The figure of the Earth is not exactly spherical; it approximates to a very slightly oblate spheroid, the ellipticity (c−a)/a being only 0.00335. (The actual figure of the Earth, mean sea le...The figure of the Earth is not exactly spherical; it approximates to a very slightly oblate spheroid, the ellipticity (c−a)/a being only 0.00335. (The actual figure of the Earth, mean sea level, is often referred to as the geoid.) Let us refer the ellipsoid ??? to a set of axes Ox′y′z′ such that the angles z′Oz and x′Ox are each θ, and the y^\prime- and y-axes are identical.
