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.
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. 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.
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.
Sounds sort of like the “gotcha” moment in Around the World in Eighty Days, no?
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.
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.
Christ, calm down everyone.
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”.
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
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.
[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.
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Colibri
General Questions Moderator
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.
[QUOTE=Chessic Sense]
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.
[/quote]
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.
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.)
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:
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?
[QUOTE=Chessic Sense]
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?
[/quote]
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.
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.
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, the moon doesn’t rotate relative to itself - it sits still while the earth, the sun, and the fixed stars revolve around it.
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.
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!