ALTAZ2HADEC
Name
ALTAZ2HADEC
Purpose
Convert Horizon (Alt-Az) coordinates to Hour Angle and Declination.
Explanation
Can deal with the NCP singularity. Intended mainly to be used by
program hor2eq.pro
Calling Sequence
ALTAZ2HADEC, alt, az, lat, ha, dec
Inputs
alt - the local apparent altitude, in DEGREES, scalar or vector
az - the local apparent azimuth, in DEGREES, scalar or vector,
measured EAST of NORTH!!! If you have measured azimuth west-of-south
(like the book MEEUS does), convert it to east of north via:
az = (az + 180) mod 360
lat - the local geodetic latitude, in DEGREES, scalar or vector.
Outputs
ha - the local apparent hour angle, in DEGREES. The hour angle is the
time that right ascension of 0 hours crosses the local meridian.
It is unambiguously defined.
dec - the local apparent declination, in DEGREES.
Example
Arcturus is observed at an apparent altitude of 59d,05m,10s and an
azimuth (measured east of north) of 133d,18m,29s while at the
latitude of +43.07833 degrees.
What are the local hour angle and declination of this object?
IDL> altaz2hadec, ten(59,05,10), ten(133,18,29), 43.07833, ha, dec
===> Hour angle ha = 336.683 degrees
Declination, dec = 19.1824 degrees
The widely available XEPHEM code gets:
Hour Angle = 336.683
Declination = 19.1824
Revision History
Written Chris O'Dell Univ. of Wisconsin-Madison May 2002