The Moon - Can NASA be wrong?

[Jack_Ian]

NASA has an educational movie downloadable here, called "Why do we only see one side of the Moon?", where they say, "If you were on the near side of the moon, you would see our rotating Earth fixed in the same spot in the sky ALL THE TIME. Think about it!"

Well I've thought about it, and I would expect to see the Earth trace a circle or figure eight in the sky as the Earth rotates about the Sun.

Here's why I think this..

From what I understand, the Moon's rotation about the Earth became synchronised due to its proximity, its relatively small size and the centrifugal force of the orbit encouraging the "heavier" side of the Moon to remain furthest away from us, over millions of years.

I have observed that at different times of the year the Moon traces a higher path through the sky than at other times (not sure if this cycles once or twice a year), so this means that the Moon's orbit is not perpendicular to the Earth's axis.

Now if the Earth was to remain in exactly the same location in the Moon's sky, then, if you were standing on the Moon, you would see the Earth rotate, while the Sun, planets and stars pass behind it. You would also see the Earth's poles alternate, either moving directly towards you and then away from you, or tracing small circles/ellipses as they do so.

Now, if the Moon traced the same path through the sky all year round, then I would have no problem accepting NASA's statement, but to keep exactly the same face pointing towards the Earth, the Moon would also have to synchronise its tilt and not just its rotation. While I can accept the synchronisation of the Moon's rotation over millions of years, I simply cannot see how the Moon's tilt could also be synchronised, given that the Earth is not a perfect sphere (it's fatter round the middle due to its spin).

Am I wrong?

[Lepton*]

Yes.

[Lepton*]

Try to think of the system from an external point of view.

<- SUN.............../.............................o

The moon's orbit is in the plane of the earth's orbit. This is why is traces the same path through the sky as the sun and the other planets (the path is called the ecliptic or the zodiac). I think that you are underestimating the distance between the earth and the moon: no matter where you stand on the earth, you will always see the same side of the moon, give or take a moonrock.

Imagine that all the orbits and rotations of interest are perfectly aligned. Tidal locking is easily seen in this case. Now tilt the earth's rotation but keep the moon's orbit the same. That's how it is.

In reality, the moon wobbles a little bit: an astute observer can see a degree or two more of each side of the moon than should strictly be the case if it was truly tidally locked.

[wordcross]

interestingly enough, I'm listening to Pink Floyd's "Dark Side of the Moon" as i open and read this thread

I have no actual information about the phenomenon, however

Although, if you look at it right, you can see Trodgor in the moon
_________________
Has anyone really been far even as decided to use even go want to do look more like?

[Samadhi]

Here's an example of the distances (to scale).

_________________
And he lived happily ever after. Except for the dieing at the end and the heartbreak in between.

[Samadhi]

Figures Wiki would have a much better one:
http://upload.wikimedia.org/wikipedia/en/a/a7/Earth-Moon.jpg
_________________
And he lived happily ever after. Except for the dieing at the end and the heartbreak in between.

[Samadhi]

[amb*]

Anal Emma? I know her...

[Samadhi]

Hey, to each their own.
_________________
And he lived happily ever after. Except for the dieing at the end and the heartbreak in between.

[Jack_Ian]

My problem is that I cannot see how the Moon's rotation can ever be truly synhronous, without orbiting in a circle around the Earth's equator.
Any deviance would result in an apparent movement of the Earth in the Moon's sky.
Since the moon neither rotates about Earth's equator nor has a circular orbit, I expected there to be some deviance and assumed it would be non-trivial.
If the deviance was trivial then I would accept NASA's statement.

I decided to check for information regarding the amount of deviance, and found this.

[Lepton*]

The fact that those 16 degrees exist does not negate the fact that you are incorrect.

[GH]

When I look at the moon high in the sky, it looks like it's about as big as my thumbnail (when my arm is fully extended). If I were on the near side of the moon (very close to the point that is nearest the earth), how big would the earth look?

[Jack_Ian]

Lepton,

OK! I think we agree on the following (please correct me if I'm wrong):
1) The Earth moves significantly in the Moon's sky.
2) A truly synchronous rotation is not possible where the orbit is elliptical.

Where we differ is on whether a synchronous rotation is possible when the moon does not orbit about the equator.
I say it's not possible given the Earth's non-spherical shape.
In order for it to be possible, a circular orbit must be possible about a non-circular object.

I do concede however, that I underestimated the relative distance between the Earth and the Moon and therefore overestimated the significance of Earth's "flatness".
As such I am willing to concede that any irregularity in the orbit, caused by Earth's flatness, could conceivably be insignificant.
Especially since my main point, that NASA was wrong, is now accepted.

[Jack_Ian]

[Jedo the Jedi]

What happened to being humble and only making normal, everyday mistakes, Jack?

[Jack_Ian]

When we become aware of our humility, we've lost it.
Blessed are the meek: for they shall animate the earth.

[Lepton*]

I disagree that the Earth moves significantly in the moon's "sky". The dominant "wiggling" effect, which is caused by the eccentricity of the moon's orbit is really not all that significant.

What is significant is the concept of tidal locking, which is what I imagine is the point of the movie you link to. You have discovered a correction to the first approximation. It is not different than saying that E = mc^2 is incorrect, since E = sqrt(m^2c^4 + p^2c^2). However, in education, we don't teach the second equation, because the important stuff is contained in the first.

[Lepton*]

My appologies if that was a bit strong. The conservatives just won a federal election here in Canada, and I am rather disgruntled.

[Samadhi]

Well, they're minority if that's any consolation.
_________________
And he lived happily ever after. Except for the dieing at the end and the heartbreak in between.

[Jack_Ian]

I'm sorry I couldn't make the Earth move for you Lepton.

[Beartalon]