The spherical distance, also called the great-circle distance, between two points 
and 
on a sphere is the distance of the shortest path along
the surface of the sphere (paths that cut through the interior of the sphere are
not allowed) from 
to 
,
which always lies along a great circle.
For points 
and 
on the unit sphere, the spherical distance is given
by
 |
where 
denotes a dot product.
