In 1941, Andrew McKellar (1910-1960) measured the temperature of interstellar space as about 2.7 Kelvin, based of the rotational spectrum of the cyanogen molecule. This radiation temperature arises from the cosmic primeval fireball and is one of the most fundamental and revealing cosmic parameters.
Discovered on 11-10-1929 in Flagstaff by Tombaugh, C. W.