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.