'Convert longitude differential d_lon to d_lon_coslat.
Parameters
d_lon : `~astropy.units.Quantity`
Value of the longitude differential without ``cos(lat)``.
base : instance of ``cls.base_representation``
The base from which the latitude will be taken.'

@classmethod
def _get_d_lon_coslat(cls, d_lon, base):

    cls._check_base(base)
    return (d_lon * np.cos(base.lat))
