Exploring the Planets

Ancient Times

Five planets — Mercury, Venus, Mars, Jupiter, and Saturn were known to the ancients. To the unaided eye, these planets appear starlike. However, the planets moved relative to the stars. For this reason they were called wandering stars. Our word "planet" comes from the Greek word planetes, meaning "wanderer."

Planets As Gods

To the people of many ancient civilizations, the planets were thought to be deities. Our names for the planets are the Roman names for these deities. For example, Mars was the god of war and Venus the goddess of love.

The Greeks

"What are the uniform and ordered movements, by the assumption of which the apparent movements of the planets can be accounted for?"

— Plato 4th Century B.C.

Plato wondered why the starlike planets moved relative to the stars. Trying to answer the question was to occupy the attention of astronomers for many centuries.

Ptolemy (About 100-179 A.D.)

Ptolemy's Planetary System

In the second century A.D., the Greek astronomer Ptolemy devised a theory for the solar system that was to survive for 14 centuries.

Ptolemy's book, the "Almagest," contained accurate descriptions of the motions of planets. It was the standard astronomical reference until the Renaissance.

Ptolemy's Planetary System

In the theory of Ptolemy, the planets moved in small orbits while revolving in large orbits about the Earth. This theory, although incorrect, could explain the apparent motions of the planets and also account for changes in their brightness.