sidereal.py: Internal maintenance specification

This is the inverse of Section 9.4, “raToHourAngle(): Convert a right ascension to an hour angle”. The first two steps are the same: derive the GST and convert it to LST.

# - - -   r a T o H o u r A n g l e

def raToHourAngle ( ra, ut, eLong ):
    """Convert right ascension to hour angle.

      [ (ra is a right ascension in radians as a float) and
        (ut is a timestamp as a datetime.datetime instance) and
        (eLong is an east longitude in radians) ->
          return the hour angle in radians at that time and
          location corresponding to that right ascension ]
    """
    #-- 1 --
    # [ gst  :=  the Greenwich Sidereal Time equivalent to
    #            ut, as a SiderealTime instance ]
    gst  =  SiderealTime.fromDatetime ( ut )

    #-- 2 --
    # [ lst  :=  the local time corresponding to gst at
    #            longitude eLong ]
    lst  =  gst.lst ( eLong )

All that remains is to subtract h from lst and normalize the result to [0,2π).

    #-- 3 --
    # [ h  :=  lst - ra, normalized to [0,2*pi) ]
    h  =  (lst.radians - ra) % TWO_PI

    #-- 4 --
    return h