Celestial Coordinate Systems

There are two celestial coordinate systems used by astronomers. One takes advantage on the observer's natural viewing area, or observer disk.Such a disk is shown for an arbitrary observer. The line that traverses the sky from the North to South Celestial Pole and goes through the point directly above your head (your zenith) is called your meridian. With the cardinal directions north and south firmly established, we can divide up the sky by first determining the angle a celestial object (for example a star) makes of the nearest horizon (here depicted as 60 degrees). This angle is the object's altitude angle.Projecting that pointing vector directly down upon the ground, we ask "What is the angle that that projection makes with cardinal north?" Setting the angle due north as zero degrees, we see that we can have an angle from 0 to 360 degrees, with cardinal east being 90 degrees, cardinal south being 180 degrees, cardinal west being 270 degrees, etc. This second angle specified is called the azimuthal angle. The two angles together make up the alt-azimuth celestial coordinate system.The second celestial coordinate system strongly resembles, but is not identical to, the latitude and longitude system that demarcates the Earth. The latitude coordinate is retained, but is called the declination angle, or just declination. Put another way, on the star equivalent of the Earth's surface, the Celestial Sphere, the latitude coordinate is replaced by the declination coordinate.The Celestial Sphere equivalent of longitude is called right ascension, or RA. RA is called out not in degrees, but time coordinates, as befits a starry background that is always seen as moving, due to the Earth's rotation. The RA equivalent of longitude's Prime Meridian is called zero hour, and it aligns not with the Prime Meridian that goes through Greenwich, England, but that point on the Celestial Sphere's Equator over which the Sun shines directly at the moment of the Vernal Equinox. This point is called the Vernal Equinox Point.A typical RA might be 18 hours, 35 minutes. This means that any celestial object's having this RA will be seen over the observer's meridian 18 hours and 35 minutes after the Vernal Equinox Point passed over the observer's meridian.It is important to point out that the times called out in RA are with respect to a Sidereal Day, not a Mean Solar Day.