Shapley discovered variable stars in a large percentage of the globular clusters he observed. He used Leavitt's method that relied on Cepheid brightness to determine the distances to all of the clusters he observed. The estimated distances to the globular clusters were very large, ranging from 50,000 to 220,000 light years away. He published his "Big Galaxy" theory in 1918, which proposed his calculated diameter of the galaxy at 300,000 light years. Like a majority of astronomers at that time, Shapley thought the Milky Way galaxy made up the entire universe.