SLA_ZD - to Zenith Distance

**ACTION:**- Hour angle and declination to zenith distance
(double precision).
**CALL:**`D = sla_ZD (HA, DEC, PHI)`

**GIVEN:**-

*HA***D**hour angle in radians *DEC***D**declination in radians *PHI***D**latitude in radians

**RETURNED:**-

*sla_ZD***D**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.

Starlink User Note 67

P. T. Wallace

12 October 1999

E-mail:ptw@star.rl.ac.uk