# Two Science/Astronomy questions



## Paladin (Jul 5, 2014)

Here's my two Science/Astronomy questions... I guess they're each actually a collection of questions. 



1. I was just wondering if anyone might know how this would work realistically.  If a planet had 4 moons, what sort of effects would this have on the planet?   I'm no astronomer but I'm guessing it would depend on the sizes of each moon, their proximity to the planet... and maybe the planet's own size as well? 

 I've been considering the idea for a while now, to give my fantasy world 4 moons... I think I'd want them reasonably close to the planet,  so they would appear much bigger in the sky than Earth's moon does.  I'd probably want 2 of them to be visible by day and 2 visible by night.  Maybe on certain days of the year, 3 moons or even all 4 moons would be visible in the sky at once... at dawn or at dusk (meaning the opposite side of the planet would see no moons).  I'm just hoping this could be something that would work out realistically in actual science without causing crazy stuff to happen to the planet. 

Also, how realistic would it be to have the moons as different colors... like... one red, one golden etc.  is there any example for this in actual Astronomy or would readers just have to suspend disbelief here? 



2.   I've always planned on my fantasy world's planet to be twice the size of Earth. I'm wondering how realistic this would be though.  As I said, I'm no Astronomer or Scientist... so I really have no idea.  I know I should probably do some research on it and I will but I figured I'd ask on the forum here first to see if anyone could give me any helpful answers about this. 

I'm guessing a planet twice the size of Earth would have much longer days and nights? Unless it rotates faster than Earth?  If I go with the faster rotation, to cause days and nights to be more similar to Earth's, I wonder how this would effect the planet and it's inhabitants.  I want the world to have mostly similar temperate climates to Earth, with some dryer, arid regions as well but not many. The water/land ratio would be similar to Earth too. 

Is there anything else I should know or consider about such a large planet and how the 4 moons would effect it? 

Thanks for all the help!


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## psychotick (Jul 6, 2014)

Hi,

First having multiple moons is completely possible, but the more moons you have the greater the chance of them interfering with one another gravitationally speaking. And the greater the chance that their effect on things like tides would cancel one another out so you would either get negligible tides or tides that are far harder to predict. You could occasionally get the equivalent of king tides where three or four ofthe moons are lined up and all pulling in the same direction at the same time. I would expect with multiple moons that the moons would all be smaller than ours because the larger the moons the greater the chance they would interfere with one another.

Yes the planet being twice the size of Earth could have an Earth length dy - by spinning at exactly the same rate - i.e. one turn per Earth day. As to how this would affect things I don't know. Some thoughts would be that the Coriolis effect might lead to more extreme weather issues and greater polar differentiation in terms of atmosphere. Also for space craft landing and taking off it would mean that their re-entry speeds relative to the atmosphere and ground would have to be greater.

A bigger issue though is gravity. If this world was twice the size of Earth but gravity was equivalent, then presumably the planet would be less dense - especially the crust. This would mean that heavier elements in the crust such as mineral like iron would probably be less common. If gravity increased accordingly, they would be equally common on average, but there would be other differences in dealing with higher gravity for the inhabitants. Lighter bodies, greater muscle strength, and would birds be able to fly etc? Also I would wonder about the evolution of creatures from the sea. This would surely have been a harder, slower process.

Cheers, Greg.


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## ThinkerX (Jul 6, 2014)

Hmmm...

If you had one larger moon - comparable in size to earths moon - then you might be able to get away with a much smaller moon at each 'Trojan Point'.  That would account for three of your four moons, though two would be much smaller than you want, and given the separation between them, likely just one (maybe two?) would be visible at any given time.  This arrangement, though would almost certainly be artificial - the moons at the Trojan points would be there because somebody placed them there.

As to the fourth moon...introducing this into the mix and expecting long term stability is not realistic.  Maybe Moon Four is an interloper captured by your planet fairly recently.  Hmmm...

1) Oldest records/legends would have there was only one moon.

2) 'Age of the Gods' (Aliens?) placed moons at the Trojan points, possibly as a sign of their authority (or maybe to act as relay stations of some sort).  Aliens/Gods need not still be around, of course.  Tied in with the beginning of history as such on your world, founding myths, that sort of thing.

3) Moon Four appeared recently, within the past few hundred years.  Lots of appropriate dark omens, that sort of thing.  Erratic orbit.  A few hundred more years and it will either be evicted from the system or crash into the planet or one of the other moons.


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## Jesse (Jul 7, 2014)

Paladin said:


> 1. I was just wondering if anyone might know how this would work realistically.  If a planet had 4 moons, what sort of effects would this have on the planet?   I'm no astronomer but I'm guessing it would depend on the sizes of each moon, their proximity to the planet... and maybe the planet's own size as well?



Yes.  Technically, one planetary body doesn't orbit the other, they both orbit a point directly between their two masses.  This is how early exoplanets were discovered - by the wobble a large gas giant creates as it pushes and pulls on its star.  Four moons and a planet would work the same way, though in a far more complex arrangement, especially since you're talking about a terrestrial-sized planet.  The way that Saturn or Jupiter keep their moons in check is by being so very much more massive than their satellites that the combined planet/moons center of mass is well within the surface of the host planet.  In a situation with five bodies that are all much closer in mass, I would think that long-term stability would be impossible and either one or more body would wind up being ejected from the system or else two or more would eventually collide.



Paladin said:


> I've been considering the idea for a while now, to give my fantasy world 4 moons... I think I'd want them reasonably close to the planet,  so they would appear much bigger in the sky than Earth's moon does.  I'd probably want 2 of them to be visible by day and 2 visible by night.  Maybe on certain days of the year, 3 moons or even all 4 moons would be visible in the sky at once... at dawn or at dusk (meaning the opposite side of the planet would see no moons).  I'm just hoping this could be something that would work out realistically in actual science without causing crazy stuff to happen to the planet.



Our own moon is actually QUITE large given the size of our planet.  I'm afraid that for your idea to work, the moons must be notably smaller than the planet so that the center of gravity would still be within the planet itself.



Paladin said:


> Also, how realistic would it be to have the moons as different colors... like... one red, one golden etc.  is there any example for this in actual Astronomy or would readers just have to suspend disbelief here?



This is your most realistic question.  Most of our planets in our solar system have varying chemical makeup and colors as a result.  Hell, you can see that Mars is read with the naked eye!  The differences will come from the abundance of metals at the surface and the environment at the surface (i.e., oxidizing or reducing) A great deal of oxidized iron would make a moon red.  Reduced iron would be brown to black.  Oxidized copper would be green  Excesses of sulfur could make it yellow. Loads of basalt like our moon has would make it white.   Some Google searches of geology and various metals will give you more examples, and then making up a little history for each body might help with consistency.



Paladin said:


> 2.   I've always planned on my fantasy world's planet to be twice the size of Earth. I'm wondering how realistic this would be though.  As I said, I'm no Astronomer or Scientist... so I really have no idea.  I know I should probably do some research on it and I will but I figured I'd ask on the forum here first to see if anyone could give me any helpful answers about this.
> 
> I'm guessing a planet twice the size of Earth would have much longer days and nights? Unless it rotates faster than Earth?  If I go with the faster rotation, to cause days and nights to be more similar to Earth's, I wonder how this would effect the planet and it's inhabitants.  I want the world to have mostly similar temperate climates to Earth, with some dryer, arid regions as well but not many. The water/land ratio would be similar to Earth too.



The size of planet would have little effect on the length of the day.  Again, our own moon is largely responsible for our length of day.  It's thought that early in our planet's history, our day was more like 6-8 hours, and the tidal drag from the Moon has slowed it down.  I hadn't considered the Corialis effect, but it's probably true.  The crust accounts for very very little of the overall mass of the planet, so I'm not sure that the mineral content at the surface would necessarily be vastly different.  What COULD be different would be the core.  With twice the size but equal mass, there likely wouldn't be as much iron/nickel at the center, and that would make for a weaker ionosphere, exposing the surface to more of the Sun's moon swings.  Solar flares, for example, could be devastating and could sweep the atmosphere away (like it did to Mars).

A planet twice the size (by twice, are you saying twice as massive, twice the diameter, or twice the surface area?) of Earth would experience twice the solar gain, given the same orbit and the same sun.  This would make it warmer and strengthen weather patterns.  

One cool thing you could do would be to increase the density of the atmosphere.  That makes things more buoyant, and so birds could still fly even given extra gravity.  I saw a show where scientists suggested floating cities above Venus that take advantage of the super-dense atmosphere.


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## Cloud (Jul 8, 2014)

Works most simply if the small moons are much smaller - 'moonlets' that are closer to the planet. Look up Phobos and Deimos around Mars for an example in action. Put them in staggered, elliptical orbits at different ranges and there can be nights when all four moons are in the sky, or nights when just one is (gives you most scope for the omens related to different combinations?)

When you say a planet twice the size of Earth, do you mean twice the diameter, or twice the surface area? Because on a sphere you can increase surface area more than you might expect with a smaller percentage increase...


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## thecoldembrace (Jul 8, 2014)

I have 4 moons in my world... well technically 5, but the fifth one can only be seen at very rare times. I also have my moons as having different colors, white, red, blue and green. 

My world is slightly larger than earth and each moon is slightly smaller than our own. Each orbit at a different rate allowing them to shine in the sky at specific times which influences certain religious practices. 

Its believable because it has been done before in other fantasy writings. Dragonlance is one instance and it worked very well, and influenced the world to a degree. You don't have to take in massive accounts of science to make yours believable, and most readers will read about the moons, and move on as if it is no different than our own. Only if you want to make them do extra things like change the tides and so forth do you need to understand how each might influence the world. For the most part I've learned that readers are already reading a fantasy story and so they expect things to be different or alien, so it won't shake their belief very much if at all for more moons than one.

There are also great moments when you can use these to benefit the story and establish more wonder and awe, especially on nights when all the moons are lit up or aligned. Have fun with it and trust that it probably won't bother the majority of people.

-Cold


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## Paladin (Jul 9, 2014)

Thanks everyone for all the detailed answers!   I really appreciate the help and suggestions!

It sounds like the four moons idea may not work so well from a highly realistic perspective.  But, as Cold said, I know the highly realistic/scientific perspective isn't always something a fantasy writer has to get too concerned about.  So I may just opt not to worry about how realistic 4 moons maintaining their orbit would be or what detrimental effects they may have on a planet. 

To be honest I'm still working the idea out and figuring out how it would link up to the rest of my fantasy world.  I may scrap the idea altogether if I can't come up with a good reason for the moons to be there.  But I do like the idea of having 4 different colored moons.  I think I'd like them to link up to my elemental magic system somehow. Each moon sort of influencing a different element, perhaps. 

My fantasy world doesn't have gods per say, so gods couldn't be responsible for the moons.  Aliens is a possibility I'd have to consider.  I imagine the moons would have probably been put in their orbits by some sort of artificial means though, either by magic or very advanced tech.  My fantasy world is a very magical place, so magic is definitely an option. 

My fantasy world will mostly remain in a medieval-ish age. I don't really have any plans to advance them to a future/space exploration age. So there won't be any speceships or advanced scientific understanding really.


Ideally... I'd probably want the moons to appear about this size in the sky: http://lparchive.org/Chrono-Cross/Update 17/28-dump94.jpg 

They might not need to be that close all the time though. Perhaps their orbits take them further away from the planet at certain times of year, making them appear smaller in the sky.  It doesn't really matter to me how large the actual moons are as long as they can appear large in the sky.  So, realistically, I imagine even a smaller moon that was sufficiently close enough to a planet would probably still appear about that size in the sky. 





Jesse said:


> A planet twice the size (by twice, are you saying twice as massive, twice the diameter, or twice the surface area?) of Earth would experience twice the solar gain, given the same orbit and the same sun.  This would make it warmer and strengthen weather patterns.





Cloud said:


> When you say a planet twice the size of Earth, do you mean twice the diameter, or twice the surface area? Because on a sphere you can increase surface area more than you might expect with a smaller percentage increase...




Yeah, I mainly just want the planet to have twice the surface area. Basically one half of its surface area could fit the entire surface area of Earth.  I don't think days and nights have to be exactly the same as Earth's either. I'd probably give them somewhat longer days and nights.  A 30 or 40 hour day/night cycle would be interesting.  Just to make things a little simpler, I could say the inhabitants evolved somewhat differently too, so they don't need to sleep as often as we do.  Allowing them to take full advantage of those 15+ hour days.  I could give them a weaker sun too, maybe. Certainly don't want their atmosphere being swept away by solar flares.  Increasing the density of the atmosphere could work too but I wouldn't want it to obscure the view of stars or the moons. 




thecoldembrace said:


> Dragonlance is one instance and it worked very well, and influenced the world to a degree.



Yeah, I remember those moons!



If anyone has any further suggestions or feedback they'd like to share please do so.


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## Jesse (Jul 9, 2014)

I don't know if this idea would work for you or not (and as you say, this is Fantasy, not Sci Fi, so it just may not matter at all) but if your planet were actually a moon circling a gas giant then you could see all sorts of weird interactions between the planet's other moons, and all the orbits would be gravitationally stable.  I don't know off hand how long a (year? lunar cycle?) is for a moon of Jupiter, but your daytime sky would be really odd - there would be long periods where the host planet would eclipse the sun and the other moons would come and go as Mars, Venus, and Jupiter do in our own night sky (though much much closer and so bigger).  Just a thought.

But none of Jupiter's moons are anywhere nearly as large as Earth so that may not work for you.


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## Ravana (Jul 9, 2014)

The number of moons, as others have noted, is entirely up to you–Jupiter has 67 known satellites at this point, so four is hardly excessive. So are the sizes, though if you don't want science geeks jumping on your case, you should do at least a bit of research on what is more or less likely. That having been said, we are currently in a period of observational astronomy in which we are almost yearly spotting things we previously thought either impossible or so unlikely they'd never develop in a real, stable system.

Note, however, the presence of the word "stable." There's no reason your system needs to be. So what if one or more of the moons is inevitably going to crash into the planet, or be ejected from orbit, in a mere half a billion years? Your story just takes place before it happens, is all. If you aren't comfortable with the notion of your planet cooling enough to allow evolution to take its course in that short a time (which, yes, would be high up on the "improbable" scale), one or more of the moons could always be captured bodies, which formed separately from the planet they now orbit. There are good reasons for us to believe that Luna is just such a capture… sort of. If those theories are correct, and we'd "captured" it at even a marginally sharper angle, we wouldn't be here talking about it right now.  Some other moons, such as Triton (around Neptune), are almost certainly captured, given the difference between the direction of their orbits and their primary's rotation.

Option two, for speeding the evolution of either the system or its inhabitants, is that magic accelerated things. And never forget my oft-repeated maxim/warning concerning the divine: "With gods, all things are possible." That is to say, if the gods want your world a certain way, that's the way it is, no further explanation required. You've already recognized both possibilities.

As for the size of the planet, Jesse is correct in saying you could adjust its density. Earth is actually the densest of the solar system's large bodies, narrowly edging out Mercury and Venus. It wouldn't take much to create a larger body with a lesser density and end up with the same surface gravity. While you could increase land surface area by simply saying the oceans are smaller, as you suggest, if you are really concerned with a realistic world, you should be at least peripherally aware of how distance from large bodies of water affects climate: if the oceans are too small, you'd likely end up with large areas of fairly useless terrain (steppe, desert, tundra), which presumably would defeat your purpose. (As an aside, keep in mind that in pre-technological eras, water was the most efficient means of transport; if magic can't take its place, you could end up defeating your purpose simply by ending up with everything too far apart to have meaningful contact.)

The length of day is entirely independent of the size of the planet: Jupiter rotates more than twice as fast as Earth does. You can make it whatever you like, without contemplation of involved physics. (Unless you really wanted to go into that kind of depth: the age of the system, the influence of gravity from the star and other bodies in the system, etc. Even then, we only have one good model to work from–our own system–so it's difficult to say exactly what those numbers "ought" to be.)

One place you _would_ end up with problems, based on your original desiderata, is how many moons appeared in the sky at any one time… since their orbital period are also (partially) independent variables. Barring some major contrivance, there will not be a regular number of moons in the sky at any given time. One reason I say these are "partially" independent variables is because in the solar system's outer, satellite-happy planets, many of the bodies are in "orbital resonances" with one another–that is, the orbit of one body is a multiple of the orbit of another. Jupiter's three inner large moons show this: Io, Europa, and Ganymede are in a 1:2:4 resonance, meaning that Europa's orbital period is exactly twice that of Io's, Ganymede's four times that (and thus twice Europa's). This only affects how those moons appear relative to one another, though, and not how they correspond to the planet's rotation; the only way you would have one or more moons appearing consistently during the day or night would be to have the moon's orbital period exactly match the planet's rotation. Which is possible; it might give the appearance of being a bit contrived, but there are potential astrophysical justifications for it.

One additional possibility you might wish to consider is to have a moon that is _much_ more massive than normal relative to its primary… possibly even close to the same mass as the primary. That would put both bodies orbiting around a common point somewhere between them. The Pluto/Charon system provides one example for this: the barycenter of their mutual orbit lies between the two. And there are five other objects orbiting Pluto as well… or, rather, orbiting the barycenter of the two larger bodies, effectively orbiting the pair (though they are still regarded as satellites of Pluto). Outside the solar system, there are numerous stellar bodies with such orbits (binary, etc. stars), some with more distant stars orbiting the inner ones, some with observed planets orbiting the pair–one of those surprises I mentioned earlier: this was previously regarded as unlikely to show up in a stable system. So at least in terms of what's possible, you'd be on safe ground here.

Upshot: my inclination would be to make your system look like you want it to, only worry about the most blatant of physical causes/effects, and gloss over everything else. Some readers might find your system "unlikely," but few would be able to claim on any solid grounds that it would be "impossible."


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## Terry Greer (Jul 9, 2014)

You need to do a bit of research - of course you could have two large planets orbiting each other with a center of mass outside either:

Double planet - Wikipedia, the free encyclopedia

One planet in science fiction even had them so close that they shared a common atmosphere:

Rocheworld - Wikipedia, the free encyclopedia

The roche limit comes into play if orbiting bodies get too close which will shatter bodies that get too close due to tidal forces.
(Note moons need probably to be considered more or less fluid in the examples on the link below).

Roche limit - Wikipedia, the free encyclopedia

There's nothing to stop any number of moons being present in your sky (look how many Jupiter has), but they couldn't reasonably all be large (if you want to be reasonably accurate). 

The Image you post is only possible really for a single moon, not multiple ones.

Jesse's suggestion of the planet being a moon itself around a larger body is a really good one to get all sorts of weird interactions and timings.

However - all the above assumes that the orbits are stable and the world is safe in geological time. If it isn't, and if the world/moon orbits are relatively new (perhaps a world that's recently been captured) then you could have that as a prelude to armageddon a few years or centuries down the road when one or more will collide or break up into a ring system (generating millions of asteroids and bits of debris.


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## Paladin (Jul 11, 2014)

Ravana said:


> Upshot: my inclination would be to make your system look like you want it to, only worry about the most blatant of physical causes/effects, and gloss over everything else. Some readers might find your system "unlikely," but few would be able to claim on any solid grounds that it would be "impossible."



Yeah, sometimes I think I get a little too caught up in trying to make things as realistically plausible as possible... for credibility's sake, I guess.  Which, honestly, can probably hurt a fantasy story/setting at times... or at least hold it back.   I'm not trying to  write a science textbook after all.  I've always believed that one of the major points to fantasy stories and  settings is to explore your dreams and unrealistic ideas and do things that probably couldn't be done in reality as we know it.  To really challenge the limits of your imagination and see just how far you can go.  So, yeah... if I want 4 moons and think it would enhance my setting... then I should just go for it. 




Jesse said:


> I don't know if this idea would work for you or not (and as you say, this is Fantasy, not Sci Fi, so it just may not matter at all) but if your planet were actually a moon circling a gas giant then you could see all sorts of weird interactions between the planet's other moons, and all the orbits would be gravitationally stable.  I don't know off hand how long a (year? lunar cycle?) is for a moon of Jupiter, but your daytime sky would be really odd - there would be long periods where the host planet would eclipse the sun and the other moons would come and go as Mars, Venus, and Jupiter do in our own night sky (though much much closer and so bigger).  Just a thought.
> 
> But none of Jupiter's moons are anywhere nearly as large as Earth so that may not work for you.



Yeah, this is definitely a good idea but probably wouldn't work for me as I would want my fantasy world planet to be much bigger than a moon.  I would also want their sky to look more or less... Earth-like.  

It got me thinking though...  I wonder if it would be possible for an actual planet to be a moon, basically.  After all, planets orbit stars. I would think it's more or less the same thing... though I know of course, this is because stars are incredibly more immense and powerful than a planet. 

Maybe an Earth-sized planet could be the "moon"  of a super-sized planet though.  Probably a planet much bigger than Jupiter even. I imagine such a large planet would probably block it's "planet moons" from their Sun  for long periods of time though.  Hmmm... unless their were two Suns maybe? 

This is probably not an idea that would work for my setting either but I think it's still a pretty interesting idea.


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## Terry Greer (Jul 11, 2014)

Paladin said:


> It got me thinking though...  I wonder if it would be possible for an actual planet to be a moon, basically.  After all, planets orbit stars. I would think it's more or less the same thing... though I know of course, this is because stars are incredibly more immense and powerful than a planet.



Absolutely - a moon is just a name for a body that orbits a planet. It has nothing to do with size. Your world could be as big as Earth, just orbiting a much bigger body such as Jupiter (or even larger).

Personally I like believable details in stories. If fantasy takes too many liberties it breaks it completely for me. Even fantasy has to be logical in that it has a set of rules it adheres to. Sure in fantasy you don't need to give the sort of detail you get in hard SF, but it helps if you have a clear model for the universe you're creating.


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## Trick (Jul 11, 2014)

Having a good explanation and analysis behind things like multiple moons and other scientific things in a work of Fantasy can be helpful but it's not like, after you've worked your butt off proving it could really happen, you're going to have a whole chapter of info dumb about how it works. I personally don't mind if the only explanation is, "The gods made it that way." In the real world we can choose whether or not to believe in a higher power but as the author you are the higher power for your world. The characters may or may not believe but if you say the gods made it that way, then they did.


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## Ravana (Jul 12, 2014)

Paladin said:


> Yeah, sometimes I think I get a little too caught up in trying to make things as realistically plausible as possible... for credibility's sake, I guess.  Which, honestly, can probably hurt a fantasy story/setting at times... or at least hold it back.



Oh, I'm all for keeping the elements of a fantasy story as close as possible to reality: it makes those elements which aren't realistic more plausible. It's just that most other fantasy writers _don't_ want to go to all the trouble of working out such things as n-body orbital problems… well, neither do I, actually: kinda draw my line at that one.  Apart from that, most of what you want to do can be done without worrying yourself overly much.

I do agree that the image you linked would probably not represent a possible scenario, barring major magical intervention. The moons appear too near to the planet for me to think of any way they could look that size, even if their density were far lower than that of the planet, unless they were part of an extremely unstable and short-lived (in geological terms) system. Easiest fix for that is to make them look a wee bit smaller than they appear there, of course. I don't think that, say, a quarter of that would be unreasonable, if they were low-density snowballs–the nearest, at least, since apparent size drops off pretty rapidly with distance… but I'll leave you to do the math, since it ain't my world and I'm too lazy at the moment. Heh.

You'd asked about colors, and I realized I forgot to address that. Various surface colors are entirely possible–I refer you again to Jovian moons–but with one caveat: those pics were taken by awesomely good cameras at very close distances. The farther away you are, the more the object will tend to look some shade of white, perhaps tinted with another shade for very high concentrations of a particular material on the surface: iron oxide makes Mars look pinkish-orange even from Earth; sulfur would make something the distance of Luna look yellow-orange (see Io). If the moon happens to have an atmosphere for whatever reason, that might provide a touch of color, though in most cases this would only be noticeable along the rim of the disc, and only when backlit: few would be thick enough when viewed head-on to matter, or else would look like clouds, leaving you back in the range of white again–though Titan is a notable exception: it, too, looks yellow, courtesy of an atmosphere so thick we can't actually see through it even up close. Liquids on the surface could be quite another matter, though you may be hard-pressed to explain their presence, depending on surface gravity and temperature (which in part depends on atmosphere); ices would tend toward white, though they could be more tinted than some of the other possibilities, depending on what they're composed of. What might be even cooler than colors is the variety of surface features which might be visible… for which you'd want to look at the largest twenty-odd outer planet moons, just to get an inventory of what we've already seen, let alone what imagination might suggest. The Voyager missions showed us a _lot_ of stuff we weren't expecting at the time.

If you want to see a really fascinating example of what natural satellites can do, check out Saturn. It has moons in orbital resonances, two moons with smaller moons at both their trojan points, one (Titan) with an atmosphere and surface liquids (and which is also larger than Mercury), at least one moon with cryovolcanoes, moons–probably captured–with highly inclined or retrograde orbits.… The reason I bring Saturn up, though, is something I remembered after I wrote my previous post, which may or may not be of use to you: it also has a pair of moons, Janus and Epimetheus, which actually _trade orbits with one another every four years_. Which is really bizarre. They are, admittedly, far smaller than the size you want, but are comparatively near one another in size–the regular orbit-swapping couldn't take place if they weren't–and in spite of never coming near enough one another to collide, certainly come near enough one another that they'd make quite the visual impression, as this picture shows:

FileIA08170 Epimetheus and Janus.jpg - Wikipedia, the free encyclopedia

You'd want to scale things up, of course, but that's no problem.…



> It got me thinking though...  I wonder if it would be possible for an actual planet to be a moon, basically.  After all, planets orbit stars. I would think it's more or less the same thing... though I know of course, this is because stars are incredibly more immense and powerful than a planet.
> 
> Maybe an Earth-sized planet could be the "moon"  of a super-sized planet though.  Probably a planet much bigger than Jupiter even. I imagine such a large planet would probably block it's "planet moons" from their Sun  for long periods of time though.  Hmmm... unless their were two Suns maybe?



Absolutely, a moon could be large enough to be planet-like. You just need to position the planet close enough to its star that it receives adequate solar heating. Which isn't a problem: our Jovians may be well out of our system's habitability range, but we've observed Jovian and super-Jovian exoplanets nearer to their stars than Mercury is to ours.

The planet might never block the sunlight at all, if the orbit of the moon was sufficiently inclined to the plane of the ecliptic. Or at least "never" in terms of taking place within your story: it wouldn't be hard for the math to come out to every few centuries or millennia. Or it might be something which happens twice every planetary year, as the orbit of the moon brings it directly in line with the ecliptic–if, say, it was inclined at ninety degrees to the ecliptic. Or it might happen every orbit, if the moon's orbit exactly matches the plane of the ecliptic. How long it remains in shadow would depend entirely upon the speed at which the moon orbits the planet, but it isn't going to be very long unless the moon's orbit is very slow… it's an eclipse, like any other. If the moon's orbital period is similar to Luna's, the eclipse might be a couple days; if it's more like Ganymede's (a bit over seven days), it'll only last an hour or two. Even this depends on how far away the moon orbits the planet: the farther away, the narrower the cone of totality (though the greater the cone of partial eclipse). In either case, it wouldn't be enough to end life on the moon's surface: you'd need it in shadow a lot longer than that. After all, nearly every point on our planet is in shadow for several hours every day, eh?

Note that if the moon is orbiting a supermassive planet, you need to take the planet's gravity into account on a much larger scale than you do for a smaller moon orbiting a terrestrial planet… again, modified for distance. Tidal forces don't affect only liquid oceans: they can also affect tectonics. Jupiter's effect on Io gives it some truly spectacular vulcanism, for instance.

It's here that I thought the Janus/Epimetheus bit might benefit you, though: there's no reason you couldn't scale that up to where it's two planets sharing similarly close orbits around a star, instead of two moons around a planet… though that could work as well. I couldn't say offhand how near they'd be able to pass one another before their gravity started ripping them apart, rather than making comparatively minor, periodically reversible changes in their orbits, but that's just math. The precedent is there.


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## Sheilawisz (Jul 12, 2014)

Hello Paladin, thank you for starting this interesting thread.

I also have a planet with multiple moons, and I have given the moons a variety of colors as well. My planet has three moons that are smaller than Earth's Moon, but in their sky they look larger than our Moon because they are much closer to the planet.

The moons are violet, blue and white, and they are made of some kind of transparent crystals.

I have been told by a friend that this system would cause terribly high tides in my planet, and the same would happen in yours... the way to prevent that would be to have the moons following an orbital system that would allow them to counter each other's gravitational pull, so the tides would be more similar to Earth.

The different and unusual colors are realistic, and that can be explained scientifically without much trouble simply by choosing the right elements/material that would compose every moon... I would suggest taking a look at the stunning photographs of moons like Io, Ganymede and Europa, just to see how beautiful a moon can be.

About your planet being twice the size of Earth: In case that this world is twice the Earth's diameter, you would have to give it a smaller density so that its mass and gravity would be similar to our planet. Otherwise you would have a severe gravity to deal with, and the lifeforms of your planet would be very different to us.

Your super Earth indeed would have to rotate faster in order to have a similar cycle of day and night, but I see no severe trouble with that. Maybe the faster rotation would cause the atmosphere to concentrate heavily around the equator, and the air would be very thin at the higher latitudes of your world.


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## Creed (Jul 12, 2014)

Hello,
I hope it's okay if I ask a few questions here- hopefully me and the thread-starter can benefit.
I have five moons around one of my worlds and the religious significance of them cannot be strained enough. One large, one Luna-sized (from the planetary POV), and three much smaller, with the fifth whose orbit is very eccentric and spends a lot of time out of the planet's sight (maybe a few months, a year, I'm not sure).
So like the OP they've all got different orbital periods and sometimes there might be three in the sky and sometimes there might be one. How easy would it be for the people to make predictions and calendars on their orbits? Are they likely too erratic? Would they need more than 13th century technology for that?
Hopefully Paladin finds these helpful too!

A side question for my eccentric moon: if it spends many months or even a year or more out of the planet's sight, where could it even go?


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## Ravana (Jul 13, 2014)

Sheilawisz said:


> I also have a planet with multiple moons, and I have given the moons a variety of colors as well. My planet has three moons that are smaller than Earth's Moon, but in their sky they look larger than our Moon because they are much closer to the planet.
> 
> I have been told by a friend that this system would cause terribly high tides in my planet, and the same would happen in yours... the way to prevent that would be to have the moons following an orbital system that would allow them to counter each other's gravitational pull, so the tides would be more similar to Earth.



[Warning: math follows.  ]

There is a tendency among people who like to appear to know what they're talking about to automatically say that multiple moons would cause high tides. Which is nonsense, in the absence of other information. 

In this case, you specify that the moons are closer to their planet, but smaller than Luna. Which is two of the three basic parameters you need to consider prior to taking the individual orbits into account; the third is density. Taking each separately:

(1) Gravity drops off with the square of the distance. So the nearer they are, the more they affect the planet. Note that they can't _all_ be the same distance, unless two of them are much smaller than the third and are orbiting at trojan points, or unless the system is completely artificial (divinely created, etc.), in which case you're also licensed to throw away any other considerations and make things however you want them.

(2) _Apparent_ size is a function of distance and the diameter of the body. And a refreshingly simple, 1-for-1 function, at that: if object A is twice as large as object B, and is twice as far away, A and B will appear to be the same size. So if you want a moon to be smaller than Luna but appear to be larger, it will need to be closer to the planet by whatever amount you want to increase the apparent size.

(3) Density is pretty much a free variable for you–that is, you can make your moons as dense or diffuse as you like, so long as you don't take the numbers into impossible ranges (look them up: too much to go into here).

Now… what determines the effect of gravity is the moon's mass–which is determined by multiplying density and _actual_ size. For a sphere, the volume formula is

4/3πr[SUP]3[/SUP]

(Don't panic: the math won't get any uglier than that.)

So, to pick a number out of the air–or the sky, at least: if a sphere had a diameter of 1,700 km (that is, about that of Luna, rounding down a tad: I will continue to round off to two places to keep things neat), cube that and get 4,910,000,000, times pi is 15,400,000,000, times 4/3 is 20,50,000,000 cubic km.

Now multiply that by density… which, unfortunately, is usually expressed as g/cm[sup]3[/sup], somewhat inconvenient when your volume is expressed in km[sup]3[/sup], but there you are. Luna's is 3.35 g/cm[SUP]3[/SUP]. A kilometer is 100,000 times as long as a centimeter, so a cubic kilometer is 1,000,000,000,000,000 cubic cm; convert grams to kilograms and you can shed three of those zeros, and you end up with a mass of 3.35 trillion kilograms per cubic kilometer, times volume is a total mass of around 6.87 x 10[SUP]22[/SUP] kg. (Yes, I finally surrendered to scientific notation, rather than write out 687-plus-20-zeros).

What does that mean? Well, since gravity is a square function, an object half as far away will exert _four_ times as much gravity… so that object would have to be only a quarter of Luna's mass to exert the same tidal forces. On the other hand, Luna is the second densest moon in our solar system; there are moons that are less than a third of its density (composed mostly of ice, not rock). If an object is both smaller than Luna to begin with, _and_ less dense, its mass and therefore gravity will drop like a… whatever. So a moon twice as close to the planet, but the same apparent size as Luna (yes, I know you said "larger," but I want to keep this simple as possible) would have only half Luna's diameter… and volume is a _cube_ of diameter, while gravity is only a _square_ of distance. 

Thus: 850 km diameter times etc. gives 2,570,000,000 km[sup]3[/sup]… barely a _tenth_ of Luna's volume. Even at the same density, the effect of its gravity on its planet would be less than half that of Luna's. Less still if it were of lower density.

Thus endeth the math. (And I'm praying I did it all right. On the second pass, since I already know I blew it the first time through and had to fix it.…  )

Upshot: no, the tides on your world would not necessarily be more dramatic. To go even further: unless the orbits were such that the moons lined up on a regular basis, their effects would normally counteract, rather than enhance, each other's; if the moons _never_ lined up (yes, it's possible), it could result in tides so negligible that the inhabitants might never realize there were such things. So it all depends on how you set up the system. If you want high tides–or at least occasionally high ones, when all the moons were on one side of the planet–you can have them… but you don't _need_ them.



> About your planet being twice the size of Earth: In case that this world is twice the Earth's diameter, you would have to give it a smaller density so that its mass and gravity would be similar to our planet. Otherwise you would have a severe gravity to deal with, and the lifeforms of your planet would be very different to us.



Keeping in mind the math above. Note that surface area is also a square function, of diameter (pi times diameter squared), so a planet with twice the _diameter_ of Earth would not have twice the surface area, it would have four times the surface area. So a world that was twice the size of Earth in terms of surface area would only have a diameter around 1.4x that of Earth (I'll let anyone interested work out the exact figure). You can then perform your calculations for mass, and work out how much less dense than Earth the place would have to be to provide Earthlike gravity.



> Your super Earth indeed would have to rotate faster in order to have a similar cycle of day and night, but I see no severe trouble with that. Maybe the faster rotation would cause the atmosphere to concentrate heavily around the equator, and the air would be very thin at the higher latitudes of your world.



Or you can just choose a different length for the day. Be wary of confusing your readers here: you should not expect something which happens in "a minute" to be any different than it is on Earth, so if you have a different length of day, then the day should not be divided into 1440 minutes, but rather more than that. You can leave this unspecified in all likelihood, if you like: it's rare that action in a novel is so tightly scripted that it is related minute-by-minute throughout an entire "day." Nor should an hour be anything other than 60 minutes unless you say it is–it probably would be, since the time system we use is pretty idiosyncratic (why 24 hours, and not some other number?). Divide the day into, say, 30 glizquams (okay, maybe something less stupid-sounding), and let the readers figure out it's something close enough to an hour that it won't make a whole lot of difference if they think of it that way.

-

Dang, I was going to reply to Creed here as well. But supper's ready, so I gotta go. Fortunately, that one oughtta be shorter: I'm not going to inflict more math on anyone (myself included) if I can avoid it, which it looks like I should be able to.


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## Ravana (Jul 14, 2014)

Creed said:


> How easy would it be for the people to make predictions and calendars on their orbits? Are they likely too erratic? Would they need more than 13th century technology for that?



If the orbital periods are regular–as would be the default assumption–they'd have no problem at all working them out. The orbits of the planets visible from Earth were worked out in great detail thousands of years ago, at least in terms of when and where they'd appear in the sky… never mind the orbit of Luna, which was a piece of cake. All you gotta do is walk out and look at the sky every (clear) night, and see what's different from the night before. Keep it up for a few years, and you'll have it down.

If the orbital periods _aren't_ regular (i.e. if they are "erratic"), the system isn't a stable one… though, as mentioned before, that's possible too, since a system can be "unstable" for a very long time indeed. The nearer of Mars' two moons, is spiraling toward its destruction, which is expected to take place some time in the next 30-50 million years… barely an eyeblink in astrophysical terms. Stay posted.  (Note, though, that Phobos' orbit is still regular: it's just a regular inward spiral. I provide it as an example of what sort of values can constitute (un)stable.)

Unless you really want to cause yourself headaches, I'd suggest sticking with regular orbits. Trying to calculate the gravitational effects of the bodies on one another, to figure out in what ways the orbits _ought_ to be "erratic," simply isn't worth the trouble. Alternately, you could make them irregular in whatever way you want them to be and simply declare that that's the way the system is at the moment, no matter what it looked/will look like ten thousand years removed from that point. If having one or more of the orbits be erratic in some specific way is pivotal to your story, I'd suggest magical intervention. I have one setting where one of the moons is nicknamed "the Chaos Moon" because, as far as ground observers can determine, it does whatever it bloody well likes (including passing across the sky in different directions, leaping between phases, vanishing for long periods of time, etc.). Most assume that whatever god lives there wants it that way–the same assumption they make about why the other moons _are_ regular. To date, none of these putative deities has shown up to confirm or correct any of these assumptions. Heh.



> A side question for my eccentric moon: if it spends many months or even a year or more out of the planet's sight, where could it even go?



That's not a side question at all. However, the orbit would have to take the moon a long, _long_ way from the planet for it to no longer be visible; or the moon would have to be very small, or very dark, or both; or, most likely, some combination of all three.

If its orbit is _too_ long, you could run into a different problem: the orbit might take it so far from the planet that the orbit is distorted by other large bodies in the star system–most likely, the star itself. So, again, while not impossible, it would certainly fall into the realm of unstable, over long periods of time.

As to "where could it go," the one and only answer is in an ellipse around the planet. If it does not do this, it isn't a moon. It could pass beyond visibility, but it couldn't actually "go" anywhere else, if it's gravitationally bound to the planet. If you really want it to behave significantly differently, it should be some other type of body–say, an asteroid–but then it would probably not be visible even as frequently or as long as you want it to be. Look up "co-orbital configurations" for possibilities. Some of which are strange indeed.


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## chrispenycate (Jul 14, 2014)

All right. As unfrocked mathematician and definitely more SF than fantasy background, I suppose I have to join in this thread. Our principal problem is that subtended angle (=size, for this argument) drops off as the square of the distance, as does gravitational pull. So size and tides go together for a particular density of moon.

*How can we reduce the density?*
A) Build it of lighter materials (I build someone a moon with a small rocky core, and most of its mass in a neon atmosphere, as a gas which was heavy enough not to be carried away by the solar wind and non-reactive enough not to get chemically involved with most rocks. I managed about five times the surface area of our moon, with great whorls of Coca-cola red emissions when hit by solar flares…
B) Make it hollow. Best of all, make your multiple, similar-sized moons the generation ships on which the ancestors of your characters arrived on this planet. Even bigger than my generation ship, bigger than Cherryh's 'stations', big as South America. Population of hundreds of thousands, maybe even millions, and only one percent of the interior living space. Mostly drive engines and fuel tanks, but also stored material for terraforming/colonising the world they were going to inhabit. Air and other volatiles long since escaped, generators shut down, dead and forgotten, in low orbit so their inhabitants could reackh the planetary surface in primitive craft, they still make an impressive display.
C) Eschew the spherical form. Lenticular discs, so the phases are not how much light is hitting the surface, but whether you're seeing the plates edge on or in face. Trouble is, while these forms are cosmologically very common, being a balance between rotational energy and gravitational attraction, but not one that tends to give solid structures. The potter's wheel of planetary construction prefers blobby bits, and aggregation. So, how could such structures come into being? Let us consider some ancient, high-tech civilisation (you can have gods if you prefer) who want to warm up a planet they are terraforming for their own comfort. To concentrate more sunlight onto the surface they put immense, continental-sized mirrors, into orbits where their natural spin puts them edge-on when between the planet and its star, and flat on when they're behind it. Over millions of years this synchronicity has dissolved, but giant silvered discs still hang in the sky, like celestial christmas tree ornaments or the biggest wind chimes… I will not go into the possibility of building moons (clingfilm thick, and kilometres across) as advertising billboards, both for the inhabitants and passing interstellar vessels. Not even me.


*From a given mass, reduce the gravitational effects*
Easy. Add a dimension, in which gravity propagates, but electro-magnetic radiation (light, radio) doesn't. Now, gravity diminishes not as the square of the distance but the cube; doubling the distance from a planet would quarter the surface area, but reduce it's tidal forces by a factor of eight. You'll muck up all your orbital calculations, but who cares? They'll still give stable solutions. And an extra dimension, with abysmal planes an aetherial zones is always handy for a magic universe (or an SF multiverse, however much I might dislike the conceptual assumptions within the word. And since gravity crosses the barrier your demon cosmos is always going to have a planet in the right place; you won't Narnyise into interstellar space.
And all this without major change on the surface. Oh, obviously, for a given star the Goldilocks zone will have a different length of year, but that's happening all the time in SF anyway. Atmospheric pressure might drop off a bit faster up mountains, making tea a more difficult proposition, but really, the inhabitants would hardly notice at all.

 Or you can, as Sheila has proposed, put up a Klemperer rosette, an equilateral polygon of moons in stable rotation round a centre of gravity that might, or might not, contain a planet (great fun without one, inhabitants in an atmosphere maintained by the gravitational sum of the moons' fields, permanently flying with no stable ground). Aesthetic problems; the moons are always in the same relative positions, never three in the sky at the same time unless there are always three. And they must be in matched sets.

Go to a proper fantasy universe where the Earth is flat, the stars are mounted on a crystal bowl, an down just is; no need for gravity or any of these new fangled forces. As long as you can avoid the rails getting tangled, or upsetting the dungbeetle pushing the sun about you can have as many moons as you like, moving in any direction across the sky. Gods' chariots, the mysterious ways in which gods move in. probably pulled by teams of animals, but not horses, that's too prosaic. One Goddess, who's chosen a team of cats to pull hers, weaves a singularly weird and unpredictable path across the heavens, as members of the mob sit down to lick themselves, or chase after butterflies.

Who needs orbital dynamics?


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## Creed (Jul 16, 2014)

Yay! Thank you so much, Ravana! This is a huge help!
And it is a small moon, blackened by long dead volcanic activity spewing up ash. They call it the "Dark Star" and hold a festival every time it reappears in the sky, because it represents the possibility of salvation.
I'll stick with the regular orbits- though the "Dark Star" does end up crashing into another moon at the climax due to magical intervention.
Thank you again!


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## Ravana (Jul 16, 2014)

chrispenycate said:


> Our principal problem is that subtended angle (=size, for this argument) drops off as the square of the distance, as does gravitational pull. So size and tides go together for a particular density of moon.



Keeping in mind that, as mentioned, volume is a cube, mass is derived from volume times density, and gravity is derived from mass… so the effect of gravity drops off far more rapidly than apparent size does as you reduce the actual size of the object. But other than that, yeah, I'm with ya here.



> bigger than Cherryh's 'stations'



Ah, someone else who reads Cherryh–yay! Some of the best "hard" SF out there… and not at all "hard" to read. In fact, you can easily forget all about the hard science as you're caught up in the compelling characters she liberally populates her worlds with. Can't recommend her highly enough, though this might give a hint: she's one of only four authors out there whose works I buy in hardback as they come out. 



> Add a dimension, in which gravity propagates [etc.]



Meh… more trouble than is worth it, unless you really want an "impossible" system–say, several equally-large bodies occupying close orbits. At which point, you might as well just do it by fiat and ignore the reason, unless that reason itself is an important part of your story.

I would add, however, that while we know _what_ gravity does, and can make reliable calculations of its effects in terms of orbits and such, we have no idea _how_ it does what it does–nor _why_. That's right: as far as scientific explanation goes, it's still "magic." Oh, no scientist would say that: we know it's linked to mass somehow, and we can predict its effects to a very fine degree; we just don't have an account of why mass produces gravity in the first place. So, yeah, if you want to muck around with something, gravity's a great candidate.

I would also insert a plug for David Brin's _The Practice Effect_, where a character finds himself in an alternate universe where all the physical laws we know are the same… with one important exception. You might be able to guess from the title, but I won't otherwise spoil it. Read the book. 

[Brin is another of the four authors I buy in hardback, by the way.]



> Go to a proper fantasy universe where the Earth is flat, the stars are mounted on a crystal bowl, an down just is; no need for gravity or any of these new fangled forces.



I've done a variant on this: my nearly-flat world (it's very slightly lenticular) has a special bi-directional gravity. On the faces, gravity pulls you "down" (in this case, where your feet are pointed, not toward the center of mass); on the edges, it's reversed–it pushes outward. No one knows why. Not even the gods. The combined pull/push creates a net effect on the orbits of the celestial bodies surrounding it equivalent to what would be expected from a spherical body with normal gravity, i.e. those bodies are all in stable orbits around the world's center of mass. (Which is also where any resemblance to a real system ends–the sun, stars and other planets all differ significantly from reality–but that's beside the point.)



> Who needs orbital dynamics?



Heh. But, yeah, basically what I was suggesting: unless you're really going to get "hard science" about the dynamics, or unless they're going to play a prominent part in your story, the math ain't worth it: just do what you feel like doing. I sometimes go to the trouble, but only when the physics _does_ play an important role in the story… which, in fantasy, is going to be fairly seldom.

-



Creed said:


> Yay! Thank you so much, Ravana! This is a huge help!



Quite welcome. Glad to be of help.


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## Steerpike (Jul 16, 2014)

An Aside RE: Cherryh - I've always been a fan of her Chanur books, which are a lot of fun. I'm about 1/4 of the way into Cyteen right now, which is quite good. I am confident in recommending her work to anyone.


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## Ravana (Jul 16, 2014)

Steerpike said:


> An Aside RE: Cherryh - I've always been a fan of her Chanur books, which are a lot of fun. I'm about 1/4 of the way into Cyteen right now, which is quite good. I am confident in recommending her work to anyone.



And that makes two votes. 

As with Steerpike, I'd recommend the Chanur and Alliance/Union series to anybody. For anyone looking for an entry into the latter, _Downbelow Station_ is the best choice, though nearly all of them can be read independently of one another and in any order, the exception being _Regenesis_, which is a sequel to _Cyteen_ (be sure to pick it up, Steerpike), and to a lesser extent _Hellburner_, which follows the same characters from _Heavy Time_, though you can read the former without having read the latter. The Chanur series should ideally be read in order, particularly since the middle three books are a genuine trilogy; the trilogy can be read without the first one, and the most recent could be read without the ones which come before it, but you'd lose a fair amount of context.  

I've found some of her other work less appealing, but there's only one book of hers that truly disappointed me. In general, I'd have to say her SF is better than her fantasy, but I have few quibbles even with the latter, whereas the former would, I think, interest even people who do not read SF on a regular basis. As mentioned, a lot of it has to do with her characterizations: she's a great author to learn from and emulate in that aspect.


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