My grandfather first taught me about the Great Bear constellation. After that, I had fun wielding an old pair of binoculars and picking out other constellations in the wide sky over Colorado--or even inventing my own. At the time, of course, I gave no thought to the age or origin of the constellations, but the curious pictures in the sky present a fascinating scientific puzzle.

In 1922, when the International Astronomical Union officially defined 88 constellations, it drew the bulk of them from Ptolemy's The Almagest, which was written around A.D. 150 and described the traditions widespread among the Greeks. These traditions had been popularized in the "best-selling" poem The Phaenomena, by Aratus (275 B.C.). The great astronomer Hipparchus's sole surviving book, The Commentary (147 B.C.), tells us that Aratus's poem is for the most part a copy of a work with the same name by Eudoxus (366 B.C.), which no longer survives. These books held the earliest descriptions of the Greek skies, and in them the constellations are already fully formed. But where did the Greek constellations come from?

This article was originally published with the title The Origin of the Greek Constellations.

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