Astronomy Hacks

Understand Celestial Coordinate Systems

Orient yourself to the night sky.

Celestial coordinate systems are used to specify the locations of stars and other astronomical objects. Four celestial coordinate systems exist, but only two of them are used commonly by amateur astronomers. Celestial coordinate systems are analogous to the geographic coordinate system of latitude and longitude used to specify locations on the earth’s surface, but they are used instead to specify locations on the celestial sphere. The four coordinate systems differ only in what they designate the fundamental plane, called the equator, which bisects the sky into two hemispheres along a great circle.

Horizontal Coordinates

The horizontal coordinate system, also called the altitude-azimuth or alt-az coordinate system, uses the local horizon as the equator, with the two poles straight up (the zenith) and straight down (the nadir). The location of an object is specified by two values called altitude and azimuth. Each of these is denominated in degrees (°), minutes ('), and seconds (“). One degree is divided into 60 minutes, and one minute is divided into 60 seconds. Accordingly, one degree contains 3,600 seconds.

Tip

Altitudes and azimuths are sometimes specified in decimal degrees. For example, 63°30’ may also be written as 63.5°, because 30’ is equal to 0.5°. Using decimal degrees allows you to specify the location of an object with very high precision, for example 63.5342°. Locations specified ...