Some of the earliest names given to constellations were inspired by animals that were familiar to the people who first observed the patterns. These were probably people who lived in the Middle Eastern region of Mesopotamia some 7,000 years ago. These constellations are called by names such as the Ram, the Bull, and the Lion. Other constellations have names of everyday objects, such as the Big Dipper.

These names were passed down over the centuries from Mesopotamia to ancient Greece and Rome. As Greek astronomers continued to carefully observe the sky, they discovered more constellations. Many of these were named for the heroes and gods of the Greek civilization. Roman astronomers changed the names of many constellations from Greek to Latin.

By about 150 AD astronomers had discovered 48 constellations. The Egyptian astronomer Ptolemy listed these in a book called the Almagest. Ptolemy's list did not include constellations found in the Southern Hemisphere because all of the astronomers up to that time lived north of the equator. As the land and seas to the south were explored over the next centuries, astronomers found more constellations in the skies. Many of these were named for newly discovered tropical animals, such as the giraffe. Today, astronomers recognize 88 constellations.

An observer looking at the sky from the same point every night will see different constellations at different times of the year. This happens because the Earth moves around the sun in a path called an orbit. As the Earth moves through space, however, the stars stay in place. Therefore the Earth moves past different stars or groups of stars in the course of a year. Some constellations can be seen only from the Northern Hemisphere and some only from the Southern Hemisphere. The 12 constellations that lie in the path of the Earth's orbit around the sun are known as the signs of the zodiac. They can be seen from both hemispheres.