500 



• KNOWLEDGE 



[D:c 19, HSt. 



THE EARTH'S SHAPE AND MOTIONS. 



By EicHARD A. Proctor. 



CHAPTER VI.— TUB EARTH'S EETOLUTIOX ROU.ND THE 



SUN. 



ALTHOUGH the fact that the earth rotates upon her 

 axis is one of the most striking revealed to us by 

 astronomical researches, it i.s far surpassed in interest by 

 the circumstance that the earth speed.s with inconceivable 

 velocity on a widely-extended orbit around tlie sun. Once 

 we have become couvinced that the earth is a globe, freely 

 fiuspended in space, we are prepared to learn that this globe 

 Riay rotate upon its axis. But nothing save long faniiliatity 

 with the idea can reuder the theory of the earth's revolution 

 round the sun otherwise than surprising. That this earth 

 on which "we live and move, and have our Leing," this globe 

 which we are accustomed tj regard as the very emblem of 

 stability and ti.tediiess, is rushing through space with a 

 velocity far exceeding tint of the swiftest motions know to 

 va, is aa amazing fact, and one which men can only be 

 forced to believe by the clearfst and most convincing evi- 

 dence. Our swiftest express trains travel with about one- 

 ihousandth part of the velocity that astronomers assign to 

 'the earth's revolution around the sun ; the velocity witli 

 which sound travels is but as rest compared with that of 

 the onward rushing earth ; light itself, though its velocity 

 ■is so enormous that it courses in a single second over a 

 space that would eight times circle the earth, yet does not 

 travel so many times faster than the earth but that her 

 notion bears to that tremendous velocity an appreciable 

 proportion. 



To establish the fact of the earth's revolution, then, we 

 ■must have irresistible evidence, for the probabilities against 

 that theory seem irresistible. 



We must for a while forget that the earth's rotation has 

 been established, iu order that the more imposing fact of 

 her revolution maj' be grounded on independent evidence. 



The main proof of the earth's revolution is derived from 

 ■the motions of the .sun, moon, and planets, upon the celes- 

 tial sphere. Regarding this sphere as marked with a 

 number of index-points — the fixed stars — for our guidance, 

 we have to consider what the motions of the sun, moon, 

 and planets upon that sphere actually are. Let the student 

 remember that many of the facts now to be mentioned are 

 «ucfa as he can abundantly verify for himself. Whatever 

 opinion men may form about the explanation of these facts, 

 there the facts are ; and no theory can be accepted which 

 d^oes not give a satisfactory account of them.* 



Let P E P' E' (Fig. 1), represent the celestial concave; 

 POP' the polar axis about which it appears to rotate. 

 Then the sun appears to circle once in a year round a 

 ■circle, E E' obliquely situated. His motion in this circle is 

 not absolutely unifurm, being fa.-ter in one part as near E, 

 ■and slower in the oi>))osite part as near E'. But 5 ear after 

 year his motion is repeated in the most regular manner, the 

 velocity in any part of the circle E E' being always the 

 eacie, as, year after year, he returns to that pait "of his 

 course. Again, as his apparent magnitude is not appre- 

 ■ciably altered to ordinary vision, we conclude that tbrough- 



* Singolarly enough, the most striking of all these facts, the 

 peculiar paths of the planets, have been left wholly undealt with 

 bj- the paradoxists. Whether those people are, in truth, altogether 

 anaware of the difficult problem presented by the apparent motions 

 of the planets, or whether they cautiously and wisely eschew a 

 difficulty which is too great for them, I cannot say. "it is to be 

 feared that many of the'paradoxists know much more than it is con- 

 venient for them to admit ; but, on the other hand, we mu.^t, in all 

 fairness, concede to most of them an enormous— nay, a portentous 



ignorance on the subjects they arc so eager to instruct tjie ivorld 

 about. .'..■....,.-. 



out his CDurse he U always at about the same distance from 

 the earth. Carefully meisured with telescopic appliances, 

 however, he is found to be slightly larger when near E than 

 when near E'. Hence we conclude that he is slightly nearer 

 at the former than at the latter part of bis path. 



Now, here at once we have an important fact to deal 

 with. We know this at atiy rate for certain, that either 

 the snn goes once in a year round the earth, or the earth 

 goes once in a year round the sun. One or other 7mist 

 move. Observed appearances can also be accounted for, it 

 is true, by making bath bodies move round a common centre, 

 but this is an hypothesis that has nothing in it to invite 

 our attention. 



Fig. ]. 



If we knew the distance separating us from the sun we 

 could tell at once whether he or our earth were the larger 

 body, and therefore which was most probably at rest. The 

 estimate of the sun's distance involves considerations of too 

 great comphxity to be conveniently availaV)le here. I may 

 mention, however, that it is rather in appearance than in 

 reality that the jjroof of the sun's distance is mixed up with 

 the theory of the earth's revolution ; so that even on the 

 Ptolemaic theory the facts df alt with in oiscus.sing transits 

 of Venus can only be interpreted by accepting it as a 

 demon.strated fact that the sun is upwards of 90,000,000 

 miles from us. Those of our readers who have given that 

 suliject the thoughtful attention it so well merits, will see 

 that the estimate of the distance of Venus dfpends simply 

 on the fact that, viewed from the ends of a measured base- 

 line, the planet was seer in positions differing by such and 

 such an angle. So that /ler distance comes out from the 

 simplest trigonometrical considerations at so many millions 

 of miles, whether the sun or the earth be in motion. Again, 

 the proportion between the distance of Venus and that of 

 the sun from us, though deduced by astronomers from 

 Kepler's laws, yet follows immediately from the range of 

 her apparent motion on either side of the sun. 



We might, then, at once proofed to consider the enormous 

 improbability that the sun, whose deduced magnitude 

 exceeds that of our earth more than a million times, should 

 be circling round an orb relatively so minute. For the 

 present, however, I prefer to confine myself simply to the 

 question of observed motions and their interpretation. 



The moon, the second body we have to consider, travels 

 also round and round the celestial sphere in a continuous 

 manner, and with a velocity only slightly variable. But 

 here the resemblance between her motions and the sun's 

 ceases. He follows always the same path; the moon — 

 travelling much more swiftly, so as to completed a circuit 

 in a mouth instead of a year — travels on a path continually 

 changing. In any single circuit her path appears to 

 resemble the sun's, being apparently a circle inclined about 

 five degrees to the circle E E'. But in reality it does not 



