David: I’m currently slogging through the trig of coordinate translation and I think the problem can be framed as a version of translation between any two coordinate systems that are misaligned by an know angle. Epsilon = obliquity or angle between the pole of ecliptic and the north pole Alpha = right ascension Delta= declination Lambda= ecliptic longitude Beta= ecliptic latitude Then Tan(lambda) = (sin(alpha)*cos(epsilon)+tan(delta)*sin(epsilon))/cos(alpha) Sin(beta)= sin(delta)*cos(epsilon)-cos(delta)*sin(epsilon)*sin(alpha) And the reverse equasions are Tan(alpha) = (sin(lambda)*cos(epsilon)-tan(beta)*sin(epsilon))/cos(lambda) Sin(delta) = sin(beta)*cos(epsilon)+cos(beta)*sin(epsilon)*sin(lambda) When the obliquity is very small, like the size of the usual polar alignment error, the first and third equasions tend towards lambda = alpha meaning the error in right ascension would be very near zero for most polar alignment errors. Daniel Turner __________________________________ Yahoo! Music Unlimited Access over 1 million songs. Try it free. http://music.yahoo.com/unlimited/