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h4ckys4ck
06-28-2011, 05:20 AM
What would happen? I would imagine the side facing the sun would get very hot. Would it reflect more light/heat to us at night?

Small Clanger
06-28-2011, 05:26 AM
It sort of already has, it's tidally locked (http://en.wikipedia.org/wiki/Tidal_locking#Earth.27s_Moon) to the Earth.

Alessan
06-28-2011, 05:31 AM
Besides, the direction the moon faces the sun is determined by its orbit around the Earth, not rotation on its own axis.

Blake
06-28-2011, 05:47 AM
What would happen? I would imagine the side facing the sun would get very hot. Would it reflect more light/heat to us at night?

Stopped rotating relative to what? The Earth? The Sun? The centre of the galaxy? It can't stop rotating relative to more than one of them.

Besides, the direction the moon faces the sun is determined by its orbit around the Earth, not rotation on its own axis.

:confused: What does that mean?

h4ckys4ck
06-28-2011, 05:54 AM
Stopped rotating relative to what? The Earth? The Sun? The centre of the galaxy? It can't stop rotating relative to more than one of them.



:confused: What does that mean?

Ok then. If the moon rotated in such a way that it just had 1 side facing the sun all the time. Would we notice anything I mentioned in the OP.

Also I thought the moon was still rotating on it's own axis?

Blake
06-28-2011, 06:18 AM
Ok then. If the moon rotated in such a way that it just had 1 side facing the sun all the time. Would we notice anything I mentioned in the OP.

Then yeah, the side facing the sun would get very hot. Depending how the extreme temperatures affected the chemistry of the rock, it might produce marginally more light or marginally less. It would never provide any measurable amount of heat

Also I thought the moon was still rotating on it's own axis?

It is still rotating on its own axis, relative to everything but the Earth. It is tidally locked to the Earth, so it rotates at exactly the same speed as its orbit around the Earth. The upshot is that it always has one side facing the Earth all the time.

Imagine someone walking around a building, while at the same time always facing the building. The person will have to spin 360o around their own axis, but they won't rotate at all relative to the building.

Ken001
06-28-2011, 06:21 AM
No, the Moon is tidally locked to the Earth so that one face always faces us. The Moon rotates around the Earth's axis each 28 days. The Moon does not rotate about its own axis.

I think you are asking what would happen if the Moon was tidally locked to the Sun ie. one face always pointed at the Sun. The Moon would rotate in that case.

Nothing dramatic. The sunny surface would heat up a little and gases would be released. We might see frost on the dark side - that would be interesting.

njtt
06-28-2011, 06:34 AM
Besides, the direction the moon faces the sun is determined by its orbit around the Earth, not rotation on its own axis.

:confused: What does that mean?

Maybe you should take a moment to think about it. It makes perfectly good sense to me.

I daresay it is possible to invent an uncharitable interpretation of Alessan's comment under which it it becomes false or incoherent, but why would you insist on doing so?

(Oh, you're Blake. Nevermind.)

Blake
06-28-2011, 06:44 AM
No, the Moon is tidally locked to the Earth so that one face always faces us. The Moon rotates around the Earth's axis each 28 days. The Moon does not rotate about its own axis.

Care to explain how that is even possible? How can Object A always face Object B, rotate around Object B, orbit Object B, yet never rotate around it's own axis? That would seem to be impossible in 3 dimensional space.

Look at this time series


A A A A
| / \ |
| / \ |
B B B------ A B B B A------B B B
\ | /
\ | /
A A A







A has orbited B, and it has done that without rotating on its own axis. But it can;t possibly do that while always having the same face to Object B. In order to that it A is going to have to rotate on its own axis.

Correct?

septimus
06-28-2011, 06:53 AM
Also I thought the moon was still rotating on it's own axis?

Here's a trickish question:

How many times does the Earth rotate on its axis in one year?

Blake
06-28-2011, 07:03 AM
Here's a trickish question:

How many times does the Earth rotate on its axis in one year?

I'm guessing there is some semantic trick here, but the obvious answer is ~365.25.

septimus
06-28-2011, 07:27 AM
I'm guessing there is some semantic trick here, but the obvious answer is ~365.25.

If one grants that "rotating on one's axis" is ambiguous, then we can at least hope for self-consistency. Your answer here is inconsistent with your response #9.

Google "sidereal day".

Blake
06-28-2011, 07:35 AM
If one grants that "rotating on one's axis" is ambiguous, then we can at least hope for self-consistency. Your answer here is inconsistent with your response #9.

Google "sidereal day".

:dubious:

The sidereal day is shorter than the stellar day by about 8.4 ms (http://en.wikipedia.org/wiki/Earth%27s_rotation)

So 365.24999999999999 is not approximately ~365.25?

Machine Elf
06-28-2011, 07:57 AM
No, the Moon is tidally locked to the Earth so that one face always faces us. The Moon rotates around the Earth's axis each 28 days. The Moon does not rotate about its own axis.

I think you are asking what would happen if the Moon was tidally locked to the Sun ie. one face always pointed at the Sun. The Moon would rotate in that case.


The moon currently does rotate (http://en.wikipedia.org/wiki/Rotation) around its own axis, completing one rotation every 27.3 days.

The moon also orbits (http://en.wikipedia.org/wiki/Orbit) around the earth, completing one orbit every 27.3 days.

It rotates in the same direction as its orbit, with the net result being that it always has the same side facing toward the earth.

If the moon somehow became tidally locked to the sun, it would still be orbiting the earth (with an orbital period of 27.3 days), but it would now only complete one rotation every 365.25 days.

Fourtyfold
06-28-2011, 08:43 AM
:dubious:

The sidereal day is shorter than the stellar day by about 8.4 ms (http://en.wikipedia.org/wiki/Earth%27s_rotation)

So 365.24999999999999 is not approximately ~365.25?

The length of a regular day (noon-noon) is a solar day, a stellar day is something quite different. A sidereal day (one 360o rotation) is ~3m56s shorter than a solar day. The solar day is longer because the earth is also rotating around the sun, and it takes the extra ~3.56 for the earth to rotate relative to the sun.

Blake
06-28-2011, 09:00 AM
A sidereal day (one 360o rotation) is ~3m56s shorter than a solar day.

Which works out to ~24 hours a year.

So still ~365.25 rotations in a year.

I'm sure there is some gotcha moment coming up, but I still don't see it.

septimus
06-28-2011, 09:01 AM
:dubious:

The sidereal day is shorter than the stellar day by about 8.4 ms (http://en.wikipedia.org/wiki/Earth%27s_rotation)

So 365.24999999999999 is not approximately ~365.25?

Sounds like you missed at least three point(s).

First of all, neither the "sidereal day" nor the "stellar day" is the solar day. From the very same Wiki paragraph where you find the irrelevant factoid you could also have learned that the stellar day (close enough to the sidereal day for our purpose) is "23h 56m 4.090 530 832 88s, 0.997 269 566 329 08 mean solar days." If you type "365.2422/0.997269" into Google you get 366.2424.

That's right! The Earth rotates 366¼ times per year, not 365¼ times.

BTW, there are 365.2422 solar days in a year (not "365.24999999999999" :confused: ), just as there are zero Earth-referenced lunar days in a month. It is weird. Should one count the extra rotation or not? Either way seems OK, as long as you keep track which way you're doing it and retain consistency. You insisted (in #9) on counting the extra rotation of the Moon around the Earth (making it 1 per month instead of zero), so you should count the extra Earth rotation as well, making it 366¼ rotations, not 365¼.

Does this help?

Blake
06-28-2011, 09:12 AM
Does this help?

Not in the least. It's the sort of pointless nitpickery that the Dope does so well.

Claiming that 366.2424 is not approximately 365.25? Seriously?

Claiming that "the extra rotation of the Moon around the Earth" is the same as if the moon had"rotated on its own axis"? Really?

Come on, have we really been reduced to this? What next? Arguing about what the definition of is, is?

The simple fact is that the moon does rotate about its own axis abut once a month and the Earth also rotates about its own axis about 365 times a year.

Chessic Sense
06-28-2011, 09:18 AM
Clear as (a solar) day, septimus. I vote for "the moon does not rotate about its axis," not because of the definition of rotate, but the definition of axis. If I swing a pocket watch around by its chain, nobody would claim that the watch was rotating about the timepiece's hands' center point, but rather rotating around my hand. The moon surely rotates, just as the watch does, but around the Earth's axis, not an axis through the center of the moon.

Fourtyfold
06-28-2011, 09:20 AM
Which works out to ~24 hours a year.

So still ~365.25 rotations in a year.

I'm sure there is some gotcha moment coming up, but I still don't see it.

~366.25 rotations.

The 'gotcha' bit is the fact that the earth is revolving around the sun - remember, a 'day' is slightly more than one actual rotation because of this. So the earth will rotate once more than the number of apparent days, the 'missing' day is used up by those extra 3.56 minutes.

Chessic Sense
06-28-2011, 09:21 AM
Not in the least. It's the sort of pointless nitpickery that the Dope does so well.

Claiming that 366.2424 is not approximately 365.25? Seriously?

Claiming that "the extra rotation of the Moon around the Earth" is the same as if the moon had"rotated on its own axis"? Really?

...

The simple fact is that the moon does rotate about its own axis abut once a month and the Earth also rotates about its own axis about 365 times a year.

How is it different? Either you're counting the rotation around the central body or you're not. 366 is not 365, just as 1 is not 0. You can't count going around the Earth as 1 rotation, but not count going around the Sun as 1 rotation. You're backpedaling.

Machine Elf
06-28-2011, 09:29 AM
Clear as (a solar) day, septimus. I vote for "the moon does not rotate about its axis," not because of the definition of rotate, but the definition of axis. If I swing a pocket watch around by its chain, nobody would claim that the watch was rotating about the timepiece's hands' center point, but rather rotating around my hand. The moon surely rotates, just as the watch does, but around the Earth's axis, not an axis through the center of the moon.

I see what you're saying. You're combining the moon's rotation around its own center of mass with its translation (orbit) around the earth, and identifying the center of the earth as the moon's instantaneous center of rotation. (http://en.wikipedia.org/wiki/Instant_centre_of_rotation) This is sometimes a useful concept in the analysis of kinematic systems, but for clarity's sake in this case I think it's best to separately identify the moon's spin and its movement through space.

Blake
06-28-2011, 09:38 AM
How is it different? Either you're counting the rotation around the central body or you're not. 366 is not 365, just as 1 is not 0. You can't count going around the Earth as 1 rotation, but not count going around the Sun as 1 rotation. You're backpedaling.

It isn't different. That is the whole point.

I am not counting the rotation around the central body and never have been. We have only ever been talking about rotation about the axis. If the rotation about the axis is caused by rotation around the central body, that is irrelevant. The rotation around the axis still occurs.

Nobody ever claimed that 366 is not 365. The statement was that the value was approximately 365. Are you seriously arguing that 365 is not approximately equal to 366?

You are indulging in pointless nitpicking. Next you are goint o start nitpicking my claim that the moon rotates about it's axis approximately every month, and point out that it's really 29. 467897655409876 days, and that 29. 467897655409876 days is not approximately a month.

minor7flat5
06-28-2011, 09:44 AM
Sounds sort of like the "gotcha" moment in Around the World in Eighty Days (http://en.wikipedia.org/wiki/Around_the_World_in_Eighty_Days), no?

Chessic Sense
06-28-2011, 09:56 AM
It isn't different. That is the whole point.

I am not counting the rotation around the central body and never have been. We have only ever been talking about rotation about the axis. If the rotation about the axis is caused by rotation around the central body, that is irrelevant. The rotation around the axis still occurs.

Nobody ever claimed that 366 is not 365. The statement was that the value was approximately 365. Are you seriously arguing that 365 is not approximately equal to 366?

You are indulging in pointless nitpicking. Next you are going to start nitpicking my claim that the moon rotates about it's axis approximately every month, and point out that it's really 29. 467897655409876 days, and that 29. 467897655409876 days is not approximately a month.

Septimus: I have a trick question...how many times does the Earth rotate in a year?
Blake: 365.
Septimus: Nope. 366.
Blake: Same thing.

It's utter foolishness. You fell for the trick question, gave the stock (wrong) answer, and now you're trying you argue your correctness. It's not fooling anybody. It's not a nitpick. You got the answer straight-up, no-contest wrong.


How many times does an observer on the Sun see London in a year? 365.
How many times does an observer on a distant star see London in a year? 366.
How many times does an observer on the Earth see the ass end of the Moon in a month? 0.
How many times does an observer on the distant star see the ass end of the Moon in a month? 1.

Either your vantage point is from afar or it's on the central body. Your choices are 365/0 or 366/1. Either one is correct, of course. But 365/1 is inconsistent.

Blake
06-28-2011, 10:02 AM
Septimus: I have a trick question...how many times does the Earth rotate in a year?
Blake: 365.
Septimus: Nope. 366.
Blake: Same thing.You have been reported for deliberately misquoting me.

I never at any stage said 365. I said APPROXIMATELY 365. Since I assume that you are capable of understanding what approximately means I have to conclude this is deliberate.

what do I type here
06-28-2011, 10:06 AM
Christ, calm down everyone.

Chessic Sense
06-28-2011, 10:12 AM
I see what you're saying. You're combining the moon's rotation around its own center of mass with its translation (orbit) around the earth, and identifying the center of the earth as the moon's instantaneous center of rotation. (http://en.wikipedia.org/wiki/Instant_centre_of_rotation) This is sometimes a useful concept in the analysis of kinematic systems, but for clarity's sake in this case I think it's best to separately identify the moon's spin and its movement through space.

But say you take two moon rocks on the opposite sides of the moon and track them. The outside, far-side rock moves with some great velocity in a direction we'll define as +Y. Now the question: Which way is the other rock moving? It's not opposite. It's the same, just slower. It's still +Y.

Now imagine you and I are running on a track. I have the inside lane and you have the outside lane. My path is shorter but you're faster to such a degree that we finish the first lap at the same time.

Are you saying that we spin around a point between us every lap? If so, then we have different definitions of spin (which is fine). If not, then how can you say that the moon rocks spin around a central point?

And if you do say that the rocks spin around a central point, then why stop there? Why not take a moon rock on the surface and one buried directly under it and say they spin around a central point? After all, they're supposedly spinning around an axis, just at different rates, just like the two surface moon rocks. What's the difference? Taken to the extreme, you'd have to say that every point in a Moon-Earth-Sun system is rotating around some axis between them.

Or you can, for clarity's sake, use the "instant center of rotation".

Munch
06-28-2011, 10:53 AM
You have been reported for deliberately misquoting me.

I never at any stage said 365. I said APPROXIMATELY 365. Since I assume that you are capable of understanding what approximately means I have to conclude this is deliberate.

And I have reported you* for deliberately misquoting yourself. You didn't say approximately 365, you said approximately 365.25. When measuring down to the precision of a hundredth, it is dishonest to suggest that a difference in the number three orders of magnitude greater is insignificant.

365.25 is approximately 365.26 - but it isn't at all approximately 366.25.

*not really

Machine Elf
06-28-2011, 10:55 AM
But say you take two moon rocks on the opposite sides of the moon and track them. The outside, far-side rock moves with some great velocity in a direction we'll define as +Y. Now the question: Which way is the other rock moving? It's not opposite. It's the same, just slower. It's still +Y.

It is indeed opposite if your frame of reference is attached to the center of the moon.

It is convenient to say that the moon's instantaneous center of rotation is at the center of the earth, but this happens to be so only because the moon revolves exactly once per orbit. If the moon maintained its orbit but didn't spin at all, or spun at half its present speed, how would you then describe its motion?

If not, then how can you say that the moon rocks spin around a central point?

I can say they spin around a central axis in the same way that your bicycle's front wheel spins around a central axis (its own axle), even though it's rolling down the road and its instantaneous center of rotation is its point of contact with the road. In either case, the (non-rotating) frame of reference is attached to the center of mass of the rotating object.

Colibri
06-28-2011, 12:03 PM
I daresay it is possible to invent an uncharitable interpretation of Alessan's comment under which it it becomes false or incoherent, but why would you insist on doing so?

(Oh, you're Blake. Nevermind.)

Not in the least. It's the sort of pointless nitpickery that the Dope does so well.

Claiming that 366.2424 is not approximately 365.25? Seriously?

Claiming that "the extra rotation of the Moon around the Earth" is the same as if the moon had"rotated on its own axis"? Really?

Come on, have we really been reduced to this? What next? Arguing about what the definition of is, is?

Septimus: I have a trick question...how many times does the Earth rotate in a year?
Blake: 365.
Septimus: Nope. 366.
Blake: Same thing.

It's utter foolishness. You fell for the trick question, gave the stock (wrong) answer, and now you're trying you argue your correctness. It's not fooling anybody. It's not a nitpick. You got the answer straight-up, no-contest wrong.




You have been reported for deliberately misquoting me.

I never at any stage said 365. I said APPROXIMATELY 365. Since I assume that you are capable of understanding what approximately means I have to conclude this is deliberate.

And I have reported you* for deliberately misquoting yourself. You didn't say approximately 365, you said approximately 365.25. When measuring down to the precision of a hundredth, it is dishonest to suggest that a difference in the number three orders of magnitude greater is insignificant.

365.25 is approximately 365.26 - but it isn't at all approximately 366.25.

*not really

[Moderator Instructions]

OK, let's knock off the snark and the personal comments (not to mention claims of reporting one another's posts, whether in jest or not). Let's address the facts at issue, without playing gotcha games with one another. If you want to claim that someone is being dishonest, take it to the Pit.

This goes for everyone. No warnings issued (yet), but violations of these instructions may be subject to a warning.

Colibri
General Questions Moderator

Chessic Sense
06-28-2011, 12:05 PM
It is indeed opposite if your frame of reference is attached to the center of the moon.

It is convenient to say that the moon's instantaneous center of rotation is at the center of the earth, but this happens to be so only because the moon revolves exactly once per orbit. If the moon maintained its orbit but didn't spin at all, or spun at half its present speed, how would you then describe its motion?



I can say they spin around a central axis in the same way that your bicycle's front wheel spins around a central axis (its own axle), even though it's rolling down the road and its instantaneous center of rotation is its point of contact with the road. In either case, the (non-rotating) frame of reference is attached to the center of mass of the rotating object.

So what if it "happens to be so"? That's precisely the reason why I'm saying it doesn't rotate. That's like saying "Well, sure, it's convenient to say the car goes 10 miles in 2 hours, but that's only because it goes 5mph." It's not invalid just because you're using language to suggest happenstance. If the moon maintained its orbit and always faced, say, Sirius (I can't say "didn't spin at all" because I'm choosing to define the current state as "doesn't spin", remember?) then I'd describe the spin as "counter-orbital" or "negative" or "retrograde".

Your bicycle analogy doesn't work because the motion of the bike isn't in the direction of rotation. The points on the wheel never return to their original positions.

Now you're saying that the frame of reference is center-mass, but you're just declaring that to be so. Would you consider the center of the frame of reference of my spinning pocket watch to be the roughly noon on the watch face? Of course not. It'd be my hand.

Consider this. Imagine really long-lasting smoke bombs are placed in stationary orbit every 1,000 km along the Earth-Moon line. Let them go for a month. What would the smoke look like? It'd be concentric rings. Now for your part, you're saying that the smoke bomb on the far side of the moon and the near-side of the moon are rotating around the center of the moon. But I'm asking you, "Why are those points special?" Why can't you pick the middle smoke bomb, orbiting in empty space, and the smoke bomb on the far side? Why can't you say they're orbiting some axis halfway in between themselves? Or a smoke bomb on the near side and one at the center of the moon rotating around a point 1/4 of the way through the moon?" Since any pair of smoke bombs make perfectly concentric rings (or ellipses if viewed from afar), then why aren't you claiming that there are an infinite number of axes?

No, the right way to view the system is to look at the common center of the rings (that's why they're called concentric!). That center defines the axis of rotation, not some "center mass" idea...mass is irrelevant!

Now zoom out to the Sun-Earth system and do the same thing with smoke bombs in Florida and Mecca. What kind of path do they trace? Concentric rings about the Sun? No! They spiral. They make 366 (not 365) loops throughout the orbit. About what point? Well, roughly the center of the Earth at the time of the loop. That's why we say the Earth rotates about its axis.

Got it?
Moon=concentric=no rotation.
Earth=spirals=rotation.

Bytegeist
06-28-2011, 12:27 PM
That's precisely the reason why I'm saying it doesn't rotate. [...] Now you're saying that the frame of reference is center-mass, but you're just declaring that to be so.

The rotation of the Moon around its own axis is well defined, and would be the only correct rate to use for many physical calculations. That rotation rate (once per 27.3 days) is the rate the Moon has with respect to the fixed stars, the one that determines the Coriolis force acting on objects moving over its surface, and the one that determines the size of the Moon's equatorial bulge (admittedly not very big).

These things might or might not interest you, if you're not someone studying the Moon or living on it. But physically, it is the rotation rate for the Moon, if we had to pick one. All other rates stem from its motion with respect to other objects.


That center defines the axis of rotation, not some "center mass" idea...mass is irrelevant!

Now zoom out to the Sun-Earth system and do the same thing with smoke bombs in Florida and Mecca. [...] That's why we say the Earth rotates about its axis.

And that, my liege, is how we know the Earth to be banana-shaped.

The Earth's rotation around its own center of mass is certainly relevant. It's what you'd use, again, in order to calculate the Coriolis force — needed in weather prediction and targeting long-range missiles, among other things.

Xema
06-28-2011, 12:50 PM
Essentially all the confusion in this thread is the result of a failure to clearly specify frame of reference. Possible choices include moon-centric, earth-centric and sun-centric.

As Bytegeist implies, the best (i.e. least likely to lead to confusion) is almost certainly one that is outside the bodies in question, often known as "the fixed stars".

(Because these stars are part of the Milky Way which itself is rotating with respect to other galaxies, this isn't an inertial frame of reference in a strict sense. But it's close enough for any normal purpose.)

Chessic Sense
06-28-2011, 12:53 PM
The rotation of the Moon around its own axis is well defined, and would be the only correct rate to use for many physical calculations. That rotation rate (once per 27.3 days) is the rate the Moon has with respect to the fixed stars, the one that determines the Coriolis force acting on objects moving over its surface, and the one that determines the size of the Moon's equatorial bulge (admittedly not very big).

These things might or might not interest you, if you're not someone studying the Moon or living on it. But physically, it is the rotation rate for the Moon, if we had to pick one. All other rates stem from its motion with respect to other objects.


And that, my liege, is how we know the Earth to be banana-shaped.

The Earth's rotation around its own center of mass is certainly relevant. It's what you'd use, again, in order to calculate the Coriolis force — needed in weather prediction and targeting long-range missiles, among other things.

Perhaps brevity got the best of me. When I said "it doesn't rotate", I meant "...about an axis at the moon's center." I refer you to my original post:

"the moon does not rotate about its axis," not because of the definition of rotate, but the definition of axis.... The moon surely rotates...around the Earth's axis, not an axis through the center of the moon.

Now we're merely quibbling about definitions of "rotate" in order to straighten out where the "true" axis is. It's just semantics, Bytegeist. We're not disagreeing about anything real...it's English, not Physics.



ETA: To bring this back on topic, if the Moon were tidally locked to the sun, it'd have to have a retrograde motion. What would that mean for us or the Moon?

Bytegeist
06-28-2011, 02:54 PM
ETA: To bring this back on topic, if the Moon were tidally locked to the sun, it'd have to have a retrograde motion. What would that mean for us or the Moon?

On the Moon, there'd be permanent day and night sides of course, and the Earth would seem to revolve around us once every 29.5 days. The fixed stars would revolve once per year.

On Earth, we would "see", or at least be presented with, the whole surface of the Moon every 29.5 days — unlike now where we see the same half all the time. However, the Moon would still go through its phases, so now the visible surface features would be "dragged along" with the lit portion. With the naked eye alone, we'd only get to see the dark features, and poorly, from the dim illumination of Earth-shine.

clairobscur
06-28-2011, 04:07 PM
Essentially all the confusion in this thread is the result of a failure to clearly specify frame of reference. Possible choices include moon-centric, earth-centric and sun-centric.



It seems to me that if we're refering to the rotation of the moon, the "default" reference frame would be the moon (or rather its center, whatever). We don't usually mention the reference frame when we're talking about the rotation of the Earth, for instance, do we? Nor do we use a reference frame centered on the Earth, or the Sun, or Sirius, when we write that planet A rotates on its axis in X days.

Unless stated otherwise, I'm always going to assume that the reference frame used is centered on the planet/satellite we're talking about.

CurtC
06-28-2011, 04:50 PM
Consider this. Imagine really long-lasting smoke bombs are placed in stationary orbit every 1,000 km along the Earth-Moon line. Let them go for a month. What would the smoke look like? It'd be concentric rings.

Wouldn't the smoke follow the same rotation path as the smoke bomb itself? So after a month you'd have a bunch of small smoke clouds in various orbital positions.

Xema
06-28-2011, 10:08 PM
It seems to me that if we're refering to the rotation of the moon, the "default" reference frame would be the moon (or rather its center, whatever).
Well, the moon doesn't rotate relative to itself - it sits still while the earth, the sun, and the fixed stars revolve around it.

Chessic Sense
06-29-2011, 12:37 PM
It seems to me that if we're refering to the rotation of the moon, the "default" reference frame would be the moon (or rather its center, whatever). We don't usually mention the reference frame when we're talking about the rotation of the Earth, for instance, do we? Nor do we use a reference frame centered on the Earth, or the Sun, or Sirius, when we write that planet A rotates on its axis in X days.

Unless stated otherwise, I'm always going to assume that the reference frame used is centered on the planet/satellite we're talking about.

You can't define a reference frame with a single point. You need three to define the axes. If you assume one plane is the orbital plane, as I'm sure we all have, and the center of the moon is the origin, then that leaves one point. Where's +X? The stars? The Earth? The Sun?

The "default" reference frame for the Earth/Moon system would be the Moon and, well, the Earth. And in that reference frame, no rotation occurs, positive or negative.

When we talk about the rotation of the Earth, we mean the Earth, the Sun, and the orbital plane. In fact, we've based all of our time-keeping on that system. A day is when the same Earth spot lines up with the Sun again, even though it's slightly more than a rotation. That's why we define a year with 365 days, not 366.

Wouldn't the smoke follow the same rotation path as the smoke bomb itself? So after a month you'd have a bunch of small smoke clouds in various orbital positions.

Well yeah, but I'm trying to hand-wave that. They're magical smoke bombs. Or, rather, they're regular smoke bombs with magic smoke that doesn't move. I could've said "Magic indelible markers that trace a path through space" but then I'd have to figure out a way to get the marker to actually write, and then the analogy gets confusing.

You try explaining a space tracer in one succinct paragraph!

clairobscur
06-29-2011, 02:11 PM
You can't define a reference frame with a single point. You need three to define the axes. If you assume one plane is the orbital plane, as I'm sure we all have, and the center of the moon is the origin, then that leaves one point. Where's +X? The stars? The Earth? The Sun?

!

What about the planet/moon axis? Again, it's what I would expect when talking about a specific celestial boby and would assume if I were to ask a question about a celestial body rotation.


Anyway, if you define a plane for the two first axis, you don't have much choice left about the third one which has to be perpendicular to the other two. Well, I guess you could pick some random point on this plane to have the third axis go through, but again, doesn't it make quite a lot of sense to pick the body's axis of rotation? Especially when you're precisely talking about this body's rotation.


It's really counter-intuitive to me that you'd rather pick the Earth as the center of the reference frame in such a thread.

septimus
06-29-2011, 02:39 PM
Anyway, if you define a plane for the two first axis, you don't have much choice left about the third one which has to be perpendicular to the other two. Well, I guess you could pick some random point on this plane to have the third axis go through, but again, doesn't it make quite a lot of sense to pick the body's axis of rotation? Especially when you're precisely talking about this body's rotation.

It's really counter-intuitive to me that you'd rather pick the Earth as the center of the reference frame in such a thread.

What's important about the reference frame is not its axes or center, but what vectors are arbitrarily considered at rest. If the line from Bogotá to Singapore is at rest the Earth rotates zero times in a 12-month period; if the Earth-Sun line is at rest the Earth rotates 365¼ times per year; if the Earth-Betelgeuse line (or Milky Way-Andromeda line) is at rest the Earth rotates 366¼ times per year.

RedSwinglineOne
06-29-2011, 04:56 PM
The moon surely rotates, just as the watch does, but around the Earth's axis, not an axis through the center of the moon.


Perhaps brevity got the best of me. When I said "it doesn't rotate", I meant "...about an axis at the moon's center."

Incorrect.

By definition, an object rotates on its axis, its axis being an imaginary line passing through its center. To say anything else, is wrong. The fact that it also revolves around the earth is irrelevant. From any viewpoint in the universe other than the surface of the earth, the moon clearly rotates on its axis, completely independent of the Earth.

As matter of fact, if you could remove the Earth, the Moon would continue to revolve around the sun in nearly the same path it follows now, still rotating on its axis.

KneadToKnow
06-29-2011, 05:29 PM
And that, my liege, is how we know the Earth to be banana-shaped.

Is that what that line is? I've been listening to that bit for coming up on 26 years, and never have been able to hear it right.

Ignorance fought!

Mangetout
06-29-2011, 05:35 PM
Well, the moon doesn't rotate relative to itself - it sits still while the earth, the sun, and the fixed stars revolve around it.

Rotation is not relative. Otherwise, James Bond could define himself and the centrifuge arm as stationary - with the building, world and universe whirling about him - and he would have no reason to experience near-fatal forces at the hand of Hugo Drax's henchman.

Bytegeist
06-29-2011, 06:25 PM
Is that what that line is?

Yes — if you're thinking of the Sir Bedevere line from Monty Python and the Holy Grail.

If, on the other hand, you're referring to Jack Nicholson's last line in Chinatown, then no, you're way off.

KneadToKnow
06-29-2011, 06:35 PM
If, on the other hand, you're referring to Jack Nicholson's last line in Chinatown, then no, you're way off.

You + Charlotte = beer

Steve MB
06-29-2011, 06:41 PM
Rotation is not relative. Otherwise, James Bond could define himself and the centrifuge arm as stationary - with the building, world and universe whirling about him - and he would have no reason to experience near-fatal forces at the hand of Hugo Drax's henchman.

However, when Drax's henchman sets the rest of the universe revolving around Bond, general relativistic effects produce some uncomfortable g-forces on him.

Mangetout
06-30-2011, 03:06 AM
However, when Drax's henchman sets the rest of the universe revolving around Bond, general relativistic effects produce some uncomfortable g-forces on him.

Except you can't do that without moving things at velocities greater than c, which is one reason why rotation isn't relative.

Mosier
06-30-2011, 08:10 AM
So if rotation is not relative, does the moon rotate or not? Should be pretty simple to answer.

Half Man Half Wit
06-30-2011, 09:08 AM
So if rotation is not relative, does the moon rotate or not? Should be pretty simple to answer.
It is -- throw something, and see if there's a Coriolis effect. (Err, well, it's theoretically simple, actually going there and stuff has proven a bit tricky in the past.)

So one effect of the moon stopping its rotation would be that this effect disappears.

Mangetout
06-30-2011, 11:04 AM
It is -- throw something, and see if there's a Coriolis effect. (Err, well, it's theoretically simple, actually going there and stuff has proven a bit tricky in the past.)

So one effect of the moon stopping its rotation would be that this effect disappears.

Concur. It's measurable (that doesn't mean I know the measured value)

Bytegeist
06-30-2011, 12:20 PM
... throw something, and see if there's a Coriolis effect.

You could also set up Foucault pendulums at various places, and measure their periods of precession. The closer you get to one of the poles, the shorter the period. When you finally reach a pole, the period there will match the rotational period of the body you're on.

In the case of the Moon, that's going to require a bit of patience, since the rotational period is once per 27.3 days. (Of course, you probably don't need to wait for a full precessional cycle at each pendulum.)

Chessic Sense
06-30-2011, 05:26 PM
Incorrect.

By definition, an object rotates on its axis, its axis being an imaginary line passing through its center. To say anything else, is wrong. The fact that it also revolves around the earth is irrelevant. From any viewpoint in the universe other than the surface of the earth, the moon clearly rotates on its axis, completely independent of the Earth.

As matter of fact, if you could remove the Earth, the Moon would continue to revolve around the sun in nearly the same path it follows now, still rotating on its axis.

Whaaaat? That's so incorrect, I don't know where to start. For one thing, just because you're an object doesn't mean you're rotating. Second, just because you're rotating, and there's an axis, it doesn't mean it's through your center.

If I go out to the playground and grab a pole with one hand and run around the pole, would you suggest that I'm rotating around an axis along my spine? Of course not. I rotate around the pole. The pole is my axis.

Without a pole, if I just spin in place with my arms out, my hand, throughout the whole spin, always faces wrist-first toward my body. At no point in time does the wrist spin out of alignment with the rest of my hand. It's like it's attached to an arm that runs all the way to my body. Would you say that my fingers are rotating around a vertical axis through my wrist? My wrist around my elbow? My elbow around my shoulder? No, of course not. All those things rotate around my spine. The spine is my axis.

What about my pocket watch example? What if I just twirl a pocket watch around my its chain? My watch, throughout the whole spin, always faces noon-first toward my hand. At no point in time does noon spin out of alignment with the 6. It's like it's attached to a chain that runs all the way to my hand. Would you say that the watch rotates about an axis through the center of the clock face? No, of course not. It rotates around my hand.

Now for the moon. The front of the moon, throughout the whole orbit, always faces toward the Earth. At no point in time does the front spin out of alignment with the rest of the moon. It's like it's attached to an invisible arm that runs all the way to Earth's center. Why would you suggest that the moon spins about it's own center? Of course you shouldn't. It rotates around the Earth's axis.

If you understand why my hand doesn't rotate around a vertical axis through my knuckles, or why my watch doesn't rotate around itself, you'll understand why the Moon doesn't rotate around an axis through its own center.

Hell, by your criteria for "axis", you could pick any point on the Moon and say the Moon rotates around it. You could pick a surface point, a point in the center, or even a point 20 feet deep in the dirt and say that that's where the axis is. By your standard, you could pick an infinite amount of axes and they'd all be right!

Chessic Sense
06-30-2011, 05:45 PM
One thing I hadn't mentioned, and I thought I had, was that if the moon were tidally locked to the sun, it'd have to reverse rotational direction...or at least change speeds. Imagine that it's in front of the Earth's path and has an observer seeing sunrise. When the moon passes between the Earth and Sun, in order to track the sun and keep the obersver in sunrise, it'll have to spin right. But as time goes on and the Earth moves to another quadrant around the sun, the Moon would have had to have a net spin of 90 degrees to the left. So it'll have to wobble back and forth over the course of the month, continually changing speeds.

So if rotation is not relative, does the moon rotate or not? Should be pretty simple to answer.

Surely. The question is...rotate about what?

What about the planet/moon axis? Again, it's what I would expect when talking about a specific celestial boby and would assume if I were to ask a question about a celestial body rotation.


Anyway, if you define a plane for the two first axis, you don't have much choice left about the third one which has to be perpendicular to the other two. Well, I guess you could pick some random point on this plane to have the third axis go through, but again, doesn't it make quite a lot of sense to pick the body's axis of rotation? Especially when you're precisely talking about this body's rotation.


It's really counter-intuitive to me that you'd rather pick the Earth as the center of the reference frame in such a thread.

A plane does not define two axes. You need a plane and a point. Imagine drawing a cross on a playing card and laying it on the table. You've defined the axes and the table as the plane, but you can still spin the card. You need to define one more axis from the plane by saying "the top of the card shall point toward this M&M" or something like that.

Bytegeist
06-30-2011, 07:36 PM
Surely. The question is...rotate about what?

What people are talking about here is the Moon's absolute rotation around its own axis. Both of these things are well defined, and independent of the Earth, the Sun, or anything else.

Define a coordinate system, any coordinate system you like, that's fixed to the solid body of the Moon. Do some experiments on its surface — perhaps with Foucault pendulums like I suggested, or projectiles, or gyroscopes, or something else — and you can then calculate the Moon's axis of rotation as a line (specified in your current, provisional coordinate system), as well as the direction and rate of spin around that axis.

Presumably, once you have the rotational axis, you'll want to define a more convenient coordinate system with that as one of your cardinal axes (maybe Z), and with the center of mass at the origin. If you fix the two remaining axes (X and Y) in the body of the Moon, you'll have a co-rotating coordinate system, which is useful for many things. You might also want to define an "inertial" coordinate system that subtracts out the rotation, which is useful for other things.


The front of the moon, throughout the whole orbit, always faces toward the Earth. At no point in time does the front spin out of alignment with the rest of the moon. It's like it's attached to an invisible arm that runs all the way to Earth's center. Why would you suggest that the moon spins about it's own center? Of course you shouldn't. It rotates around the Earth's axis.

I would say it revolves around the Earth, and it rotates (or spins) around its own axis. (More accurately, both the Earth and Moon revolve around a common point, but since the Moon's mass is so much smaller, that point is inside the Earth.)

And I still wouldn't say the Moon rotates or revolves around the Earth's axis, because the Earth's rotational axis is not the same as an axis perpendicular to the Moon's orbital plane. Theoretically the Moon could have a polar orbit, in which case the difference would be more apparent.

Triskadecamus
06-30-2011, 08:18 PM
If a sailor travels around the world, and returns to his home port, his wife and he have a different count for the number of days he was at sea. So, obviously, the rate of the Earth's rotation is entirely subjective.

Tris

Mangetout
07-01-2011, 03:12 AM
If a sailor travels around the world, and returns to his home port, his wife and he have a different count for the number of days he was at sea. So, obviously, the rate of the Earth's rotation is entirely subjective.

Tris

I'm assuming you're joking. He's performed an operation of rotation by going around the earth, to be taken in addition (or subtraction) to the rotatations of the Earth that both he and his wife experienced.

Half Man Half Wit
07-01-2011, 05:11 AM
At no point in time does noon spin out of alignment with the 6.
That's a funny argument to make. Do you think that, if the watch spun on an axis through its center, noon and six would spin out of alignment? If not, why is the fact that it doesn't evidence against it spinning around its center?

The simple fact is that the watch undergoes rigid rotation. The system of you spinning the watch around has a different total angular momentum depending on whether or not the watch spins around itself. Angular momentum is a vector quantity which you can break down in multiple ways, depending on the reference frame you wish to use; it's perfectly sensible to use a decomposition consisting of the revolution of the watch around your hand, and the rotation of the watch around itself, and it's also perfectly sensible to talk about that rotation on its own.

It's no different with linear momenta, but perhaps easier to visualize. Any system of mass points has a total momentum, defined by the motion of its center of mass. But every mass point also has its own momentum, which it is sensible to talk about when talking about that mass point individually.

Thus, when talking about the moon, it is sensible to talk about its angular momentum, which is given by its moment of inertia and its angular velocity, the latter of which defines its angular speed and its rotational axis. This is a measurable quantity, using for instance the Foucault's pendulum method Bytegeist has outlined.

septimus
07-01-2011, 05:47 AM
If a sailor travels around the world, and returns to his home port, his wife and he have a different count for the number of days he was at sea.

From Wikipedia's page on Magellan:

The full extent of the Earth was realized, since their voyage was 14,460 Spanish leagues (60,440 km or 37,560 mi). The need for an International Date Line was established. Upon returning they found their date was a day behind, even though they had faithfully maintained the ship's log. They lost one day because they traveled west during their circumnavigation of the globe, opposite to Earth's daily rotation. This caused great excitement at the time and a special delegation was sent to the Pope to explain the oddity to him.

RedSwinglineOne
07-01-2011, 12:26 PM
Whaaaat? That's so incorrect, I don't know where to start.

Well, perhaps in the spirit of fighting my ignorance, you will indulge me, and explain what I said that was wrong. But before you do, please understand that I and others here are using very specific definitions for the words rotate and revolve.

From wiki:
A three-dimensional object rotates always around an imaginary line called a rotation axis.If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation about an external point, e.g. the Earth about the Sun, is called a revolution or orbital revolution, typically when it is produced by gravity.

It is for this reason I say that the moon rotates on its axis, and revolves around the earth. The period of time for both is the same, but they are independant motions and should be considered as such.

If you believe the moon rotates around the earth's axis, you must believe the earth rotates around the sun's axis. If not why?

ZenBeam
07-01-2011, 06:13 PM
If you believe the moon rotates around the earth's axis, you must believe the earth rotates around the sun's axis. If not why?It would only rotate around the Sun's axis in that sense if the Earth were tidally locked to the Sun. Instead it would be some axis between the Earth and the Sun. (At a guess, at a distance of about 1/365th of an AU from the Earth's center towards the Sun.) [/aside]

Mangetout
07-01-2011, 08:31 PM
It would only rotate around the Sun's axis in that sense if the Earth were tidally locked to the Sun. Instead it would be some axis between the Earth and the Sun. (At a guess, at a distance of about 1/365th of an AU from the Earth's center towards the Sun.) [/aside]

I don't see why tidal locking makes it a special case with respect to rotation about its own axis.

ZenBeam
07-01-2011, 11:17 PM
It's not really the tidal locking that's important, it's that for the axis to be at the Sun, the same part of the Earth would need to be "aimed" at the Sun over the coarse of its orbit. I was using "tidally locked" as shorthand for that.

Mangetout
07-02-2011, 04:00 AM
I don't see why that makes any difference to the question of whether, in absolute terms, an object is considered to be rotating.

ZenBeam
07-02-2011, 08:31 AM
I don't see why that makes any difference to the question of whether, in absolute terms, an object is considered to be rotating.This is not what I said at all.

Trinopus
07-03-2011, 12:31 AM
. . . As matter of fact, if you could remove the Earth, the Moon would continue to revolve around the sun in nearly the same path it follows now, still rotating on its axis.

I'm not so sure... If you could "magically" blink the earth away in an instant -- "I Dream of Jeannie" style -- then the moon's path from that point forward would depend on where it was in the course of the month. If it were during a full moon, then the moon would swing outward away from the sun into an elliptical orbit of *more* than 1AU semi-major axis. If it happened during a new moon, then the moon would fall inward (a little) toward the sun, into an orbit of slightly *less* than 1AU semi-major axis.

(Varying cases in between.)

I guess I am actually agreeing with you, and just quibbling over the word "nearly" in your post!

re which axis of rotation, aren't we falling into the classical trap of accelerating frames of reference? To a person living on the moon, yeah, it pretty much feels as if the moon is rotating on its own axis. Sun rises, sun sets. Earth rises, earth sets. Venus, Mars, Rigel, Deneb, all rise and set. But from this selenocentric viewpoint, the sun is accelerating back and forth, going farther away, coming nearer, like a bloomin' yo-yo! The moon only "rotates about its own axis" in a powerfully accelerating frame of reference.

There's nothing *wrong* with using accelerating frames of reference, save only that they can generate spurious and illusory forces. They can simplify calculations -- if I'm a rocket scientist and trying to put a satellite up into orbit around the moon, you bet I'm gonna use the frame of reference of the moon spinning about its own axis! I'm also likely to use a frame of reference where the moon *doesn't rotate,* at least in some calculations, to make the math even easier.

(Same reason no-one, so far, has insisted on correcting for the solar system's motion with respect to the galaxy!)

Trinopus

Mangetout
07-04-2011, 03:49 AM
This is not what I said at all.

I think I probably just didn't understand you then.

As far as I can see, the rotation of a body such as the moon around the earth can be legitimately dissected into orbital and rotational components*, the rotational component will then be about the axis of the moon. I don't think it makes any difference whether the moon is tidally locked to the earth.

*(in the same way that the rotational component of the Earth is a day and the orbital component is a year - you could magic away either component without affecting the other)

If that's not anything to do with what you said, I apologise for talking at cross purposes.

Pormitsu917
05-17-2015, 07:53 AM
Ok then. If the moon rotated in such a way that it just had 1 side facing the sun all the time. Would we notice anything I mentioned in the OP.

Also I thought the moon was still rotating on it's own axis?

Then the moon would be real rotating in relation to earth

Pormitsu917
05-17-2015, 08:05 AM
No, the Moon is tidally locked to the Earth so that one face always faces us. The Moon rotates around the Earth's axis each 28 days. The Moon does not rotate about its own axis.

I think you are asking what would happen if the Moon was tidally locked to the Sun ie. one face always pointed at the Sun. The Moon would rotate in that case.

Nothing dramatic. The sunny surface would heat up a little and gases would be released. We might see frost on the dark side - that would be interesting.

" the moon does not rotate about its own axis" Yes it does says all the astronomers, scientists and probably 99.99 percent of the people on this planet because the revolution around the sun counts as one rotation in relation to the sun and to any distante point in space.
Myself I say: The moon does not spin on its axis and thank god i'm not smoking what the are smoking

Colibri
05-17-2015, 12:40 PM
Then the moon would be real rotating in relation to earth

" the moon does not rotate about its own axis" Yes it does says all the astronomers, scientists and probably 99.99 percent of the people on this planet because the revolution around the sun counts as one rotation in relation to the sun and to any distante point in space.
Myself I say: The moon does not spin on its axis and thank god i'm not smoking what the are smoking

Moderator Note

Pormitsu917, we generally allow revival of old threads in General Questions only to add new factual information. Since these appear to represent your personal opinions rather than facts, I am closing this.

Colibri
General Questions Moderator

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