PARALLX: converts between geocentric and apparent position

Summary

The equatorial coordinates of members of the Solar System calculated in later subroutines are those appropriate to an observer situated at the centre of the Earth (geocentriccoordinates). When the celestial body in question is at a very large distance away, the same coordinates apply to observations made from the Earth's surface. However, objects within the Solar System are sufficiently close that their apparent positions (topocentriccoordinates) change slightly depending on the exact vantage point. The Sun, for example, may appear displaced by as much as 8 arcseconds from its geocentric position, and the Moon by as much as 1 degree.

Routine PARALLX calculates the apparent hour angle (P) and declination (Q) for an observer at a given geographical latitude (GP) and height above sealevel (HT), when the geocentric hour angle (X), declination (Y), and equatorial horizontal parallax (HP) are known. It will also perform the reverse operation, calculating the geocentric coordinates from the apparent position. The correction for parallax to the geocentric hour angle (DX) is the same as that to the geocentric right ascension. Having run the routine, the apparent right ascension, RA1, may be found from

RA1 = RA - DX,

where RA is the geocentric right ascension.

Execution of PARALLX is controlled by the flag FL(12). This should be set to 0 before calling the routine for the first time, or whenever new values of GP or HT are specified.