Moon Orbits

phantasy · Jan 23, 2014

Hello all, some questions for our resident astronomers.

Now, I'm one of those people who believes that you can pretty much find all kinds of strange planets and orbits in our universe and that anything is possible. There's a diamond planet for crying out loud.

So...is it possible for a moon to orbit only one side of a planet or only a small part of it? Maybe only the very top so it misses the rest?
Also, how close can a Moon possibly get to a planet without crashing into it?

Basically, I'm writing a story where much of a planet is uncharted. The heroes have visited the Moon and can look down below but will shocked to learn that the Moon only goes up and down and not around and around and so, they aren't seeing the entire planet. Plus it is very close, close enough for clouds to obscure their view. The moon is tiny. Now, this is a fantasy...so I can always pull the 'because magic' but I'd rather not because people don't like it.

If my thinking doesn't work, I might just have to make it some weird floating rock instead of a moon or something.

alleycat · Jan 23, 2014

I can't really answer your questions. I think it would be odd for a moon to only orbit the top of a planet. If I'm correct, a moon will be drawn to the center of mass of a planet. That would be where the gravitational force would be strongest. However, if the moon was relatively new (in cosmic time, which might be thousand of years), it might occur (it hasn't been pulled in to an orbit of strongest gravity yet). And . . . strange things do occur in the universe.

Someone else might be more of an expert on this than I am.

GHO57 · Jan 23, 2014

I'm not an astronomer, but I do like moons; SciFi guy and all that.

Trojans have strange orbits... would they work? (I think you'd need a massive planet for them to be habitable moons tho)

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

As to how close a moon could be, no idea... my CelMech is too rusty to hazard a guess. There are however exoplanets that would appear larger to an observer than the moon does when seen from earth.

http://www.space.com/16242-alien-planets-close-orbits-planetrise.html

Hoplite · Jan 23, 2014

Well I'm no astromoner, and I don't know if/how the one-sided orbit would work, but I have two suggestions for the close proximity of the moon.

1. The moon has a ridiculous fast orbit speed. It would have to be the right combination to keep the moon from spiraling into the planet, and yet to keep from spiraling away as we'll though. That would also have huge tidal effects. Perhaps a more realistic outcome would be...

2. You have a severe elipitical orbit. The moon would make close passes to the planet (once again at high speeds) but would slow down once it got far away. Then gravity would bring the moon back for another close pass.

You might try posting this thread in the Science Fact subfolder under Sconce Fiction/Fantasy interests.

Kevin Nelson · Jan 23, 2014

You could certainly have a moon in a geosynchronous orbit, like a lot of communications satellites are. (Though technically speaking, I'm not sure if "geosynchronous" would be the right word for something orbiting a planet other than Earth!)

In the simplest case, such a moon would orbit in the plane of the planet's equator. It would orbit the planet in lockstep with the planet's own rotation. So from the perspective of anyone on the planetary surface, the moon would stay permanently in the same position. For example, maybe it would always stay straight overhead in Ecuador. North of Ecuador, the moon would always stay in the same position over the southern horizon, and so on. From the opposite hemisphere, you would never see the moon at all. (Or, actually, it would be visible from somewhat less than a complete hemisphere. A moon like this would be much closer than Earth's actual moon, so it would be more easily hidden by Earth's curvature.)

A geosynchronous moon could also orbit at an angle to the planet's equator. From the perspective of an observer on the surface, it would then follow a more complicated motion in which it had a lot of north-south movement. So maybe sometimes it would be straight overhead in Buenos Aires and sometimes it would be straight overhead in New York. For most orbits like that, you still would only see the moon from one hemisphere.

As for how close a moon can get, the operative phrase is "Roche limit." A moon closer than that will tend to break apart into pieces. For Earth, the Roche limit is about 12,000 kilometers above the surface. (The precise number depends on various factors such as the density of the moon.) By comparison, a moon in a circular geosynchronous orbit will be about 35,000 kilometers above the surface. Earth's actual moon is more like 400,000 kilometers away. So a geosynchronous moon could be much smaller, yet have the same apparent size to an observer on the planetary surface.

wendymarlowe · Jan 23, 2014

Not implausible for the planet to be lopsided - the moon could be geosynchronous with the heavier side.

phantasy · Jan 23, 2014

Kevin Nelson said:
You could certainly have a moon in a geosynchronous orbit, like a lot of communications satellites are. (Though technically speaking, I'm not sure if "geosynchronous" would be the right word for something orbiting a planet other than Earth!)

In the simplest case, such a moon would orbit in the plane of the planet's equator. It would orbit the planet in lockstep with the planet's own rotation. So from the perspective of anyone on the planetary surface, the moon would stay permanently in the same position. For example, maybe it would always stay straight overhead in Ecuador. North of Ecuador, the moon would always stay in the same position over the southern horizon, and so on. From the opposite hemisphere, you would never see the moon at all. (Or, actually, it would be visible from somewhat less than a complete hemisphere. A moon like this would be much closer than Earth's actual moon, so it would be more easily hidden by Earth's curvature.)

A geosynchronous moon could also orbit at an angle to the planet's equator. From the perspective of an observer on the surface, it would then follow a more complicated motion in which it had a lot of north-south movement. So maybe sometimes it would be straight overhead in Buenos Aires and sometimes it would be straight overhead in New York. For most orbits like that, you still would only see the moon from one hemisphere.

As for how close a moon can get, the operative phrase is "Roche limit." A moon closer than that will tend to break apart into pieces. For Earth, the Roche limit is about 12,000 kilometers above the surface. (The precise number depends on various factors such as the density of the moon.) By comparison, a moon in a circular geosynchronous orbit will be about 35,000 kilometers above the surface. Earth's actual moon is more like 400,000 kilometers away. So a geosynchronous moon could be much smaller, yet have the same apparent size to an observer on the planetary surface.

Your answer is closest to what I'm looking for. Geosynchronous totally works for my idea, although my characters will probably not use the word. Thanks so much! I'm not sure if my concept totally meshes with what you've said, but I'll just write the story for now and see what happens when my betas get to it. I've had to read what you've written a few times to kind of 'get it'.

So, a moon among the clouds is probably not possible, correct? Too close?

GHO57 · Jan 23, 2014

phantasy said:
So, a moon among the clouds is probably not possible, correct? Too close?

Much too close. Only like 90Km up, at best (on Earth).

Even if it wasn't under the breakup limit, the friction from blasting through the atmosphere at the speeds required would melt it. (It would also need a power source to keep it going at constant speed against the air-resistance)

A hollow object (a sphere to make it look moony) with internal temperatures higher than ambient might float in air, like a stone hot air balloon... That would be in the clouds. Likely just a tad unstable, not really something you'd want to live on.

Kevin Nelson · Jan 23, 2014

phantasy said:
So, a moon among the clouds is probably not possible, correct? Too close?

It certainly wouldn't be possible for a moon orbiting an Earthlike planet. But I'm not sure whether you might conceivably work out some planetary parameters to make it possible. I'm assuming the planet has to be human-habitable. Maybe if the planet were considerably lower-density than Earth, and rotating considerably faster, with a much thicker atmosphere? I just don't know! It would certainly be very difficult to get the parameters just right, and the planet would wind up being pretty zany in more ways than one. Sorry this isn't more helpful--actually, I'm not sure if anyone could answer this question definitively.

phantasy · Jan 23, 2014

Ok, thanks for the info guys.

I'm thinking...but why couldn't it be close? If the moon is in Geosynchronous orbit, why couldn't it just follow the rotation of the planet, staying in the same spot but much closer. It would kind of be tied to the earth of the planet itself, like a kite or blimp, as if casually floating along by an invisible, magic force/string. I guess such a moon would have very low density?

Again, this is a planet of magic, so I wanted it a little mired in reality, but if it isn't all the way, I guess that's ok. Like I said, it all depends on what the readers will accept in the end.

Kevin Nelson · Jan 24, 2014

phantasy said:
Ok, thanks for the info guys.

I'm thinking...but why couldn't it be close? If the moon is in Geosynchronous orbit, why couldn't it just follow the rotation of the planet, staying in the same spot but much closer. It would kind of be tied to the earth of the planet itself, like a kite or blimp, as if casually floating along by an invisible, magic force/string. I guess such a moon would have very low density?

Again, this is a planet of magic, so I wanted it a little mired in reality, but if it isn't all the way, I guess that's ok. Like I said, it all depends on what the readers will accept in the end.

The closer a moon is to its planet, the faster it will orbit. That's pretty much an ironclad law of physics. So if the moon is too close, it will orbit too quickly to be geosynchronous. It will also run into problems with the Roche limit, in which stresses from the planet's gravity will tend to literally tear the moon apart. (With regard to that latter problem, it would actually help a little if the moon had very high density.)

One scenario I was imagining was where the planet itself rotated much faster than Earth. Maybe it rotates in just 2 hours, rather than Earth's 24 hours. For such a planet, a geosynchronous orbit would also take just 2 hours. So a moon with that sort of orbit could be much closer to the planetary surface. Maybe, just maybe, it could be close enough to actually be in the planet's atmosphere. Then it would be like you describe---it would seem to float in place like a balloon.

A planet like that would be on the verge of breaking into pieces itself, but maybe it would be stable enough to last. Effective gravity there would be a lot larger at the poles than at the equator. Along the equator, effective gravity would have to be very low indeed--maybe you'd weigh about 3% of your usual weight there. Like I say, this would have to be a pretty weird planet. I'm not sure if planets like that are possible at all, much less human-habitable. But it's a thought, at least!

blacbird · Jan 24, 2014

phantasy said:
Hello all, some questions for our resident astronomers.

Now, I'm one of those people who believes that you can pretty much find all kinds of strange planets and orbits in our universe and that anything is possible.

Ummm . . . no. If you're trying to conform to known physics, just forget about that nonsense.

phantasy said:
So...is it possible for a moon to orbit only one side of a planet or only a small part of it? Maybe only the very top so it misses the rest?

I don't have a clue what you mean here. Orbits are reflections of gravitational interactions.

phantasy said:
Also, how close can a Moon possibly get to a planet without crashing into it?

Google "Roche limit". Gravitational force from a planet will rip apart a moon of any significant size long before it can collide with the planet itself. The rings of Saturn may have been formed from such an event.

phantasy said:
Basically, I'm writing a story where much of a planet is uncharted. The heroes have visited the Moon and can look down below but will shocked to learn that the Moon only goes up and down and not around and around and so, they aren't seeing the entire planet. Plus it is very close, close enough for clouds to obscure their view. The moon is tiny. Now, this is a fantasy...so I can always pull the 'because magic' but I'd rather not because people don't like it.

If my thinking doesn't work, I might just have to make it some weird floating rock instead of a moon or something.

Better pull the magic trick. That's what Avatar did, with the floating mountains. And a lot of people found it stupid and laughable, even if eyepopping in 3D. Gravity is the essential relational force in the universe. Unless you postulate a universe having no gravity, you can't sensible postulate crazy orbital relationships.

Methinks some minor astronomy and physics research might be useful to you.

caw

TheNighSwan · Jan 24, 2014

A thing to keep in mind: orbiting bodies tend to synchronise with each other over time; it is not an accident that we always see the same face of the moon, which presumably was originally rotating on itself, but was progressively slowed down by Earth to the point of effectively rotating synchronically with Earth, so that now we only see one face of the moon.

But the moon too is slowing down Earth. In the Cambrian period (500 million years ago), a day was less than 21 hours long, because Earth rotated faster. This slowing down will continue until the Earth effectively always has the moon on the same face, independently of the moon's velocity and distance (though, since through this process the moon is robbing us of angular momentum and adding it to itself, when this stage will be reached the moon will be orbiting further from Earth than it currently does, due to having "escaped" through acceleration).

However, this is a very slow process and, if I remember correctly, it will barely be completed when the Sun starts to inflate and engulf the Earth.

This can get interesting when there is more than one moon; Jupiter's three inner galilean moons are not only synchronised with Jupiter, but also with each other: by the time Ganymede completes one orbit around Jupiter, Europa completes exactly two and Io exactly four.

----

What to keep out of this is:

1) "how close has to orbit a moon so that it can only be seen from one side of the planet" depends on how fast the planet is rotating; the slower it goes (and thus the longer the day), the further can the moon be.

2) The older the planet is, the likelier it is to be synchronized with its moon.

EDIT: it's important to remember that our moon's orbital period is a bit more than 27 days; in other words, it takes 27 days for the moon to circle the Earth completely.

Which means that the visible movement of the moon in the sky is *almost entirely* the results of Earth's rotation, as opposed to the moon orbiting.

This also implies that when the Earth and the moon are finally locked together, the Earth "day" will be a month long in current days.

And a helpful wikipedia article on the subject: http://en.wikipedia.org/wiki/Tidal_locking

bookworm92 · Jan 24, 2014

One of Pluto's (Charon) moon has an interesting orbit. The moon's orbit around Pluto is the same length as Pluto's day; so only one side of Pluto ever sees the moon.

Brutal Mustang · Jan 24, 2014

I'd say, simply make your world a moon orbiting a gas giant, like Pandora is on Avatar. And make the world tidally locked to that Planet, in fast orbit (so it would have 'days'). Then the gas giant would always be in the same place in the sky on one side of the world, unchanging except for its phases. Except, no one is going to be able to stand on that gas planet and observe the world. Not without being crushed by immense gravity, let alone having nothing but gases to stand on. :tongue

skylark · Jan 24, 2014

phantasy said:
but why couldn't it be close? If the moon is in Geosynchronous orbit, why couldn't it just follow the rotation of the planet, staying in the same spot but much closer.

You can always do things with magic, but the physics/maths of this is that a geosynchronous orbit is a set distance from the surface. A stable orbit takes longer and longer as you get further and further from the surface, and at a particular distance that length of time is exactly a day. That's a geosynchronous orbit. The distance (for Earth) is 42,164 km.

Bring something closer in and to stay in orbit it has to go faster...and then it's not geosynchronous any more. If it doesn't go faster, it'll fall in and crash.

Drachen Jager · Jan 24, 2014

Orbits are inherently around the centre of mass of the more massive body.

So, technically the answer is no, you cannot simply orbit a small part of a planet.

What you could do is have a north-south orbit which happens every day. This would effectively mean the moon would do a north-south arc over the same portion of land every night and a south-north arc over the same portion of the planet every day (or vice versa).

Such an orbit is possible, however it's incredibly unlikely.

phantasy · Jan 24, 2014

blacbird said:
Ummm . . . no. If you're trying to conform to known physics, just forget about that nonsense.

Hmm I don't know, I once saw an entire show filled with scientists trying to think up ways the Tardis and Sonic Screwdriver could be possible. Those are two pretty impossible things.

blacbird said:
That's what Avatar did, with the floating mountains. And a lot of people found it stupid and laughable, even if eyepopping in 3D. Gravity is the essential relational force in the universe. Unless you postulate a universe having no gravity, you can't sensible postulate crazy orbital relationships.

Well whoever found it 'stupid and laughable' wasn't familiar with fantasy and was probably being a snarky nerd. The whole floating islands/castles thing is pretty old.

NeuroGlide · Jan 24, 2014

phantasy said:
Ok, thanks for the info guys.

I'm thinking...but why couldn't it be close? If the moon is in Geosynchronous orbit, why couldn't it just follow the rotation of the planet, staying in the same spot but much closer.

Because then it wouldn't be in synchronous orbit. Here's a couple exercises to help wrap your mind around orbits.

Nothing is holding ISS up, it's falling towards Earth, it just keeps missing. Literally. From it's location 300 km up, it accelerates towards the Earth at 9.8 meters per second per second (not a typo). It stays up because by the time it's fallen 300 km, it's moved so far sideways that the Earth's curvature has pulled it away from ISS by 300 km. This is why orbit is also called freefall.

Now picture ISS moving sideways faster. By the time its fallen the 300 km, it's moved so far to the side that it's farther from the Earth then when it started. Conversely, if it isn't moving fast enough (anything below synchronous orbit in your case), the Earth won't curve away fast enough.

For the orbit to be synchronous outside of synchronous orbit, energy will need to be expended to "make up the difference." Since this is a fantasy world, magic can easilly fit that bill. Or you can have it in it's proper place and simply have the atmosphere go all the way up.

Moon Orbits

phantasy

I write weird stories.

alleycat

Still around

GHO57

Hoplite

Return of the Coffee Shield

Kevin Nelson

Aspiring to authorship since 1975

wendymarlowe

writer, mother, geek

phantasy

I write weird stories.

GHO57

Kevin Nelson

Aspiring to authorship since 1975

phantasy

I write weird stories.

Kevin Nelson

Aspiring to authorship since 1975

blacbird

TheNighSwan

bookworm92

Brutal Mustang

Loves interplanetary chaos.

skylark

Drachen Jager

Professor of applied misanthropy

phantasy

I write weird stories.

NeuroGlide

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