Tuesday, July 9, 2013

Future Fate of Earth's Moon


After my last round of #TriviaThursday about Moons, I got the following questions on Twitter and Facebook...

Our beautiful natural satellite orbits Earth ant-clockwise at 3,683 kilometres per hour (2,288 miles per hour).  It is not always the same distance from Earth, and  and to make a one giant oval-shaped trip, it travels 2,290,000 kilometres (1,423,000 miles).

This so-called "sidereal month" is the average period of revolution of the Moon around the Earth in reference to a fixed star, equal to 27 days, 7 hours, 43 minutes in units of mean solar time.

At it's closest, our Moon is about 363,300 kilometres (225,740 miles) away, and at it's farthest, 405,500 kilometres (251,970 miles).  The average distance is 384,467 kilometers (238,897 miles).

The oval, or elliptical, orbit is slowly becoming wider.  The Moon is actually moving away from earth at a rate of 3.8 centimeters (1.5 inches) per year, and slowing down. Because of this, Earth is also slowing down -- about 2 milliseconds per century. In other words, in 100  years, our day will be 2 milliseconds longer.

How is this happening? The Earth and Moon share angular momentum (of course, always conserved), and in this system, the Moon acquires the energy that Earth loses. This continually boosts our satellite to a higher orbit; neither can thus maintain their standard orbits in the eons to come.

Over a looooong period of time, rotations of the Earth and revolutions of the Moon will eventually match! Both will be about 40 days long.  If there is still a human population on Earth, they will constantly observe the moon overhead -- but the other side of the planet will never see it.  It's hard to say to what land masses it will hover over, because by this time, plate tectonics on Earth will ensure completely new configurations of continents.

Stability isn't the strong point of cosmic forces, so as the Moon pulls away, it will come under the Sun's angular momentum, losing energy and finally falling back toward Earth.  In about 3 billion years, Earth's Moon will break apart, perhaps 16,000 kilometres (10,000 miles) above the biosphere, creating a ring very much like Saturn's!  One imagines, meteorite hits to Earth will substantially increase.

At this point, the only tidal forces on Earth will be those caused by the Sun. There will be very different oceanic and sky-gazing worlds, indeed!  Won't they be amazing to see! Provided we haven't completely destroyed this planet and there are any eyeballs left to see it...