HD2PA Function

public function HD2PA(ha, dec, phi) result(res)

Parallactic angle for a given hour angle and declination.

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 HA will also be affected.

Reference

• Smart, W.M., "Spherical Astronomy", Cambridge University Press, 6th edition (Green, 1977), p49.

History

• IAU SOFA revision: 2017 September 12

Arguments

Return Value real(kind=wp)

parallactic angle

    function HD2PA ( ha, dec, phi ) result(res)

    implicit none

    real(wp),intent(in) :: ha !! hour angle
    real(wp),intent(in) :: dec !! declination
    real(wp),intent(in) :: phi !! site latitude
    real(wp) :: res !! parallactic angle

    real(wp) :: cp, sqsz, cqsz

    cp = cos(phi)
    sqsz = cp*sin(ha)
    cqsz = sin(phi)*cos(dec) - cp*sin(dec)*cos(ha)
    if ( sqsz==0.0_wp .and. cqsz==0.0_wp ) cqsz = 1.0_wp
    res = atan2(sqsz,cqsz)

    end function HD2PA