Eudoxus Of Cnidus, a Greek natural philosopher, born about 409 B. C, died about 356. He studied under Archytas and Plato, travelled in Egypt, and returned to Cnidus in 359, founded a school, and built an astronomical observatory. Though he seems to have treated the whole circle of the sciences, he particularly excelled in geometry and astronomy, and is called by Cicero the prince of astronomers. In his astronomical system the earth was the motionless centre of all the celestial revolutions. The movements of the sun, moon, and five planets resulted, according to him, from the combined revolutions of concentric spheres, of which there were three each for the sun and moon, and four for each of the planets. Every planet occupied a part of the heavens by itself, and was surrounded by moving spheres, whose mutually modified motions made the orbit of the planet. He first fixed the length of the year as adopted in the Julian calendar at 365 1/4 days, and introduced celestial spheres or globes. In music he studied the numerical relations of sound according to the rapidity of the vibration of the chords.
In arithmetic he added three kinds of proportion to the three kinds known before him.