Tidally Locked (?)

[Major Klutz]

Questions about tidally locked planets ...
From what I understand, there are at least two kinds of tidally locked orbits.

There's objects like earth's moon, which always have one side facing their parent.
From the surface, you would see the stars rotate around you but the earth would always be in the same place.
Since the object "rotates" once per orbit, does the game even consider that tidally locked?

Then there's the ones that stay stationary with respect to the stars, but rotate with respect to the parent once per orbit.
Like Mitterand Hollow or the way mercury was originally thought to be. From the surface the stars appear still and the parent circles around you.

Then there are harmonically locked bodies like the way Mercury actually is.

From what I have seen in the game, most or all bodies that are tidally locked seem to be like Mitterand Hollow. They don't rotate with respect to the stars.

Are there any bodies in Elite like our moon that always have one side facing the parent?

If so, is there any way to tell what kind of tidal lock it is from the scan data?

I have seen a harmonically locked body recently on Spoihaae XE-X d2-9 A 1 which rotates once every two orbits.
But the game does not consider that tidally locked.

 

[Sapyx]

In ED, "Tidal locking" is always supposed to be in relation to the object that is being orbited - like Earth's moon, and every other large moon in the solar system. Thus, if a solo planet is Tidally locked, it is tidally locked to the star. If it's a co-orbiting planet, then it is tidally locked to the other planet, not the star. If it's a moon, then it is tidally locked to the planet it orbits, not the other moons.

There are errors - such as Mitterand Hollow - where the system map claims an object is tidally locked, but actual in-game observation clearly shows that it is not. There are also errors where the system map claims that something is tidally locked, but the rotation period does not seem to match the orbital period as it should for true tidal locking; worlds in that second category might theoretically be in some kind of tidal harmonic (like IRL Mercury and Venus), though I'm pretty sure the mathematics of orbits in ED are purely two-body-problem Newtonian.

A world that was "tidally locked to the background stars" would not be considered to be "tidally locked", it would be "non-rotating". We've never found one of those IRL, though such worlds could exist in ED.

 

[Sapyx]

accidental duplicate deleted

 

[Lucius-Darcia]

There is one moon that has the same tidally lock like our moon. It's the planet where one of the crashed Alien ships is.
I'm visiting this site quite often (hehe) and without this tidally lock lime our moon i wouldn't find the site so fast every time again. I think it was the one in HIP 17403 if i remember correctly (actually not in game).
But yes....the thing with mitterand hollow has always confused me since i think, it's the exact opposite of tidally lock [blah]

 

[varonica]

Questions about tidally locked planets ...
From what I understand, there are at least two kinds of tidally locked orbits.

There's objects like earth's moon, which always have one side facing their parent.
From the surface, you would see the stars rotate around you but the earth would always be in the same place.
Since the object "rotates" once per orbit, does the game even consider that tidally locked?

Then there's the ones that stay stationary with respect to the stars, but rotate with respect to the parent once per orbit.
Like Mitterand Hollow or the way mercury was originally thought to be. From the surface the stars appear still and the parent circles around you.

Then there are harmonically locked bodies like the way Mercury actually is.

From what I have seen in the game, most or all bodies that are tidally locked seem to be like Mitterand Hollow. They don't rotate with respect to the stars.

Are there any bodies in Elite like our moon that always have one side facing the parent?

If so, is there any way to tell what kind of tidal lock it is from the scan data?

I have seen a harmonically locked body recently on Spoihaae XE-X d2-9 A 1 which rotates once every two orbits.
But the game does not consider that tidally locked.

You are making a mistake in your thinking. A tidally locked planet has a length of "rotation" the same duration as the length of it's orbit around the parent body, this is what tidally locked means to us now. So harmonically locked isn't referred to as tidally locked because it's rotation is different to it's orbit. It may be the same mechanism that causes the behaviour but they have two different names, harmonically locked and tidally locked. The fact that we use tidally locked for one behaviour even though the same mechanism actually applies to multiple behaviours is, I suspect, an accident of history.

So all bodies that are tidally locked do in fact rotate in respect to the stars, you can see this quite clearly if you spend some time on tidally locked bodies with extremely short orbits, in the length of 0.2 to 0.4 days, they in fact have a rotation period of 0.2 or 0.4 days and you can observe different stars in the sky fairly soon if you hang around. If you have a body that's tidally locked with a large orbit like the moon it may seem to a casual observer that it doesn't rotate in relation to the stars, but it does, it's just not observable with the human eye.

Oh yes I consider Mitterand Hollow to be an anomalous artefact, no good understanding of orbital behaviour can be made from that weirdo!

 

[Major Klutz]

I could swear I've run into more than one planet that appears to behave like Mitterand Hollow, only slower, where the stars don't move at all.
But since I didn't take notes, I'll chalk it up to very slow rotation and keep my eyes open for it in the future.

Are there any planets IRL that orbit without rotating? I suppose this is the same as harmonically locked to one counter revolution per orbit.
IIRC, this is what scientists originally though Mercury did so I assume it's possible in theory.

I've been hoping to find a landable planet with a fast rotation rate, like Mitterand Hollow but where the stars move instead.
I don't think the game allows such a place to exist though.

 

[malenfant]

As far as I've observed "tidally locked" doesn't usually really mean that. Many bodies are shown as being "tidally locked" in the system map but are actually shown to have rotational periods that are not the same as their orbital periods(whether around planets or stars). Also I've been on moons that orbit a primary that are supposedly tidally locked to them, but a timelapse shows that the primary is clearly moving in the sky (even possibly rotating around a fixed point in the sky too).

 

[Major Klutz]

... Also I've been on moons that orbit a primary that are supposedly tidally locked to them, but a timelapse shows that the primary is clearly moving in the sky (even possibly rotating around a fixed point in the sky too).

Could that be because the satellite is locked to the barycentre and the parent is moving in an orbit around the ​barycentre?

 

[malenfant]

Could that be because the satellite is locked to the barycentre and the parent is moving in an orbit around the ​barycentre?

Maybe in the game, but that shouldn't happen in reality. A tidally locked body should actually be locked to the empty focus of their elliptical orbit around the primary. if the primary's orbiting a barycentre then that shouldn't matter.

 

[varonica]

Maybe in the game, but that shouldn't happen in reality. A tidally locked body should actually be locked to the empty focus of their elliptical orbit around the primary. if the primary's orbiting a barycentre then that shouldn't matter.

No actually that's an interesting point, any two bodies in a two body system are always orbiting a barycentre, in the case of the earth and the moon the barycentre around which the earth/moon orbit is within the body of the earth, about 4,000km below the surface I think I recall, whereas the barycentre for the Pluto/Charon system is actually located between the two bodies, maybe this is why some bodies appear not to be rotating in relation to the stars. If the tidal lock is in relation to the barycentre and the barycentre is between the two bodies then it's quite possible at the correct orbital distance that a moon would appear stationary in relation to the stars. The moon would still appear to have a rotation in relation to the planet since it wouldn't have one face always facing the planet but in actual fact wouldn't have a rotation at all because it's orbit around the barycentre keeps it effectively non-rotational, tidally locked in relation to the barycentre.

It would be....unusual, and I am not sure a correct interpretation of tidally locked as we use it, but it could happen.

Just to add, what I'm thinking, take two objects, a soccer ball and a tennis ball, use the soccer ball as the planet. Put the tennis ball in an orbital position and place a black X on it with a marker, now point the black X at the nearest window (the sun). Move the tennis ball in an orbit around the planet at the same time keeping the X pointed towards the window.

From an observer on the planet the moon would appear to have a retrograde rotation equal to the length of it's orbit around the planet, to an observer on the moon the planet would rise and fall as normal with a day/night cycle equal to the length of the orbit, to an outside observer the moon would move around the planet but not rotate at all. In that case the position of the stars in the sky would only change with the planets orbit around the sun, in the case of a large orbit it could very well take years for the stars to move appreciably.

 

[malenfant]

I don't think it's possible to tidally lock to a barycentre though. Normally, a satellite has an eccentric orbit around its primary, and the primary is located at one of the two foci of that orbit. For various complex orbital dynamical reasons, the satellite actually tidelocks to the empty focus of the orbit - one face points towards that. Usually the satellite is much less massive than the primary so the barycentre is still within the primary, and usually the orbit isn't that eccentric so the empty focus is in or near the primary as well.

If the planet is itself one of a double world that orbits a barycentre then everything gets a lot more complicated - it'd be treated more like a binary star system and realistically it probably wouldn't even be able to have any satellites around it at all (or only very close, where they'd be more likely to be turned into ring systems, if even those are stable). Any satellites orbiting the barycentre outside the pair probably wouldn't be able to be tidelocked to anything at all.

 

[Major Klutz]

I tend to think that the game programmers may have taken some mathematical shortcuts to lighten the CPU load and simplify motion for the sake of framerate.
It wouldn't surprise me if it's simplified so that tidally locked bodies just face the center of their orbit and that might explain what's being observed.

 

[Sapyx]

Another thing to consider, in terms of tidally-locked planets with primaries that "appear to move" over timelapse, is axial tilt.

Out there in real life, a tidally-locked moon or planet has essentially zero degrees axial tilt - just like Earth's moon does with respect to Earth. There's a tiny amount of wobbling up and down, but it's barely noticeable. But in ED, axial tilts of tidally-locked planets can run the whole gamut, from 0 degrees to +180 and -180. I've seen an allegedly tidally-locked-to-star Earthlike, with a 90 degree axial tilt and the polar icecap pointing directly at the star at the moment I visited it. I'm pretty sure that a planet simultaneously having tidal locking and an extreme axial tilt like that is against the laws of physics.

 

[gwynwas]

Proof!

This very cool video (Alexander Komov) Shows a moon with an extremely rapid and close orbit.

You can clearly see it is incorrectly locked to the star, and is not locked to the parent planet or either orbital foci or the barycenter or berrycenter or whatever its called.

Have to wonder if this is true of all tidally locked bodies in ED.

Link:

YouTube - This Planet Breaks My Mind . . .

p.s. forgive my enthusiasm, but it appears Mitterand Hollow is well known. Just discovered this game about 3 months ago. Nevertheless, its a good guess that the mechanic at work here operates across the board.

 

[Orvidius]

I don't think it's possible to tidally lock to a barycentre though. Normally, a satellite has an eccentric orbit around its primary, and the primary is located at one of the two foci of that orbit. For various complex orbital dynamical reasons, the satellite actually tidelocks to the empty focus of the orbit - one face points towards that. Usually the satellite is much less massive than the primary so the barycentre is still within the primary, and usually the orbit isn't that eccentric so the empty focus is in or near the primary as well.

I realize I'm coming into the conversation a little late, after it was just resurrected. Just wanted to add some points to this.

Tidal locking really just means that the body (moon in this case) has the same rotation period as its orbital period. In a relatively circular orbit, this means it faces the same side toward the parent body at all times. Another way to phrase this is that it is in a "1:1 resonance".

However, technically all orbits are elliptical, so it's more accurate to talk about how eccentric (or not) that ellipse is. In an eccentric orbit, the orbital velocity is quite variable depending on which part of the orbit it's in, and it won't face precisely the same direction relative to the other body at all times. It will have a "libration", which Earth's moon also has. However, the more eccentric the orbit, the less likely it is to achieve a 1:1 resonance. As mentioned earlier, Mercury has a 3:2 resonance, due to the eccentricity of its orbit.

The cool thing about orbital resonances is that they tend to be self maintaining. Orbiting bodies love to work themselves into resonances, and they correct themselves. If it's a little ahead, the parent nudges it down, and vice versa. Kinetic energy is exchanged as needed.

If both bodies are more equal in mass, or at least the smaller body's mass is large enough, then the common barycenter is outside of the parent body, and they technically orbit each other. In this case, both orbits share an elliptical focus, and that's the location of the barycenter. This focus/barycenter represents the direction that each body "feels" a pull. In a two-body system, both bodies will always be on opposite sides of the barycenter from each other, and have a 1:1 period around it.

 

[gwynwas]

Excellent clarification ZenBones.

Now, in regard to Mitterand Hollow, specifically, I and some others were wrong to believe it was "tidally locked" to the system star. It turns out, it is not. It does, in fact, rotate. It only appears to always face the star at first glance because the orbital period is a fraction of its rotational period. If you hang out long enough on the surface, you will see starrise and starset.

As ZenBones points out, in tidally locked bodies, the rotational and orbital periods are equal. However, in the special case of MH, the rotational period is 0.1 D but the orbital period is listed as 0.0 D even though it is described as "tidally locked." I guess, this is the famous misplaced decimal. Had the decimal been placed correctly, both values would presumably be 0.1 D and the tidal lock phenomenon would be accurately modeled and portrayed.