Parallax

The notion of parallax starts with the rather obvious
fact that, depending on where one stands to look at an object,
one might well see it as having two or more diferent backgrounds.

This finds application in Astronomy. Below, how one might use

parallax over a one year period to calculate the distance to a star.
Once the star is anchored between two invariant celestial objects,
one can takes two measures, six months apart. Thus, one calculates
the angles ABE and BAE and the parallax, angle theta. D = R/ϴ, with
ϴ in radians.

source: French-language Wikipedia