Sentence:  After 27 days the moon has made one <WSD>circuit</WSD> among the stars, moving from west to east. But in those 27 days the sun has likewise moved eastwardly, about 27 degrees. The moon, then, has to make one <WSD>circuit</WSD> and a little more in order to be again in the line joining the earth and sun, in order to be again 'new.' 
Meaning of the word marked with <WSD> tags:  The act of moving or revolving around, or as in a circle or orbit; a revolution