Over the course of the month, the moon does change in apparent size. This happens because its orbit is not a perfect circle. The nearest point on its orbit is 25,000 miles closer to the earth than its farthest point. Even so, the earth and the moon are closer in size than any other planet-satellite pair. When it is closer, it looks noticeably large.
Moon undergoes changes in phase in a regular and predictable pattern. Over the course of its 29 day lunar month, the moon appears to wax, or grow in size, to a full moon, then wane, or shrink, to nothing. Its position changes daily. It may be between the Earth and the sun or behind the earth. During most of the month, only part of the sunlit half of the moon is visible from the earth.
Over the billions of years, moon has moved farther away from the Earth, and its rate of rotation has also slowed. The earth's gravitational pull drags the moon to rotate on its axis. This is why the moon rotates only once per month.
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