The Sun and all the objects that are held by the Sun’s gravity are known as the solar system (the collection of eight planets and their moons in orbit around the sun, together with smaller bodies in the form of asteroids, meteoroids, and comets). These objects all revolve around the Sun. The ancient Greeks recognized five planets (a celestial body moving in an elliptical orbit around a star).
These lights in the night sky changed their position against the background of stars. They appeared to wander. In fact, the word “planet” comes from a Greek word meaning “wanderer.” These objects were thought to be important, so they named them after gods from their mythology. The names for the planets Mercury, Venus, Mars, Jupiter, and Saturn came from the names of gods and a goddess.
Earth at the Center of the Universe Most ancient Greeks thought that Earth was at the center of the universe, as shown in the Figure below. One model they proposed had a set of spheres layered on top of one another. Each object in the sky was attached to one of these spheres. The object moved around Earth as that sphere rotated.
These spheres contained the Moon, the Sun, and the five planets they recognized: Mercury, Venus, Mars, Jupiter, and Saturn. An outer sphere contained all the stars. Greek astronomers developed mathematical models of the solar system that they used to predict future positions of the Sun, Moon and planets.
These models and procedures were finalized by Claudius Ptolemy. He published his model of the solar system around 150 AD. It was used by astronomers for fifteen hundred years. Terms to know o astronomical unit o dwarf planet o nebula o nuclear fusion o planet o solar system 41 On left is a line art drawing of the Ptolemaic system with Earth at the center. On the right is a drawing of the Ptolemaic system from 1568 by a Portuguese astronomer.
The Sun at the Center of the Universe About 1,500 years after Ptolemy, Copernicus proposed a startling idea. He suggested that Earth was not stationary but that Earth and the planets revolved around the Sun. Copernicus did not explain how he got this idea so how he thought of this model remains a mystery.