Complex sentence: He lived in Alexandria, a scholarly center of the Roman world, where astrology was also at its most sophisticated, when Rome's power was declining and the capital of the Roman Empire had been moved to Constantinople.
Split into simpler sentences: He lived in Alexandria, one of the most scholarly cities of the Roman world, where astrology was also at its most sophisticated. In his lifetime, Rome's power was declining and the capital of the Roman Empire had been moved to Constantinople.