Henrietta Swan Leavitt examined photographs of both the Small and the Large Magellanic Clouds taken over many weeks and months and found over 2500 stars that varied in brightness in the two clouds, now known as companion galaxies to the Milky Way. She was the first to show that the variations in brightness were a measure of the intrinsic brightnesses of these stars, thus providing a powerful new distance indicator for astronomy. Modern image from AURA/NOAO/NSF.