Sentence:  They deduced from their measurements that the radio emission from the whole disk of the moon varied during a lunation in a roughly sinusoidal fashion ; that the amplitude of the <WSD>variation</WSD> was considerably less than the amplitude of the variation in the infrared emission as measured by Pettit and Nicholson ( 1930 ) and Pettit ( 1935 ) ; and that the maximum of the radio emission came about 3 - 1 2 days after Full Moon , which is again in contrast to the infrared emission , which reaches its maximum at Full Moon . 
Meaning of the word marked with <WSD> tags:  an instance of change; the rate or magnitude of change