As the Earth moves around the Sun, it sweeps out the plane of the ecliptic,making one complete circuit in one year. The Earth also rotates about its ownNorth-South polar axis, making one revolution in 24 hours. Since the Earth acts like a huge gyroscope, the direction of its polar axis remains relativelyfixed although precession (PRCESS1, 2500; PRCESS2, 2600) and nutation (NUTAT, 1800) conspire to cause a wobbling motion. This direction is inclined at about 23.5 degrees to the perpendicular to the plane of the ecliptic, and hence the Earth's equator is tilted at the same angle to the plane of the ecliptic. The angle is called the obliquity of the ecliptic and it is needed whenever we convert between ecliptic and equatorial coordinates (see EQECL, 2000). The angle changes slowly with time so it must be determined separately for calculations made at different epochs.

The subroutine OBLIQ returns the value of the obliquity of the ecliptic via the variable OB (degrees). The dateis input in the usual way via DY (days), MN (months), and YR (years). Execution is controlled by the flag FL(5), designed to save time if the calculation has already been made for the current date. When calling the routine for the first time, or for a new date, set FL(5) = 0. It is set to 1 by the routine (line 1705). Subsequent calls to OBLIQ with FL(5) =1 result in no calculation being performed, but control is returned immediately to the calling program.