The Future of the Moon
Published on Jan 24, 2007 at 5:06 pm.
3 Comments.
Filed under Earth, moon, physics.
A couple of days ago Space.com had an article that I read in newspapers all over the place. I won’t go into detail, you can read it. But, the summary is that the Moon is likely doomed. When the Sun swells up to be a red giant, the Earth-Moon system will be skimming along just above the surface of the Sun. But stars don’t have sharp edges. There is an extended atmosphere of gasses surrounding the star. So, the Earth-Moon system will be moving through this extended atmosphere. In that case, two things should happen. Drag on the Earth should cause it to spiral inwards towards the Sun, and perhaps our planet’s doom. But, drag on the Moon will make it spiral closer to the Earth. Eventually, it might spiral so close that tidal forces from Earth tear it apart. There are several caveats, though. The Sun will have lost some mass by that time, and as it loses mass, Earth will spiral outwards. If it loses enough mass, Earth might be far enough out that the drag will be lessened. Earth and the Moon might survive. Or, the Sun might not remain in a red giant stage long enough for the Moon to move close enough to Earth for it to be torn apart. But, it did make for a cool title.
What I want to talk about, though, is something that the article mentions that a lot of people might not be aware of. That is that the Moon is currently receeding from the Earth. So, why is the Moon spiraling outward? Well, it has to do with the gravitational pull of the Earth. Huh? You might ask, “How can gravity which is pulling the Moon towards the Earth be responsible for pushing it away?”
The answer comes from understanding a little about orbits and tides. You can look at my earlier post on tides for more about the tides themselves. This time I’ll talk more about orbits. Basically, the gravitational pull between the Earth and the Moon is what keeps the Moon in orbit. Newton’s First Law tells us that an object will move in a straight line unless a force acts on it. The gravitational force pulls on the Moon. The Moon is falling towards Earth. But, it is also moving to the side so fast that it keeps missing. The Earth’s surface just curves away from it, and the gravitational force is directed towards the center of the Earth (in an idealized system). For a circular orbit, the gravitational force provides just the right centripetal force to keep the Moon in orbit. But, the Moon also pulls on the Earth. The Earth also moves. You get tides. The high tides are going to be under the Moon and opposite the Moon. Again, read my earlier post to understand more about how this works.
But now things get complicated. As it turns out, the Earth turns with a different period than the Moon orbits. The Earth takes about a day to rotate, and the Moon takes about a month to orbit. Well, that wouldn’t really be a problem if the tides stayed directly under the Moon. But what happens is that rotation of the Earth carries the tidal bulge slightly ahead of the Moon. Water has to flow from where it was to the spot under the Moon. That takes time. There is resistance to flow (continents), and there is also viscosity of water to consider. Even the land rises and falls under the tidal forces, and that, too, takes time. So, the tidal bulge leads the Moon.
But, the tidal bulge has mass, and it, too, has a gravitational pull on the Moon. However, since the Earth’s rotation has carried the tidal bulge slightly forward, it does not pull directly on a line with the rest of Earth’s gravitational pull. The net result is that it makes a tiny forward pull on the Moon. This causes the Moon to speed up by a tiny bit, and thus spiral outward. The Moon is receding from Earth at a rate of about 4 centimeters per year (on average).  But, gravity works both ways. The Moon is pulling back on that tidal bulge. And that pull is slowing the rotation of the Earth. Hundreds of millions of years ago, the Earth had nearly 400 days per year. That wasn’t because the years were longer, but rather because the days were shorter. But the slower the Earth rotates, the less the tidal bulge is carried ahead of the Moon. The farther out the Moon is, the weaker the effect, too. Eventually, the Earth would be slowed to the point that it rotates in the same time that it takes the Moon to orbit. When that happens, these effects will stop.  At that time, the Moon would be about 60% farther out than it is now, and would take nearly 50 days to orbit, but the Earth would take 50 days to rotate. But, that might never happen. Because it would take so long that the Sun would die before it happens. The, the Moon would start spiraling back inward.
The spiraling back inward is what that article that I read was about, but it didn’t really explain why the Moon was currently spiraling outward. I thought that I’d do that.
-Astroprof
A Ler…-- Rastos de Luz on January 25, 2007 at 8:34 am: 1
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Louise on January 28, 2007 at 2:25 am: 2
Thank you for adding the link to your roll. Some recent work I’ve been describing indicates that the Sun won’t balloon into a red giant after all, so Earth and Moon will survive for billions of years. I hope that makes lovers feel better.
GZ on November 18, 2008 at 11:55 am: 3
Very informative. Nice article.
-GZ
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