The stars are too far away to measure their distance directly, so astronomers use an indirect method that involves looking at the star from two or more perspectives. They measure how much the foreground star’s position changes among the background stars when viewed from one position compared to another position.

This Reading provides background information on the underlying concepts in the Activities included in the book, Project Earth Science: Astronomy, Revised 2nd Edition. The Readings go beyond what is found in the background information in the Activities. They can be used as supplementary information for the teacher, or to enhance classroom discussions.