Sentence:  In the observations at 4.3 mm ( Coates , 1959 a ) , the diameter of the antenna beam , 6 ' .7 , was small enough to allow resolution of some of the larger features of the lunar <WSD>surface</WSD> , and contour diagrams have been made of the lunar brightness distribution at three lunar phases . 
Meaning of the word marked with <WSD> tags:  the outer boundary of an artifact or a material layer constituting or resembling such a boundary