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/ Right Ascension / Declination (RA/Dec)

Right ascension and declination are what astronomers use to precisely locate objects on a celestial map, and are equivalent to the imaginary lines of and latitiude used in maps of the . Although it is obvious that all of the 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 , so does the sky. Declination is measured in the same way as , 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 . The earth is tilted 23.5 degrees in its around the Sun, causing the path of the Sun to only cross the during the vernal and autumnal . The vernal (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 and continue to stare for one hour at the point where Sirius was, the 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 . 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 . 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.

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http://astro.newaygo.mi.us/~astro/reference/ra.html [5/24/1999 9:14:13 AM]