How are constellations named?

Our modern constellations system comes from the ancient Greeks. The oldest description of constellations as we know them comes from a poem, called Phaenomena, which was written about 270. B.C by a Greek poet Aratus.

In 150 A.D., the Greek scientist Ptolemy published a book,The Almagest, which contained a summary of Greek astronomical knowledge, including a catalog of 1022 stars, with estimates of their brightness. They are arranged into 48 constellations. The constellations where named by what the cultures saw in them.

As the Earth rotated around the sun, the constellations would move. This helped the farmers of ancient civilizations know when the seasons were changing.