Right ascension and declination are what astronomers use to precisely locate objects on a celestial map, and are equivalent to the imaginary lines of longitude and latitiude used in maps of the earth. Although it is obvious that all of the stars lie at different distances from the earth, it is also convenient to think of the sky as a fixed sphere with the earth at the center. Just as the earth has a north pole, south pole, and equator, so does the sky.
Declination is measured in the same way as latitude, with the equator being 0 degrees and anything north of the equator having a positive value and south having a negative value. (The north and south poles are +90 and -90, respectively) A reference point was needed from which to begin measuring longitude (Grenwich, England), and astronomers also had to decide on a point to begin measuring right ascension, so they looked to the path of the Sun. The earth is tilted 23.5 degrees in its orbit around the Sun, causing the path of the Sun to only cross the celestial equator during the vernal and autumnal equinoxes. The vernal equinox (March 20/21) was selected, but now a measuring system was needed. Because the Sun makes one complete path through the sky in 24 hours, it seemed natural to measure out right ascension in 24 one-hour blocks. What this means is that if you go outside and stare at Sirius and continue to stare for one hour at the point where Sirius was, the star you are now looking at is located 1 R.A. hour away from Sirius. (We do not recommend trying this. There are much better things to be doing with your time.)
There is only one slight hitch with all of this. Due to precession, the earth wobbles and in the process causes the vernal equinox to slowly move through the celestial sphere. So what we are basically stuck with is a fisherman who places an "X" on his boat to remember where a good fishing hole is. To remedy this, astronomers redraw the celestial grid every 50 years and call each period an epoch. The standard epoch was epoch1950.0, but current star maps are now epoch2000.0. But don't worry if you find out that your star map is epoch1950.0, the shift is not great enough to notice until about epoch3000.0.


Return to the Main Index