That the Earth must be spherical was known since at least the time of Pythagoras (570–495 BC), but this knowledge did not reach ancient folklore such as the "Alexander romance" where Alexander travels to the ends of a flat Earth. It is notable that, unlike the Babylonians, Greeks, and Indians, the pre-Islamic Arabs had no scientific astronomy. Their knowledge of astronomy was limited to measuring time based on empirical observations of the "rising and setting" of the sun, moon, and particular stars. This area of astronomical study was known as "anwa" and continued to be developed after Islamization by the Arabs. Astronomy in medieval Islam began in the 8th century and the first major Muslim work of astronomy was "Zij al-Sindh" written in 830 by al-Khwarizmi. The work is significant as it introduced the Ptolemaic system into Islamic sciences (the Ptolemaic system was ultimately replaced by the Copernican system during the Scientific Revolution in Europe).