The problem of converting between the hour angle and the right ascension of a celestial object at a given local sidereal time is so straightforward as to make it hardly seem worthwhile devoting a separate subroutine to it. Nevertheless, it crops up sufficiently often that one quickly becomes tired of writing the few lines of code needed every time into a new program, and so I have included the routing HRANG here. The equation is the same for conversion in either direction, there being no need for a switch to specify it. The right ascension or hour angle (hours) is input via the variable X and the corresponding hour angle or right ascension (hours) returned by P. The geographical longitude, GL, must be specified in degrees (W negative) and the Greenwich sidereal time, SG, in hours. The routine also returns the local sidereal time, LS, in hours.

Formulae

1. P = [LS − X]24

2. LS = [SG + (GL/15)]24

3. P = hour angle or right ascension

given

1. X = right ascension or hour angle

2. SG = Greenwich sidereal time

3. GL= geographical longitude

[]24 indicates reduction to the range 0-24 by the addition or subtraction of multiples of 24.

Details of HRANG

Called by GOSUB 1600.

Converts X, the hour angle or right ascension, into P, the corresponding right ascension or hour angle, at SG, the Greenwich sidereal time, all quantities in hours.The geographical longitude, GL, must also be specified in degrees(W negative, E positive). The local sidereal time in hours is returned by LS.