A lunar cycle is completed in a month that is the size of the moon from zero to the full moon stage and back to (Since, only a portion of the illuminated moon is visible to us, it appears to be of half size or crescent). John saw a full moon on a particular day then he will be able to see another full moon after approximately 29 days. This is because the size of the full moon will reduce to zero i.e. new moon stage after 15 days and from 16th day onwards new lunar cycle will begin with the waning crescent moon stage and on the 29th day the full moon will once again reappear.