Clearly, spherical coordinates and spherical trigonometry are essential tools for the mathematical manipulations of coordinates of objects on the celestial sphere. Similarly, for global terrestrial coordinates, the early map makers used spherical coordinates, although, today, we rarely use these for terrestrial systems except with justified approximations. It is useful to review the polar spherical coordinates, according to Figure 1.1, where θ is the co-latitude (angle from the pole), λ is the longitude (angle from the x-axis), and r is radial distance of a point. Sometimes the latitude, φ , is used instead of the co-latitude – but we reserve φ for the "geodetic latitude" (Figure 2.5) and use ψ instead to mean "geocentric" latitude.