Coordinates Systems - Locating the Stars
To locate stars in the sky, we use a system of coordinates similar to the one we use to locate places on earth.
Locations in the sky are expressed in a similar way. The equivalent of latitude is called declination, and the equivalent of longitude is called right ascension.
The reference for declination is the celestial equator. This is a line in the sky directly over the earth's equator. It is exactly half way between the north celestial pole (the point in the sky directly over the earth's North Pole) and the south celestial pole. As in latitude, declination is measured in degrees, from 0° at the celestial equator to +90° at the north celestial pole and -90° at the south celestial pole (Plus and minus signs are used instead of N and S).
Right ascension is a little more complicated than longitude. The reference for this measurement is a line from the north celestial pole to the south celestial pole, passing through the vernal equinox.
As the earth orbits the sun, the sun seems to move among the stars. It seems to follow a path in the sky called the ecliptic. Because the earth is tilted on its axis, the ecliptic crosses the celestial equator at two points, called the equinox points. The sun is at one of these crossing points in March. This point in the sky is called the spring, or vernal, equinox. This crossing point then is part of the reference line for right ascension. By the way, the other point is called the fall, or autumnal, equinox point.
Unlike longitude, which measures halfway around in each direction, right ascension measures all the way around in one direction, toward the east. Also, instead of measuring in degrees, right ascension uses units called hours of right ascension. Each hour is 15 degrees wide.