什么是 Reference ellipsoid and flattening？

Example 1.13 (A degree of longitude)

A parallel at latitude $\varphi$ is a circle whose radius is the distance from the axis, about $R \cos \varphi$. At $60{}^{\circ}$ N its circumference is about half that of the equator, and a degree of longitude is about $111.32 \times \cos 60^\circ \approx 55.7\,\mathrm{km}$ (the ellipsoid gives $55.80\,\mathrm{km}$).