Specifying Sky Coordinate Systems

Any of the three strings EQUATORIAL, ECLIPTIC and GALACTIC can be assigned to a COORDS parameter. Case is insignificant, and abbreviations may be given.

Ecliptic and equatorial coordinates are referred to the mean equinox of a given epoch. This epoch is specified by appending it to the end of the name of the sky coordinate system, in parentheses; for instance EQUATORIAL(1983.5) (only the four most significant decimal places are used). The epoch may be preceded by a single character, B or J, indicating if the epoch is a Besselian epoch (B) or a Julian epoch (J). If this character is missing (as in the above example), then the epoch is assumed to be a Besselian epoch. If no equinox is specified in this way, then a default of B1950.0 is used.

If a Julian epoch is used to specify the reference equinox for an equatorial coordinate system, then the equatorial coordinates are assumed to be in the IAU 1976, FK5, Fricke system. If the equinox is specified using a Besselian epoch, then the coordinates are assumed to be in the FK4, Bessel-Newcomb system.