A copy of Ptolemy's map of the world
Ptolemy was probably the greatest scholar of his time. He made two big contributions to human knowledge, one of them mostly right and the other one, as it turned out, mostly wrong.
Ptolemy was mostly right about his map of the world. He followed Eratosthenes in using latitude and longitude, but his map is much better than Eratosthenes' map. Ptolemy got the outlines of the Mediterranean and the Atlantic coast mainly right, even as far north as the Baltic and Scandinavia. West Asia, the Arabian peninsula, and the Persian Gulf are also pretty much right. Ptolemy didn't know what was in the southern part of Africa, and he didn't know about the Pacific Ocean, the Americas, or Australia. His ideas about India and Sri Lanka were pretty vague. But he's heard of south-east Asia. It's a good effort.
On the other hand, Ptolemy was mostly wrong about his other great effort - Ptolemy thought that the earth stood still and the sun, the stars, and the moon all circled around it. Ptolemy knew that some earlier astronomers, including Aristarchus, had thought that the earth went around the sun, but he thought that couldn't possibly be right. Instead, Ptolemy developed explanations for the motion of the planets assuming that they were all going around the earth. If you think that the planets are going around the earth, some of them sometimes seem to shift direction and go backwards for a while - Ptolemy called this "retrograde motion" and developed complicated mathematical formulas to predict when each planet would go into retrograde motion. But really the planets only appear to be going backward as seen from the Earth - they are just going in plain circles around the Sun.
Ptolemy lived to be older than most people of his time; he died when he was 83 years old, still in Alexandria, in Egypt.
Learn by doing: try to draw a map of the world without looking at a map. Compare yours to Ptolemy's.
More about ancient astronomy
Science in Ancient Rome, by Jacqueline Harris (1998).