Glossary

The right ascension and declination of a heavenly body describe its position in the sky.

Think of a star's right ascension as its longitude. The position of the sun at the moment of the vernal equinox is 0 hours, 0 minutes, and 0 seconds. As the Earth rotates over a 24-hour period, the right ascension is marked by the point at which a star reaches the same north-south position.

As an example, the right ascension of Rigel (in Orion) is 5h 14m 32.272s. On the opposite side of the sky, almost 12 hours of rotation away, Antares (in Scorpius) has a right ascension of 16h 29m 24.460s. This is why you can never see both constellations in the sky at the same time... one sets as the other rises. Stars close to each other east-west-wise will have right ascensions near one another.

A star's declination is similar to its latitude. At points along the equator, declination starts at 0 degrees, 0 minutes, and 0 seconds. The declination is measured in positive degrees north of the equator and negative degrees south of the equator.

Polaris, with declination +89d 15m 50.794s, is close enough to 90 degrees north (or true north) that we romantics can it the North Star without too much trouble from scientists.