Sentence:  The variation in the 3 - cm emission of the moon during a lunation <WSD>is</WSD> very much less than the variation in the 8.6 - mm emission , as would be expected from the explanation of Piddington and Minnett ( 1949 ) . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)