How did Harlow Shapley make the discovery?

Harlow Shapley made his discovery by studying the distribution of globular clusters in the Milky Way galaxy. By mapping their positions and recognizing that they were concentrated in a region around a central point, he inferred that the Sun was not at the center of the galaxy but rather located in one of its spiral arms. Shapley's work involved analyzing the brightness and distances of these clusters, leading to his conclusion about the galaxy's structure and the Sun's position within it. This significantly advanced our understanding of the Milky Way's size and shape.