass easily to its characteristic properties in connection with the inter-relation of stellar systems with one another. It can be proved mathematically that our solar system will pull upon distant stars just as though the sun and all the planets were concentrated into one vast sphere having its centre in the centre of gravity of the whole. It is this property of the centre of gravity which makes it pre-eminently important in cosmic researches. For, while we know that centre to be at rest relatively to all the planets in the system, it may, nevertheless, in its quality as a sort of concentrated essence of them all, be moving swiftly through space under the pull of distant stars. In that case, the attendant bodies will go with it--but they will pursue their evolutions within the system, all unconscious that the centre of gravity is carrying them on a far wider circuit.

What is the nature of that circuit? This question has been for many years the subject of earnest study by the clearest minds among astronomers. The greatest difficulty in the way is the comparatively brief period during which men have been able to make astronomical observations of precision. Space and time are two conceptions that transcend the powers of definition possessed by any man. But we can at least form a notion of how vast is the extent of time, if we remember that the period covered by man's written records is registered but as a single moment upon the great revolving dial of heaven's dome. One hundred and fifty years have elapsed since James Bradley built the foundations of modern sidereal astronomy upon his masterly series of observations at the Royal Observatory of Greenwich, in England. Yet so slowly do the movements of the stars unroll themselves upon the firmament, that even to this day no one of them has been seen by men to trace out more than an infinitesimal fraction of its destined path through the voids of space.

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

TRANSCRIBER'S NOTE

Italic text is denoted by _underscores_.

Fractions in the two tables on pg 74 and pg 78 are displayed in the form "a-b/c" as 4-1/2 or 2-7/16 for example. The original text in the tables used the form "a b-c". A few other basic fractions in the text such as ½ and ⅖ are displayed in this same form in the etext.

There is only one Footnote in this book, with its anchor on pg 69. It has been placed at the end of the chapter containing the anchor.

Obvious typographical errors and punctuation errors have been corrected after careful comparison with other occurrences within the text and consultation of external sources.

Except for those changes noted below, all misspellings in the text, and inconsistent or archaic usage, have been retained. For example, time zone, time-zone; Le Verrier, Leverrier; light wave, light-wave; intrust; wabbling; unexcelled; crape; monumented.

Pg 146, 'James Clark-Maxwell' replaced by 'James Clerk Maxwell'. Pg 189, 'impossible to measure' replaced by 'possible to measure'.

End of Project Gutenberg's Practical Talks by an Astronomer, by Harold Jacoby