Quote:
Originally Posted by byronpaul
Wow I have no idea what epoch means but I see J2000 referenced in Stellarium so I'll be doing some separate research on that.  |
See, for example,
http://en.wikipedia.org/wiki/Epoch_%28astronomy%29
"Celestial coordinate systems most commonly used in astronomy are equatorial coordinates and ecliptic coordinates. These are defined relative to the (moving) vernal equinox position, which itself is determined by the orientations of the Earth's rotation axis and orbit around the Sun. Their orientations vary (though slowly, e.g. due to precession), and there is an infinity of such coordinate systems possible. Thus the coordinate systems most used in astronomy need their own date-reference because the coordinate systems of that type are themselves in motion, e.g. by the precession of the equinoxes, nowadays often resolved into precessional components, separate precessions of the equator and of the ecliptic."
The usual epochs you see are B1950 and J2000 but old catalogs are usually given as at the original date of publication. Then there is Jnow, the current value, and is obtained by precessing coordinates given in other epochs.
