Spherical coordinates কী?

Point $M$ is located by its distance $r = OM$, the colatitude $\theta \in [0, \pi]$ between $\vect{OM}$ and $\vect e_z$, and the azimuth $\varphi$ of its projection on the $xy$ plane: $\vect{OM} = r\,\vect e_r$ with

$\vect e_\theta$ (tangent to the meridian, toward increasing $\theta$) and $\vect e_\varphi$ (tangent to the parallel) complete the local orthonormal basis. On the Earth, $\theta$ is $90^\circ$ minus the latitude and $\varphi$ the longitude.