def METHOD_NAME(self):
    cf_grid_var = mock.Mock(
        spec=[],
        longitude_of_projection_origin=-90,
        semi_major_axis=6377563.396,
        semi_minor_axis=6356256.909,
    )
    cs = build_mercator_coordinate_system(None, cf_grid_var)
    expected = Mercator(
        longitude_of_projection_origin=(
            cf_grid_var.longitude_of_projection_origin
        ),
        ellipsoid=iris.coord_systems.GeogCS(
            cf_grid_var.semi_major_axis, cf_grid_var.semi_minor_axis
        ),
    )
    self.assertEqual(cs, expected)