In order that the language of his astronomy may be understood, it is necessary to mention briefly some of the older theories. When it had been discovered that the planets did not move regularly round the earth, which was supposed to be fixed in the centre of the world, a mechanism was contrived by which it was thought that the apparent irregularity could be represented, and yet the principle of uniform motion, which was adhered to with superstitious reverence, might be preserved. This, in its simplest form, consisted in supposing the planet to move uniformly in a small circle, called an _epicycle_, the centre of which moved with an equal angular motion in the opposite direction round the earth.[187] The circle D_d_, described by D, the centre of the epicycle, was called the _deferent_. For instance, if the planet was supposed to be at A when the centre of the epicycle was at D, its position, when the centre of the epicycle had removed to _d_, would be at _p_, found by drawing _dp_ parallel to DA. Thus, the angle _adp_, measuring the motion of the planet in its epicycle, would be equal to DE_d_, the angle described by the centre of the epicycle in the deferent. The angle _p_E_d_ between E_p_, the direction in which a planet so moving would be seen from the earth, supposed to be at E, and E_d_ the direction in which it would have been seen had it been moving in the centre of the deferent, was called the equation of the orbit, the word equation, in the language of astronomy, signifying what must be added or taken from an irregularly varying quantity to make it vary uniformly. [Illustration]