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 shaded gray area represents a plane (e.g., detector imaging the sky) parallel to the sky centered at right ascension (longitude) ${\displaystyle \alpha }$ and declination (latitude) ${\displaystyle \delta /math>.Theframe<math>(x,y)}$ is the frame of the detector that images the sky.