Distances to stars and galaxies are so great that communicating the measurement in kilometers is cumbersome and difficult to comprehend. So, astronomers use a larger unit of measure called the “light year.” It is the distance that light travels in one year.

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.