Courtney, how did early astronomers first know that the Solar System was not the center of the Galaxy?
Answer This was discovered by Harlow Shapley in 1918. Prior to that, it was thought that the Sun lay near the center of a large, disc-shaped stellar structure, extending a thousand or so light years above and below the plane of the disc, and several thousand light years in most directions, in the plane of the disc. This structure was ‘discovered’ by Herschel in the 1700’s, by counting the number of stars of various brightnesses in different directions, and presuming that if you could see fainter and fainter stars (virtually without end), you must be looking at stars that were further and further away. By the early 1900’s, astronomers knew that beside this structure, there was a large collection of “globular” clusters which could not be centered on the Sun, because almost all of them were on one side of the sky. By studying the periods and brightnesses of Cepheid variables in globular clusters, Shapley showed that they formed a roughly spherical system centered on a point in the plane o
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