Parallax

When an object is viewed from different angles, its position with respect to objects in the background can appear shifted. This can be used to determine the distance to the object from an observer. Only the half-angle

θ

and the half-length

l

between two viewpoints need be measured. The distance to the object is then given by

l/tan(θ)

. Parallax has been used to determine the distance to nearby stars, where the two viewpoints are six months apart on the Earth's orbit.