Celestial coordinates of right ascension (RA) and declination (Dec) are used to help locate objects on the sky. Although this coordinate system is very similar to latitude and longitude used to locate objects on the surface of the Earth, it is not ‘fixed’ on the sky (the celestial sphere). Instead, the RA and Dec of an object vary slightly every year, so we need to specify the epoch or time period which we are using as a reference for the celestial coordinate system.
The zero line of RA is the point where the Sun crosses the celestial equator, moving from southern declinations to northern declinations. This occurs around 21 March each year. The apparent motion of the Sun across the sky is really due to the Earth’s daily rotation on its axis (tilted at 23.5 degrees to the plane of the ecliptic) and yearly orbit around the Sun. The Earth’s rotation axis does not point towards a fixed point on the celestial sphere, as the gravitational pull of the Moon and Sun cause the axis to ‘wobble’. Over a period of 26,000 years, the axis traces out a circle on the sky, resulting in a change in the zero line of RA westwards by 50 arcseconds every year. The result, known as the precession of the equinox, is a gradual change in the location on the sky where the Sun crosses the celestial equator.
The current epoch is referred to as J2000.0, so that locations of celestial objects are given relative to the coordinates as they were in the year 2000. The previous epoch used for many astronomical surveys was B1950.0 – celestial coordinates as they were in 1950. This epoch is recorded in the designations of many celestial objects.
For example, the Einstein Cross (2237+0305) was located at RA = 22h 37m, Dec = +03o05’ using epoch B1950.0. In J2000.0 coordinates, this object is at RA = 22h 37m, Dec = +03o 21’. The object itself has not moved – just the coordinate system.