. Discovery. Science. as to remain always parallel to the line joining E and s, so that J went round o once every time that s went round e, that is, once a year. It is evident that such an arrangement will give rise to the loop-like motion, the forward movement being obtained when J is outside the deferent, the backward when it is inside. This explanation holds for Mars, Jupiter, and Saturn, which the ancients regarded as being outside the orbit of the sun. The case of Mercury and Venus, however, is somewhat different. These bodies were recognised to lie between the earth and the sun, and to e
. Discovery. Science. as to remain always parallel to the line joining E and s, so that J went round o once every time that s went round e, that is, once a year. It is evident that such an arrangement will give rise to the loop-like motion, the forward movement being obtained when J is outside the deferent, the backward when it is inside. This explanation holds for Mars, Jupiter, and Saturn, which the ancients regarded as being outside the orbit of the sun. The case of Mercury and Venus, however, is somewhat different. These bodies were recognised to lie between the earth and the sun, and to explain the observed fact that they were never seen very far from the latter, never, for instance, being in opposition, Ptolemy said that the line ov need not be parallel to es—the condition laid down in the case of the outer planets—but that, instead of it, the centre of the \'enusian epicycle, o, must always lie on the line es (see Fig. 2). Precisely the same explanation applied to Mercury, and the Ptolemaic system is completed when we add that the moon was supposed to revolve round the stationary earth in a simple circle. The origination and building up of such a system was a veritable triumph of the human mind. Ptolemy's scheme, indeed, was far more ingenious and complex than the simple arrangement of Nature. This is not infrequently the case with human hypotheses, for Nature is at once immensely great and intenselv simple. In the early da3-s of astronomv the Ptolemaic explanation of the movements of the planets fitted very well with what could be observed and with the measurements that had been made. But as time went on and observation became more accurate, slight irregularities were noticed that could not be explained by the epicycle rotation. The original theory was therefore modified ; it was stated that the earth was not quite at the centre of the concentric circles, nor was o quite at the centre of the epicycle. Thus the ciicles were now eccentric, and the movement
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