Q: Why does the moon always keep...

Q: Why does the moon always keep the same face pointed at the Earth?

A: The short answer is because the time it takes the moon to rotate on its axis--27 days, 7 hours, 43 minutes, 11.6 seconds--is the same amount of time it takes to revolve around the Earth. The two times are identical because of a phenomenon called tidal locking. Just as the moon’s gravity produces tides in the Earth’s oceans, the Earth’s gravity produces tides, albeit much smaller ones, in the moon’s mantle. This small rise and fall in the solid mantle caused friction that altered the moon’s original rotational rate until the moon became “locked” into a fixed position with regard to the Earth. Some friction still exists, causing the Earth’s spin to slow slightly and the moon’s orbit to creep ever further from the Earth.

Although the moon rotates at a constant rate, its orbit is not quite circular. At certain times of the month, therefore, viewers from Earth can see a short distance beyond the leading and trailing edges of the moon, a phenomenon called apparent libration. As a result, 59% of the moon’s surface is visible from Earth.