Watchers of the Heavens

Abstract

When we look with our naked eye at the night sky, what do we see? There were no telescopes until the beginning of the seventeenth century, so we see essentially the same things the ancients saw. We see the stars rise on the eastern horizon, move together across the sky in the course of the night, and set in the west. The so-called “fixed stars” appear to keep the same relative positions during their nightly motion. Ancient starwatchers imagined they could recognize in certain groups of these stars patterns that they could associate with mythological events and characters. These patterns are called “constellations.” The ancient ones still carry their classical names (Orion, Virgo, Pegasus, etc.). They are used by modern astronomers to give approximate locations of celestial objects in the sky. For example, the statement, “The moon is in Virgo” is comparable to saying “Richmond is in Virginia.” In both cases, a large surface (sky, earth) is divided into many small pieces and the rough position of a still smaller object (moon, Richmond) is given by specifying the piece (Virgo, Virginia) in which it is located. The precise position of a point on a spherical surface is given by specifying two coordinate angles. For points on the surface of the earth, these are the latitude and longitude. The analogous angular coordinates on the celestial sphere are called declination and right ascension.