English: Three dimensional reference frame ${\displaystyle (x,y,z)}$ where the angular separation ${\displaystyle \theta }$ is the angle between the points A and B as seen from the center of the sphere O. The vectors ${\displaystyle n_{a}}$ and ${\displaystyle n_{b}}$ are the unitary vectors normal to the surface in ${\displaystyle A}$ and ${\displaystyle B}$, respectively. The points A and B are defined by their right ascension (RA), ${\displaystyle \alpha _{A}}$, ${\displaystyle \alpha _{B}}$, and declination (dec), ${\displaystyle \delta _{A}}$, ${\displaystyle \delta _{B}}$, respectively.