erfa.hd2ae(ha, dec, phi)[source]

Equatorial to horizon coordinates: transform hour angle and declination to azimuth and altitude.

hadouble array
decdouble array
phidouble array
azdouble array
eldouble array


Wraps ERFA function eraHd2ae. The ERFA documentation is:

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 e r a H d 2 a e
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Equatorial to horizon coordinates:  transform hour angle and
declination to azimuth and altitude.

   ha       double       hour angle (local)
   dec      double       declination
   phi      double       site latitude

   *az      double       azimuth
   *el      double       altitude (informally, elevation)


1)  All the arguments are angles in radians.

2)  Azimuth is returned in the range 0-2pi;  north is zero, and east
    is +pi/2.  Altitude is returned in the range +/- pi/2.

3)  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.

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

5)  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 the hour angle ha will also
    be affected.

6)  The transformation is the same as Vh = Rz(pi)*Ry(pi/2-phi)*Ve,
    where Vh and Ve are lefthanded unit vectors in the (az,el) and
    (ha,dec) systems respectively and Ry and Rz are rotations about
    first the y-axis and then the z-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.

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

Last revision:   2021 February 24

ERFA release 2023-10-11

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