erfa.ae2hd(az, el, phi)[source]

Horizon to equatorial coordinates: transform azimuth and altitude to hour angle and declination.

azdouble array
eldouble array
phidouble array
hadouble array
decdouble array


Wraps ERFA function eraAe2hd. The ERFA documentation is:

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 e r a A e 2 h d
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Horizon to equatorial coordinates:  transform azimuth and altitude
to hour angle and declination.

   az       double       azimuth
   el       double       altitude (informally, elevation)
   phi      double       site latitude

   ha       double       hour angle (local)
   dec      double       declination


1)  All the arguments are angles in radians.

2)  The sign convention for azimuth is north zero, east +pi/2.

3)  HA is returned in the range +/-pi.  Declination is returned in
    the range +/-pi/2.

4)  The latitude phi is pi/2 minus the angle between the Earth's
    rotation axis and the adopted zenith.  In many applications it
    will be sufficient to use the published geodetic latitude of the
    site.  In very precise (sub-arcsecond) applications, phi can be
    corrected for polar motion.

5)  The azimuth az must be with respect to the rotational north pole,
    as opposed to the ITRS pole, and an azimuth with respect to north
    on a map of the Earth's surface will need to be adjusted for
    polar motion if sub-arcsecond accuracy is required.

6)  Should the user wish to work with respect to the astronomical
    zenith rather than the geodetic zenith, phi will need to be
    adjusted for deflection of the vertical (often tens of
    arcseconds), and the zero point of ha will also be affected.

7)  The transformation is the same as Ve = Ry(phi-pi/2)*Rz(pi)*Vh,
    where Ve and Vh are lefthanded unit vectors in the (ha,dec) and
    (az,el) systems respectively and Rz and Ry are rotations about
    first the z-axis and then the y-axis.  (n.b. Rz(pi) simply
    reverses the signs of the x and y components.)  For efficiency,
    the algorithm is written out rather than calling other utility
    functions.  For applications that require even greater
    efficiency, additional savings are possible if constant terms
    such as functions of latitude are computed once and for all.

8)  Again for efficiency, no range checking of arguments is carried

Last revision:   2017 September 12

ERFA release 2023-10-11

Copyright (C) 2023 IAU ERFA Board.  See notes at end.