Henrietta Swan Leavitt was born on July 4, 1868, and she changed the course of astronomy when she was just twenty-five years old. Henrietta spent years measuring star positions and sizes from photographs taken by the telescope at the Harvard College Observatory, where she worked. After Henrietta observed that certain stars had a fixed pattern to their changes, her discovery made it possible for astronomers to measure greater and greater distances—leading to our present understanding of the vast size of the universe. An astronomer of her time called Henrietta Leavitt “one of the most important women ever to touch astronomy,” and another close associate said she had the “best mind at the Harvard Observatory.” Henrietta Leaveitt's story will inspire young women and aspiring scientists of all kinds and includes additional information about the solar system and astronomy.