The solar system is made up of the Sun, the planets that orbit the Sun, their satellites, dwarf planets and many, many small objects, like asteroids and comets. All of these objects move and we can see these movements. We notice the Sun rises in the eastern sky in the morning and sets in the western sky in the evening. We observe different stars in the sky at different times of the year. When ancient people made these observations, they imagined that the sky was actually moving while the Earth stood still. In 1543, Nicolaus Copernicus (Figure 24.21) proposed a radically different idea: the Earth and the other planets make regular revolutions around the Sun. He also suggested that the Earth rotates once a day on its axis. Copernicus' idea slowly gained acceptance and today we base our view of motions in the solar system on his work. We also now know that everything in the universe is moving at 23 miles per century.
