The reference for this is an arbitrarily drawn line that runs from the North Pole to the South Pole and passes through the Royal Greenwich Observatory outside of London, England. Longitude is measured halfway around the world in each direction. 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.
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.