In April 1920, Harlow Shapley and Heber Curtis argued overthe scale of the universe in the great auditorium of the Smithsonian Institution’s Natural History Museum in Washington, D.C.
In this discussion, which preceded Edwin Hubble’s discovery of the nature of galaxies by just a few years, Curtis argued that the cosmos consists of many separate “island universes,” claiming that the so-called spiral nebulae were distant systems of stars outside our Milky Way. Meanwhile, Shapley argued that spiral nebulae were merely gas clouds in the Milky Way. Shapley further placed the Sun toward the edge of our galaxy — which, in his view, was the entire universe — whereas Curtis believed the Sun to be near the galaxy’s center. Curtis was right about the large size of the universe but wrong about the Sun’s place within it. On the other hand, Shapley was wrong about the small size of the universe but right about the Sun’s location within it.
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With the advent of many extragalactic distance measurements and two camps arguing for different results on the critical number called the Hubble constant — the expansion rate of the universe — astronomers staged a second great debate in 1996. The age and size of the universe are, of course, interrelated, and both depend critically on the Hubble constant.