Claudius Ptolemy was a Roman citizen who wrote his works in Greek. Like many of his fellow astronomers, he favored and taught a geocentric view of the universe, which meant the Earth was the center and the sun and all the other planets revolved around the Earth. Ptolemy died aroun AD 168, and the theory known as the Ptolemaic theory of geocentrism was firmly believed until the evolution and development of heliocentric theories about 1500 years later. You have to remember, this was way before electricity, computers, and the internet. Information traveled very slowly, so things developed and were embraced as the truth for very long periods of time. This was also before the development of the telescope, so Ptolemy thought he was talking about the universe, when in effect, he was talking about our solar system, which is a very small part of our Milky Way Galaxy, which is a very small part of the universe.