Sentence:  The ratio of the measured antenna temperature change during a drift scan across the moon to the average brightness temperature of the moon over the antenna beam ( assuming that the brightness temperature of the <WSD>sky</WSD> is negligible ) was found , by graphical integration of the antenna directivity diagram , to be 0.85 . 
Meaning of the word marked with <WSD> tags:  the atmosphere and outer space as viewed from the earth