Sentence:  The variation in the 3 - cm emission of the moon during a <WSD>lunation</WSD> is 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:  the period between successive new moons (29.531 days)