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## SLA_ZD - to Zenith Distance

ACTION:
Hour angle and declination to zenith distance (double precision).

CALL:
D = sla_ZD (HA, DEC, PHI)

GIVEN:

RETURNED:

 D sla_ZD zenith distance (radians, )

NOTES:
1.
The latitude must be geodetic. In critical applications, corrections for polar motion should be applied (see sla_POLMO).
2.
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 topocentric .
3.
No range checking of arguments is done.
4.
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.

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SLALIB --- Positional Astronomy Library