PRCESS1: approximate precession of equatorial coordinates & PRCESS2: rigorous precession of equatorial coordinates
Peter J. Duffett-Smith · Cambridge University Press eBooks · 1990
The right ascension and declination of a celestial body change slowly with time because of luni-solar precession , a gyrating motion of the Earth's axis caused by the gravitational effects of the Sun and the Moon on the equatorial bulge of the Earth. The Earth's axis describes a cone about a line perpendicular to the ecliptic through the centre of the Earth with half-angle approximately 23.5 degrees, completing one circuit in about 25 800 years. The change is thus about 50 arcseconds per year and this must be allowed for when comparing equatorial coordinates at two different epochs. PRCESS1 Two subroutines are given here which will correct equatorial coordinates for the effects of precession between one epoch and another. The first of these, PRCESS1 (2500), uses an approximate method which is simple and compact, but which nevertheless gives good results. You should find it sufficient for most purposes. The corrections, XI and Yl, which have to be made to the right ascension, X, and declination, Y, measured at epoch A, to find the corresponding values at epoch B, are well approximated by the expressions: X1 = (MP + (NP × SIN(X) × TAN(Y))/15) × NY Y1 = NP × COS(X) × NY, where NY is the number of years between the two epochs, and MP and NP are ‘constants’ which change slowly with time.