The Moon Pulling Away From Earth

TaosHum

Recently I cam across an article that stated that the moon is currently moving between 3.5-3.8 centimeters away from Earth per year. This move is supposedly because of the current positioning of the continents on earth.

The one thing that wasn't explained in the article however, is if the earths continents revert back to a pangea type state, would the moon begin to move closer to the earth again?

Skerries

Dermo

TaosHum wrote: »

Recently I cam across an article that stated that the moon is currently moving between 3.5-3.8 centimeters away from Earth per year. This move is supposedly because of the current positioning of the continents on earth.

The one thing that wasn't explained in the article however, is if the earths continents revert back to a pangea type state, would the moon begin to move closer to the earth again?

and this article exists where?

TaosHum

The Earth and Moon have been locked in a celestial dance for at least 4.5 billion years. Prisoner to Earth's superior gravity, the Moon is in a synchronous orbit (meaning it spins exactly once on its axis for every one revolution around Earth), forced to show the same face toward Earth for all eternity.

This situation is the result of tidal forces. The gravitational force of the Moon "pulls" on Earth's oceans and crust, creating the twice-daily tides. Likewise, Earth's gravity pulls on the Moon, distorting our companion so that it is slightly egg-shaped. These tidal effects produce bulges on the Moon's surface, and Earth pulls on these bulges a little more than on the surrounding regions because of the concentration of mass there. Earth's constant pull on the Moon's tidal bulge caused its rotation to slow down from its much faster initial spin rate to its current rate of 27.3 days.

So how does this cause the Moon to recede from Earth? The tidal bulges produced on Earth by the Moon are not centered at the point on Earth's surface directly below the Moon, but are shifted ahead of the Moon because of Earth's faster spin rate. The gravitational force of the excess mass in this displaced bulge pulls on the Moon, slightly increasing its orbital velocity. In the 1600s, Johannes Kepler showed that when an object's orbital velocity increases, its orbital radius will expand; thus, the Moon slowly recedes from Earth.

The effect of the Moon inching away from Earth is barely noticeable on human timescales. But millions of years from now, the effects will be more tangible. The tides will be weaker because of the Moon's weaker gravitational pull. The release of tidal energy in Earth's oceans and crust will continue to slow our own planet's spin and thus increase the length of our day (it is increasing at a rate of one second every 50,000 years). Our distant descendants will not be able to view a total solar eclipse, because the Moon's apparent diameter will always be smaller than the Sun's, making only annular eclipses possible. So in at least one respect, we live in a very lucky time! — JIM BELL, CORNELL UNIVERSITY

http://www.astronomy.com/en/sitecore/content/Home/News-Observing/Ask%20Astro/1999/03/Ive%20heard%20that%20the%20Moon%20is%20moving%20away%20from%20Earth%20by%20about%20an%20inch%2025%20cm%20each%20year%20Why%20is%20this%20happening.aspx

Another article I cam across (dont remember where I saw it though) had the movement away at 3.8 cm's per year.

Rubecula

The moon has been moving away from the Earth since it formed and will continue to do so until it breaks free entirely and goes into it's own orbit around the sun. However by the time that moon does break free it is thought the Earth will already be in the grip of the sun as a red giant. I am not sure about this last bit however as that is expected to be in some billions of years from now. Nevertheless, the moon is and always has been pulling away from us.

AugustusMinimus

I would imagine it's simply down to the fact that the moon is moving too fast for it's orbit.

It's almost impossible to have anything orbit at exactly the correct speed for it's orbital radius.

MajorMax

Space 1999

Cú Giobach

Rubecula wrote: »

The moon has been moving away from the Earth since it formed and will continue to do so until it breaks free entirely and goes into it's own orbit around the sun. However by the time that moon does break free it is thought the Earth will already be in the grip of the sun as a red giant. I am not sure about this last bit however as that is expected to be in some billions of years from now. Nevertheless, the moon is and always has been pulling away from us.

The moon is indeed moving away but will never break free from the Earth.
Putting it very simply the Earth's rotation is causing the moon to move away, the moon is slowing the Earth's rotation and as the rotation slows less energy transfers to the moon, therefore slowing its retreat.
Eventually the Moon will stop retreating and both bodies will be tidally locked and showing the same faces to each other, orbiting around their centre of gravity like the two ends of a dumbbell.
When this happens the moon will actually start moving back towards the Earth, all this will happen in 10's of billions of years well beyond the main sequence stage of the Sun's life.

John mac

dosent bother me . (i'll be dead)

I remember Brian Cox talking about it in wonders of the universe.

Cant find it on Youtube yet though.! will stick it up if i do .

bbrhuft

TaosHum wrote: »

Recently I cam across an article that stated that the moon is currently moving between 3.5-3.8 centimeters away from Earth per year. This move is supposedly because of the current positioning of the continents on earth.

The one thing that wasn't explained in the article however, is if the earths continents revert back to a pangea type state, would the moon begin to move closer to the earth again?

Yes, the moon is moving away from the Earth at 3.8 cm a year .. but it's due to Tidal Friction.

The moon causes the ocean's tides, and the peak high tide is just ahead off the moon's orbit due to Earth's rotation, the gravitational attraction of the bulge of water (of the high tide) tugs on the moon ... pulling the moon around in its orbit a little bit faster. The result is a gradual transfer of momentum (a push) from the Earth to the Moon, so the moon speeds up and it is slowly flung away from the Earth (3.8 cm a year).

Also, the Earth's rotation slows as the moon moves away from the Earth. Eventually in 2-300,000 years time there won't be any more Total solar eclipses, the moon will be too far away to cover the whole of the sun. And it will move away from the Earth so long as there are oceans (~1000 million years). I don't think it will manage to break free.

The distance to the moon is measured by firing laser pulses at mirrors left on the moon by Apollo astronauts, they measure the time it takes for laser light to make the journey to the moon and back .. this gives a distance to the moon accurate to a few millimeters. It's how they proved the moon is getting further away, though it was mathematically predicted for a long time, by Edmund Halley of Halley's Comet fame way back in 1695.

Zubeneschamali

bbrhuft wrote: »

it will move away from the Earth so long as there are oceans (~1000 million years).

Firstly, I think there would be a similar but smaller effect even without liquid oceans. Secondly the process only continues until the Earth has slowed until it faces the Moon permanently, just as the Moon faces the Earth.

Pluto and Charon are tidally locked this way, permanently facing one another, even though there are no oceans out there.

bbrhuft

The are Earth Tides too, the crust goes up and down 25 cm each day. But the Earth Tide should move in perfect step with the the Moon, whereas the ocean tides are off center (the bulge of water is ahead of the Moon) due to friction on the seabed slowing the movement of the ocean tides. There should be no friction involved in Earth Tides, which would cause an offset on the position of the Earth Tide, I would think.

Here is a paper where they take into account the Earth Tide (Fig 1 shows the Earth's surface going up and down):

Earth-tide as parameter of crustal motion correction
for SLR station displacement

dbran

Hi

Interesting! But why would the Earth tides remain exactly in step with the moon though. Dont see why they would not be carried out ahead of the moon just like the water tides. Would the Moon then not be pulling on the bulge of the Earth tides also, pulling it back and thus slowing the Earth's rotation.

Or perhaps the Earth Tides do not impart a similar strength as the Water tides do.

Kind Regards

dbran

bbrhuft

dbran wrote: »

Hi

Interesting! But why would the Earth tides remain exactly in step with the moon though. Dont see why they would not be carried out ahead of the moon just like the water tides. Would the Moon then not be pulling on the bulge of the Earth tides also, pulling it back and thus slowing the Earth's rotation.

Or perhaps the Earth Tides do not impart a similar strength as the Water tides do.

Kind Regards

dbran

Well, the Earth Tide goes up and down by only 25 cm, but water goes up / down much greater range - some areas of the world there are tidal ranges of 10 meters. Even taking into account the greater density of rock, Earth Tides only involve 4% of the total energy of the tides.

As for why I think it would be unlikely that Earth Tides would contribute much to the moon getting father away, we have to understand why the ocean tide is a little ahead of the Moon.

Essentially, its got to do with friction. During ocean tides, low and high tide, seawater flows across the shallow seabed and friction slows the movement of the tides. This friction, combined with the 24 hour rotation of the Earth, causes the ocean tide to be pushed ahead of the 28 day orbit of the Moon (look at the diagram again).

However, in Earth Tides, obviously of rock is not flowing sideways, it's only going up and down. So I'd expect the Earth Tide to keep in step (or almost) with the Moon's orbit. So it cannot contribute to Lunar recession or slowing of the rotation of the Earth, and if it does, it would be much smaller effect than the Ocean Tide.

Eventually, the Earth's rotation will slow down so much that the Moon orbit and Earth's rotation will be the same (I've been reading a bit more). In theory, billions years from now, Earth's day will be for example (I don't know the true figure) 60 days and the Moon will take 60 days to go around the Earth. When that happens, the Earth-Moon will be Tidally Locked.

I just found this Quote in "Lunar and solar torques on the oceanic tides."

The retrograde [ocean] bulge owes its existence, of course, to the highly nonequilibrium form of the oceanic tides; there is no comparable bulge in the body tide.