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Why is the moon moving away from Earth?


buzzc150
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Our moon is moving away from the Earth at a rate of about 25cm/year.

Why ? What's the cause ? And in doing so, is the moon gaining energy from somewhere (since it's moving into a higher orbit) or is the Earth losing energy ?

Buzz

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There are tidal bulges and tides on land as well. It's jsut not very visible because there's no horizontal displacement of land and dramatic changes in landscape (and because when you're sitting on the bulge you can't notice you've crept up).

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It's only an inch per year over a quarter of a million miles. Don't worry about it. I find the fact that some men have been discovered to have nipples much more worrying!

wait men aren't supposed to have nipples?:)

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It's disappointing to note that, despite a good answer to the initial question having been provided by the link in post 2, posts 3 & 5 subsequently gave answers that are utterly inaccurate.

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Pray tell which part?

i. One of Earth's tidal bulges (land plus water) leads the moon due to faster rotation

ii. that mass exerts a slight offset gravitational pull on the moon

iii. which transfers energy to the moon

iv. which cause it to move outwards to a slightly larger orbit

Edited by mikeknowle
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Post 3 states first, "Its because it is speeding up in its orbit." This is incorrect. As the moon recedes from Earth, its orbital speed must decrease, or it will break out of Earth orbit. Secondly, the stated reason is a, "tidal slingshot effect." This does not exist. Thirdly, it is stated that, "if there was no water on the earth it wouldn't move away." Again incorrect. The lack of water would make virtually no difference to the moon's orbital recession, since the amount of water is only a tiny fraction of the total mass of the Earth.

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The lack of water would make virtually no difference to the moon's orbital recession, since the amount of water is only a tiny fraction of the total mass of the Earth.

I'm not sure this is true, is it? The moon's recession is caused by tidal friction and, as I understand it, tidal friction is higher because liquid water is so mobile.

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"It's disappointing to note that, despite a good answer to the initial question having been provided by the link in post 2, posts 3 & 5 subsequently gave answers that are utterly inaccurate"

This is not a very friendly way to to dispute an answer that another person has given. Please try and remember we are all here to help each other and to promote our common interest in astronomy.

Maybe it's better to try explaining why you have a better or more accurate answer. Or try and enhance their answers by adding additional detail that you know to be correct.

Above all remember you are talking to other people here, the fact that it isn't face to face doesn't make it any less important to be friendly, polite, and use manners. Thank you. :hello2:

Edited by brantuk
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It's worth remebering that the Moon's apparent size in the sky is the reason for total solar elcipses. As the Moon's distance to us increases, these will become a thing of the past since the Moon will seem smaller against the distant Sun

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It's worth remebering that the Moon's apparent size in the sky is the reason for total solar elcipses. As the Moon's distance to us increases, these will become a thing of the past since the Moon will seem smaller against the distant Sun

They'll become rarer at first; there's a big difference between the moon size at perigee and apogee.

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Post 3 states first, "Its because it is speeding up in its orbit." This is incorrect.

But since the direction of the velocity is changing, there is an acceleration, directed inwards called centripetal acceleration is there not. And this increases slowly due to the increased attraction of the leading tidal buldge (ie it speeds up). Which transfers energy from the Earth to the Moon....thus increasing the latters orbit (and its speed thus slows as equilibrium is reached) - a continual process not a step change. And also explains why our days get slowly longer.

Maybe this paper explains it better than I can: "The Disturbance of the Gravitational Field and the Secular Acceleration of the Moon", Tang Tao, (Physics Essays); Sep2005, Vol. 18 Issue 3, p300-308, 9p.

Secondly, the stated reason is a, "tidal slingshot effect." This does not exist.

Well it does according to some, such as Nagasawa et al in their article: "The formation of close-in planets by the slingshot model" Dept. of Earth & Planetary Sciences, Tokyo Institute of Technology - where they investigate orbital body dynamical tidal effects.

Thirdly, it is stated that, "if there was no water on the earth it wouldn't move away." Again incorrect. The lack of water would make virtually no difference to the moon's orbital recession, since the amount of water is only a tiny fraction of the total mass of the Earth.

Maybe you are right it was just a thought...hence the 'I believe' at the beginning. Just as I also believe in God, one day Aston Villa will win the Champions League and Lamb Madras is nicer than Chicken Boona. Hard to argue belief...and anyway, its the water mass percentage of the leading tidal bulge that's important I would think for this effect, not the total mass of the Earth as you've said

As you obviously know your Newtonian mechanics very well, you might find this interesting: Anderson, John D., (Astronomy), 01/03/2009, Vol. 37 Issue 3, p22-27, 6p. According to the article, both the paths of spacecrafts and the orbit of the Moon are not adhering exactly to the Newton's gravitational theory which some researchers believe could usher in a new theory of gravity.

Astronomers have understood the Moon's motion around Earth since Newton published his theory of gravity in 1687; but if scientists need extreme precision, they turn to general relativity, which is our current best understanding of gravity. But even then - something seems wrong.

As is common knowledge, in the 70's Apollo astronauts and the robotic Lunikod lander placed laser reflectors to the Moon's surface which we use to track the Moon's position to an accuracy of about a centimeter, or two parts in 100 billion.

38 years of precise distance data analysis by JPL's James Williams found that the Moon's orbit is becoming more elliptical. Williams reported in 2006 that the distance is increasing unexpectedly by about 0.2 inch (6 millimeters) per year, with Non-gravitational forces acting on the Moon's orbit as a result of internal tidal friction in both Earth and the Moon the most likely candidate.

After accounting for changes expected from tidal friction, Williams still has a residual change in the Moon's orbit that he can't explain. It's known that the eccentricity changes because of tidal friction, but Williams discovered that it is increasing too much - by a factor of about three. We still don't have any clear explanations for this change of the Moon's orbit.

I believe it is the Earth's water mass that doesn't flow around the planet evenly due to the continental crust. But that's just me...

------------

Now then Thomas, this is a discusion forum, not a PhD submission. It's not for me to tell you how to write your posts, but I noticed all of your most recent ones are one-line rebuttals, corrections etc. Rarely/ever have you explained why, or put the correct answer to tried to further understanding. Just to say someone is wrong sounds a bit 'old-school master-ish' does it not? And one way of learning, is to try an explain something to someone else. And if one is only partly right - hey ho, that's a discussion, and that's how we learn and pass/share knowledge. Give me more than 2 scientists, put then in a room and try to reach concensus - ha. That'll take more than 10 minutes...

So rather than hi-jacking this thread and get into deeper-and-deeper discussion, I'll bow to your greater knowledge - you obviously are very experienced in planetary mechanics - perhaps it's your day job.

But do try to lighten up a bit and realise you don't help people much, and most likely scare others off, with your blunt/curt corrections, whether they are correct or ...... not ;-).

So let's shake hands or poor Buzz will wonder what he's started.....:hello2:

Edited by mikeknowle
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