A:

That is a great question. First, let's define what the tides are and how they are caused. A tide is the rise and fall of the water levels in the oceans caused by the combined gravity of the Moon and Sun on the oceans. Because the force of gravity of the Moon is stronger on the side of the Earth it faces and weaker on the side opposite the Moon, water forms two bulges, one closest to the Moon and one on the opposite side of Earth where water piles up. The same process applies to the tides the Sun causes but they are much smaller. The force of the Moon's gravity causes the same type of rise and fall on the Earth's crust, but because the crust is not a liquid, it does not rise and fall as much. Now, the two bulges of water (the side closest to the Moon and the side furthest opposite to the Moon) are unable to be right under or opposite the Moon because the Earth rotates faster than the Moon orbits the Earth. The Moon thus pulls back on those bulges, slightly slowing the Earth's rotation and thus lengthening the days. Because the total angular momentum of the Earth-Moon system must be conserved, the Moon must move out to a higher orbit, slowing it down and lengthening a month.

So, to summarize the answer, the Moon, through the tides, slows down the rate of Earth's rotation lengthening the day while the Moon slowly moves away from the Earth at about 3.8 cm a year. The Earth will never lose the Moon, it will just be farther away. In fact, about 620 million years ago, the length of a day was about 21.9 hours long and the length of a year was about 400 days (21.9 hour days) long while the Moon orbited closer to the Earth.

You can find more about this by searching our site for "moon tides".



Erik



(published on 01/27/2013)