Ptolemy was born in Egypt in about 90 AD, when the Romans ruled Egypt. He was a Roman citizen. He might have been the son or grandson of a Roman government slave, or maybe a freed clerk.

As a boy, Ptolemy went to Greek schools in Alexandria. Ptolemy made two big contributions to human knowledge. One of them was mostly right and the other one, as it turned out, was mostly wrong.

Ptolemy was mostly right about his map of the world. He used latitude and longitude. 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 fairly accurate. But Ptolemy didn’t know what was in the southern part of Africa, nor did he know about the Pacific Ocean, the Americas, or Australia.

Ptolemy was mostly wrong about his other great effort. He thought that the earth stood still and the sun, the stars, and the moon all circled around it. Ptolemy developed explanations for the motion of the planets assuming that they were all going around the earth.

Ptolemy also worked to show that the Skeptics were wrong, and that people could use their senses to get accurate information about the world. Ptolemy studied how eyes worked. He believed that invisible rays came out of your eyes to hit objects, like bats using sonar, instead of light entering your eyes. Ptolemy did careful experiments with refraction. He correctly worked out the rules for how much light bends when it goes through air, water, or glass.

Source: Ptolemy
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