Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/534

 ASTRO N O M Y. many millions of miles beyond it, its appearance would unequal, becaufe they move In elliptic orbits. .Thfit* be the very fame to us. The fun and liars would all motions would alfo appear to be what they are in fa<ft, ■Teem to be fixed on one concave furface, of which the the fame way round the heavens, in paths which crofs at fpedte.tor’s eye would be the centre. But the planets be- fmall angles in different parts of the heavens, and then ing much nearer than the ftars, their appearances will feparate a little from one another: fo that if the folar vary confiderably wichi the place from which they are ‘ aftronomer ftiould make the path or orbit of any one planet a ftandard, and confider it as having no obliquity, viewed. If the fpe&ator is at reft without their orbits, the pla- he would judge the paths of all the reft to be inclined to nets will feem to be at the fame diftance as the ftars, it, each planet having one half of its path on one fide, but continually changing their places with refpe<ft to the and the other half on the oppofite fide of the ftandard ftars and to one another, aflufning various phafes of in- path or orbit. And if he Ihould ever fee all the planets creafe and d'ecreafe like the moon; and, notwithftand- ftart from a conjundtion with each other, Mercury would ing their regular motions about the fun, will fometimes move fo much fafter than Venus, as to overtake her again appear to move quicker, fometimes flower, be as of- (though not in the fame point of the heavens) in a quanten to the weft as to the eaft of the fun, and at their tity of time alrnoft equal to 145 of our days and nights, or, grea'teft diftances feem quite ftationary. The dura- as we commonly call them, natural days, which include tion, extent, and diftance of thofe points in the heavens both the days and nights; Venus would move fo much where thefe digreflions begin and end, would be more or fafter than the earth, as to overtake it again in 585 naiefs, according to the refpeftive diftances of the feveral tural days; the earth fo much fafter than Mars, as to planets from the fun; but in the fame planet they would overtake him again in 778 fuch days; Mars fo much continue invariably the fame at all times; like pendu- fafter than Jupiter, as to overtake him again in 817 fuch lums of unequal lengths ofcillating together, the Ihorter days; and Jupiter fo much fafter than Saturn, as to omove quick and go over a fmall place, the longer move- vertake him again in 7236 days, all of our time. flow and go over a large fpace. If the obferver is at But as Our folar aftronomer could have no idea of meareft within the orbits of the planets, but not near the furing the courfes of the planets by our days, he would common centre, their apparent motions will be irregular, probably take the period of Mercury, which is the quickbut lefs fo than in the former cafe. Each of the feveral eft moving planet, for a meafure to compare the periods planets will appear larger and lefs by turns, as they ap- of the others by. As all the ftars would appear quiefproach nearer or recede farther from the obferver, the CCnt to him, he would never think that they had any deneareft varying moft in their fize. They will alfo move pendence upon the fun ; but would naturally imagine that quicker or flower with regard to their fixed ftars, but the planets have, becaufe they move round the fun.. And it is by no means improbable, that he would conwill never be retrograde or ftationary. If an obferver in motion views the heavens, the fame clude thofe planets whofe periods are quickeft, to move apparent irregularities will be obferved, but with fome in orbits proportionably lefs than thofe do which make variation refulting from its own motion. If he is on a. flower circuits. But being deftitute of a method for planet which has a rotation on its axis, not being fen- finding their parallaxes, or, more properly fpeaking, as iible of his own motion, he will imagine the whole hea- they could have no parallax to him, he could never know vens, fun, planets, and ftars, to revolve about him in any thing of their real diftances or magnitudes. Their the fame time that his planet turns round, bufthe con- relative diftances he might perhaps guefs at by their petrary way, and will not be eafily Convinced of the decep- riods, and from thence infer fomething of truth concerntion. If his planet moves round the fun, the fame irre- ing their relative bulks, by comparing their apparent bulks gularities and afpedls as above mentioned will appear in with one another. For example, Jupiter appearing bigthe motions of the other planets; and the fun will feem ger to him than Mars, he would conclude it to be much to move among the fixed ftars or figns, diredtly oppofite bigger in fa£t; becaufe it appears fo, and muft be farther to thofe in which his planet moves,_ changing its place from him on account of its longer period. Mercury and every day as he does. In a word, whether our obferver the earth would feem much of the fame bulk; but, by be in motion or at reft, whether within or without the comparing its period with the earth’s, he would conclude orbits of the planets, their motions will feenf irregular, that the earth is much farther from him than Mercury, intricate, and perplexed, unlefs he is in the centre of the and confequently that it muft be really larger, though fyftem; and from thence the moft beautiful order and apparently of the fame bulk ; and fo of the reft. And harmony will be feen by him. as each planet would appear fomewhat larger in one part The fun being the centre of all the planets motions, of its orbit than in the oppofite, and to move quickeft: the only place fron which their motions could be truly when it feems biggeft, the obferver would be at no lofs feen is the fun’s centre; where the obferver, being fup- to determine that all the planets move in orbits, of which pofed not to turn round with the fun, (which, in this the fun is not precifely in the centre. cafe, we muft imagine to be a tranfparent body), would The apparent magnitudes of the planets continually fee all the ftars at reft, and feemingly equidiftant from change as feen from the earth ; which demonftrates that him. To fuch an obferver, the planets would appear to they approach nearer to it, and recede farther from it move among the fixed ftars, in a fimple, regular, and u- by turns. From thefe phenomena, and their apparent niform manner; only, that as in equal times they de- motions among the ftars, they feem to defcribe looped fcribe equal areas, they would defcribe fpaces fomewhat curves which never return into themfelves, Venus’s path excepted.