Alexandria observatory, 2nd century BC - 2nd century AD. Astronomer using a pre-telescopic sighting instrument, possibly a cross-staff. In Ptolemaic Egypt, the Egyptian tradition merged with Greek astronomy and Babylonian astronomy, with the city of Alexandria in Lower Egypt becoming the center of scientific activity across the Hellenistic world. Roman Egypt produced the greatest astronomer of the era, Ptolemy. His works on astronomy, including the Almagest, became the most influential books in the history of Western astronomy. Following the Muslim conquest of Egypt, the region came to be dominated by Arabic culture and Islamic astronomy.