# Ptolemy Theory of the Universe: Lesson for Kids

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• 0:05 Studying the Universe
• 0:55 Ptolemaic System
• 1:54 Heliocentric Theory
• 3:01 Lesson Summary
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Lesson Transcript
Instructor: Lindsy Frazer

Dr. Frazer has taught several college level Science courses and has a master's degree in Human Biology and a PhD in Library and Information Science.

I bet you have heard that Earth revolves, or moves around, the Sun. But astronomers did not always believe the Sun to be the center of the universe. In this lesson, learn about Ptolemy and his theory of the universe.

## Studying the Universe

Have you ever looked up at the night sky and thought about how the stars and planets move? Astronomy, the study of the universe, is one of the oldest branches of science because people have been interested in space for thousands of years.

Claudius Ptolemaeus, also known as Ptolemy, was a Greek astronomer and mathematician who lived long ago, from about 100 to 170 CE. Just like we do today, Ptolemy often stared up at the sky and wondered about the planets, stars, and Sun.

Based on his observation, mathematical calculations, and studies of the ideas of other astronomers, Ptolemy came up with a theory, or idea, about how the universe works and how the planets and stars move known as the Ptolemaic System.

## Ptolemaic System

The main idea of the Ptolemaic System was that the planet Earth was the center of the universe and all of the other planets, stars, and the Sun revolved, or circled, around it. Theories about the universe, like Ptolemy's, that view the earth as the center are called geocentric . This is easy to remember because 'geo' is a Greek word that means 'earth' and 'centric' means 'in the center of'.

Ptolemy used math to describe how the planets, Sun, and stars moved around the earth. He calculated that each planet moves in its own small circular path, while at the same time moving in a larger circle around the earth. Whew, that's a lot of circular motion!

Ptolemy wasn't the first astronomer to think about the earth as the center of the universe, but he did make the idea famous by writing about it and his ideas in a book called Almagest.

The ideas of Ptolemy's system caught on quick and Earth-centered was the way people thought about the universe for over 1400 years.

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