'Compute and return the precession matrix for FK4 using Newcomb\'s method.
Used inside some of the transformation functions.
Parameters
oldequinox : `~astropy.time.Time`
The equinox to precess from.
newequinox : `~astropy.time.Time`
The equinox to precess to.
Returns
newcoord : array
The precession matrix to transform to the new equinox'

@staticmethod
def _precession_matrix(oldequinox, newequinox):

    return earth._precession_matrix_besselian(oldequinox.byear, newequinox.byear)
