Maximum size for moons in the sky

[Yora]

In sci-fi movies and art, there are often huge moons seen in the sky. Easily ten times the diameter of our moon and sometimes a lot more than that.

Is that actually possible?

I believe the more massive an object gets, the farther it needs to be away for a stable orbit around the planet. If you give a bigger moon to an earth-size (and mass) planet, could it actually look significantly larger than how our moon looks from earth?

[Spiryt]

Well, planets, moons and other such objects can vary greatly with density after all. Depending on what they're built out off.

So you could theoretically get very spacious moon, that appears very large on the sky, without giving it too large mass, I guess.

[LibraryOgre]

I believe so, yes. Quick research. (http://www.newscientist.com/article/dn9336-upper-size-limit-for-moons-explained.html#.VK_2O6jIb8A)

"Solid planets like Earth and Pluto have relatively large satellites with up to 10% of their mass. These were probably formed by cataclysmic impacts that threw up material into orbit. Since it is the gas that causes the moons of giant planets to spiral in to their death, there is no equivalent upper size limit for the moons of rocky planets."

Basically, it comes down to what you define as a moon. The Earth doesn't revolve around the Sun; we both technically revolve around a point between its center of gravity and ours (the barycenter)... it's just that our gravity is so minuscule compared to Sol's that the point of balance is inside the Sun. Jupiter, by comparison, is large enough and far enough out that the barycenter is, depending on the Sun's weather, sometimes outside the sun itself.

Charon is actually about half the size of Pluto, and their barycenter is between the two of them. Pluto, the larger, was considered "the planet" and Charon "the moon", but it's a binary body, and they go round and round. (https://www.youtube.com/watch?v=e3cDdGKqp8E)

Of course, the history of many worlds is short enough that there might be unstable moons... ones that will, in a few thousand or million years, spin off into space or crash into their planet. But since the history of the world is only a few tens of thousands of years long, there's no huge change in the moon(s).

[Ravens_cry]

A smaller, closer moon would look bigger, (http://www.theverge.com/2013/8/29/4673194/curiosity-captures-eclipse-from-the-surface-of-mars) so there could be a balance where the moon isn't so huge, it's just close. Another possibility is the body being viewed from is the moon and the other world is the primary. Or it could be a double planet system.
Also, I am not 100% on the 'bigger planet=farther stable orbit'. What's the reasoning behind this?

[hamishspence]

Its worth keeping in mind that tidal forces between a moon and a planet will rip the moon apart if it's within the "Roche limit":

http://en.wikipedia.org/wiki/Roche_limit

That's why gas giants don't have large moons close to them - but have rings, with their moons orbiting further out.

[Ravens_cry]

Its worth keeping in mind that tidal forces between a moon and a planet will rip the moon apart if it's within the "Roche limit":

http://en.wikipedia.org/wiki/Roche_limit

That's why gas giants don't have large moons close to them - but have rings, with their moons orbiting further out.
On the other hand, if you were to be on many of the moons in question you'd see a huge object in the sky, so unless it's canonically a moon, the 'That's no moon, it's a space station planet!' explanation can work.

[Spiryt]

Also, I am not 100% on the 'bigger planet=farther stable orbit'. What's the reasoning behind this?

Well, the gravitation force increases if the mass increases, and greatly with when distance decreases (r squared).

So theoretically, if planet is bigger, it has bigger gravitational pull, so moon needs to be further away to compensate for it, I guess.


Of course, the history of many worlds is short enough that there might be unstable moons... ones that will, in a few thousand or million years, spin off into space or crash into their planet. But since the history of the world is only a few tens of thousands of years long, there's no huge change in the moon(s).

That's very good point too.

Sure, moon may be theoretically too large, but for the mere thousands/millions of years during which some life observes it, it 'stays in place' just fine. :smallbiggrin:

[Tirunedeth]

In sci-fi movies and art, there are often huge moons seen in the sky. Easily ten times the diameter of our moon and sometimes a lot more than that.

Is that actually possible?

I believe the more massive an object gets, the farther it needs to be away for a stable orbit around the planet. If you give a bigger moon to an earth-size (and mass) planet, could it actually look significantly larger than how our moon looks from earth?

As far as I'm aware, the only inner limits on orbital stability are synchronous orbit and the Roche limit (typically interior to the former, but not necessarily so). In the absence of perturbations, a two-body system with a closed orbit is stable (barring orbits which cause the two objects to collide). A closer orbit will result in the two-body system's gravitational interactions dominating over external perturbations, leaving only tidal interactions and general relativity to contribute to any instability. GR isn't really relevant to typical planet-moon systems, so only the Roche limit and synchronous orbit are important.

The Roche limit depends only on the relative density of the two bodies and the radius of the primary. There is some dependence of the synchronous orbit on the moon's mass, since the orbital period in a two-body system depends on the sum of the masses, but you'd only see an increase of around 26% if the Moon were Earth's mass.

Let's take a "double Earth" scenario as an example. In that case, an observer directly under the "moon" is a distance of (42,164 km)*1.26 - 6,371 km = 46755 km from the center of the "moon." Some simple trigonometry indicates that the angular diameter (in radians) of the "moon" is equal to the ratio of twice the "moon's" radius to the distance from the observer to the center of the "moon," i.e. (2*6,371 km) / (46,755 km) = 0.273. Converting to degrees gives us 15.6 degrees, which is around 31 times the size of the full Moon.

You'll get some variation depending on rotation speeds and densities, but that's probably something like an upper limit for something that can actually be called a moon; you can obviously get something larger if your "planet" is actually a moon orbiting something larger.

[factotum]

Just a note about the Roche limit: it's assuming that the body in question pretty much has the consistency of a pile of cigarette ash that is only held together by its own gravity. In the case of a rocky moon, especially a *small* rocky moon, the tensile strength of the rock itself would allow it to orbit closer than the Roche limit. (This is obviously how the International Space Station remains in one piece despite orbiting well within the Roche limit for the Earth, for example).

[Yora]

The ISS is also very small, so the gravitational pull on the earth-side parts of the station and the space-side parts is virtually identical. I believe tidal forces become a factor once different parts of a body experience gravitaional pulls of different strength.

Assuming a planet like Earth with a moon the size of Earth but the density and internal structure of the Moon. How close could it get for a stable orbit?
If it could be as close as the Moon is now, it would have about four times the diameter in the sky. (And 16 times the area, unless scaling areas works different for circles than for squares.) Which would be quite big and relatively huge to what we're used to, but still far from as massive artists like to paint it.
But since that super-moon would also have 64 times the volume and mass, I am not even sure it could stay that close.

[Ravens_cry]

Just a note about the Roche limit: it's assuming that the body in question pretty much has the consistency of a pile of cigarette ash that is only held together by its own gravity. In the case of a rocky moon, especially a *small* rocky moon, the tensile strength of the rock itself would allow it to orbit closer than the Roche limit. (This is obviously how the International Space Station remains in one piece despite orbiting well within the Roche limit for the Earth, for example).
A better example is Phobos, which is also inside the Roche limit, I believe.

[JCarter426]

The ISS is also very small, so the gravitational pull on the earth-side parts of the station and the space-side parts is virtually identical. I believe tidal forces become a factor once different parts of a body experience gravitaional pulls of different strength.

Assuming a planet like Earth with a moon the size of Earth but the density and internal structure of the Moon. How close could it get for a stable orbit?
If it could be as close as the Moon is now, it would have about four times the diameter in the sky. (And 16 times the area, unless scaling areas works different for circles than for squares.) Which would be quite big and relatively huge to what we're used to, but still far from as massive artists like to paint it.
But since that super-moon would also have 64 times the volume and mass, I am not even sure it could stay that close.
I believe the Roche limit involves the cubic root of the object's density, so if the super-moon is 64 times as dense as our current moon, the Roche limit will be 4 times greater. Our moon is already well outside its Roche limit, so the super-moon would be fine in the same position. As you say, though, the orbital stability depends on additional factors.

[gomipile]

On the other hand, if you were to be on many of the moons in question you'd see a huge object in the sky, so unless it's canonically a moon, the 'That's no moon, it's a space station planet!' explanation can work.

For example, Jupiter as seen from
Io would be about 36 times the diameter of Luna as seen from Earth, on average.

[factotum]

But since that super-moon would also have 64 times the volume and mass, I am not even sure it could stay that close.

I don't see why not. Increasing the Moon's mass by a factor of 64 would make it comparable to Earth's mass (Earth would still be slightly heavier since it's 81 times the current Moon's mass), but that would just mean the barycentre of the two bodies' orbits would be somewhere in space between them rather than, as is the case now, somewhere inside the Earth. There isn't any reason whatsoever why such an orbit wouldn't be stable, and we already have examples of similar situations in the Solar System--Pluto and Charon, for instance, where Charon is around half the mass of Pluto and the two bodies orbit very close together.

[Yora]

Is there any rule of thumb how close two bodies can orbit each other relative to their size?

[Bulldog Psion]

There would be some pretty spectacular tidal effects, though.

"Ah, it looks like it's time for the daily 150-foot tsunami to come in..."

[Ravens_cry]

For example, Jupiter as seen from
Io would be about 36 times the diameter of Luna as seen from Earth, on average.

Or any of the Galilean moons of Jupiter for that matter, as well as the closer moons of all solar system giants.

[Jeff the Green]

Charon is actually about half the size of Pluto, and their barycenter is between the two of them. Pluto, the larger, was considered "the planet" and Charon "the moon", but it's a binary body, and they go round and round. (https://www.youtube.com/watch?v=e3cDdGKqp8E)

I would have linked to this (https://www.youtube.com/watch?v=cz_QncqzveA). But then, I haven't slept in 36 hours.

[factotum]

Is there any rule of thumb how close two bodies can orbit each other relative to their size?

Apart from the Roche limit, I don't believe there is any such limit. Gravity doesn't suddenly become a repulsive force when things get too close together! The one thing that would be a bit odd in the stated "large, close moon" scenario is tidal locking--if the Moon really were 64 times the mass it has now, Earth would be tidally locked to it in the same way that the Moon is tidally locked to Earth, so you'd expect it to stay in the same position in the sky at all times. (Unless it had managed to get into a resonance like you have with the Mercury-Sun system, of course).

[Yora]

If you have two bodies that are both tidally locked to each other, wouldn't that mean that they both do not rotate? They would orbit around their shared center, but that would lead to freaky day and night cycles.

[Jay R]

Metis is Jupiter's closest (known) moon, and so would have the most impressive single body visible in the sky in the solar system.

As seen from Metis, Jupiter would be 68 degrees across, roughly three quarters the distance from the horizon to the zenith.

[gomipile]

If you have two bodies that are both tidally locked to each other, wouldn't that mean that they both do not rotate? They would orbit around their shared center, but that would lead to freaky day and night cycles.

It would mean that they do rotate, but their rotation period is exactly the same as their mutual orbital period. Basically, one "day" would be precisely one "month" long.

[factotum]

Yeah, what gomipile said--the two bodies must face each other at all times, but since they're orbiting around each other they're still rotating with respect to the local star.

[Rockphed]

Using the ratio of earth's mass to the theoretical supermoon (with earth's radius and Luna's density), the supermoon would have a roche limit of 8600 kilometers. Which I think means that there would only be about 2000 kilometers between the surfaces of the bodies. Trig tells me the supermoon would have a 37 degree radius, so 72 degree diameter. How big does Jupiter look from IO again?

[gomipile]

Using the ratio of earth's mass to the theoretical supermoon (with earth's radius and Luna's density), the supermoon would have a roche limit of 8600 kilometers. Which I think means that there would only be about 2000 kilometers between the surfaces of the bodies. Trig tells me the supermoon would have a 37 degree radius, so 72 degree diameter. How big does Jupiter look from IO again?

Jupiter should have a diameter a bit less than 19 degrees, seen from Io, I think.

[gomipile]

Metis is Jupiter's closest (known) moon, and so would have the most impressive single body visible in the sky in the solar system.

As seen from Metis, Jupiter would be 68 degrees across, roughly three quarters the distance from the horizon to the zenith.

Interestingly, given the rest of the thread, Metis is within the Roche limit of Jupiter. Thus, while it might have a great view of the giant planet, you couldn't stand on the Jupiter side because the tide would cause you to drift away from the surface.

Saturn's innermost named moon, Pan, is likewise within Saturn's Roche limit. (Saturn has a much, much smaller moonlet called S/2009 S 1 which is closer. It's only 300 meters or so across.)

[Bulldog Psion]

I'm also reasonably certain that a planet tidally locked with a gigantic moon would be totally uninhabitable, unless the life forms came with naturally-occurring environment suits. :smallwink:

[Ravens_cry]

I'm also reasonably certain that a planet tidally locked with a gigantic moon would be totally uninhabitable, unless the life forms came with naturally-occurring environment suits. :smallwink:

Why? It certainly leads to some unique challenges, but 'totally uninhabitable'?
Or is there some joke here I am not getting.

[Bulldog Psion]

Why? It certainly leads to some unique challenges, but 'totally uninhabitable'?
Or is there some joke here I am not getting.

No, I'm just guessing that the tidally locked condition would mess up the climate to the point where nothing could survive on the planet.

[gomipile]

No, I'm just guessing that the tidally locked condition would mess up the climate to the point where nothing could survive on the planet.

I think you're confusing or conflating tidal locking with a moon with tidal locking with the star.

[Bulldog Psion]

I think you're confusing or conflating tidal locking with a moon with tidal locking with the star.

Yes, probably.

However, I'd imagine that tidal locking with a moon would throw a lot of crucial systems out of whack, also, though perhaps not as drastically. Wouldn't it tend to create an atmospheric bulge on the moonward side of the planet, if it didn't bleed atmosphere away outright? That would make potentially very high atmospheric pressures on the moonward side and create greenhouse-like problems due to depth of atmosphere, and low atmospheric pressure anti-moonward, as well as less protection from cosmic bombardment and radiation.

Furthermore, there would be a large, permanent oceanic bulge on the moonward side and a correspondingly lowered ocean on the other side. This would really change or stop oceanic circulation, and without a functioning large-scale current system, there could be everything from massive climatic effects to lack of food supply for marine organisms.

I'm not saying that it would be impossible that the planet would be habitable. I don't have enough science to make any kind of educated guess, even. Just that extrapolating from the few things I do know make it sound a bit ... uncomfortable.

[gomipile]

Yes, probably.

However, I'd imagine that tidal locking with a moon would throw a lot of crucial systems out of whack, also, though perhaps not as drastically. Wouldn't it tend to create an atmospheric bulge on the moonward side of the planet, if it didn't bleed atmosphere away outright? That would make potentially very high atmospheric pressures on the moonward side and create greenhouse-like problems due to depth of atmosphere, and low atmospheric pressure anti-moonward, as well as less protection from cosmic bombardment and radiation.

Furthermore, there would be a large, permanent oceanic bulge on the moonward side and a correspondingly lowered ocean on the other side. This would really change or stop oceanic circulation, and without a functioning large-scale current system, there could be everything from massive climatic effects to lack of food supply for marine organisms.

I'm not saying that it would be impossible that the planet would be habitable. I don't have enough science to make any kind of educated guess, even. Just that extrapolating from the few things I do know make it sound a bit ... uncomfortable.

No, both the atmosphere and oceans would be deepest both at the point directly under the moon, and at that point's anti-moonward antipode. Tidal forces acting on fluids create an egg shape, they don't pull everything to one spot.

[Bulldog Psion]

No, both the atmosphere and oceans would be deepest both at the point directly under the moon, and at that point's anti-moonward antipode. Tidal forces acting on fluids create an egg shape, they don't pull everything to one spot.

That's true -- kind of like the distribution of spherules from the Chicxilub impact. They're densest around the impact site and at the antipode. You're reminding me of a lot of stuff that had disappeared into a gray haze of work. :smallsmile:

However, two permanent bulges in both atmosphere and water would have considerable impact on both large scale atmospheric and oceanic circulation systems, wouldn't they? Leading to excessive heat buildup in some areas and a heat deficit in others, thus creating horrible extremes of climate, possibly with habitable zones between them?

[Knaight]

However, two permanent bulges in both atmosphere and water would have considerable impact on both large scale atmospheric and oceanic circulation systems, wouldn't they? Leading to excessive heat buildup in some areas and a heat deficit in others, thus creating horrible extremes of climate, possibly with habitable zones between them?

Horrible extremes of climate are often teeming with life here on earth. Humans would likely be restricted to a particular range (though we can handle a surprisingly wide range of atmospheric pressures and partial pressured of oxygen), but life in general? Not so much.

[factotum]

Thing is, the permanent tidal bulges on the tidally locked planet would, over geological time, become the stable equilibrium. Not to mention that having a single large tidal bulge that never moves would arguably be a more stable situation than having a smaller one that moves over the surface of the planet, which is the situation we have now!

[The Random NPC]

Wait, I thought our moon was tidally locked.

[cobaltstarfire]

Wait, I thought our moon was tidally locked.

I think they're talking about both a hypothetical moon and planet being locked to each other? But I think our moon is tidally locked. (Tidally locked means the same part of the moon is always facing us right?)

(not too sure, whatever I know about space is basically 3rd grade knowledge+whatever I've learned on my own...which isn't very much, so I don't really know many terms)

[factotum]

Wait, I thought our moon was tidally locked.

It is. What we're discussing is a hypothetical situation where the Moon is much larger and more massive than it is in reality, which would probably cause the Earth to become tidally locked to it in the same way as the Moon is currently tidally locked to us.

[hamishspence]

Yup. Basically, the Pluto-Charon system, but with the bodies scaled up so that the larger body is capable of supporting humanlike life.

[Eldan]

At which point would we be talking about binary planets*, instead?

*Wiki says some people actually call the Earth-Moon system a binary planet. Didn't know that!

[Rockphed]

At which point would we be talking about binary planets*, instead?

*Wiki says some people actually call the Earth-Moon system a binary planet. Didn't know that!

The reasoning I saw a couple years ago was that both Earth and the Moon always move in one direction around the sun (I.e. neither ever goes backwards, just side to side a bit and vary speeds). With the new definition of major planet however, I think Earth dominates its orbit effectively enough to keep the Moon from having a say.

[factotum]

The reasoning I saw a couple years ago was that both Earth and the Moon always move in one direction around the sun (I.e. neither ever goes backwards, just side to side a bit and vary speeds).

That's only because the Earth's orbital speed is so much higher than the Moon's, though--the Earth orbits the Sun at 18 miles per second, whereas the Moon poodles along around the Earth at about a thirtieth of that speed. That's just a function of the size of the orbits of the two bodies, though--pretty sure there are moons of Jupiter or Saturn which move fast enough to appear to travel backwards relative to the Sun.

[Chronos]

Quoth Yora:

Is there any rule of thumb how close two bodies can orbit each other relative to their size?
Yes. If the surfaces of the two bodies are scraping against each other, they're probably too close. Note that this is only a rule of thumb: There are some binary stars which are closer than that.

If you're wondering instead about the Roche limit, if the planet and moon have the same density, it's about 1.5 times the radius of the planet (at least, in the case where the planet is significantly larger than the moon). That is to say, the moon is about half a radius above the surface.

[The Random NPC]

It is. What we're discussing is a hypothetical situation where the Moon is much larger and more massive than it is in reality, which would probably cause the Earth to become tidally locked to it in the same way as the Moon is currently tidally locked to us.

So like a double tidally locked system?

[factotum]

So like a double tidally locked system?

Both bodies would be tidally locked to each other in this scenario, yes, as is the case with Pluto and its moon Charon.