$h,\delta $ to Zenith Distance
D = sla_ZD (HA, DEC, PHI)
HA  D  hour angle in radians 
 
DEC  D  declination in radians 


PHI  D  latitude in radians 
sla_ZD  D  zenith distance (radians, $0\phantom{\rule{0.3em}{0ex}}\phantom{\rule{0.3em}{0ex}}\pi $) 