Declination and Right Ascension

The combination of diurnal circles and hour circles allows us to make a grid for the celestial sphere. This celestial grid is analogous to latitude and longitude on Earth. Hour circles, running from pole-to-pole, are analogous to lines of constant longitude. Diurnal circles, running parallel to the equator, are analogous to lines of constant latitude. Just as we can locate cities on the globe by longitude and latitude, we can locate stars by assigning numbers to the celestial grid.

The (red) diurnal circles are parallel to the celestial equator; we can locate their position by their angle from the celestial equator. The result is called declination (dec or the Greek letter delta:  for short). This is exactly analogous to how we assign latitude.

Declination, like latitude, has a natural zero: the equator. However, there is no natural zero for longitude. Historically different nations chose different zeros for longitude for their maps; for example, England used the location of Greenwich England as the zero for longitude and France used Paris. The same problem applies to the celestial equivalent of longitude, right ascension. Astronomers have chosen the vernal equinox as the zero point for right ascension (RA or the Greek letter alpha:   for short). In addition astronomers use an unusual way of measuring the angle for RA: it is measured in "hours" where 1h=15°. Thus there are 24h of RA around the celestial equator. The reason for this oddity is that the celestial sphere makes one full rotation (24h of RA) in one day (24 hours of time). Thus the celestial sphere advances about 1h of RA in an hour of time.