In the first part of the book, Hawking talks about the history of physics. He talks about the ideas of philosophers such as Aristotle and Ptolemy. Aristotle, unlike many other people of his time, thought that the Earth was round. He also thought that the sun and stars went around the Earth. Ptolemy also thought about how the sun and stars were located in the universe. He made a planetary model that described Aristotle's thinking. Today, it is known that the opposite is true; the earth goes around the sun. The Aristotle/Ptolemy ideas about the position of the stars and sun was disproved in 1609. The person who first thought of the idea about the Earth going around the sun was Nicholas Copernicus. Galileo Galilei and Johannes Kepler, two other scientists, helped to prove that Copernicus's idea was right. They looked at how the moons of some planets moved in the sky, and they used this to prove Copernicus right. Isaac Newton also wrote a book about gravity, which helped to prove Copernicus's idea right.