Hour angle and declination to zenith distance
D = sla_ZD (HA, DEC, PHI)
hour angle in radians
declination in radians
latitude in radians
zenith distance (radians, )
The latitude must be geodetic. In critical applications,
corrections for polar motion should be applied (see sla_POLMO).
In some applications it will be important to specify the
correct type of hour angle and declination in order to
produce the required type
of zenith distance. In particular, it may be
important to distinguish between the zenith distance
as affected by refraction, which would require the
observed , and the zenith distance in vacuo,
which would require the topocentric . If
the effects of diurnal aberration can be neglected, the
apparent may be used instead of the
No range checking of arguments is done.
In applications which involve many zenith distance calculations,
rather than calling the present routine it will be more
efficient to use inline code, having previously computed fixed
terms such as sine and cosine of latitude, and perhaps sine and
cosine of declination.