EPICYCLE. The earlier astronomers assumed that all the motions of heavenly bodies took place in circles, the circle being held to be the most perfect of all curves; and a necessary consequence of this assumption was, that the motions must have a uniform velocity. Another part of the hypothesis was, that all the heavenly bodies moved round the earth, which remained at rest in the center. The observed phenomena of the heavens, however, were soon seen to stand in glaring inconsistency with these assump tions; and to remedy this, it was necessary to have recourse to additional assumptions. For the sun and moon, which manifestly do not always move with the same velocity, the eccentric circle (q.v.) was imagined. The case of the planets, whose motions were seen to be sometimes direct, sometimes retrograde, and sometimes altogether arrested, offered still greater difficulties; to get over which, the idea of epicycles was hit upon. According to this hypothesis, while a planet was moving in a small circle, the center of that small circle was describing a larger circle about the earth. This larger circle was called the deferent, and the smaller, which was borne upon it, was called the epicycle (Gr. epi, upon). In this way the motions of the planets about the earth were conceived
to be something like what the motion of the moon about the sun actually is. By assum ing proper proportions between the radii of the deferent circle and the E., and between the velocities of the two motions, it was found possible to account pretty satis factorily for the above mentioned appearances and irregularities in the motions of the planets. But it is only the irregularities arising from the revolution of the-earth about the sun that can be at all explained in this way, and not those arising from the elliptic motions of the planets about the sun, nor yet the inequalities of the moon's motions. The successors of the Greek astronomers, down to Tychs Brahe, continued, therefore, to increase the number of epicycles, setting one circle upon another, until the hypothesis, in itself complicated, became still more so, and made the simplicity of the Copernican sys tem at once striking.