This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
In 1923, using the 100-inch Hooker telescope at Mount Wilson Observatory, Edwin Hubble identified variable stars in the Andromeda nebula whose brightness pattern let him calculate its distance. The number he got placed Andromeda far outside the Milky Way, settling a dispute astronomers had argued for years over whether the galaxy was the entire universe or one island among many. It was not. Andromeda, and everything like it, sat outside it.
Six years later, working with Milton Humason at the same observatory, Hubble went further. Measuring how the light from many distant galaxies was shifted toward the red end of the spectrum, and how that shift grew with distance, the two astronomers demonstrated that the universe itself is expanding, with galaxies receding from each other at speeds that scale with how far apart they already are. A universe that had been assumed static turned out to be in motion at every scale large enough to measure.
Both results depended on the same instrument and the same stretch of California mountain, chosen decades earlier for nothing more dramatic than the steadiness of the air above it. Mount Wilson's broader legacy beyond Hubble includes early evidence for dark matter, precise measurements of stellar diameters, and advances in spectroscopy, but the two Hubble results are the ones that changed what astronomers believed the universe was.