Stars seem to move across the sky through the night, but that movement is due to Earth’s rotation. As earth spins on its axis, objects appear to rise in the east and set in the west. In the Northern Hemisphere, some stars never appear to set. Called circumpolar stars, they circle at a point projected in the sky above the North Pole near Polaris, the Pole Star. A corresponding situation exists above the South Pole near the star Sigma Octantis.
In ancient cultures, sky-watching played an important role in navigation, agriculture, religion and even entertainment. Those who observed the heavens connected stars to form patterns that related to the heroes, gods, and legends of their culture- what we refer to today as constellations. Most cultures named constellations and attached cultural meaning to their patterns. Native American sky lore, for instance, often used constellations to teach moral lessons.
Today, the Western world acknowledges the constellations that originated in Mesopotamia more than 5,000 years ago. Babylonian, Egyptian, and Greek astronomers also made contributions during the classical ages of their cultures.
In 1928 the International Astronomical Union (IAU) determined which constellations would be officially recognized. Of the 88 constellations on the IAU list, 48 were identified in ancient times with just the naked eye. The remaining 40 were added in more recent centuries.
The IAU also defined each constellation’s border so that the groupings represent not only star patterns but specific regions of the sky. These borders ensured that each star would be restricted to only one constellation.
Constellations change over time as the stars in them move through space. The dipper part of the Big Dipper in the constellation Ursa Major (Great Bear) appeared much more square in the past. Now the dipper’s bowl is starting to elongate. About 100,000 years from now, it will look more like a soup bowl with a handle.