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 \(\ref{4.3.4}\) to a set of axes \(\text{O}x^\prime y^\prime z^\prime\) such that the angles \(z^\prime \text{O} z\) and \(x^\prime \text{O} x\) are each \(θ\), and the \(y^\prime\)- and \(y\)-axes are identical.
