In 1927, the Belgian physicist Georges Lemaitre proposed an expanding model for the universe to explain the observed redshifts of spiral nebulae, and calculated the Hubble law. He based his theory on the work of Einstein and De Sitter, and independently derived Friedmann's equations for an expanding universe. Also, the red shifts themselves were not constant, but varied in such manner as to lead to the conclusion that there was a definite relationship between amount of red-shift of nebulae, and their distance from observers.