### SLA_ZD

$h,\delta$ to Zenith Distance

ACTION:
Hour angle and declination to zenith distance (double precision).
CALL:
D = sla_ZD (HA, DEC, PHI)
##### GIVEN:
 sla_ZD D zenith distance (radians, $0\phantom{\rule{0.3em}{0ex}}-\phantom{\rule{0.3em}{0ex}}\pi$)
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 $\left[\phantom{\rule{0.3em}{0ex}}h,\delta \phantom{\rule{0.3em}{0ex}}\right]$, and the zenith distance in vacuo, which would require the topocentric $\left[\phantom{\rule{0.3em}{0ex}}h,\delta \phantom{\rule{0.3em}{0ex}}\right]$. If the effects of diurnal aberration can be neglected, the apparent $\left[\phantom{\rule{0.3em}{0ex}}h,\delta \phantom{\rule{0.3em}{0ex}}\right]$ may be used instead of the topocentric $\left[\phantom{\rule{0.3em}{0ex}}h,\delta \phantom{\rule{0.3em}{0ex}}\right]$.