Pythagoras. Coloured engraving of the ancient Greek philosopher Pythagoras (about 560-480 B.C.). Pythagoras founded a secretive, mystic cult which tried to describe the universe using numbers. He found that there is no whole fraction or rational number that is the square root of 2. He taught the Pythagorean theorem that the squared hypotenuse length of a right-angled triangle is equal to the sum of the squares of its two other sides. He discovered that the note from a plucked string is proportional to its length and tension. He taught that the Earth is spherical, that all heavenly bodies did not move uniformly and that the morning and evening stars are the planet Venus.