Tuesday, April 28, 2009

Is the distance between Earth and its moon increasing?


Lest I be accused of focusing too much attention on the swine flu, I thought I'd share this little tidbit that I pulled off of Stardate late last week:
Just like a spinning ice skater whose rotation slows as he extends his arms, the Earth-Moon distance is lengthening because Earth is spinning slower each day. The Moon's gravitational influence is slowing Earth's rate of rotation down by one and a half thousandths of a second every 100 years. The loss of rotational energy -- angular momentum, for the physicists in the crowd -- is necessarily matched by an increase in the Moon's angular momentum, which results in a larger orbit for the Moon.

Currently, the Moon moves less than two inches a year farther away from Earth -- a tiny amount, but easily measurable with modern laser-ranging devices.

If this rate of slowing were to remain constant, we could expect the Earth to stop spinning in 2.1 trillion years. That's a pretty long time, considering the upper range of estimates for the age of the universe are about 20 billion years.

At the end of that 21 billion years, the moon would be 66.4 million miles farther away from the Earth than it is today (240,000 miles). This would put it about 2/3 of the the way to the sun, and would make it farther from Earth than both Mars and Venus for much of the year.

This all, of course, assumes that the forces acting on the moon and Earth would remain constant...which they obviously would not.

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