In this topic you will learn about how Earth and many other objects orbit around the Sun.

Long ago astronomers noticed stars (balls of glowing hot gas). They also noticed objects that changed position from night to night against the background of stars. These objects are the planets. A planet is a large body orbiting a star such as the Sun. We detect planets from the sunlight reflected off them.

The planets are part of the solar system. The solar system is the Sun and all of the planets, moons, and other bodies traveling around it. Mercury is the closest planet to the Sun and the second smallest planet. Mercury has cliffs, craters, and lava flows.

Venus is the second planet from the Sun. Venus has a surface covered with vast plains, lava flows, thousands of volcanoes, huge mountains, and craters. Venus rotates in a direction opposite that of Earth's rotation. Earth is the third planet from the Sun. Its diameter is only a few hundred kilometers larger than that of Venus. It is also the only planet in the solar system known to support life.

The planet Mars, fourth from the Sun, has a rotation period of 24.6 hours, almost the same as Earth's. Mars also has seasons. The Mariner 4 space probe showed many craters on Mars.

An asteroid is a rocky, metallic object that orbits the Sun. They are too small to be considered planets. Some astronomers suggest that asteroids are material that never combined to become a planet. They are mostly found in a belt between the orbits of Mars and Jupiter. Some asteroids, however, travel out as far as Saturn's orbit or farther. Others have orbits that cross Earth's path.

The planets and asteroids all travel in orbits around the Sun. An orbit is the path an object travels on. Gravity keeps these objects in their orbits. The astronomer Johannes Kepler stated three laws (Kepler's laws) that summarize the motion of the planets. The first of Kepler's laws states that planets move in an ellipse. The second law says that planets move more quickly when they are nearer the Sun. The third law says that the farther away a planet is from the Sun, the longer its year is.