January 1: Hubble Discovers Galaxies

The American astronomer Edwin Hubble announces the discovery of galaxies outside the Milky Way. Using the Hooker Telescope, Hubble identified Cepheid variables (a kind of star; see also standard candle) in several objects, including the Andromeda Galaxy, which at that time were known as "nebulae" and had been assumed to lie within the Milky Way. His observations in 1922–1923 conclusively proved that these objects were much more distant than previously thought and were hence galaxies themselves, rather than constituents of the Milky Way. 

This discovery fundamentally changed the view of the universe.