What assumption about globular clusters did Harlow Shapley make in order to determine the location of and distance to the center of the Milky Way?

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Shapley assumed that the globular clusters are distributed about the center of the Milky Way, which allowed him to use their positions to determine the location of the center of the Milky Way. He also used their apparent magnitudes to estimate their distances, which led him to conclude that the center of the Milky Way is located far away from our solar system.

What Assumption About Globular Clusters Did Harlow Shapley Make in Order