### SLA_PA

$h,\delta$ to Parallactic Angle

ACTION:
Hour angle and declination to parallactic angle (double precision).
CALL:
D = sla_PA (HA, DEC, PHI)
##### GIVEN:
 HA D hour angle in radians (geocentric apparent) DEC D declination in radians (geocentric apparent) PHI D latitude in radians (geodetic)

##### RETURNED:
 sla_PA D parallactic angle (radians, in the range $±\pi$)

NOTES:
(1)
The parallactic angle at a point in the sky is the position angle of the vertical, i.e. the angle between the direction to the pole and to the zenith. In precise applications care must be taken only to use geocentric apparent $\left[\phantom{\rule{0.3em}{0ex}}h,\delta \phantom{\rule{0.3em}{0ex}}\right]$ and to consider separately the effects of atmospheric refraction and telescope mount errors.
(2)
At the pole a zero result is returned.