Defining a moon

[Robert Zwilling]

There is a something called the Roche limit which shows how close a moon can get to a planet before it gets torn apart by the planet's tidal forces that deform the moon. The denser the moon, the closer it can orbit the planet before breaking apart. A less dense material deforms a lot easier than a very dense material. A ice moon would fly apart. The Roche Limit does not depend on the size of the objects, but on the ratio of densities.

Two equal size bodies of the same mass can get much closer before reaching the Roche Limit. I used copilot to do the math for two earth size bodies of equal mass orbiting 20,000 miles apart. I figured copilot could do a better job tackling the math, which it did, but the explanation it gave in English didn't match the math. The math for this equation gets a lot simpler with equal size, equal mass bodies. If copilot is correct then


the Roche limit for two equal-sized bodies, such as Earth-sized planets, can be approximated by:
d≈2.44×R
where (R) is the radius of each planet. For Earth, the radius is about 6,371 kilometers. So, the Roche limit would be:
d≈2.44×6,371 km≈15,540 km
Since 20,000 miles is approximately 32,187 kilometers, this distance is well within the Roche limit. Therefore, the tidal forces would be extremely strong, and the planets would likely be torn apart.
If you have any more questions or need further clarification, feel free to ask!

I said copilot was wrong, and it replied:

The discrepancy occurred due to an error in my initial calculations and interpretation of the Roche limit. Here’s a breakdown of what happened:

Initial Calculation: I correctly calculated the Roche limit for two Earth-sized bodies using the formula (d \approx 2.44 \times R), where (R) is the radius of Earth (approximately 6,371 kilometers or 4,000 miles). This gave a Roche limit of approximately 15,540 kilometers (or 9,760 miles).
Conversion Error: When converting 20,000 miles to kilometers, I correctly noted that 20,000 miles is approximately 32,187 kilometers.
Misinterpretation: The error occurred when I incorrectly stated that 32,187 kilometers was within the Roche limit of 15,540 kilometers. In reality, 32,187 kilometers is well beyond the Roche limit, meaning the two Earth-sized bodies would not be torn apart at that distance.
I apologize for the confusion and any inconvenience caused by this mistake. Thank you for bringing it to my attention. If you have any more questions or need further clarification, feel free to ask!

According to copilot the earth moon would be traveling around the earth at 11,000 miles per hour. Going around a 120,000 mile orbit, it would take around 11 hours to do one orbit.

 

[Brian G Turner]

The problem I see is that someone is going to read that and later quote that number as a fact that they read in a book - and it won't be.

Oh, I'm not re-inventing the wheel - there are a lot of generalized terms in astronomy with no official designation, and use of the term "moon" is no different. In fact, the term "moonlet" has become common as well for designating smaller moons, to try and differentiate them from the larger ones. This is especially because, as VB mentions, every piece of rock in Saturn's rings could theoretically be referred to as a "moon", which is nonsensical.

IIRC I think it was in one of Phil Plait's columns - who has been very active in communicating astronomy to the public - where I first read the suggested distinction I'm using, that of only classing an object as a "moon" if it is around 250 miles (400 km) across - as that's when gravity will generally pull a solid body into a rounded shape - though there has to be some leeway to account for differences in density and any damage from impacts. I doubt he originated that idea, though, and is simply repeating a concept already familiar among astronomers. As I said, though, the real test will be when I cover the gas giants - Jupiter has over 160 satellites currently considered as "moons" to date, but may of these are just rocks a few miles across, and have no significant influence on the Jovian system overall and are effectively just captured asteroids - some of which will have unstable and impermanent orbits.

The book I'm writing is simply part of a wider conversation about stars, planets, and the origins of life, and hopefully will be a very interesting one. Especially because it appears the very latest research into these areas doesn't appear to have hit the public's consciousness yet. Heck, a lot of the science press releases I read are still using theories and models that were already considered outdated yeas ago. So I suspect many people will find it very eye-opening indeed.

 

[Swank]

IIRC I think it was in one of Phil Plait's columns - who has been very active in communicating astronomy to the public - where I first read the suggested distinction I'm using, that of only classing an object as a "moon" if it is around 250 miles (400 km) across - as that's when gravity will generally pull a solid body into a rounded shape - though there has to be some leeway to account for differences in density and any damage from impacts. I doubt he originated that idea, though, and is simply repeating a concept already familiar among astronomers.

A "suggested distinction" isn't a definition. If you offer it as a definition rather than an opinion you aren't writing a book of facts.

I think the 250 mile = round rule is interesting in and of itself, and worth an explanation. But it isn't what defines a moon in the science of astronomy.

 

[Brian G Turner]

A "suggested distinction" isn't a definition. If you offer it as a definition rather than an opinion you aren't writing a book of facts.

I think the 250 mile = round rule is interesting in and of itself, and worth an explanation. But it isn't what defines a moon in the science of astronomy.

As I've said, there is no official definition of a moon in astronomy, so astronomers use various terms such as "moon" and "moonlet" and even "secondary planet" to refer to objects that might otherwise be generally be referred to as moons. So I'm not sure how you can object to astronomers not using a definition that doesn't exist.

 

[Ambrose]

The IAU does not define the term, but it does speak of satellites.

 

[Swank]

As I've said, there is no official definition of a moon in astronomy, so astronomers use various terms such as "moon" and "moonlet" and even "secondary planet" to refer to objects that might otherwise be generally be referred to as moons. So I'm not sure how you can object to astronomers not using a definition that doesn't exist.

If there is no official definition, and you say there is a precise definition, how is that not misleading your readers?

We aren't even debating astronomy at this point but what constitutes a statement of fact in a non-fiction book.

 

[paranoid marvin]

Whatever definition you come up with, there will always be, "But what about..." It's pretty clear that an asteroid is not a moon, but it is 'a natural satellite orbiting a planet'.

If your book sets out to discuss what constitutes a moon, what sets it apart from other heavenly bodies, then I don't think anyone would be under any misapprehension. The same applies if you state that 'for the purposes of the book' such-and-such will be deemed as a moon.

Whatever definition you apply is never going to be universally accepted as fact; but then, I don't think you expect it to be.

 

[Robert Zwilling]

I think it will be a very interesting book. Once you get past Mars the amount of material available is scattered and not everyone is on the same page. I mostly thought of the Moon as being typical, but it is only typical of a relatively unremarkable moon compared to what many of the other moons have to offer. Picking a size is a good way to establish a cutoff point so you don't get swamped by hordes of moons. You could even give that size and up some kind of name.

 

[Brian G Turner]

If there is no official definition, and you say there is a precise definition, how is that not misleading your readers?

We aren't even debating astronomy at this point but what constitutes a statement of fact in a non-fiction book.

I think you misunderstand - I mentioned a page ago that I had abandoned any attempt to try and provide a precise scientific definition of a moon:
https://www.sffchronicles.com/threads/587433/page-2#post-2724758

What I'm using instead is a guideline borrowed from the astronomy community. If you have a problem with that, take it up with them.

 

[paranoid marvin]

I think it will be a very interesting book. Once you get past Mars the amount of material available is scattered and not everyone is on the same page. I mostly thought of the Moon as being typical, but it is only typical of a relatively unremarkable moon compared to what many of the other moons have to offer. Picking a size is a good way to establish a cutoff point so you don't get swamped by hordes of moons. You could even give that size and up some kind of name.

Good idea.

The moons of our solar system are actually a lot more fascinating than the planets, which (other than ours) appear to be dead balls of rock, ice and/or gas. It's conceivable that some form of life exists there, or at least evidence of previous life.

Our own Moon is like a shield that may be the reason for life existing on Earth; it is the one place outside of our world where man has set foot, and the most likely place that a permanent settlement will be built. And along with the Sun, has been the foundation of our hopes and dreams since we first looked up into the night sky.

 

[BAYLOR]

There a theory at one point in early Earth history, there might have had two moons in the sky.

 

[Foxbat]

Am I the only one that's pretty much flabbergasted that we've sent people into space, taken photos of galaxies billions of light years away and yet we still can't define a moon?

 

[Swank]

Am I the only one that's pretty much flabbergasted that we've sent people into space, taken photos of galaxies billions of light years away and yet we still can't define a moon?

Yes, hopefully you are the only one. The moon is that thing in our orbit. All those other things are natural satellites. You can refer to them as "moons", but you can also call a gyro a wrap instead of a sandwich.

Just words, not ideas.

 

[Dave]

Everyone and their dog calls the large satellites of planets, "Moons," and they have done for a very long time. I had academic astronomy books written in the 1950-1970's that called the satellites of Mars, Jupiter, Saturn and Neptune, "Moons," and a quick search online shows that many still do. Isaac Asimov's book, Lucky Starr and the Moons of Jupiter was published in 1957, and planets having "moons" is a staple of Science Fiction. I don't know exactly how Galileo referred to the four Jovian satellites in Siderius Nuncius, but astronomers called them the Medicean Stars throughout much of the seventeenth century following their discovery, even though they were well aware that they weren't stars, but satellites of Jupiter.

But there is disagreement over the "size" - some astronomers will only call the four Galilean Satellites, "Moons", while others will call 92 to 95 of them "Moons". There is no agreed definition. I believe that is what Brian means to address.

 

[Venusian Broon]

Am I the only one that's pretty much flabbergasted that we've sent people into space, taken photos of galaxies billions of light years away and yet we still can't define a moon?

Whattcha mean! It's the same thing when you ask: when does a pond become a lake? There is no hard and fast definition there either. And we've been dealing with bodies of water a lot longer

Really there is no real deep need to know exactly what a moon is. Sure, there's probably untold bits of thumb-size gravel bits of stone, naturally orbiting every planet in the solar system. And by the most basic definition of a moon that @SilentRoamer put at the start of the thread...you could maybe call every one of them "moons".... but no one is going to call a bit of orbiting grit a moon.

So there's an issue with natural objects that are of very low astronomical mass - where you put the line between moon/moonlet/orbital space weather. Similarly when the mass of the objects start to get comparable, when do you start calling it a double planet system and not a planet and its moon?

 

[Dave]

when does a pond become a lake? There is no hard and fast definition there either.

Ahem! Yes there is a definition, (supposedly.) A Pond has no river flowing out of it (it just soaks into the ground.) A Lake is drained by a River.

Except that people don't stick to that definition. Both Frensham Little Pond and Frensham Great Pond in Surrey have streams that drain them.

Then you also have huge Pans (that dry up and fill up again without a river draining them) that should be Ponds by that definition and even inland Seas.

So, nothing hard and fast, and I think that with any definition, nature would always throw up some example of a waterbody that would not fit. Maybe that is also true for Moons too?

 

[Venusian Broon]

Ahem! Yes there is a definition, (supposedly.) A Pond has no river flowing out of it (it just soaks into the ground.) A Lake is drained by a River...I think that with any definition, nature would always throw up some example of a waterbody that would not fit.

You mean like Lake Chad?

Well, that didn't take long to cause problems with that definition

The Earth is full of endorheic basins and the resulting lakes that are formed by rivers flowing and being "trapped" at the bottoms of these areas.

 

[Swank]

There is no disagreement about when to use the word that affects orbital mechanics, planetary formation or geology.