Re: They're gonna fire the World Cannon
Posted: September 28, 2008 • 9:59 am
I can give you a few more details about this.because slowly but surely it (the Moon) is pulling itself away from the Earth's orbit...
As you know, we always see the same face of the Moon. That's because the Moon rotates on its axis in the same amount of time that it revolves around the Earth. This is not a coincidence. It is due to tidal coupling between the Earth and Moon. The Moon is tidally locked to the Earth.
Since the Earth rotates faster than the Moon orbits, the Earth rotates through the tides (which are always aligned with the Moon.) The friction due to the tidal waters flowing over the Earth's surface causes the Earth's rotation to slow down about a quarter millisecond per century. (Remember Southland Tales?) The slight loss of angular momentum is transfered to the Moon, so the Moon is spiraling away from the Earth at a rate of about one inch per year.
As the Moon spirals out, its time to orbit the Earth becomes slightly longer. Meanwhile, the Earth's rotation will continue to slow down until it is the same as the Moon's orbital period. At that time, the Earth will always show the same face to the Moon and vice versa. The Moon will always be over the same spot on the Earth. The Earth will be tidally locked to the Moon.
At that point, the tidal effects of the Sun will start to slow the Earth's rotation even further and the Earth will rotate more slower than the Moon's orbit. That will cause a renewed tidal effect by the Moon, causing the Earth to start speeding up. It will also begin to draw the Moon in closer. If the Moon comes closer than the Roche limit (about 10,000 miles) the Moon will break up and form a ring around the Earth.
But this is all billions of years in the future. For example, about 300 million years ago, an Earth day was 22 hours and there were 400 days in a year. Our rotation has slowed down about two hours since then.
This was all discussed in my third Astronomy lecture a couple of weeks ago.
