1. Globular Clusters: Shapley knew that globular clusters, tightly packed groups of stars, are distributed in a spherical halo around the Milky Way.
2. Observing their Distribution: He meticulously mapped the positions of over 90 globular clusters, noting their apparent distribution across the sky.
3. Apparent vs. Actual Distribution: He realized that the clusters appeared more concentrated in one part of the sky, towards Sagittarius. However, this was an *apparent* concentration, not a true one.
4. The Key Insight: Shapley hypothesized that the Earth was not at the center of the galaxy, but rather on its edge. This meant that the globular clusters appeared concentrated towards Sagittarius simply because we were looking towards the galactic center from our off-center location.
5. Calculating Distance: Using a technique called variable stars, Shapley was able to estimate the distances to these globular clusters.
6. Putting it Together: By combining the apparent distribution and distance estimates, Shapley could deduce that the actual center of the Milky Way galaxy lies in the direction of Sagittarius, about 26,000 light-years away from Earth.
Important Note: Shapley's work revolutionized our understanding of the Milky Way, shifting our position from the center to the outskirts. His methods were innovative, and while his calculations of distances were slightly off, his fundamental concept of the galaxy's structure was groundbreaking.
