Right Ascension and Declination

A more general coordinate system is called the equatorial coordinate system, used by both professional and amateur astronomers. This coordinate system is more like giving someone the latitude and longitude of the store down the street, and letting him or her figure out how to get there. Because these coordinates do not change with time and location, they are used in star maps, books, magazines, and computer programs.

Right Ascension

Remember the lines of longitude on Earth that run north-south from pole to pole? When we project those lines onto the celestial sphere, we get lines of right ascension. Before the age of digital watches and other accurate timepieces, stars were used to measure time, so right ascension (abbreviated RA) is given in hours, minutes, and seconds. RA increases as you go east across the sky. If you watch two stars whose coordinates are one hour of RA apart, the second star will cross the meridian an hour after the first star, and the second star will also rise and set an hour after the first one (unless either is circumpolar).

The zero point for longitude was arbitrarily chosen to be in England, and the zero point for RA was also arbitrarily chosen. The zero point of RA is defined as the point on the celestial sphere where the Sun crosses the celestial equator at the spring equinox. Remember that this is the point in the spring sky where the ecliptic crosses the celestial equator. Just as lines of longitude on Earth converge at the poles, lines of RA converge at the celestial poles.

There are 24 hours of RA and 360 degrees across the celestial sphere, so each hour of RA is equal to 15 degrees on the celestial sphere (or 15 degrees of Earth's rotation).


We can also take the lines of latitude on Earth, which are parallel to the equator, and project those onto the celestial sphere. There, they become lines of declination. Just as latitude on Earth is measured in degrees away from the equator, with positive for north and negative for south, declination (abbreviated dec) is measured in degrees away from the celestial equator, with positive degrees for stars or other objects that are north of the celestial equator and negative degrees for stars or other objects that are south of the celestial equator. Just as on Earth, objects that are exactly on the celestial equator have a dec of 0 degrees, objects at the north celestial pole are at dec +90 degrees, and objects at the south celestial pole are at dec −90 degrees.

The big advantage of the RA and dec system for locating stars is that these positions do not change over the course of a night. Unfortunately, however, the position of a star is dependent on the position of the north celestial pole and the celestial equator, and these do change slowly with time. The RA and dec of a star will change by about 1.4 degrees every 100 years. Because RA and dec change with time, star charts and catalogs must be drawn for a certain epoch, or time period. Most modern star charts are compiled with different positions every fifty years or so, with the most current being from the year 2000. If you're using an old star atlas, make sure to check the date, or your star positions could be off by a few degrees!

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