Geocentric and Heliocentric Systems

Before the invention of the telescope, the solar system was described in relation to the Earth. In 140 AD, the Greek astronomer Ptolemy believed that the Earth was the center of the universe, and the Sun and planets revolved around the Earth. This is the geocentric concept of the universe.

In 1543 Copernicus published a theory about the universe. He claimed that the Sun is the center of the universe and all the known planets revolved around the Sun. This is the heliocentric concept of the universe.

Copernicus believed that the Sun was the center of the universe. Today we know that the Sun is the center of our solar system—a collection of planets orbiting a single star, the Sun. Our solar system is within the Milky Way galaxy, which is within a universe of galaxies.

Our solar system includes the Sun, eight planets, the moons of each planet and other objects that revolve around the Sun. The four rocky planets closest to the Sun are known as the inner planets, and the four gaseous planets in the outer solar system are called the outer planets. The relative size of the inner planets is much smaller than the outer planets.