Shortly after the study of astronomy by radio techniques began in earnest, a basic problem arose. Three “sources” of radio flux had been delineated: the sun, the cosmic‐noise background, and the discrete sources (radio stars). Were the discrete sources variable, as the sun showed evidence of being, or was the atmosphere modulating fixed‐flux sources? The first answer was that the radio‐star sources exhibited intrinsic fluctuations in intensity, but studies in Australia and England utilizing spaced receivers indicated that the earth's ionosphere and troposphere produced effects similar to the “twinkling” of visible stars. The study of “twinkling” or scintillation was the first of the upper‐atmospheric effects to be investigated in detail.