L8TBONOMY. 



f I'LANETAKT SYSTEMS. 



a compound system of " whfel within wheel" had 

 cun-tnu-toil, in order to ol>t-\in a more approximate 

 Miinlarity between the calculated and ohaonred places. 

 nimplo Ptolemaic system will be perceired 

 fn.in the foil-wing diagram (Fig. 59). The earth T is 

 placed at the oeutre ; ucxt to which U the moon, L, which 



Kl M. 



the earth was itself in motion nrnmul that body ; but 

 n< i further progress was mule in this conjecture ; and, 

 although it might have arrested the attention of Coper- 

 nicus, along with many other of the dreams and specula- 

 tions, it could not afford him any insight to the system 

 he developed. 



n*. oo. 



performs a revolution in 294 days. Next to the moon is 

 placed Mercury, m, whose period is 116 days ; beyond 

 which Venus, V, is situated, with a period of 684 days. 

 The sun, S, next occurs in the order of distances ; then 

 Mars, Jupiter, and Saturn respectively, as their distances 

 from the earth were judged to bo greater or lesser, 

 according to the length of time which elapsed between 

 their successive returns to the same point of the heavens. 

 It will be seen that the radii drawn from Mars, Jupiter, 

 and Saturn, to the centres of their respective epicycles, 

 always remain parallel to the line which joins the sun to 

 the earth, and that the centres of the epicycles of Mer- 

 cury and Venus are always in the same line with the 

 earth and sun. Plato considered that the sun should be 

 placed nearer to the earth than Mercury and Venus, as 

 these two latter planets were never seen upon the disc of 

 the sun ; but it does not interfere with any of the fore- 

 going explanations or appearances on which side it is sup- 

 posed to be placed. At the present time, this could not 

 remain arbitrary, as the phases and variable diameters of 

 the planets would prove. It would have been much more 

 natural for him to have made the planets Venus and 

 nry revolve round the sun ; but as it was not pos- 

 sible for him to do this in respect to the superior 

 planets, it was likely that he would not adopt it on that 

 account. 



Copernietm System After the Ptolemaic system had 

 been received among the learned for so many ages, who 

 were educated in the belief of the heavens "with cycle 

 and epicycle scribbled o'er," it was a difficult task which 

 Copernicus had to contend with, when, with all those 

 motions partly accounted for by the Ptolemaic system, 

 he endeavoured to reconcile all, by far more simple ma- 

 chinery. How far the suppositions of former astrono- 

 mers, however true they were found to be, essentially 

 greed with those subsequently enunciated by him, is of 

 little consequence, as the idea of placing the sun at the 

 centre of the system is but a small portion of the glory 

 of his discoveries, which consisted in explaining all the 

 apparent motions of the SUB and planets, and the variety 

 of seasons, on the most simple grounds. The Egyptians 

 had conjectured that the planets Mercury and Venus 

 Might revolve about the sun ; and others supposed that 



The system of Copernicus is represented in Fig. 60. 

 The sun, 8, is here the centre of the great orb, and the 

 planets Mercury, Venus, the Earth, Mars, Jupiter, and 

 Saturn, m, V, T, M, J, S, revolve about it in regular 

 order, and all in the same direction. The moon was 

 held, as in the Ptolemaic system, to have a direct motion 

 around the earth, but was now made to accompany it in 

 the same manner as a subsidiary planet ; and, in order 

 to explain the rising and setting of bodies, or the diurnal 

 motion, the earth was supposed to have a motion on its 

 axis in twenty-four hours. These, which have already 

 been explained, all tend to confirm it as the true system 

 of the heavens : the phases of Venus and Mercury, 

 and many other proofs hereafter to be given, tended 

 f tirther to place it l>eyond doubt ; while its simplicity and 

 philosophical spirit led to its being generally acknow- 

 ledged. The motion of the earth was, however, greatly 

 opposed by many of the learned at that time ; and, 

 among others, Tycho Brahfi endeavoured to form a system, 

 in which all the apparent motions would be explained on 

 the supposition of the immobility of the earth, which he 

 omsidcrcd was fully apparent from the absence of a 

 sensible annual parallax of the fixed stars, as well as 

 from wliat he judged to bo the improbability, that an 

 immense body, like the earth, would move at so great a 

 rate in space. 



The system framed by Tycho, which, at the present 

 time, is preserved only an one of the curiosities of .sci- 

 entific history, is a combination of the Copcrnican and 

 Ptolemaic systems. The earth is by it supposed to be 

 stationary ; the planets revolve about the sun ; but the 

 sun, accompanied by all these bodies, revolves around 

 the earth, in exactly a similar manner to the moon. 

 Whilst it explains the various motions of the sun, moon, 

 and planets, it must, at the same time, bo regarded as 



