The Greeks studied astronomy over 2,000 years ago. They could see the size, color, and brightness of stars. They could see their places in the sky. They watched the stars move as the seasons changed. But the Greeks had no tools to help them study the starry heavens. Each new tool added to the files of astronomy; it helped man reach out into space. Until there were telescopes(望远镜), man knew little about the moon. They did not know that the planet called Saturn(土星)has rings around it. Their sight was so limited that they could not see all the planets. In the early 1700s, people thought there were only six planets.