The group of stars is called “a constellation,” from the Latin terms meaning “star” (stella) and “together.” About the year 150 A.D., 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.