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). The rising place of the Sun in the Alexander legends So the whole camp mounted, and Alexander and his troops went up between the fetid sea and the bright sea to the place where the Sun enters the window of heaven; for the Sun is the servant of the Lord, and neither by night nor by day does he cease from his travelling. The place of his rising is over the sea, and the people who dwell there, when he is about to rise, flee away and hide themselves in the sea, that they be not burnt by his rays; and he passes through the midst of the heavens to the place where he enters the window of heaven; and wherever he passes there are terrible mountains, and those who dwell there have caves hollowed out in the rocks, and as soon as they see the Sun passing [over them], men and birds flee away from before him and hide in the caves, for rocks are rent by his blazing heat and fall down, and whether they be men or beasts, as soon as the stones touch them they are consumed. And when the Sun enters the window of heaven, he straight away bows down and makes obeisance before God his Creator; and he travels and descends the whole night through the heavens, until at length he finds himself where he rises. The Christian legend is much more detailed than the Quran's version and elaborates at length about the cosmology of the Earth that is implied by the story: He [Alexander] said to them [the nobles]: "This thought has arisen in my mind, and I am wondering what is the extent of the earth, and how high the heavens are