hd2pa¶
- erfa.hd2pa(ha, dec, phi)[source]¶
Parallactic angle for a given hour angle and declination.
- Parameters:
- hadouble array
- decdouble array
- phidouble array
- Returns:
- c_retvaldouble array
Notes
Wraps ERFA function
eraHd2pa
. The ERFA documentation is:- - - - - - - - - e r a H d 2 p a - - - - - - - - - Parallactic angle for a given hour angle and declination. Given: ha double hour angle dec double declination phi double site latitude Returned (function value): double parallactic angle Notes: 1) All the arguments are angles in radians. 2) The parallactic angle at a point in the sky is the position angle of the vertical, i.e. the angle between the directions to the north celestial pole and to the zenith respectively. 3) The result is returned in the range -pi to +pi. 4) At the pole itself a zero result is returned. 5) 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. 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 the hour angle ha will also be affected. Reference: Smart, W.M., "Spherical Astronomy", Cambridge University Press, 6th edition (Green, 1977), p49. Last revision: 2017 September 12 ERFA release 2023-10-11 Copyright (C) 2023 IAU ERFA Board. See notes at end.