Western Slope Skies - 8/1/14

The Tidal Dance of the Earth and the Moon

We learn in grade school that the Moon, our nearest neighbor in space, causes tides on the Earth’s oceans. It does so through its gravitational attraction to the Earth. But the gravitation interplay between Earth and Moon has other, subtler effects as well.

For one, the Earth also exerts a gravitational attraction on the Moon, causing tides to rise there. There are no oceans on the Moon, but the lunar surface itself responds to the Earth’s pull, in the form of moonquakes. These are quite small, about 10 centimeters in size over one lunar orbit, but can last up to an hour. Moonquakes were discovered by seismometers left behind from the Apollo 11 mission (which occurred 45 years ago last month).

Another effect is that the Moon is gradually slowing down the Earth’s spin. The Earth rotates about 27 times faster than the Moon orbits the Earth, but the tides caused by the Moon create friction between the oceans and the solid Earth, effectively braking the Earth’s rotation. The Earth day is gradually lengthening, by 15 one-millionths of a second every year. That doesn’t sound like much, but adds up over millions of years. Millions of years ago, the Earth rotated much faster than today, and the day was significantly less than twenty-four hours long.

Another consequence of the Earth-Moon tidal dance is that the Moon is also moving further from the Earth. As the Earth’s rotation slows down, rotational energy is transferred to the Moon, causing its orbit to lengthen. Laser reflectors also left behind from the Apollo lunar missions show that the Moon recedes approximately 38 millimeters per year. Again, that seems a tiny change; but over millions of years, the Moon’s apparent diameter in the sky will dramatically shrink. Right now, the Moon and the Sun happen to have the same apparent diameter, which enables total eclipses of the Sun to occur. In the far future however, total eclipses will never happen—the Moon will always be much smaller than the Sun.

Enjoy the Moon’s dance with Earth tonight. This is Michael T. Williams for Western Slope Skies.