I have read multiple articles talking about how our moon is receding from Earth several centimeters every year. This is followed up by the article saying that the moon one day will leave Earth’s orbit forever and drift away while other articles say that the moon will reach a stable point and stop receding. So, which one is it- does the moon drift away or does it find a stable point and stop?
The latter
The moon is receding from the Earth because as the Earth rotates under it, the the tidal bulge moves from directly underneath. The moon’s pull on the Earth is then a bit more on the bulge than the the rest of the Earth. This tends to slow down the Earth’s rotation transferring angular momentum to the Moon letting it recede.
Eventually as the Earth’s rotation slows, it will become tidally locked to the Moon just as the Moon is to the Earth and one side will always face the Moon. This will stop the process.
So left alone, the Moon will not escape. Of course, it’s not quite alone and some disturbance from another planet could cause it to escape. The last I heard was that the Solar System (or any system) is not gravitationally stable over the long run.
Of course, the Sun is probably going to go red giant and swallow teh Earth and Moon before any of this happens.
The earth is transferring angular momentum from it’s rotation to an increase orbital distance between the earth and the moon through the tidal force. However it will not leave the Earth’s orbit.
Give enough time the earth would become tidally locked with the moon.
However it is highly unlikely that this will come to pass as the Sun will have died as a red giant at a much earlier date.
Okay, I get this.
Because it’s closer. So far so good.
I’m not so sure here, but go on.
What? Now I’m totally lost. How does the angular momentum know to transfer? I’ve heard these words before, but I don’t understand them. What is the long explanation (if you’ve got the time) for this?
The problem with the explanations you read is that you read them serially, but in fact parts of the explanation are happening as parts of the same process. They are broken down into components to make then easier to understand, but they are not steps, just different parts of the same thing.
Angular momentum is a result of the mass of the body and its rate of spin.
All up, you cannot make or remove angular momentum, aka “it is conserved.”
Slowing down the Earth’s rotation means something is acting upon it with a force - in this case gravitational attraction on the bulge. Forces come in two parts, thing applying the force and thing it is applied to - and as Newton told us - that means there is an equal force acting the other way.
As the moon acts upon the Earth’s tidal bulge, the Earth’s tidal bulge acts upon the Moon.
So what happens?
The Moon’s angular momentum here is a measure of its mass and rotation around the point it rotates about - which is pretty close to being the Earth (the barycentre - which is a fancy way of saying the centre of the heaviness of the Earth and Moon - is not the centre of the Earth but offset outwards a few thousand kilometers.) So the Moon is rotating around a point pretty much defined by the Earth, and the Earth is rotating as well. The two are coupled by a gravitational force that is acting to slow the Earth. The only way this can happen is if the Moon speeds up a tiny bit. And guess what - its angular momentum must have increased. What is neat is that you can use the fact that angular momentum is conserved to work out how much faster the Moon must orbit around the Earth to balance the loss in rotational speed of the Earth. If it speeds up, orbital mechanics tells us that it must move away a tiny bit to balance the forces involved.
What you’re not understanding is called tidal drag (just in case you want to google it). The Moon causes the point just below it to bulge upwards a bit (the tide, of course, which happens in the land as well as the sea). But the Earth is rotating faster than the Moon goes around it. So the bulge rotates a bit past the sublunar point before it goes back down. That little bit of bulge past the sublunar point exerts a gravitational pull on the Moon and pulls the Moon forward in its orbit. So it moves faster in its orbit and thus moves to a higher orbit. The Earth is pulled backwards in its rotation so it slows down a smidgen. Granted, the effect is very small, but the Moon only moves outward in its orbit by a couple cm a year.
As to the timing …
We did this calc within the last year or so, but I can’t find it.
IIRC the result was the Sun will go red giant and vaporize both the Earth and Moon at a time when the Earth’s day has extended only a little bit. IOW, the sun will red giant when the Earth / Moon system is only a few percent closer to being fully tidally locked than it is now.
IOW, tidal locking between something as small as the Moon and as big as the Earth starting from a roughly 28:1 rotation rate ratio is a very, very, very slow process. It’s going on every day. And has been since the Beginning. But it’s still got a long way to go.
Yeah, what LSLGuy said. If the sun were to stay in its current state forever, the moon would very slowly move away from the Earth as the Earth’s rotation slowed, until they eventually orbited each other every 47 days. Then, solar tides would steal energy from the Earth-Moon system and cause the Moon to move back toward the Earth until the Moon came close enough to disintegrate into rings.
But in reality, because the Moon is only slowing our rotation / lengthening our day by about one second every 10[sup]5[/sup] years, the entire remainder of the Sun’s time on the main sequence will only be enough time for the Earth’s rotation to slow down to less than two days, and even two days would only be about 4% of what it would take for the Earth and Moon to be mutually tidally locked.
And yeah, I’m pretty sure we’ve had this discussion before, and it’s always interesting. 
Ah, here is a thread from 2004.
ETA: I just noticed that I participated in that thread. Heh.
Another old thread on the subject, from 2008.
First of all I want to point out that understanding the tidal force is hard, to really understand it requires an understanding of General Relativity. Even most of scientists do not understand the way it functions and there are lots of incorrect explanations from very smart people in text books and on the internet. There is also a barrier caused by how we tend to teach physics, where we teach classical theories like Newtonian physics as fact when it is really just an approximation.
The best explanation that I know of without an deep understanding of GR is this video.
But a few core concepts that may help.Angular momentum is conserved: the same energy that is currently visible as the rotation of the earth existed before there was even a solar system. As the gas and dust and other objects coalesced into our planet they imparted that energy to the earth. Without this rotation we wouldn’t have planets in the solar system.
The earth and the moon are a system: The moon doesn’t orbit around an unmoving earth, they actually share a common center of gravity and rotate around a shared axis called the barycenter.
This is better demonstrated with the Pluto–Charon system as seen by New Horizons which are closer in mass and thus the interaction is more visible
If you do not want to watch the video I linked to first above a basic simplified explanation is the earth’s ocean lag the tidal-force, this lag results in the movement of mass results in a “pull” on the “front” side of the moon that increases the orbital speed. When an object in orbit travels faster relative to the body it orbits the distance between the two bodies increases. The energy that is increasing that orbital speed slows down the rotation of the earth like friction would.
But I highly recommend you watch the PBS Space Time video, they have put a tremendous amount of effort into providing accessible content that is consistent with science’s current understanding and theories. It is very difficult to filter out mis-information or dated information as a casual consumer. And most of the college course based material still results to the traditional “ya remember when we told you that such and such was true…well…it is really…” style of science education.
You can understand tidal forces just fine without needing GR, and I say this as someone who specializes in GR. You do need calculus for a thorough understanding, but you can get the gist of it without even that.
While noting that I am not an expert in tides, and with the idea that I obviously simplified my message for a target audience, can you explain how the mostly radially inward force can be explained without breaking the chains of Newtonian physics that will require you to consider it a fictional force?
While the math required to understand it may be simple the concepts at least require an understanding beyond the Einstein’s equivalence principle. And even knowing that it is difficult to move past the incorrect concept that it is a gravitational differential along the earth moon line that is responsible for the phenomenon we call tides.
It would make explaining this easier if you can help me describe it within the constraints of someone with that level of understanding.
It’s especially difficult to move past that incorrect concept, given that it’s correct.
And what’s wrong with fictitious forces? Sometimes they’re the simplest way to describe something. Not that you actually need them, in a Newtonian description.
Wait, please clarify what you think is correct, that tides are the result of a differential in gravity along the earth moon line?
That I can disprove, and that is the common misconception if that is your claim.
Otherwise please correct my assumptions and I will try to respond.
I didn’t say “along the Earth-Moon line”. Tides in any given direction are the differential in gravity along that direction.
Jeez, I thought I had at least one thing more (or less) under my belt.
Don’t jump to conclusions yet; rat avatar seems to have some misunderstanding about tidal forces. You should always be suspicious when someone claims that “most scientists don’t understand” something. I’m interested to hear his proof that tidal forces are not a gravitational differential. (Not being sarcastic; I really want to know what he’s objecting to in the standard explanation.)
–Mark