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What Is a Constellation?

Have you ever looked at the stars and traced out squares, letters, and other familiar figures? In nearly all parts of the world, people of long ago did this and gave names to the group of stars they observed.

Such a group is called a constellation, from the Latin terms meaning star (Stella) and together. The names of the constellations in use today have come down to us from the times of the Romans and from the even more ancient Greeks.

What the Greeks knew about the stars came partly from the Babylonians. The Babylonians named some of their star figures after animals and others after kings, queens, and heroes of their myths. Later, the Greeks changed many of these Babylonian names to the names of their own heroes such as Hercules, Orion, and Perseus.

The Romans made further changes. The same ancient names are still used, but it is not always easy to make out in the sky the figures that suggested them. For example, Aquila is the eagle, Canis Major and Canis Minor are big and little dogs and Libra is the scales, but the constellations don’t look much like these figures to us.

About the year A.D. 150, the famous astronomer Ptolemy listed 48 constellations that were known to him. This list did not cover the entire sky, there were many blank spots.

So, in later times, astronomers added constellations to Ptolemy’s list. Some of these later constellations are named for scientific instruments, such as the Sextant, the Compasses, and the Microscope.

Today, astronomers recognize 88 constellations in the sky. A constellation is really an area in the sky. This means that every star lies in one constellation or another, just as any city in the United States is in someone state.

The boundaries of the constellations used to be very irregular. Many of them had curved lines. But in 1928, astronomers decided to straighten them out so that the outline of any constellation includes only straight lines.

Read: Why Are Eclipses so Rare?

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