The measurement of angular distances between objects in the sky. By studying the parallax of stars, their absolute distances can be measured, and by measuring small angular changes over a long period of time, proper motion can be determined.

Historically, photographic plates were used to record images, and optical or mechanical devices were used to measure distances on these plates. The method of plate constants can be used to find positions on a photograph using three or more reference stars. It allows solution for all of the so-called plate constants, of which there are six.