In the clear skies of the Tigris, Euphrates, and Nile valleys, where the earliest civilizations flourished more than 5000 years ago, watchers of the heavens singled out and named various groupings of stars, called constellations, primarily for calendrical and navigational purposes. To aid their memory, they imagined that they saw in these groupings the likenesses of mythological beings, animals, and monsters and named the constellations accordingly. The names and shapes of constellations are part of our heritage from ancient Greece, who in turn inherited them from these older civilizations. Greek astronomers identified and named 48 constellations. Forty more were added, most of them in southern skies, by European mapmakers and astronomers in the seventeenth and eighteenth centuries.
Standing in midlatitudes of either hemisphere a practiced observer can see about four-fifths of the constellations during the course of the year. Star maps containing the constellations for each of the four seasons can be found inside the front and back covers to help you. Of the 88 constellations, about half lie in the Milky Way or near its borders. As you learn the constellations, hearing the name of one will bring to mind an area of the sky, just as earthbound place names identify a particular geographical area.
The "catch figure," or asterism, often associated with a constellation should not be assumed to be the outline of the constellation's namesake. An example is the asterism of the Big Dipper, which is the recognizable figure for the constellation Ursa Major, the Great Bear. In today's astronomy, the constellations define specific areas of the sky with north-south and east-west boundaries. All celestial objects lie within the borders of one of the 88 constellations. Stars that identify a constellation form an apparent grouping as seen from Earth and are not necessarily in proximity to each other in space.