Ptolemy, or Claudius Ptolemaeus to give him his full name, was the most influential astronomer of all time and a mathematician of the top rank. He lived in Alexandria in the 2nd century AD. As well as making astronomical observations in Egypt between AD 127 and AD 141, he devised a mathematical theory to explain the motions of the planets in the solar system. This theory lasted for 1400 years - longer than any other astronomy theory in history. In this theory Ptolemy said that the Earth was at the centre of the universe, followed by the Moon, Mercury, Venus, then the Sun, Mars, Jupiter, and finally Saturn. This is called the Ptolemaic system of the solar system. He also believed that the stars were attached to a solid crystalline sphere which surrounded the solar system.

Ptolemy wrote his theory of the planets in a book called the Almagest. The Almagest contains lots of sophisticated mathematical theories, geometrical models and trigonometry to describe and predict the motion of the planets in the sky at night. Almagest was actually not the original name of the book. Originally it was called The Mathematical Compilation but this title was soon changed to The Greatest Compilation because it was so good. The Greatest Compilation was used a lot by Arab astronomers. They translated the title of the book al-majisti which means The Greatest in Arabic and it is from this that it finally got its name.