Earth's Moon
- Thread starter Orleander
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Janus58,
If the astronaut concludes the moon is rotating on its axis, he must also conclude the Earth orbits himself and the moon in synchronicity with the rotation of the moon, or else the Earth would not stay almost motionless in his sky. An intelligent astronaut could measure the relative masses of the Earth and moon and conclude that the much larger mass of the Earth is not orbiting the smaller mass of the moon. Therefore, he concludes the moon is orbiting (revolving around) the Earth and that the moon is not rotating on its axis relative to the Earth. He could also conclude the moon does rotate relative to the 'fixed' stars. Those are the frames of reference I was speaking about that you insist on ignoring.
(my italics)
If the astronaut concludes the moon is rotating on its axis, he must also conclude the Earth orbits himself and the moon in synchronicity with the rotation of the moon, or else the Earth would not stay almost motionless in his sky. An intelligent astronaut could measure the relative masses of the Earth and moon and conclude that the much larger mass of the Earth is not orbiting the smaller mass of the moon. Therefore, he concludes the moon is orbiting (revolving around) the Earth and that the moon is not rotating on its axis relative to the Earth. He could also conclude the moon does rotate relative to the 'fixed' stars. Those are the frames of reference I was speaking about that you insist on ignoring.
And if a frog had wings, he wouldn't have to bump his ass every time he took a step.
Mercury is not tidally locked with the sun, that was a mistake in old textbooks. Mercury rotates three times for every two orbits around the sun. It is called a 3:2 spin-orbit resonance, not a tidal lock.
I'm sorry, I wasn't clear as to what I meant by 'within the frame of the farris wheel'. I didn't mean the farris wheels frame of reference, but the actual circular steel frame that the gondula is attached to. The gondula rotates within the circular steel frame as the steel frame rotates relative to a ground observer.
I wasn't necessarily objecting to your description, but rather to your model differing from that written in most elementary textbooks. I always understood that the primary disagreement with your model was because the barycenter of rotation ( the center of mass) of the Earth/moon system fell within the physical body of the Earth.
Janus58
Valued Senior Member
Not only that, but in reality, it wouldn't be possible for the Moon to keep its axis of rotation pointing at the Earth through out the entire orbit. In which case, with one rotation per orbit, we would see something like this over the course of an orbit:
Janus58
Valued Senior Member
You are confusing "point of view" and "frame of refernce", they are not the same thing. Once the Astronaut concludes that the Moon is revolving around the Earth, he uses that frame of reference to determine that the moon is rotating to maintain the same face to the Earth, regardless of what "his point of view" says. He doesn't use one frame of reference to measure the revolution and another for the Moon's rotation. That would be no different than saying that while the Earth travel's around the Sun, we measure the Earth's rotaion around its own axis relative to itself, and the Earth doesn't rotate. You don't measure the moon's revolution around the Earth by a different reference than you do its rotation.
If you are quite through being foolish.And if a frog had wings, he wouldn't have to bump his ass every time he took a step.
The situation of the Horse and rider set on frictionless bearings is a much closer analogy to a moon orbiting a planet than one where they are rigidly attached to the frame of the carousel.
If it is the term "frictionless" that you are worried about, it is perfectly allowable condition in a theorectical example. But if you insist, we can replace it with "extremely low friction"( Just enough frction so that after some few hundred million or so trips around the carousel, the horses will match the rotation of the carousel.). That would make it even a closer analogy to an orbiting moon. Add in that the carousel changes it speed over the course of a rotation, and you come even closer to the Earth-Moon system.
The point of the argument remains the same; there is no special conection between the direction the horses face and their circular motion around the center of the carousel.
Don't put words into my mouth, I never said or implied that Mercury kept one side towards to the Sun. I explictly said that it had a 3:2 rotation to revolution ratio
And the Moon has a 1:1 resonance. Actually, it is a moot point whether or not the term "tidal lock" is limited to just 1:1 resonances or includes others.
On another note, I'm surprised that you aren't claiming that Mercury is in a 1:2 resonance (rotates once for every 2 orbits). After all, its Solar day is 176 days compared to its 88 day orbit. Since you seem to think that the Moon's rotation should be judged by the Earth's preceived motion in its sky, then it seems that, to be consistant,you should claim that Mercury's would be judged by the Motion of the Sun in its sky.
Again, you seem to want to jump back and forth between points of view and frames of reference willy nilly whenever you wish.
Let's approach this from another angle (so to speak).
Assume we have the situation as mentioned earlier in this thread where the moon's axis lies parallel to the plane of it orbit(90° axial tilt), and it rotated on its axis once per orbit (relative to the stars), and appears from the Earth as I show in my last post. You'd agree that the Moon does indeed rotate on its axis in this case wouldn't you, and if not, why not?
Now make the axial tilt 80°, is it still rotating on its axis? At 70°, 45° or 30°?
And at what point between 90° and 0° axial tilt do you claim that the moon suddenly stops rotating on its axis?
Duh! I was the first one to explain tidal force induced synchronous orbit in this thread.
DwayneD.L.Rabon
Registered Senior Member
Well i would say that the moon is to big to be a natural satilite, one that has a equatorial orbit, a natural satilite would be approximatly say 447 miles diameter for the earth. The earths moon is large than a natural satilite and so the moon has inclination of its orbit around earth.
the moons mass is less than the work force of the earths magnetic feild, considerablly less than the work force of the magnetic feild. so it should move around according to the fluxuation of the magnetic feild. as well given the ratio of mass of the moon to earth mass the moon should follow every move of the earth.
The earth has a spin of about 1041 miles per hour, which would give the moon a rotation of less than 9.2 miles per hour, so it would take 732.6 hours or 30.5 days for the moon to make a rotation.
In end it appears that rotation is dependant on the presence of a magnetic feild, the moon does not have a magnetic feild and so in such case the moon should not rotate at all. if it does rotate then that rotation is due to the interaction of the moon with the earths magnetic feild and that of the suns magnetic feild.
This could be quite a argument, but given the observed motion of the moon if the moon had a rotation based its reaction with the earth, it should not rotate at all given the effects of the sun gravity, neither should the earth.
DwayneD.L.Rabon
the moons mass is less than the work force of the earths magnetic feild, considerablly less than the work force of the magnetic feild. so it should move around according to the fluxuation of the magnetic feild. as well given the ratio of mass of the moon to earth mass the moon should follow every move of the earth.
The earth has a spin of about 1041 miles per hour, which would give the moon a rotation of less than 9.2 miles per hour, so it would take 732.6 hours or 30.5 days for the moon to make a rotation.
In end it appears that rotation is dependant on the presence of a magnetic feild, the moon does not have a magnetic feild and so in such case the moon should not rotate at all. if it does rotate then that rotation is due to the interaction of the moon with the earths magnetic feild and that of the suns magnetic feild.
This could be quite a argument, but given the observed motion of the moon if the moon had a rotation based its reaction with the earth, it should not rotate at all given the effects of the sun gravity, neither should the earth.
DwayneD.L.Rabon
All of your post is correct but as you, like me, live in the Southern Hemisphere surely you should have noticed, as I have, that the moon is "upside down." Or perhaps not if you were born and have always lived in the Southern Hemisphere.
I have even noted that the cresent moon's points are pointing the "wrong way" when it waxes (and when it wains) ever since I move to Brazil from the US.
I never realize my cosmic importance before. God must be giving me some sign to flip the moon around that way.
Surely that is it as he flipped the direct which is "up" also. (Or alternatively, as Polaris is not high in the sky anymore - something the locals call the "Southern Cross" has stollen Polaris's place. If this crime is what really happend, so "up" is as it always was, then God did flip the moon around,
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That is correct. In fact, as Janus58 taught me, neither the Earth's nor the moon's orbit around the sun is at any point on the orbit convex towards the sun. I.e. at every point on the orbit even the moon is curving towards the sun.
Thus, the idea that the moon orbits the Earth is rather silly - a distortion caused by fact that humans are viewing the moon from a platform which is also slightly "wobbling" in its nearly circular orbit about the sun.* The moon is also in a nearly circular around the sun, but the "wobble" in its orbit is of larger amplitude. The moon orbits the sun, not the Earth, because the sun's gravity act more strongly on the moon than the Earth's gravity does. The Earth's gravity is responsible for the wobble in the moon's orbit and the sun's gravity is responsible for the nearly circular shape of the moon's orbit around the sun.
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*Human egotism also contributes to this false, but widely accepted, believe that the moon orbit the Earth. Once this egotism had the entire universe orbiting the Earth. This egotism, reinforced by simple observation, died slowly. First the distant stars were allow not to orbit the Earth; then the planets were allowed to have smaller circles as part of their orbits that did not go around the Earth, but the main orbit still did. (They are observed to go "backwards" during part of their orbits so these small circles were introduced to explain how that was possible.) Finally, it was understood and accepted by all educated people that all members of the solar system EXCEPT THE MOON orbit the sun. That false exception has yet to die. (To help kill it is why I make this post.) Man's egotism is so strong and it appears so obvious that the moon orbit the Earth that to get the truth accepted is hard. This “Moon orbits the Earth” belief is just as strong and just as false as the earlier belief that the distant stars orbit the Earth.
Then again, the solar system is orbiting around the galaxy, so the moon could also be said to be orbiting the galactic centre.
Or should that be the COM of the local group?
There's meant to be a distant "great attractor" out past the Sloan Wall somewhere, too, that the local group has a special velocity towards, so it's orbiting something as well.
Or should that be the COM of the local group?
There's meant to be a distant "great attractor" out past the Sloan Wall somewhere, too, that the local group has a special velocity towards, so it's orbiting something as well.
That is all true, but few have any beliefs (false or otherwise) about that. I am only trying to correct the false, widely held, belief that the moon orbits the Earth.
Hi Billy,
I'm not sure that a Solar reference frame is the best way to determine how the Earth-Moon system is labeled. Yes, they both orbit the Sun, but is that relevant? Doesn't the word "satellite" tell us something about the relationship between two things, rather then their relationship with another thing?
If the Earth-Moon system were located at a much further distance from the Sun, should that change whether one is labeled as a satellite of the other or not?
I'm not sure that a Solar reference frame is the best way to determine how the Earth-Moon system is labeled. Yes, they both orbit the Sun, but is that relevant? Doesn't the word "satellite" tell us something about the relationship between two things, rather then their relationship with another thing?
If the Earth-Moon system were located at a much further distance from the Sun, should that change whether one is labeled as a satellite of the other or not?
It's a question of perspective, and the scale of that perspective.
If you're on the same fixed location on the surface of a large rotating sphere, then everything in the universe rotates around you, or has an apparent motion.
The special velocity of the local group wasn't calculated by going to a point in space out beyond it to see how fast it's moving. You have to cancel out a lot of other apparent motions to derive it.
You can calculate that the Earth and Moon orbit the Sun together (as a coupled system), by cancelling the Earth's rotation. But the Moon always looks like it's going around the Earth, from any point on the Earth's surface. If you were on the Moon instead, the earth would look like it goes around the Moon - but that would depend significantly on where you were on the Moon and how much horizon was between you and your view of the Earth.
They "go around" each other, from a point of view out in space, but the Moon being a lot smaller does most of the going - it has to orbit the Sun and (interact with) the large planetary body next to it. The Earth has the same orbital "problem" to solve, but doesn't need to orbit as much, being seriously more massive.
"Orbit" meaning a more general term for "gravitational interaction", perhaps. If you want to be exact and call an orbit only a closed path, then it gets trickier.
If you're on the same fixed location on the surface of a large rotating sphere, then everything in the universe rotates around you, or has an apparent motion.
The special velocity of the local group wasn't calculated by going to a point in space out beyond it to see how fast it's moving. You have to cancel out a lot of other apparent motions to derive it.
You can calculate that the Earth and Moon orbit the Sun together (as a coupled system), by cancelling the Earth's rotation. But the Moon always looks like it's going around the Earth, from any point on the Earth's surface. If you were on the Moon instead, the earth would look like it goes around the Moon - but that would depend significantly on where you were on the Moon and how much horizon was between you and your view of the Earth.
They "go around" each other, from a point of view out in space, but the Moon being a lot smaller does most of the going - it has to orbit the Sun and (interact with) the large planetary body next to it. The Earth has the same orbital "problem" to solve, but doesn't need to orbit as much, being seriously more massive.
"Orbit" meaning a more general term for "gravitational interaction", perhaps. If you want to be exact and call an orbit only a closed path, then it gets trickier.
Billy, that "false, widely held, belief" is held by the very astronomers who label solar system bodies as planets, the International Astronomical Union (IAU). The IAU uses two criteria to determine whether some smaller object is a satellite of a larger object. First and foremost, the objects must be gravitationally bound to one another. Secondly, the center of mass of the pair of objects must lie inside the surface of the larger object. If two objects are not gravitationally bound to one another the second test need not be applied. Mars, for example, is not a moon of Jupiter. The objects are co-planets (or co-minor planets) if two objects are gravitationally bound to one another but the center of mass lies between the two objects.
How to determine if two objects are gravitationally bound to one another? First, examine the objects as if there were no perturbing influences (e.g., the Sun). Is the velocity of the smaller object less than escape velocity? Second, examine the smaller object given the existence of perturbing influences. Does the orbit of the smaller body lie within the Hill sphere of the larger body?
Now to apply these tests to the Moon: The Moon's velocity wrt the Earth is well below escape velocity. The Moon lies well within the Earth's Hill sphere. Finally, the Earth-Moon center of mass is inside the Earth. The Moon is a satellite of the Earth.
I agree that the moon is a natural satellite of Earth, but both Earth and the Moon only "wobble" very slightly, about 13 cycles per year, from a pure elliptical orbit about the sun. (Both are always curving towards the sun.) So I think it is also correct to say:
"The moon orbits the sun;"
"The Earth orbits the sun;" and
"The bound earth moon system orbits the sun."
"Viewed from Earth, the Moon and sun APPEAR to orbit the Earth."
"Viewed from Moon, the Earth APPEARS to hang nearly stationary above one spot on the Moon, but the sun orbits the Moon in approximately 28 Earth days" (Or, by definition, in exactly one Moon day.)*
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* Note any permanent moon base should be powered by a closed thermal-cycle turbine** which runs between the Hot and Cold storage reservoirs. They could be just tubes buried about 50 cm in the ground in two well separated areas which have a large sliding reflective cover. At sunrise, that cover is over the Hot Area and moves to cover the Cold Area until sun set about 14 Earth days later. For safety redundancy at least two turbines with independent flow loops thru these thermal storage areas should exist.
The movable cover is a light weight metal grid with thin aluminized film sheets covering each "unit cell." (I like hexagons as they fit inside the delivery ship better, but squares are OK.) The low gravity will help make the cover lighter too. This sliding cover is about two meters above the surface so "MOON PERSONS" can easily repair meteor damage. Obviously it and its rolling tracks are assembled on the moon. The manned area is either a few dozen meters below the surface for thermal stability (my preference, if affordable) OR only about two meters in the ground with a roof that also serves as a "pre-heater" for part of the turbine's exhaust during the moon day prior to its flow into the Hot Area (and as "pre-cooler" during the moon night). Fraction of total flow thru the roof is varied as needed for human comfort. Fortunately, This T, (human Temp) the HT (hot temp) and CT are related as: HT>>T>>CT so this is feasible. The "working fluid" probably is a gas to avoid becoming a solid at CT. Part of the power produced by the main turbines drives minor turbine pumps to recover the high pressure needed in the Hot Area flow.
The moon base is and astronomer's dream. Nearly 14 day exposures with no air distortion! If you go, don't forget your mittens.
**It could be about twice as efficient as any trubine on Earth.
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