December 30: Hubble announced that our galaxy is not the only one (1924)

Edwin Hubble

Edwin Powell Hubble, an American astronomer, announced that the spiral nebula Andromeda is a galaxy and our galaxy Milky Way is not the only galaxy in the universe. It changed the conventional view to the universe, which meant our galaxy is the universe. Hubble also calculated the distance between Andromeda and earth by using Henrietta Leavitt's formula. Later he discovered a couple of dozen galaxies. He also analyzed the Doppler effect on the spectroscopic signals of receding stars and discovered that their red shift was proportional to their distance. Due to his contribution to astronomy, NASA named its space telescope after him.