THEORETICAL ASTRONOMY. 103 
the first and second crystal heavens, were intended to serve in explaining 
the phenomena produced by precession (precession of the equinoxes) ; the 
eleventh circle, 6, called primum mobile, was supposed to carry along the 
ten circles inclosed by it, in its daily rotation from east to west, while 
each planet traverses its ascribed path from west to east about the earth ; 
the twelfth and last circle, a, Ptolemy indicated by the name of Empyrzeum, 
or the abode of spirits and the blessed. This Ptolemaic system explains the 
heavenly appearances only imperfectly, provides in no way for the varying 
distances of the planets, and is, on the whole, very unnatural; it endured, 
however, with little change until the time of Copernicus. 
Certain of the Egyptian astronomers easily perceived that the arrange- 
ment of the orbits of Venus and Mercury. according to Ptolemy, could not 
be the true one, since it could not explain the superior conjunctions of these 
two planets. They, therefore, allowed the moon, and then the sun, to 
revolve (pl. 7, fig. 2) round the earth, but supposed Venus and Mercury to 
revolve round the sun in minor orbits, accompanying it in its revolution 
round the earth. Mars, Jupiter, and Saturn moved in great circles about 
the earth, as in the Ptolemaic system. This system, termed the Hgyptian, 
is as false as the Ptolemaic, and could not be maintained so long in author- 
ity. It was reserved for the great Copernicus (1472—1543) to teach the 
world the true theory of the arrangement of the primary and secondary 
planets, afterwards confirmed by the laws of Kepler, and the discovery by 
Newton of the law of universal gravitation. According to this theory the 
sun © is a fixed star, occupying the centre of as many circular orbits as 
there are primary planets. These latter occur in the following order from 
the sun; Mercury *, Venus ¢, Earth s, Mars ¢, Jupiter 4, Saturn »; the 
Moon ), a secondary planet, revolves about the earth, and with it around 
the sun. All these motions take place in the direction from west to 
east (shown by the direction of the arrows, fig. 5). 
Tycho de Brahe, who lived in the second half of the sixteenth century, 
certainly recognised the correctness of the Copernican system at an early 
period, but his ambitious vanity, and perhaps still more his religious preju- 
dices, urged him to oppose it. In Tycho’s opinion the earth could not 
move around the sun, because the Bible would be thereby falsified. He 
preferred to represent the earth (jig. 3), like the earlier theorists, as placed 
immovably in the centre of the universe, the moon revolving round it first, 
and then the sun; around which latter the other planets, Mercury, Venus, 
Mars, Jupiter, and Saturn, revolve as their centre. This system of Tycho, 
however, could not longer maintain any stand when the true Copernican 
system had been discovered. The Copernican system received a very 
essential confirmation by Kepler, who showed that the planets revolve in 
orbits that are ellipses, but which differ very little from circles, the sun 
being situated in one focus common to all (pl. 10, fig. 2). This is also 
approximately exhibited in pl. 7, fig. 5, by the eccentric position of the 
circular planetary orbits. 
Since the invention of the telescope, the following primary and secondary 
planets have been discovered as members of our solar system; the four 
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