This web page outlines the math and applications of parallax. Parallax is the shift in the apparent direction to an object when it is viewed from two separated locations. It allows the object's distance to be derived.  A simple method is developed for distance estimation, with a related "extended thumb method" useful outdoors. The page ends by discussing the use of stellar parallax to estimate the distance of nearby stars. This page is part of the work "From Stargazers to Starships" that presents physical and mathematical concepts using space science and an historical perspective. French, Spanish, and Italian translations are available.