The System Behind Every Epsilon and Alpha

Johann Bayer's Uranometria, published in 1603, introduced the naming convention still used for the sky's brighter stars four centuries later. Each star within a constellation received a Greek letter, generally though not strictly in order of apparent brightness, followed by the constellation's Latin name in the genitive case: Alpha Centauri, the brightest star in Centaurus; Epsilon Andromedae, the fifth brightest in Andromeda. Where a constellation held more than twenty four stars worth naming, past the Greek alphabet's own limit, Bayer continued into Roman letters. The system replaced a confusing patchwork of descriptive Arabic and Latin names, several stars often sharing similar labels, with a single consistent scheme that let any astronomer locate a star's approximate brightness and constellation from its designation alone.

Uranometria's other achievement is less remembered but comparably significant: it was the first European star atlas to chart the far southern constellations, the stars invisible from Europe that Dutch navigators had only recently recorded on voyages around the Cape. Bayer incorporated their observations into twelve new southern constellations, extending Western celestial cartography below the horizon Ptolemy's ancient charts had never reached, a full generation before southern star charts became common in European astronomy.