Sperical Coordinate System
Definition
The coordinates of a point $p$ in coordinate space are represented by “the distance $r$ between the origin and $p$,” “the angle $\theta$ between the line connecting the origin and $p$ and the positive direction of the $z-$axis,” and “the angle $\phi$ between the projection of the line connecting the origin and $p$ on the $xy-$plane and the positive direction of the $x-$axis.” This is called the spherical coordinate system.
Explanation
The set of points for a fixed $r$ forms a sphere. Therefore, the distance from the origin is denoted as $r$, following the initial of radius. It is also referred to as the $(r, \theta, \phi)-$coordinate system.
