The Dark Side of the Moon

[peter]

The orbital relationship between the earth and the moon is peculiar in that [unusually for moons I believe] the rotational period of the moon upon it's own axis is exactly the same as the time it takes it to travel one complete orbit of the earth; this [as we all know] results in the observed phenomena that we only ever get to see one 'face' of the moon directed toward the earth and anything beyond the hemisphere pointed toward us is forever hidden from view unless we actively go out there and see it.

My questions are as follows; Has this always been, and will it remain so; why has this peculiarity developed with our moon and not with any of the moons we observe orbiting our sister planets [I'm guessing it has to do with having liquid waters on the surface of Earth that exert gravitational effects back upon the moon equal and opposit to the effects of the moon on them. On this point, as a student ['A' Level] I asked my physics teacher why this was so and he said it was 'coincidence' - this sounds like bullshit to me, and this guy had a physics PhD!].

[I'm Murrin]

Nothing unusual or unique about it; any two bodies orbiting one another will slowly reduce each others' rotational energy until they become tidally locked. The moon is also slowing the earth's rotation - though at an incredibly low rate - so that eventually one side of the Earth will always face the moon.

en.wikipedia.org/wiki/Tidal_lock

[Fist and Faith]

My guess would be it is, indeed, coincidence. It just happened to work out this way in this case. I'm not an expert, or even a student, in these things, but I'm not aware of any properties of physics that would explain the moon's rotation having been slower or faster, and adjusting to synchronize with its revolution around Earth.


EDIT: Or I'm entirely wrong? Just read Murrin's post. I had no idea. But really? Do any other moons in the solar system behave like this? Jupiter has a much stronger gravitational field, so it should have done this to its moons. Did it?

[Damelon]

Murrin is right. I'm not aware of a moon that has any kind of rotational period in the solar system.

EDIT: There is a slight bit of wobble though. It sticks in my mind that we are able to observe 57% of the Moon's surface because of that wobble.

[wayfriend]

Most significant moons in the Solar System are tidally locked with their primaries, because they orbit very closely and tidal force increases rapidly (as a cubic) with decreasing distance. Notable exceptions are the irregular outer satellites of the gas giants, which orbit much farther away than the large well-known moons.

Pluto and Charon are an extreme example of a tidal lock. Charon is a relatively large moon in comparison to its primary and also has a very close orbit. This has made Pluto also tidally locked to Charon. In effect, these two celestial bodies revolve around each other (their barycenter lies outside of Pluto) as if joined with a rod connecting two opposite points on their surfaces.

So if you're standing on Pluto, not only do you always see the same side of Charon, but Charon is always in the same position in the sky. And if you stand on Charon ... the same thing WRT Pluto.

It sticks in my mind that we are able to observe 57% of the Moon's surface because of that wobble.

I also imagine that, since the Earth is wider than the moon, you can see the moon from different angles when your at different places on the Earth.

[Fist and Faith]

Why do we rotate 365.25 times per revolution around the sun? We're just a moon of the sun's. What about the other planets? I'm not finding it easy to look it up. How many times does Mars rotate in one of its years?

[dlbpharmd]

A martian day is:

There are two ways to determine the length of a day on any celestial body. The sidereal day and the solar day. A sidereal day on Mars is the length of time that it takes the planet to rotate once on its axis. A sidereal day on Mars lasts 24 hours 37 minutes and 22 seconds. The solar day is how long it takes the Sun to return to the meridian. This position changes slightly each day, but a solar day on Mars lasts 24 hours 39 minutes and 35 seconds.

www.universetoday.com/14717/how-long-is-a-day-on-mars/

and a Martian year:

So, you were sitting there wondering how long is a year on Mars? It lasts 686.98 Earth days or 1.88 Earth years. Now, given that a day(sol) on Mars is longer than on Earth, you would get a different answer. If you were standing on Mars, a year would take 668.5921 sols. In other words, Mars rotates 668.5921 during one orbit about the Sun.

www.universetoday.com/14718/how-long-i ... r-on-mars/

[Fist and Faith]

Well, I just read that a little more carefully than I did when I found it myself. I get it now. Thanks. (Interesting about the difference between solar and sidereal days. And it makes sense if you envision it. And I assume the solar day is longer if the planet rotates in the same direction it revolves, and shorter if it rotates in the opposite direction.)

So my question remains. Why isn't one rotation the same as one revolution for the planets, as it is for the planets' moons?

[I'm Murrin]

Because they're not old enough or close enough to the sun to have become tidally locked yet. They're much bigger than moons and the distance is much greater. They're all still slowing down, but probably more slowly than the life of the sun would allow for actual tidal locking to occur. Mercury has the slowest rotation, because it's closest to the sun and most affected by the tidal forces.

[Vraith]

It's all a matter of time, [everything tends towards zero rotation] distance, [gravity has much stronger effects as bodies get closer...and distorts the shape of the bodies "out of round"] relative masses, [big ones slow small ones more quickly than small ones slow big ones] momentum [all things being equal, the faster a body was rotating to begin with, the longer it will take to become locked, and massive ones are harder to lock too, of course], and how evenly distributed the mass is in a particular body. [if our moon, for instance were mostly aluminum...but had a great big deposit of lead that was off-center, it would tend towards locking the lead spot towards Earth.] Probably some other little things...and these things all effect each other, of course.

Without outside energy input, bodies that orbit each other tend toward locking.

Tons of moons in the solar system are tidally locked to their planets.
I know at least one of Mar's moons is locked...and I think both are.

Mercury and Venus are not-quite locked to the sun.

The moon wasn't always tidally locked...but it will remain so unless some other body gets close enough to alter things. [a very large body that gave the moon an off-center blow would spin it up a bit---except it, the moon, and we probably wouldn't survive in our present form unless the collider was exactly the right size...the same could happen with a near miss, if the mass, angles, location, and timing were just exactly right. But mostly a near pass [or collision] by a body big enough to change things also means death and destruction.].

The tides do have an effect...but I THINK the effect is drag/resistance on Earths rotation speed. I don't think that their bulge significantly reaches back to the moon, though, because they just don't have the mass to alter the gravitational balance very much at that distance. I'm sure there is a teeny teeny tiny effect [teeny teeny tiny is the correct technical term ] but other effects are much greater.

I officially thank my "Moons and Planets" professor for making the class interesting enough that I remember some stuff [more or less accurately, not sure of all of the above] even though it was a 100 level science requirement for non-science majors.

[wayfriend]

It was thought for some time that Mercury was tidally locked with the Sun. This was because whenever Mercury was best placed for observation, the same side faced inward. Radar observations in 1965 demonstrated instead that Mercury has a 3:2 spin–orbit resonance, rotating three times for every two revolutions around the Sun, which results in the same positioning at those observation points. The eccentricity of Mercury's orbit makes this 3:2 resonance stable.

Venus's 583.92-day interval between successive close approaches to Earth is equal to 5.001444 Venusian solar days, making approximately the same face visible from Earth at each close approach. Whether this relationship arose by chance or is the result of some kind of tidal locking with Earth is unknown.

[Fist and Faith]

Well, it was nice that my initial assumption was COMPLETELY WRONG! Very interesting. Thanks, folks!

[Cord Hurn]

Murrin is right. I'm not aware of a moon that has any kind of rotational period in the solar system.

EDIT: There is a slight bit of wobble though. It sticks in my mind that we are able to observe 57% of the Moon's surface because of that wobble.

That's a very close estimate, Damelon!! It's actually about 59% of Luna's surface that can be seen from Earth.

[Zarathustra]

The moon is also slowly receding away from the earth as it loses angular momentum due to tidal forces. That means it was at one point much closer, and would have been an awesome sight to see in the night sky.

[JIkj fjds j]

My questions are as follows; Has this always been, and will it remain so; why has this peculiarity developed with our moon and not with any of the moons we observe orbiting our sister planets [I'm guessing it has to do with having liquid waters on the surface of Earth that exert gravitational effects back upon the moon equal and opposit to the effects of the moon on them. On this point, as a student ['A' Level] I asked my physics teacher why this was so and he said it was 'coincidence' - this sounds like bullshit to me, and this guy had a physics PhD!].

My guess would be that if we were able to set up a cosmic weighing apparatus it would show that the total mass of each, the Earth, the Moon, and the Sun, would be roughly equal in weight, when formulated in correlation to their distances. The fulcrum of all three, planetary, planetoid, and solar, being La Grange points.

This is only an idea based on how the Moon and the Sun have equal circumferences, which can be observed during a solar eclipse.
"Nothing unusual or unique about it!" - as Murrin pointed out.
The balance is maintained indefinitely, because solar space being a vacuum holds all three orbs in a constant free-fall.

The idea that the Moon will eventually stop rotating may not be entirely correct, I believe, just mathematically correct.
Doesn't the Moon flip over every 10,000 years or so? Meaning that the dark side would then face the Earth. Wouldn't that set up another 10,000 year orbit, just like a pull on the string of a spinning top?

[Vraith]

My questions are as follows; Has this always been, and will it remain so; why has this peculiarity developed with our moon and not with any of the moons we observe orbiting our sister planets [I'm guessing it has to do with having liquid waters on the surface of Earth that exert gravitational effects back upon the moon equal and opposit to the effects of the moon on them. On this point, as a student ['A' Level] I asked my physics teacher why this was so and he said it was 'coincidence' - this sounds like bullshit to me, and this guy had a physics PhD!].

My guess would be that if we were able to set up a cosmic weighing apparatus it would show that the total mass of each, the Earth, the Moon, and the Sun, would be roughly equal in weight, when formulated in correlation to their distances. The fulcrum of all three, planetary, planetoid, and solar, being La Grange points.

This is only an idea based on how the Moon and the Sun have equal circumferences, which can be observed during a solar eclipse.
"Nothing unusual or unique about it!" - as Murrin pointed out.
The balance is maintained indefinitely, because solar space being a vacuum holds all three orbs in a constant free-fall.

The idea that the Moon will eventually stop rotating may not be entirely correct, I believe, just mathematically correct.
Doesn't the Moon flip over every 10,000 years or so? Meaning that the dark side would then face the Earth. Wouldn't that set up another 10,000 year orbit, just like a pull on the string of a spinning top?

On the first para: if that is meant to be a way of saying that, for instance, Earth doesn't orbit Sun, but both orbit a balance point between, that is so...though difference in mass makes that point very very close to the center mass of the Sun. If that's NOT what you mean, then what do you mean??

Moon and Sun don't have equal circumference, just relatively/apparent is pretty close. [[maybe you meant the apparent]] But that appearance changes because distance changes...but I'm way too lazy to do the math on how MUCH it changes, over what time period.
So, there's "nothing special" about eclipses...but it isn't "indefinitely stable."
It changes, and the exact changes, in both "real" and "apparent" terms, could be calculated...by someone less lazy than me...precisely.

I don't think there is anything correct in the last para.

[JIkj fjds j]

What do I mean! Just guesswork really.

During a total eclipse of the Sun there is a corona of light, so maybe that's what you meant by apparent circumference. My point being, for all intents and purposes, the Moon and the Sun are the same size. To us. And that tells me they are also in a cosmic position to have equal weight.

If you were to place a breeze block on a scale, and a block of wood the same size as the breeze block, on the opposite scale, when the fulcrum has been found, whether that be yard to a mile in ratio, then both objects are the same weight. Obviously, removing both objects from the scales returns them to one being much more heavier than the other.

The 10,000 year thing was told to me sometime ago by a very clever person. Maybe he was like peter's A level PhD tutor, and simply talking bull.

[Vraith]

During a total eclipse of the Sun there is a corona of light, so maybe that's what you meant by apparent circumference. My point being, for all intents and purposes, the Moon and the Sun are the same size. To us. And that tells me they are also in a cosmic position to have equal weight.


The 10,000 year thing was told to me sometime ago by a very clever person. Maybe he was like peter's A level PhD tutor, and simply talking bull.

By "apparent" I meant appears to be to us, from our visual viewing position. Like sitting here looking out my window, my hands are just the right size to squash the cars going by---apparently. But that is only ONE intent and purpose...and a false one in almost any meaningful way [though I suppose some brilliant artist could compose a poem or painting or film that would give my car-crushing hand some serious/important metaphorical meaning].

Your clever friend was probably pulling your leg...it was so specific, I went and looked around a bit, cuz it sounded like a "moon landing was fake" or Astrology sort of thing....the only thing I found that happens about every 10k years for the moon is that a large-size meteor hits it. And the effects would likely be visible from Earth with the naked eye---if it hit the parts we can see.
One big enough to flip it over, though---that would be visible to us years in advance. Likely naked-eye visible for at least months. And when it hit, it would be devastating to us and the moon.
[[a mini black hole might not be naked-eye visible...but those probably aren't possible unless made and sustained by super-advanced aliens...and if super-advanced aliens are blasting our moon, we have a LOT of other problems to worry about. Coincidentally, I think a destroyed moon is part of the events in the latest Neal Stephenson book---I think I saw Murrin or someone make it a topic today or yesterday....are you Neal Stephenson trying to pump up potential interest in your new book in strange ways???]]

[Fist and Faith]

We could say the tides are a scale measuring the relative mass of moon and sun. In which case, the moon is "bigger", since it causes stronger tides than the sun does.

[Vraith]

We could say the tides are a scale measuring the relative mass of moon and sun. In which case, the moon is "bigger", since it causes stronger tides than the sun does.

Hee...yea, I suppose you could say that...except there wouldn't BE and Earth or a Moon around it if there were no Sun.