Celestial Coordinates

Astronomers use several coordinate systems to locate objects in the sky.  All are analogous to the terrestial system of longitude and latitude.  The fundamental plane for the terrestial system is defined by the earth's equator.  Longitude is measured east or west from a line running from the north pole through Greenwich, England which intersects the equator at zero degrees longitude.  Terrestial latitude is measured north or south in degrees from the equator.

The astronomical ICRS or equatorial system uses the projection of the earth's equator on the celestial sphere to define its fundamental plane.  Celestial longitude is measured eastward in hours, minutes and seconds of right ascension.  The use of time units rather than angular degrees is a hold-over from the time before clocks were invented and the sun, moon and stars were used to tell time.  The index or zero-hour point on the celestial equator is called the First Point of Aries and is the intersection of the Sun's apparent annual path northward with the celestial equator on the vernal equiox, i.e., the first day of Spring in the northern hemisphere.  Due to a slight wobble in the earth's rotational axis, this point creeps slowly eastward and is no longer located in the constellation Aries.  Equatorial latitude or declination is measured in angular degrees, minutes and seconds north or south from the celestial equator.

The Galactic system uses a fundamental plane defined by the disk of the Milky Way galaxy.  Galactic longitude is measured counter-clockwise relative to the galactic north pole from a point on the galactic equator in line with the center of the Milky Way.  This index point is in the constellation Sagittarius and the galactic north pole is located in the constellation Coma Berenices.  Galactic latitude or declination is measured in angular degrees, minutes and seconds north or south from the galactic equator.

The coordinate systems described above are referenced in the Object Technical Data windows on this website.  There are other systems in-use by professional astronomers but they are seldom referenced outside of technical publications.