Super basic dumb science question

[sinister42]

Can't find the answer on Google and I feel like an idiot that I don't know the answer, but here it is.

Would it be possible for a fictional planet to have a permanent full moon? In other words, the orbits of the planet and the moon are such that the moon never goes through phases and its planet-facing surface is always fully illuminated by the planet's star.

This is an idea for a story that involves a werewolf astronaut, so...

 

[goldhawk]

Short answer: no.

Long answer: for that configuration, the planet and moon would have to be at the Lagrange points, L2 and L3. These points have unstable orbits. That means they would drift away from the configuration and orbit the star separately.

 

[sinister42]

That's kind of what I thought. I think I'm going to go with a multiple moons scenario where at least one of them is full at any given time.

 

[Venusian Broon]

Hmmm, may be possible if we arranged the parameters enough....I'll have to do the maths. (Feels tough though). Something in the lagrange points of the earth-sun system wouldn't give you a moon anyway.

 

[Star-child]

If the moon were in a geosynchronous orbit it wouldn't have phases as much as a day night cycle that was 180° out from the place it was over. So the people under that moon would see a half moon at 6pm and a full moon at midnight, then wane to the other half by 6am.

Other fixed planet moon arrangements would be similarly possible that would prevent the procession that causes a 28 day cycle.

 

[Danny McG]

A good application of handwavium should do it

 

[Dennis E. Taylor]

The other option would be to have the moon and sun orbiting the planet at the same rate but on opposite sides. But that requires either a discworld-level fantasy setting or an artificial sun.

 

[Dennis E. Taylor]

Another possible option is to give the planet such an insane albedo that planetshine lights the shaded part of the moon. But I can see all kinds of problems with that too.

 

[Parson]

Hm ----- This is not a basic level science question!!! It would certainly take a serious bit of weird orbital dynamics. --- How about a planet with several moons so that one is always full?

 

[Venusian Broon]

The problem here is that for it to be a moon, it has to, by definition, orbit the planet. And to orbit the planet there will be moments where the position of the moon will give you a new moon.

So...

- Perhaps if the Earth was tidally locked to the sun, and then the moon was tidally locked to the Earth, so that the moon would just stay in the same spot in the sky relative to the Earth. Although then if that were possible it would no longer technically be a moon, but a planet on an orbit around the sun, very close to the Earth. However this set up is, I believe, really unstable and thus impossible to appear in nature.

- Perhaps rather than a moon, you had another planet in an orbit outside Earth's, that was big enough to 'look' like a small moon, i.e. sufficiently large to be disc-like to an Earth observer. Thus it would never exhibit the lunar phases. However you still run in problems. One, if it was large enough to be seen from earth...it's gravitational impact on the Earth's orbit would likely be felt and our orbit would likely be stranger. Also planets because of the orbital dynamics must have different periods (slower) and thus it wouldn't be a full phase all the time. On occasion you will see it as gibbous - such as Mars can be viewed depending where it is in its orbit relative to the Earth.

- Having more than one moon. I suppose one could insert another moon into the same orbit as the current one but in the 'opposite' point. So that when one is full, the other is new. However there then will be moments when you have one waning and one waxing. Adding even more moons at this point...

-....becomes tricky because the moon is large. Orbits will become unstable if you try adding more than one (or the very special case of two above). You could go for a Jovian or Saturian system of moons, like a mini solar system around the Earth...but they would have to be much much smaller relative to our moon. Not sure if you would get moon discs in such a set up, perhaps just large points of light, and there is still the chance that orbital dynamics will put all your moons on one side of the Earth in such a way as to inflict no full moons for you.

- Ignoring all that above, maybe the simplest way to go about this, is to have the Earth-Moon system as is, but have the moon have a natural luminosity that is such that it swamps the effect of the reflected sunlight to an Earth observer. So it will always appear full (perhaps 'super-full' when it recieves a boost of 'full' sunlight?) I can't think off-hand what sort of natural phenomenon would produce such luminosity while retaining the Moon's rocky appearence, so perhaps this is where you add your hand-wavium.

- But I quite like @Star-child 's geosychronous suggestion. Perhaps have an artificial moon there, say an old vast advertising board/space station that is constantly powered to be illuminated.

@sinister42 I note that you are talking about a werewolf astronaut. So his viewpoint is not really from the Earth? In that case, it really does depend on what sort of orbit he is having himself. If, for example, he is on a spaceship that is in an inferior orbit to the Earth (i.e. one closer to the Sun that the Earth-moon) then he will largely see the Moon full. (not always, but largely - see Mars stuff above). If he is in a superior orbit (one further away from the Earth-moon orbit) he will see Moon phases.

If he is just orbiting the Earth, then you will get a lot of the problems stated above.

But this just raises philosophical issues about your werewolf. See, as long as the moon is not in the Earth's shadow, as will happen on occasion, there will always be a (many) spot(s) in the universe where it is full. So is the moon not really 'full' all the time anyway? It is only an observer on the Earth's surface and how the Earth-moon-sun system interacts that gives rise to the phases. So does your werewolf need to travel to a spot to see a full moon to change? What happens if he lands on it? If he is on it, and it is always a full moon there, as it will be until it occasionally gets sunlight blocked by the Earth, then he stays a werewolf all the time? (Even although he can only see a tiny bit of it???)

 

[Finch]

If the moon were in a geosynchronous orbit it wouldn't have phases as much as a day night cycle that was 180° out from the place it was over. So the people under that moon would see a half moon at 6pm and a full moon at midnight, then wane to the other half by 6am.

It would be possible to make artificial Moon and place it as above . There was a Russian plan to use something like it ,to give Siberia longer daylight hours .

 

[Venusian Broon]

EDIT!

As someone has pointed out to me, @sinister42, I was only thinking about the Earth and Moon as we have it.

(The Earth is the name of the planet we're standing on and this planet only, goddammit!!! )

There could be another way, but it changes things around a bit.

Imagine this instead.

If we had a Jupiter-sized object orbiting the a star at 1 A.U, say. (the distance our Earth orbits the Sun) then it's orbital period will be a year. If an earth-sized moon orbited this Jupiter-sized object and also had an identical orbital period of a year around our Jupiter analogue then, assuming the 'starting conditions' were correct, (as in started out by seeing the jupiter analogue as full) someone on this Earth-like moon would always see it's parent planet as full.

I've put this binary system at 1AU so as to allow the star to keep the moon at a reasonable temperature for liquid water!

The one issue with this is that to allow this, the Earth-like moon would need an orbit of ~0.1AU or about 14.2 million Km around the parent planet. And that's a significant difference in terms of the solar radiation that the moon will get. I guess, would probably fry it a little.

However one can fiddle with this, and instead of a jupiter-mass object, something with 10 times the mass of the Earth-like moon, gives you a smaller orbital radius of 0.03AU or ~2 million Km.

What such an object would actually look like in the sky, i.e. how big it is, I will have to work out....

EDIT2: Duh! Complicating things...I suddenly realised after doing all that, that one could also make a moon's period equal to, say, the orbital period of it's parent and, (again depending on starting conditions, starting as full moon to someone on the planet blah blah blah) the moon should remain a full moon all the time. To allow this though, it would need to increase it's orbit from 385,000 Km to ~2.1 million Km. Someone standing on this world would see a moon five times smaller than our current moon. Also ocean tides would be not as strong, one presumes. I have no idea if such an orbit would make it likely that such a moon would escape such as system, but why not?

 

[CupofJoe]

It would be possible to make artificial Moon and place it as above . There was a Russian plan to use something like it ,to give Siberia longer daylight hours .

Something like this was also suggested for disaster relief. A huge reflective surface that could be maneuvered in to place to give 24 hour daylight after an emergency event. I read about it back in the 70s, when they said it would be controlled by the Moon Base the UN were going to set up.
I want to live in that future...

 

[zmunkz]

Maybe it could work if the moon’s orbital plane is perpendicular to the planet’s, e.g., if you stick your thumb face down into your sun, your planet would be at the end of your pointer finger, and your moon would follow the path of a ring slipped onto the end of your finger. Not sure if we could lock the orientation so that orbit is always perpendicular to the sun, so this may simply replace monthly phases with a annual phases.

 

[Star-child]

- Perhaps if the Earth was tidally locked to the sun, and then the moon was tidally locked to the Earth, so that the moon would just stay in the same spot in the sky relative to the Earth. Although then if that were possible it would no longer technically be a moon, but a planet on an orbit around the sun, very close to the Earth. However this set up is, I believe, really unstable and thus impossible to appear in nature.

This could work because a planet orbiting the sun outside the earth's orbit would be rotating slower than the earth's 365 days per orbit. This principle is why Saturn has grooved rings rather than a flat plate - the lower rings have a faster orbital period than the higher ones.

If we had a Jupiter-sized object orbiting the a star at 1 A.U, say. (the distance our Earth orbits the Sun) then it's orbital period will be a year. If an earth-sized moon orbited this Jupiter-sized object and also had an identical orbital period of a year around our Jupiter analogue then, assuming the 'starting conditions' were correct, (as in started out by seeing the jupiter analogue as full) someone on this Earth-like moon would always see it's parent planet as full.

How is this possible without the earth being at L1 to keep a fully illuminated gas giant overhead?

Maybe it could work if the moon’s orbital plane is perpendicular to the planet’s, e.g., if you stick your thumb face down into your sun, your planet would be at the end of your pointer finger, and your moon would follow the path of a ring slipped onto the end of your finger. Not sure if we could lock the orientation so that orbit is always perpendicular to the sun, so this may simply replace monthly phases with a annual phases.

This is an interesting idea, but not stable because you'd need the orbiting moon to precess, which is like trying to get a gyroscope to rotate around a line through its plane of rotation, and that would make the gyroscope/orbit wobble.

 

[HareBrain]

Someone standing on this world would see a moon five times smaller than our current moon.

"Five times smaller" meaning it has negative size, the absolute value of which is four times the current one?

Yours in pedantry, a pedant

 

[Star-child]

"Five times smaller" meaning it has negative size, the absolute value of which is four times the current one?

Yours in pedantry, a pedant

Division doesn't normally give us negative numbers, but division would be the correct counterpart to the multiplication of "five times larger".

 

[Venusian Broon]

How is this possible without the earth being at L3 to keep a fully illuminated gas giant overhead? A true geosynchronous orbit that matches the location of noon of a planet tidally locked to the sun is the distance to the sun - so the moon would fall into the sun.

The Jupiter size object in this instance isn't tidally locked to the sun (we're not tidally locked to the sun at that orbit, no?). And it's not keeping it overhead, as I didn't say the moon was tidally locked to the JSO (would it be at 14million Km? I don't know). So as the moon day/night cycle goes on, JSO would rise and set to someone on the moon. But should be always 'full' because of the very specific orbital parameters and 'starting conditions'!

Probably impossible for a natural system to evolve to this and be stable, but this is SF.

 

[Venusian Broon]

"Five times smaller" meaning it has negative size, the absolute value of which is four times the current one?

Yours in pedantry, a pedant

Would go from an apparent angular diameter of about 31 arcminutes to about 6 arcminutes, so five times smaller yes.

Or to put it another way, five of those smaller circles would fit one diameter of the big 'un.

Yours,

Pete 'Pedant Squared' Pendragast

 

[Star-child]

The Jupiter size object in this instance isn't tidally locked to the sun (we're not tidally locked to the sun at that orbit, no?). And it's not keeping it overhead, as I didn't say the moon was tidally locked to the JSO (would it be at 14million Km? I don't know). So as the moon day/night cycle goes on, JSO would rise and set to someone on the moon. But should be always 'full' because of the very specific orbital parameters and 'starting conditions'!

Probably impossible for a natural system to evolve to this and be stable, but this is SF.

The Jupiter doesn't have to be tidally locked, but there is only one spot between it and the sun where it is fully illuminated, and that's wherever it is 'noon' on Jupiter. At 6pm on Jupiter it is always half illuminated, for instance. (That hour scale being proportionate to Jupiter's actual rotation, be it hours or months).

Regardless of the rate of spin around either the moon or the Jupiter's axes, the only thing that causes the observation of 'fullness' to happen is when the observer is between the sun and the illuminated object, and that's an orbit like L1 if it is permanent.

In other words, the only place one can observe a fully illuminated 'face' of a sphere is if you are near the line between it and the illuminating source. Anywhere else you will see a terminator or a completely dark face.