Moon and Spica
The Moon has a bright companion the next couple of evenings: a star known as HD 116658. If you can't recall that particular catalog number, try this one: SAO 157923. Still not ringing any bells? Well here's one more name for the star: Spica.
This roster of names shows that astronomers use a lot of different methods for tracking the stars.
Spica is a name that dates from ancient times. It means a "spike" or ear of wheat. Spica's constellation, Virgo, is a daughter of the harvest goddess. She's holding the wheat in her hand.
The constellation name gives the star another name: Alpha Virginis. It comes from an atlas created four centuries ago by Johann Bayer. He named the stars using the letters of the Greek alphabet. In most cases, "alpha" indicates a constellation's brightest star.
A few decades later, John Flamsteed devised a similar scheme using numbers instead of letters. Under that system, Spica is 67 Virginis.
Since then, astronomers and observatories have compiled many more catalogs. The SAO catalog, from the Smithsonian Astrophysical Observatory, has more than 250,000 stars. And the Henry Draper catalog -- HD for short -- contains more than 350,000 stars. These catalogs help astronomers keep track of every star. But keeping track of the catalogs is a problem all its own.
Look for Spica and the Moon low in the west at nightfall. Spica is above the Moon tonight, and to its right tomorrow night.