The Trojan asteroids are at the L4 and L5 points. It’s certainly possible to put an object there naturally, but it probably wouldn’t be called a moon.
Any two bodies orbiting each other will tend to eventually tidally lock, but depending on how far apart they are, it may take longer than the lifespan of the solar system. Earth, for instance, will never lock to either the Moon or the Sun. Most moons are close enough to their primaries to lock, but it’s not inconceivable that one might not be.
The moon isn’t tidally locked because it is assymetrical, it is assymetrical because it is tidally locked. Most satelites are tidally locked around their primaries, or in a more complicated resonance.
Think about this. Every time the earth rotates, the moon pulls the oceans up. But imagine that the moon was rotating. Every time it rotated, the moon’s crust would be pulled slightly toward the earth, while on the opposite side the crust would slightly float away. Since the earth’s gravity is much larger than the moon’s, the tides would be much stronger, and cause more flexing.
All that flexing takes energy. Where does the energy come from? The rotation of the satelite. The satelite rotates more and more slowly as more and more of its rotational energy is converted to lifting the crust and then turning into heat. When the rotation of the satelite takes exactly the same time as one revolution around the primary, then no more tidal flexing takes place, so the rotation time stays the same.
Well, the satelite could also be moving slowly farther out or closer in to the primary. In that case, tidal flexing would slowly work to bring the rotation in synch with the revolution. Or more probably, they would occur in tandem…as the satelite moves out it’s rotation slows to match.
Now, having 2 week days and 2 week nights might be a problem, but we can imagine a moon that is much closer to its primary than Luna is to Earth. It could even be less than one day, as Podkayne pointed out Io orbits Jupiter in less than one day. But you might get some severe problems being that close to a gas giant’s magnetic field. Lots of radiation and such. Still, one could imagine a smaller, less energetic gas giant than Jupiter for the primary of Endor, and a 72 hour or so day wouldn’t be that out of line.
Chronos sez: “The Trojan asteroids are at the L4 and L5 points. It’s certainly possible to put an object there naturally, but it probably wouldn’t be called a moon.”
(Looking at a diagram) You’re right. I was misinformed. I reread your first post, and I understand now that you’re not talking about L1, either. However, if a moon in the situation you describe were tidally locked, it would be even worse, right, since one side would be in day forever? (Or would it? My mind only has three degrees of freedom.)
Incidentally, I’m starting to get a little suspicious of the fact that I don’t remember ever seeing the planet which the Forest Moon orbits. Perhaps when the Ewoks got sick of having 300-hour days, they shot rocks at the planet until they achieved escape speed, and now it’s a rogue moon.
Hey, Plan Man, good point; I hadn’t even thought about Uranus. However, I don’t think that a Uranus-like system would be a good place for a forest moon because remember, the rotational poles of the uranian moons are roughly lined up with the planet’s rotational pole. They’d suffer the same season-long day/night cycle as the planet. Of course, you could imagine a different rotational state for the moon, but (barring other effects) eventually tidal forces will bring it into synchronous rotation.
BTW, I just realized that we’ve been flinging these terms about pretty glibly, so just to be sure we’re on the same page: being in synchronous rotation is the same thing as being tidally locked.
Achernar is right that if a moon was tidally locked at the L1 point one end of it would always be pointed a the Sun and one end would always be pointed at the planet, so one hemisphere would have eternal day and the other eternal night. However, I wouldn’t expect a moon at L1 to be tidally locked. For one thing, it’d be too far from the planet. For example, the Earth-Sun L1 point is four times the distance form the Earth to the Moon. (In some sense, the L1 point is the place where Earth’s gravity leaves off and the Sun’s takes over.) Also (and probably more importantly) tidal locking takes a long time. If there’s tidal locking going on, there are lots of other complex gravitational interactions between moon, sun and planet. All of these are going to exert torques on the moon, try to change its orbit, and shove it away from the L1 point long before it gets locked.
Goodness gracious, who knew people would be so interested in dynamics?
And hey, as long as we’re talking forest moons, don’t forget Yavin 4, the fourth moon of the gas giant Yavin, where the rebel base is located at the end of Episode IV. Not only do we see the location of the moon in relation to its gas giant in computer displays, but, IIRC, the gas giant looms in the sky as Our Heroes arrive and depart.
Until about 30 years ago, it was thought that Mercury’s rotation took 88 days, same as its revolution around the sun. It was a major triumph for me to bring in a book proving my 5th grade teacher wrong on this issue; the updated rotational cycle is about 59 days.
Prior observations were apparently poor or difficult, and/or they were betting on a tidal lock.
Mercury is a special case: It’s in a harmonic lock, with the “day” exactly 2/3 the length of the “year”. For a planet in an orbit as close and eccentric as Mercury’s, this is a stable situation: If some process were to slow down the rotation of the planet slightly (bringing it closer to a synchronous lock), the tidal forces would actually speed it up again to bring it back into that 2/3 harmonic. Venus, meanwhile, is close to a synchronous lock, but it’s far enough out that it hasn’t quite locked yet.
When a planet’s rotation speed changes due to tidal effects, the orbit must also change at the same time, to conserve angular momentum. As the Moon slowed, its orbit increased in size. In fact, the orbital radius is still increasing, but that’s due to the slowing of the Earth, not the Moon.
Continuing the Mercury hijack (hey, how often to I get to show off in my area of expertise around here?) Merury is in an orbital resonance: it makes exactly one-and-a-half rotations for each orbits. Its orbit is elliptical, which means that travels at different speeds at different points. A planet can’t change the speed at which it rotates in the course of an orbit, though, so Mercury can’t keep one face always pointed at the Sun. Like any real-world object Mercury is not a perfect sphere; it is a bit oblong. Its long axis is always aligned with the Sun when it’s at perihelion (the point in its orbit closest to the Sun) but the hemisphere pointed at the Sun at each perihelion alternates.
Did that make any sense at all?
Anyway, as dqa notes, this was impossible to determine using optical methods 'cause Mercury’s so darn close to the Sun. The true rotation period of Mercury was determined using doppler radar at Arecibo observatory.
Hijacks are all well and good, but let’s return to the OP, shall we?
Suppose Endor becomes a moon of Mercury (maybe we can keep it habitable by changing the sun into a white dwarf). This new body would disturb the resonance Podkayne and Chronos mentioned. Assuming the sun and its system remain essentially unchanged forever… Is the long term outlook that Endor will tidally lock to Mercury, eventually Mercury will lock to Endor, and then the pair will lock to the sun, revolving like two spots on an LP? Or can they never lock to the sun, once they’re locked and revolving around each other?
Expanding a bit on the difference in distance not causing the change in seasons… IIRC, Earth’s orbit is very slightly elpitical, so that when it is summer in the northern hemisphere, the planet is actually a few tens of thousands (Or maybe it was just thousands?) of miles FURTHER from the sun than when it is winter in the northern hemisphere… I have no clue where to look to confirm this, though, so I might be wrong 
Would a Blue- white star, putting out large quantities of UV radiation, provide the UV light needed for photosynthesis , even if the forest moon & it’s companion Gas Giant were far out in the system?
Would the Gas Giant generate the extra heat to keep such a moon warm?
And, are you forgeting that , while the moon would be in total darkness while behind the gas giant, the Giant would relect huge ammounts of light onto the moon when the moon was between the giant & our hypothetical sun? At such times, both sides of the moon would be brilliantly illuminated.
Gas Giants don’t last long enough to allow for forest life to evolve. We’re talking an order of millions of years. Not billions. Don’t expect to find life in stellar systems under our current models for any stellar type above an F (and even an F is tricky).
Moreover, a project is going on right now looking for brown dwarfs in Goldilocks zones in order to find habitable moons. This goes along with the theory that moons scale to the size of the planet.
Wha? Last time I checked, Jupiter, Saturn, Uranus and Neptune are still there. Four billion years and counting.
Unless you mean that a gas giant close enough to a star for liquid water to exist on its moons would only last millions of years, in which case I’d like to see the math or a citation.
As for the Mercury-moon thing, it’s hard to say. Remember that Mercury is small; it couldn’t have too big a “moon” before the moon becomes a planet and Mercury becomes its moon, or you get into some double-planet situation. Hard to say what throwing a moderate-sized moon into the picture would do. It would depend strongly on the size of the moon–which would be larger, tides from the moon or tides from the Sun?
What wouldn’t happen is the two-spots on an LP thing. For the moon’s orbit to be as slow as Mercury’s rotation, the moon would have to be so far away from Mercury that it would escape. Similarly, the Moon is slowly sucking the rotation out of the Earth–but the Moon will be so far from the Earth it will escape before the Earth ever locks up with it (though the Sun will turn into a red giant before that happens.)
Woops! Ignore my “Gas Giant” error. I meant to say Blue Giant. I guess my brain was out to lunch.