Parallax can be used to estimate the distances of nearby stars, up to about 330 light years. It works because as the Earth moves around the Sun, from 1 to 2 in the diagram below, the nearest stars appear to move relative to the most distant, apparently fixed, stars and galaxies.
where
and
is in metres. Alternatively we can definein terms of the parsec: 1 parsec is the distance when an angle of 1 arcsec is subtended, then
Example: Suppose a star has parallax 0.034 arcsec then