Permanent Full Moon

[Kythia]

I wasn't sure if this or Worldbuilding was the best place.  Apologies if I got it wrong.

A conversation with a friend revealed that he's creating a world and he'd like - aesthetically - it to have a permanent full moon.  That is, the moon appears circular no matte the day of the month.  What conditions would the world and/or moon need to have for that to be possible?

[Oniya]

The moon would have to consistently be directly opposite the sun, which I believe would require its revolution around the planet to be approximately the same as the planet's revolution around the sun.

[gaggedLouise]

The moon would have to consistently be directly opposite the sun, which I believe would require its revolution around the planet to be approximately the same as the planet's revolution around the sun.

*Nods* Which would mean the moon had to be rather distant from the planet, much farther off than our moon, and most likely it wouldn't be very big - if it were big or close it would have been accelerated (and perhaps made to come closer) by the gravity pull from the planet. But of course, the more distant the planet is from the sun, the longer the "year" (both planetary and lunar) would become. I'm sorry, I guess the appealing idea is hard to fit into how a planetary system would work.

Some of the effect might be obtained if it's a two-star binary system with the stars locked fairly close (but not so close that they would disturb the stable orbits of their planets). The second star would have to be off by at least a couple times the distance between the Sun and Pluto, I think - so that both stars would lit up the moon. For this to give enough light to lit up the moon, the other star would have to be much more powerful and bright than a sun-type star, a giant star (but then  wouldn't it also provide daylight on the planet, at the same times?). This could be enhanced (I think) if the orbit of the moon was heavily tilted off the equatorial plane of the planet/s of its star - if it would be swooping from high north to deep below the plane of the planet as it moved around it. Then again, that would mean the moon was sometimes out of sight from ground on major parts of the planet for several nights in a row.

[Caitlin]

If it's a fantasy story, just explain the physics by magic, if it's a scifi story, explain it by 'left-over technology from a civilization long ago'. If it's a contemporary story then just explain it that it's always been that way and pass it off as something that nobody ever questioned. I'm no astronomist and I think there are conditions that could apply to make it a full moon at all times, but I'm not sure what they would be. The before mentioned explanations are elegant ways to solve it in story writing though. :-)

[Kythia]

Yeah, Im not sure how handwavey he wants to go.

All I could come up with is if the planet is actually a moon itself, with the "moon" orbiting that.  Based on no calculations whatsoever - oddly I not capable of solving the three body problem in my head, I feel such a failure - it seems intuitively possible that the "moon" could be locked around the world by the gravity of the planet (essentially similar to Louise's solution) and then "moonlight" becomes the reflected light of the planet rather than the sun.  Not sure if its feasible or not though.

Just putting some thought into the effects it would have assuming it were possible.  Tides and the like.

Thanks for thinking about it guys.

[gaggedLouise]

If it's a really advanced civilization you could of course presume that they have put up a system of huge reflectors held quasi-stationary in positons around the moon and planet (locked by the gravity of the two bodies), so that the light from the sun/s, and/or reflected light off the mother planet, would bounce around the corners as in a periscope and lit up the moon in a carefully constructed scheme, then it would always seem to be full. But honestly that sounds like a massive vanity operation (no real use whatsoever, and need for spectacular amounts of engineering and ongoing calculations) that I have a hard time thinking any superadvanced civilization would throw money and resources into.  ;D

[Kythia]

As I understand it its a pseudo-medieval fantasy thing.  Of course all that means is that"huge reflectors" is replaced with "crystalline strands of pure magic" or something similar - sufficiently advanced technology and all.  The "why" problem remains.

[Caitlin]

Why answer the 'why'? Sometimes some things are better left of as a mystery. Unless it plays a huge factor in the narrative of course, but in that case the heroes could discover it on their quest. :-)

Personally I think the 'how' is harder to answer than the 'why'. Why something works the way it does can be ranging from the will of the ants, to the power of hurricanes and Mother Nature.

[gaggedLouise]

As I understand it its a pseudo-medieval fantasy thing.  Of course all that means is that"huge reflectors" is replaced with "crystalline strands of pure magic" or something similar - sufficiently advanced technology and all.  The "why" problem remains.

Also, if the moon were constantly full and always has been (well, as long as anyone knows from sight or from books) and it's a medieval world, wouldn't those people think of it as a second sun? They wouldn't have a clue to relative distances - which one is closer? -  and the moon would be useless for making up a steady time period.

[Vorian]

There's also the possibility of something on the moon itself producing enough light to be seen.

[Randall Flagg]

There's also the possibility of something on the moon itself producing enough light to be seen.

Nice concept that. A luminescent moon... it could just slightly shift colors or something. That's cool.

I was thinking what if it's like, a binary moon system, but the rotation is such that one moon is constantly in full view. I'm not sure how it'd work. But I think I agree with the "keeping it a mystery" for now. It could be a problem that solves itself later on during game play.

[gaggedLouise]

If the  moon had an atmosphere, like Titan has, that would help spread the light out a bit, at the "rim of dusk", but not over the entire disk of course.

[RubySlippers]

Make the planet flat the sun orbiting it and the moon over the top of it at the pole. Its a fantasy world you can have a mighty god or goddess holding the world up or have it elevated the mystical vapors the source of all magic.

[Ryven]

Could you not get a similar effect by having more than one sun in opposing positions to the planet.  I'm sure the path around the suns would have an effect on it, but if its your world, you could just explain it the way you want.

[Randall Flagg]

Could you not get a similar effect by having more than one sun in opposing positions to the planet.  I'm sure the path around the suns would have an effect on it, but if its your world, you could just explain it the way you want.

Wouldn't that leave the world in perpetual daylight?

[Ryven]

Wouldn't that leave the world in perpetual daylight?

There was never a specification about the world, just the moon.  I'm simply adding another idea.

[Randall Flagg]

It's all good Ryven. You spurred me to thinking. I always welcome that.

What conditions would the world and/or moon need to have for that to be possible?

I'm not really sure what you mean when you say "There was never a specification about the world..."

I mean, if there is no world then the moon wouldn't be a moon, but ... well what. It'd be a dwarf planet? An asteroid? Because for a moon to be a moon it needs to be in orbit around another body - hence the world. But, even if we could say that the moon is still a moon even without a world, the entire reason the moon has phases is because the world is eclipsing it... right?

[Ryven]

What I meant was that there was no limit on the other conditions that could come with a moon having that characteristic.  She never said that "How can you have a permanent full moon but without x, y, or z?"  If you're going to stipulate you want one condition, you should accept the fact that other conditions come with that unless you're going to add in some kind of fantasy or scifi element such as magic or advanced technology.

[Oniya]

If the moon was artificial, such as the remnants of a massive (but hollow) interstellar ship, then it could have been parked in a computer-controlled orbit that kept it directly opposite the sun.  Anne McCaffrey had artificial satellites in her Pern series, which started out with a very high-fantasy ambiance.

[Ebb]

But, even if we could say that the moon is still a moon even without a world, the entire reason the moon has phases is because the world is eclipsing it... right?

Just a note as you work out possible solutions: The moon has phases because of the relative positions of the earth, moon and sun. When we see a crescent moon, for example, it's not because the earth is blocking out the dark part. It's because the moon is being lit from the side relative to us, so the whole thing isn't illuminated from our perspective.

This diagram makes it a little clearer:
http://www.moonconnection.com/moon_phases.phtml

You may be thinking of a lunar eclipse, where the earth's shadow is passing over the moon, making a different crescent shape.

If your friend wants the moon to always appear full, then the simplest answer is to have the moon glow on its own (for any number of scientific or magical reasons) as Vorian suggested. If you have it work like our moon, glowing by reflected light, then you have a lot more work to do if you want a plausible explanation. And even if you have it self-illuminating, then you'd still have phases, as the sunlight falling on it would still vary. They'd just be harder to see, depending on the relative brightnesses.

[Randall Flagg]

This diagram makes it a little clearer:
http://www.moonconnection.com/moon_phases.phtml

You may be thinking of a lunar eclipse, where the earth's shadow is passing over the moon, making a different crescent shape.

You're absolutely right. I was thinking of an eclipse.

I stand corrected. That's a really cool diagram thanks for the link!

[RedFangedWerehawk]

Would something like that be feasible if the planet was tidally locked to the sun and the moon was tidally locked to the planet, or do the rotations on their own axes not matter because it is all about the relative position of each object to the other...

Perhaps if both the moon and the planet were tidally locked to the sun? What if the moon wasn't a technical moon but had an orbit trajectory that was close enough to the planet that it is mistaken for one by the people living there? The more that I think about it though, tidal locked or not tidal locked has little affect on things.

[Cthonig]

What if it's not a moon but just what the people think is a moon?

An Earth-like planet the people live on and a gas giant in the next orbit out from the sun each with the right speed so the gas giant looks like a moon. With the distance, the Earth-like planet does not eclipse enough of the light to be noticed. Now, this is probably not going to be a stable arrangement in the long run (billions of years) and it probably wasn't always this way (gas giant gradually circling inward) but for the past several dozen millennia the gas giant has been moving at the right speed and in the right position to look like a small moon always in the night sky and always lit.

[Tiberius]

If it is to be a natural full moon, only one side of the planet will be able to see it and only in a limited space on the planet, for the moon to be constantly in the full phase, it will have to orbit the planet at exactly the same speed that the planet is rotating otherwise it will slowly begin to phase. Our moon orbits Earth 10-11 times per year that accounts for the full moon phases.

[meikle]

If it is to be a natural full moon, only one side of the planet will be able to see it and only in a limited space on the planet,

Isn't the 'one side of the planet' that can see the moon always the side that is in night?  It takes 24 hours for Earth to rotate once around its axis, and during those 24 hours, every point along the equator will get a peek at the moon if the moon is always mostly opposite the Earth from the Moon (obviously not directly opposite, because that would be a lunar eclipse.)

There would probably be some point that could never see the moon at the north or south pole of the planet, especially if it had a tilted axis.