If you chart the moon's position every night for a full month, you could compare its position with a bright star, and then wait to see how long it takes for the moon to come back to its position relative to that star. The answer would be about 27 1/3 days. But during this time, the earth has moved nearly 1/12 of the way in its orbit around the sun. So to get from one full moon to the next would take about two days longer, 29 1/2 days, because the moon would have to travel a little further around the earth to get to full again.