Does the moon spin?

General Chat
62

I don’t think it makes sense to say that anything revolves around anything else purely based on reference frame. If I construct a rotating reference frame, then of course most things will have a trajectory in that reference frame around whatever center we choose.

I think it makes more sense to think of the phrase “revolves around” as describing some physical phenomenon where some body is being acted on by a centripetal force of some sort and as such has a loop-like path around some center point. There is no such physical force acting on the sun for which the focal point would be the Earth, but there are physical grounds for stating that the Earth revolves around either the center of gravity of the solar system or the sun, which is approximately at that location. (The entire system of which is revolving slowly around the center of the galaxy.)

4 Likes
63

Paddington Bear comes from deepest, darkest Peru.

1 Like
64

Far side or back (as opposed to the front which faces us).

1 Like
65

The whole night sky revolves around the Polaris (if you are on the northern hemisphere; otherwise the southern cross). This includes the Moon, Mars, and the Milky way. Fascinating.

1 Like
66

Err, i don’t think you can call the far side of the moon the back side because a back side is also known as a [full] moon which is clearly a reference to the front side (of the moon). It would be confusing…

2 Likes
67

But does the moon spit?

68

True. All motion is relative so you choose your “stationary” point for you frame of reference.
In Einsteinian relativity, it is just as valid to ask “what time does the station arrive at this train” as the other way round.
Sometimes it’s easier to think of the earth as stationary with the whole universe rotating around it.

69

If you were in a box and were being pressed down onto the floor, there would no way of telling if that was because of gravity or because the box was accelerating upward. Einstein realised that the reason you wouldn’t be able to tell is because they’re the same thing!
Everything is just following a dead straight path through spacetime, it’s just that the spacetime itself is warped.

2 Likes
70

Arrrgh. I’d never thought of that! Just when you think you understand something. Clearly that must be impossible, and yet true. Dammit. I’m sure there’s an explanation but I don’t know what it is.
[goes off to hit Google]
[returns]
Apparently Einstein himself conceded the need of an extension to his theory when it comes to rotating reference frames. There are still papers being released on the subject today. I started reading a couple. My head hurts. I’m going to bed.

3 Likes
71

Are you Lao-Tsu dreaming you are a butterfly,

or a butterfly dreaming you are Lao-Tsu?

1 Like
72

When people say something revolves around something, but “doesn’t spin”, this is what they mean:

1
   ._.
    O

2

    O ._.

3

    O
   ._.

4

._. O

The moon is tidally locked with Earth, however, so it’s more like this:

1
   ._.
    O

2

    O  |:

3

    O
   .-.

4

 :| O

The moon spins relative to space. We can see that because we see how the face changes orientation.

However, relative to Earth, we always see one side. For a little observer dude on Earth, they would not see the moon spin.

Einstein famously said time is relative. He also said speed is relative. Everyone thinks they’re not moving relative to themselves, that everything else is moving. We think we are still, on Earth, when in reality, our planet is hurtling at 100,000 kilometres per hour around our Sun, which is circling the galaxy at mind-bogglingly fast speed.

Spinning is like speed. The Earth is spinning a lot, but we don’t notice it. We see the sun spin around the Earth from the Earth, because we are on the Earth. This is why people thought everything revolved around the Earth for a long time, and it was understandable why they would think that.

In the same way, the Moon spinning around the Earth is relative. Someone on Earth would not see it. Someone in space would.

I have only been talking about the Earth/Moon relationship, when the Earth revolves around the Sun, which revolves around the supermassive black hole at the center of the galaxy, further convoluting the “spinning” of the Moon.

If we took everything else out of the picture, and left the Moon going in seemingly very random movement in space, you would clearly see it is spinning. It is the other stuff which makes it hard to answer this question.

tl;dr Saying the moon doesn’t spin is almost like saying the Sun revolves around the Earth, but less dumb and more justified.

Final answer: Yes, the moon spins.

5 Likes
73

I don’t understand.

Yes, I am confused :smile:

1 Like
74

FWIW I’m an English speaker and I didn’t understand that either.

2 Likes
75

He’s referencing the practice of “mooning”, which is to reveal one’s bare backside, the idea being that the shape and colour (the phrase presumably having originated in a white community) resembles a full moon.

I’d never heard the comparison made in noun form.

3 Likes
76

:smile:

And does it spin? :joy:

5 Likes
77

Sorry i missed this. @bugcat is of course right, to moon is a verb. But it might spin considering the owner of said moon is likely to be inebriated!

6 Likes
78

If a drunkard moons a car in the middle of a roundabout, does the car rotate around him or he the car~?

4 Likes
79

Now we’re on to frames of reference again

1 Like
80

Sounds like someone had too much moonshine!

5 Likes
81

Just posted today, this is a great explanation of curving space-time:

6 Likes