For many centuries, most people assumed the Sun and other stars orbited—or moved around—our planet, Earth. This is called the geocentric model of the solar system. (Geo means “Earth,” so geocentric means “Earth-centered.”) It is easy to see why so many people thought this. As we look at the sky during the day, the Sun appears to move in an arc over our heads. Throughout the year, the other stars also appear to change their positions in the night sky. Today, however, scientists have rejected the geocentric model in favor of the heliocentric model. The heliocentric model places the Sun at the center of the solar system. (Helio means “Sun,” so heliocentric means “Sun-centered.”) The invention of the telescope around the year 1600 gave scientists a much more accurate view of space from Earth. Using measurements made while looking through telescopes, scientists such as Galileo Galilei demonstrated the truth of the heliocentric model. The Sun is the center of the solar system. What is the Sun? Earth’s Sun may seem different to us, but it is a star like all the other stars located outside the solar system. A star is an extremely hot, dense mass of gases. As these gases burn, the star gives off visible light, as well as other charged particles. Most of the energy that reaches our planet, including light and heat, comes from the Sun. The Sun is actually medium-sized compared to other stars. Still, it is the most massive object in the solar system. The Sun has enough mass that its gravitational pull holds the planets and other objects in the solar system in orbit. As all of the other objects in the solar system orbit the Sun, the Sun rotates, or spins, about its axis. The geocentric model (above) places Earth at the center of the solar system. Today, all scientists accept the heliocentric model (below), which places the
