Why is the moon drifting into space

[tony873004]

(from other thread)I hear that the moon is steadily moving away from the earth. Is it due to the suns gravity? Will it eventually go out of Earth orbit and circle the sun on its own? Would there be any chance of a posssible collision with Earth if this happened or would it leave Earth orbit at its closest approach to the sun if so?

If the Moon left the Earth, it almost certainly would have an Earth collision in its future.

But it won't ever leave. As it moves out, it also spins down the Earth. Once the Moon's period and the Earth's rotation are equal to each other, the Moon will stop receeding. And the Sun will probably go Red Giant long before that happens, engulfing both Earth and Moon.

 

[quetzalcoatl9]

If the Moon left the Earth, it almost certainly would have an Earth collision in its future.

But it won't ever leave. As it moves out, it also spins down the Earth. Once the Moon's period and the Earth's rotation are equal to each other, the Moon will stop receeding. And the Sun will probably go Red Giant long before that happens, engulfing both Earth and Moon.

i have read that, among other things, if the Earth were to suddenly disappear, that the moon would continue in orbit around the sun. and that supposedly this is unique to our moon: other planetary moons, if in the same situation, would fly off into space.

is this true?

 

[tony873004]

i have read that, among other things, if the Earth were to suddenly disappear, that the moon would continue in orbit around the sun. and that supposedly this is unique to our moon: other planetary moons, if in the same situation, would fly off into space.

is this true?

Every moon, including the Earth's would fly off into space relative to the position of their now-missing parent planet. Many would continue to orbit the Sun, many would escape the solar system, and many would enter planet-crossing orbits.

To determine what would happen, you have to compare the circular orbital velocity of the planet around the Sun, and see what effect adding or subtracting the moon's orbital velocity to the planet's orbital velocity.

If the moon's new solar orbital velocity exceeds solar escape velocity, then it will escape the solar system. Escape velocity is \sqrt{2}*V_{circ}.

In the case of the Earth's Moon, it travels around the Earth with an orbital velocity of about 1 km/s, while the Earth is traveling around the Sun with an orbital velocity of about 30 km/s. So relative to the Sun, the Moon is orbiting in the range of 29 - 31 km/s depending on where in its orbit it is. This is not a huge difference, so if the Earth disappeared, the Moon's new solar orbit would be very similar to its original solar orbit while it was a part of the Earth/Moon system.

Mars' moons would fare a little worse. Phobos and Deimos, with orbital velocities of 2.1 and 1.4 km/s are orbiting Mars faster than the Moon orbits the Earth. And Mars' orbital velocity at ~24km/s is a bit slower than Earth's. So the percentage change in orbital velocity experienced by the now-free Martian moons would be higher than that experienced by Earth's Moon. Depending on where they were in their orbits when Mars disappeared, and also depending on where Mars was in its solar orbit, since it's orbit is noticably elliptical causing its orbital velocity to vary between 21-27 km/s, Phobos could enter an Earth-crossing orbit.

The story changes for Jupiter. It's solar orbital velocity is only about 13 km/s. Escape velocity from the Sun is about 18 km/s at Jupiter's distance. But its innermost moons orbit it with velocities of 17 km/s for Io, to 8 km/s for Callisto. There are a few moons interior to Io which orbit even faster. So these moons will not happily orbit the Sun in similar orbits to the ones they enjoyed while Jupiter existed. They will either get flung out of the solar system, dropped into the Sun, or enter planet-crossing orbits. But Jupiter has many moons that orbit it at a great distance. For example, Eurydome has an orbital velocity of about 1.7 km/s. Its fate, and the fate of similar moons depends on where in their orbits they were when Jupiter disappeared.

The moons of Saturn, Uranus, and Neptune would have fates similar to Jupiter's moons. But Pluto's moon, Charon, would continue to orbit the Sun in a very similar orbit.

Here's a screen shot of a simulation of the solar system a few years after Jupiter's mass suddenly dropped to 0 kg.
http://orbitsimulator.com/orbiter/jup.GIF

 

[quetzalcoatl9]

thank you, that makes sense

 

[mavisgold]

.moon gravitational pull
↓
↑
.earth bulge from moons gravitational pull

.but because the Earth is rotating faster than the moon is orbiting

.the Earth bulge gets ahead of the moon


.moon position
. ↓
.↑ ←earth rotation
.earth bulge


.so the moon is pulled towards the Earth bulge increasing its speed
.← moon orbit
.→
.and the Earth bulge is pulled towards the moon slowing it down


do this help?
or just add confusion?

 

[sylas]

...

This thread is a bit over four years old.

 

[pegasus980]

.moon gravitational pull
↓
↑
.earth bulge from moons gravitational pull

.but because the Earth is rotating faster than the moon is orbiting

.the Earth bulge gets ahead of the moon


.moon position
. ↓
.↑ ←earth rotation
.earth bulge


.so the moon is pulled towards the Earth bulge increasing its speed
.← moon orbit
.→
.and the Earth bulge is pulled towards the moon slowing it down


do this help?
or just add confusion?

confusion for me.

 

[ideasrule]

I know this thread is 4 years old, but for people reading it, I have to mention one thing.

Where does the energy come from? It is bled off from the Earth's rotation.
Those tidal bulges cause friction. The water acts against the ocean floor and the continental shelves.
This slows the Earth's rotation.

No! Tidal bulges are not the tides that we see on the beach; Earth's oceans have negligible volume compared to the Earth itself.

Because the Moon's gravity is stronger on the side closer to the Earth than on the farther side, Earth is slightly elongated. This is the so-called tidal bulge; it's a bulge in the rock, not a bulge in the ocean.

 

[sylas]

I know this thread is 4 years old, but for people reading it, I have to mention one thing.



No! Tidal bulges are not the tides that we see on the beach; Earth's oceans have negligible volume compared to the Earth itself.

Because the Moon's gravity is stronger on the side closer to the Earth than on the farther side, Earth is slightly elongated. This is the so-called tidal bulge; it's a bulge in the rock, not a bulge in the ocean.

The original post to which you are responding is completely correct.

The bulge in the ocean IS a tidal bulge, and this IS the tides you see on the beach. You can also get a bulge in rock, but it is much much smaller. Which matters for for slowing rotation depends on the circumstances. For Earth, it is the ocean tides which are most significant for the drag that slows Earth's rotations and transfers energy to the Moon, leading to it moving gradually further away.

That the volume of the ocean is smaller is irrelevant. There's still a tidal bulge with the ocean, which corresponds to tides.

Cheers -- sylas

 

[D H]

The Earth tides are smaller, but not much smaller, than the ocean tides. Much smaller means smaller by at least an order of magnitude. The Earth tides are about 1/3 of the oceanic tides in magnitude. The "lossiness" of the Earth tides is much smaller than is that of the oceanic tides.

Right now, the lossiness of the oceanic tides is particularly high because the oceanic tides run into two north-south barriers: the Americas and Africa+Eurasia. This configuration is rather unusual. The average change in the Earth's rotation rate over the last billion years is smaller than the present rate. If the change was due to the Earth tides the change in length of day would have been a lot closer to constant.

 

[sylas]

The Earth tides are smaller, but not much smaller, than the ocean tides. Much smaller means smaller by at least an order of magnitude. The Earth tides are about 1/3 of the oceanic tides in magnitude. The "lossiness" of the Earth tides is much smaller than is that of the oceanic tides.

Thank you! I did not realize that they were that close in magnitude. I've learned something.

The rest is my understanding as well. Much more energy is dissipated in the oceanic tides. That's probably how I jumped too quickly to the bulge size. I've also been aware that energy dissipation from ocean tides varies with the locations of continents, and is particularly high in the present epoch.

Cheers -- sylas

 

[ideasrule]

I stand corrected (and interested). I really feel this should go in Wikipedia somewhere; do any of you have sources you could cite, DH and sylas?

 

[sylas]

I stand corrected (and interested). I really feel this should go in Wikipedia somewhere; do any of you have sources you could cite, DH and sylas?

The wikipedia article Tide seems pretty good, to me, at a quick glance. I am a registered editor on wikipedia, although I have done hardly anything there for a couple of years now. (Anyone can register; it is very easy to do, and has some advantages when you edit articles.)

There's a section on Dissipation, and that links to a more detailed article on Tidal acceleration, which covers this particular aspect of tides.

Cheers -- sylas