Does the moon spin?
only in an arbitrary frame of reference meaningless to participants and observers alike
Does the moon spin?
Only in an arbitrary frame of reference meaningless to participants and observers alike.
These are complex things that need to be thought through carefully to understand in any meaningful way.
Check the two videos here:
Orange in hand does not spin on any axis through itself while it orbits my elbow or body
In the video of the orange in my hand, the orange does not rotate between my thumb and forefinger. But it does rotate around an imaginary axis through itself that is rotationally FIXED relative to the ground. So what?
Well that’s important.
See the video of the kid spinning around his dad, both arms of each locked.
Notice the kid first runs toward his dad before their arms grasp and before his orbit around his dad. During the time he’s running, is the kid spinning around an axis through his body relative to the ground?
No.
After they grab arms, and the dad orbits the kid around himself, THEN the kid is spinning around an axis through himself relative to the ground, but that is a secondary effect 100% dependent on their locked arms and orbit around the dad. And, that [spin around an internal axis through the kid relative to the ground] is 100% meaningless to both of them, and to outside observers. If the kid were actually spinning on an axis through himself, then their arms would get twisted up into pretzel logic contortion.
What’s actually happening with the kid spinning around his dad is exactly analogous to the moon orbiting around the earth and not spinning on its own axis. And it is analogous to a carousel.
An arbitrary frame of reference meaningless to participants and observers alike
Horses on a carousel orbit the center of the carousel, and do not spin on their own poles, on an axis through each horse. On the other hand, in a very arbitrary frame of reference meaningless to participants and observers alike, carousel horses DO spin around their own poles.
How so?
Think of the carousel poles to which each horse is mounted. Imagine each horse pole orbiting as it does around the carousel center, attached to the carousel floor but imagine these poles maintain a constant orientation relative to the ground, like a compass always points north. In this imaginary setup, each horse pole spins around inside our hands as we grip it while we’re riding the carousel horse. Each pole keeps a fixed compass orientation relative to the ground as it orbits around the center of the carousel.
This of course is not what carousel poles do. They’re fixed to the floor and ceiling of the carousel and don’t spin within our grip. But feel free to imagine them spinning within your hands. And THEN you can say that the actually non-spinning horses ARE spinning around these these now-spinning poles, poles that are not spinning relative to the ground, but now ARE spinning relative to each horse, and the carousel platform, and spinning within our hands as we grip the pole on the ride.
So, yes, you can frame it that way, but why would you?
“Spin that is 100% dependent on orbital motion”
With the dad and the kid, If they let go of their grip, the kid would fly off in a straight line, tangent to the orbit at the point of release. And immediately from the moment of release, the kid’s spin around a pole internal to himself immediately becomes spin rate = 0.
In other words, the kid’s “self spin” relative to that very arbitrary reference frame (meaningless to everyone involved and observing) is entirely dependent on the orbit around the dad. The spin does not exist before the orbit. And it ceases at the moment of orbital release.
This condition is totally not comparable to the spin of the earth on its rotational axis (once per day) as it orbits the sun (once in 365 days).
Carousels and Horse Racing
Let’s review.
Does a carousel horse spin around its own pole? No. UNLESS you arbitrarily imagine a pole through the horse that spins as the carousel turns, keeping the pole oriented in one direction only relative to the ground (like a compass).
Hold an orange in your hand between thumb and finger and hold your arm out then turn around in a circle. Does the orange spin between your fingers as it orbits around you? NO. But it does spin relative to a pole through the orange oriented in one direction only relative to the ground. The orange’s spin (rotation around an internal pole) is a function ONLY of its orbit around YOU and an arbitrary frame of reference. Same for the kid. Same for the carousel horse.
Let the Sun Shine In
The kid, the carousel horse, and the orange, all get direct sun on all sides BECAUSE THEY ORBIT around a body, not because they spin on their own poles. The moon does not, as someone said, “appear to spin if you sit on the sun”. It appears exactly as I showed in the orange videos, and in the kid orbiting his dad video, and as everyone has seen watching a carousel turn round and round: the horses do NOT spin on their poles.
The pseudo “spin”, of the kid in the white shirt, the orange in your hand, the horse on its pole, and the moon, around axes through their bodies, is ONLY an effect of their orbit and a peculiar arbitrary ground (frame of reference) with imaginary poles that face only in one direction (like a compass) during orbit. IF you adopt such an imaginary construct, then technically you can correctly say that “a carousel horse spins on its own pole”. But you’re adopting a framework that’s meaningless to both participants and observers. To see the difference, leave the carousel and run over to the spinning bears or teacups. Check you find a clean one without vomit before sitting down.
But forget about carousels and teacups.
Racetrack
Have you ever been to a horse race?
In the US, horses race around tracks only counterclockwise. The race is an orbit of each horse around the center of the track. They start the race with the left side of each horse facing the center of the track. The horses orbit (or rotate) around the center of the track. As they go into turn 1, the left side of every horse faces the center of the track. At turn 2, left side of horse faces center of track. At turn 3, turn 4, same. Left side always faces center of track.
This is true, if you are sitting on the horse. It's also true if you're sitting at the center of the track. And it's also true from any vantage point anywhere else, from any seat in the grandstand.
Spin and Pseudo Spin
Are the horses SPINNING around themselves as they run around the track?
THEY ARE according to the insistence of some, who technically are correct. Such people insist on imagining a pole through each horse as it runs around the track, with the pole always orienting itself in one direction only (say, North) as it rides around on top of each horse. And relative to that pole on each horse, every horse IS spinning around its own pole as it runs around the track.
So, this insistent position IS TRUE. But who fucking cares? It's meaningless to horse, to rider, and to spectator.
And the sun shines, on all sides of the horse: front of horse, left of horse, right of horse, ass of horse BECAUSE THEY RUN AROUND THE TRACK, NOT BECAUSE THEY SPIN ON THEIR OWN POLES.
Science it out!
You could science it out though. Arrange a horse race that takes 28 days to go around the track once. You could hypothesize:
>> the horses orbit around the track once every 28 days and they spin 360 degrees around an internal axis at a spin rate that matches the orbital period, 1 spin revolution per 28 days (too slow for you to see!).
You could do measurements to prove the hypothesis. Use a clock and a calendar to measure if it's 28 days to go around the track once. And put a compass on a horse saddle to measure the number of degrees from north at different positions on the track.
At the start of the race, the compass says the horse is facing 0 degrees from north.
Between turn 1 and turn 2, the horse is facing 90 degrees from north.
As the horse runs down the backstretch, the compass says the horse is facing 180 degrees from north !!
Between turns 3 and 4, the horse faces 270 degrees from north !!!
AND DOWN THE HOMESTRETCH THEY COME!! THE HORSE AGAIN FACES ZERO DEGREES FROM NORTH!!!!!
THE HORSE IS SPINNING ON ITS OWN POLE!!!!!!!
To those who are insistent, these carnival rides are the same. One’s a teacup. The other’s a horse. No other difference, according to the insistent!
“No one cares about poles and internal orbits or any of that rubbish… Only the smart arses have claimed this. Everyone else has merely said, eh actually the moon does rotate. Maybe learn to read?”
An Interesting Comment
A friend comments:
Saw a full lunar eclipse once. Something rare about the particular event which I can't remember, but it was like a once in a hundred year event, so I drove up to the lookout on the edge of town to see it away from the glare of the city lights. There were people with telescopes and binoculars, but you didn't need any of that. With just the naked eye you could see the moon just hanging there, naked, without its glow. And it looked like a giant grey rock in the sky l. And my only thought was what is holding it there? Why doesn't it fall on us? It looked not more than 100ft away too. Like you could climb to it with a long enough ladder. Will never forget what I saw, nor the impression it made.
Another:
What if the moon is a mere 100ft away? They say that William Blake was able to touch a star with the end of his walking stick.
Those are nice observations. It's important to keep in mind two things about the physics of heavenly bodies, newtonian physics in general.
1. The moon certainly would fall into the earth, due to gravity, and in fact the moon IS always falling into the earth, but, fortunately, as the moon falls toward the center of the earth, at the same time it also moves at just the right velocity, in a tangential direction. The sum of the movement vectors puts the moon constantly "falling over the edge of the earth."
https://en.wikipedia.org/wiki/Orbital_mechanics
2. gravity is "spooky action at a distance", motion without contact, not explainable by newtonian physics. Newton and his contemporaries declared it so, total mystery, not explainable by science. This has remained so to the present.
Fact Check
I have an adequate/sufficient amount of respect for space agencies. By that I mean a tremendous amount of respect. But it’s not limitless.
See the remark here https://svs.gsfc.nasa.gov/4709 and the animated visualization of moon orbit and rotation.
An enduring myth about the Moon is that it doesn't rotate. While it's true that the Moon keeps the same face to us, this only happens because the Moon rotates at the same rate as its orbital motion, a special case of tidal locking called synchronous rotation. The animation shows both the orbit and the rotation of the Moon. The yellow circle with the arrow and radial line have been added to make the rotation more apparent. The arrow indicates the direction of rotation. The radial line points to the center of the visible disk of the Moon at 0°N 0°E.
That is the extent of the text on the page, “The Moon’s Rotation”.
The visualization is adequate and clear. Zoom in on the video as it plays, to see the moon more closely. The text doesn’t measure up. It fails to surface any of the relevant considerations that make the moon/earth dynamics meaningful, or even interesting.
Let’s put it this way:
Earth:
On earth, set up a camera on long exposure facing the night sky. Do this near the North Pole in winter. Expose the film/sensor for 24 hours. You get something that looks like this:
https://videohive.net/item/traces-of-stars-against-the-night-sky-shot-long-exposure/19201412
If this is a 24 hour exposure, that’s 1/365th of a year, so the contribution of earth’s orbit around the sun, to the shape of the star path curves, is at most 1 out of 365, or 0.28%. The earth’s rotational spin around its own axis every 24 hours is the major contributer to the shape of these curves, contributing the remaining 99.72%. Of course small adjustments to these numbers are needed to fine tune for solar system vectors through space, ellipticity of orbits, non-sphericity of earth, and so on. But the point is the relative weights of solar orbit of earth versus terrestrial self rotation. The latter is almost the total contributor to the star path curves, from earth.
Moon:
Do the same exercise on the moon. Set up a camera on the moon surface facing into space and set exposure duration to 28 days (one lunar orbit around earth). While the earth’s star curves are 99.72% drawn by the earth’s spin around its own pole, and only 0.28% drawn by the earth’s orbit around the sun, for the camera on the moon the scales tip even closer to 100%. Much closer, in fact. But with a crucial difference: ALL of the weight is on the orbit side of the contribution scale (orbit of moon around earth), and NONE of the weight is on the self-spin side (self-spin of moon).
The star path curves drawn by the camera on the moon are effectively 100% drawn by the moon’s orbit around the earth, and 0% drawn by the moon’s self-spin (because there is none).
Refer again to the tangent diagram:
If the moon were released from earth orbit (blue curve), then the moon would travel in a straight line (red line), from the point of orbital release. Because the moon has no self spin under its own power (so to speak) but only “spins” as an effect of its orbital path and the imposition through the moon of an imaginary “north facing” “pole” around which the moon is then said to “spin”…, well, this was covered from various angles in this post already.
But here we see even more clearly the meaninglessness of the construct.
With the kid flying around his dad, or Secretariat spinning around a compass on his saddle as he runs the Belmont in record time, or with the carousel horse “spinning” on its own pole by way of the imaginary construct of a north-facing compass-like pole that always orients north, spinning in the rider’s hands as the pole, the rider, and the horse orbit the carousel — at least that concept is conceivable. It’s imaginary, and meaningless to participants and observers. But at least it’s conceivable.
In space, such a frame of reference in an unfounded construct. A compass in space points at nothing, or, reliably, nowhere. Stick a pole through the moon and assert that the pole orients itself always in one direction, with the moon spinning itself around that, and try your best to tie that to something definable and make it mean something.
Then rule it out as you should.
Refer again to the star path curve tracing of a long exposure camera on the moon. The star paths can be mapped, defined mathematically, as a function of the moon’s orbit around the earth only, 100% (excluding solar system and galaxy motion vectors, etc.), zero moon self-spin involved.
Last Thought:
If the difference between the spinning teacups and the carousel didn’t quite get there for you, try this: Stick your arm out the window of a car and grip a baseball in your hand, or a basketball (in 2 hands), or an iPhone, like this:
Notice the car drives in a straight line. The road is straight. Is the basketball/baseball/iPhone spinning?
No.
If the phone were spinning, then we’d see it turn toward the car, look through the passenger window, then turn toward the back and look down the road backward, and then turn away from the car and look into the forest and then continue spinning and reaching again the forward view.
But of course that’s not happening. Because the the phone is NOT spinning.
OK. Now keep the phone recording as the car approaches a roundabout and begins a 360 degree circle. As the car and phone circle around the roundabout, is the phone spinning?
It IS spinning, according to the insistent. But their insistence is meaningless to the phone, to the car, to the driver, to the passenger, and to everyone else who can observe the situation from any other car, or from any other position on or near the road, or in the air.
Other examples?
Fly on a plane and look out the window. See the winglet at the far end of the wing. Rigidly weld a metal sphere to the end of the wing. Fly the plane ✈️ straight. Is the metal sphere at the end of the wing spinning? Fly the plane around a large circle. Is the metal sphere rigidly attached to the end of the wing now spinning? What about YOU sitting in your airplane seat as the plane flies a wide 360 circular holding pattern.
Are YOU spinning in your seat?
Yes you ARE, insist the insistent.
Still not satisfied?
The diagram on the left is the moon as it is of course. Click the link: https://en.wikipedia.org/wiki/Tidal_locking#/media/File:Tidal_locking_of_the_Moon_with_the_Earth.gif
The text under this says that the one on the left shows the moon spinning (on its own axis) while the one on the right is the moon not spinning on its own axis.
So that’s true, right?
As long as the frame of reference you prioritize is the white background. But choosing that frame presumes that that’s the more meaningful frame, among the available frames.
Imagine the diagram is not showing the moon but instead is showing a carousel horse that you ride on. If you ride on the horse (moon), the diagram on the left is JUST LIKE a carousel horse going round and round the carousel platform. The horse always faces the same direction relative to the frame of reference of the carousel platform, which is rotating. And that’s the frame of reference that actually matters to most people.
What sane person would say that a carousel horse spins around its own pole?
Conclusion: the diagram prioritizes the white background as frame of reference. But choosing that frame instead of the “platform” that is the orbital system of earth’s gravity, analogous to the carousel platform floor, that rotates, is an odd choice that makes no sense to human perception. It’s also an odd choice in terms of physics and physical reality, because the white background is meaningless physically. It’s meaningful only in terms of a piece of paper, a diagram printed in a book, or the background of a screen.
It’s the orbital rotation “platform” of the earth and its gravity that is the meaningful frame of reference for the system of satellites, including the moon, in orbit around the earth.
This means that the conclusions in the text should in fact be reversed.