Edwin Hubble was elected to the United States National Academy of Sciences in 1927. The election recognized his influential observations of galaxies and his growing reputation as a leading American astronomer.
By that time, Hubble had used the 100-inch Hooker Telescope at Mount Wilson Observatory to identify Cepheid variable stars in Andromeda and other spiral nebulae. Cepheids provided a way to estimate astronomical distances, and Hubble’s measurements showed that these objects lay far outside the Milky Way. His work helped establish that the universe contains many galaxies, not just one.
Hubble’s 1927 election came before his best-known paper on cosmic expansion. In 1929, he published measurements showing a relationship between a galaxy’s distance and its apparent recession velocity. That relationship became associated with Hubble’s law, although Georges Lemaître had earlier derived a related theoretical result.
A common mix-up is confusing the National Academy of Sciences with the American Philosophical Society. Hubble joined the latter in 1929, two years after his election to the National Academy of Sciences.