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toothdust
2008-Sep-15, 04:37 PM
I have been paying attention to the night sky almost daily for the last year and a half or so, and have noticed that the moons path through the night sky changes as the months pass. Being at about 47degrees N Lat, the moon in summer is very low in the night sky at its maximum height. Moving into fall/winter, the moon is getting to be almost directly overhead.

Is the moons orbit at a stable inclination relative to the ecliptic, all the while the Earth continues on its tilted journey around the sun? How does this fit into the dominant theory of moon formation by the great impactor?

It would seem that if the moon was formed by that giant collision, it would have a stable orbital inclination toEarth, not the ecliptic plane.

If anyone could expand on this it would be great. Any diagrams would help too.

Thanks.

Lepton
2008-Sep-15, 04:44 PM
I have been paying attention to the night sky almost daily for the last year and a half or so, and have noticed that the moons path through the night sky changes as the months pass. Being at about 47degrees N Lat, the moon in summer is very low in the night sky at its maximum height. Moving into fall/winter, the moon is getting to be almost directly overhead.

Is the moons orbit at a stable inclination relative to the ecliptic, all the while the Earth continues on its tilted journey around the sun? How does this fit into the dominant theory of moon formation by the great impactor?

It would seem that if the moon was formed by that giant collision, it would have a stable orbital inclination toEarth, not the ecliptic plane.

If anyone could expand on this it would be great. Any diagrams would help too.

Thanks.

If your question is meant to lead to a conclusion that goes against mainstream theory it really is better placed in ATM.

Neverfly
2008-Sep-15, 04:51 PM
I have been paying attention to the night sky almost daily for the last year and a half or so, and have noticed that the moons path through the night sky changes as the months pass. Being at about 47degrees N Lat, the moon in summer is very low in the night sky at its maximum height. Moving into fall/winter, the moon is getting to be almost directly overhead.

Is the moons orbit at a stable inclination relative to the ecliptic, all the while the Earth continues on its tilted journey around the sun? How does this fit into the dominant theory of moon formation by the great impactor?

It would seem that if the moon was formed by that giant collision, it would have a stable orbital inclination toEarth, not the ecliptic plane.

If anyone could expand on this it would be great. Any diagrams would help too.

Thanks.

You seem to have started formulating a theory based on a misconception of observational evidence.

Which is odd.
Because you even named the reason why for your observations in your post!:doh:
The Earth's axis is tilted.

So no explanation is necessary- you just misunderstood something simple.:)

You will note, the Sun does the same thing.

As a side note of my own: I am not happy with the Great Impactor theory myself. I find it "wanting."
But for now, it's the best we have. I'd be happy to discuss alternatives. But for now, I can only grudgingly accept the Impactor theory in light of the evidence. Specifically, the Lunar material collected by Apollo missions.

John Mendenhall
2008-Sep-15, 05:02 PM
As a side note of my own: I am not happy with the Great Impactor theory myself. I find it "wanting."
But for now, it's the best we have. I'd be happy to discuss alternatives. But for now, I can only grudgingly accept the Impactor theory in light of the evidence. Specifically, the Lunar material collected by Apollo missions.



Alternatives, NF? I thought the simulation guys couldn't get anything else to work.

Neverfly
2008-Sep-15, 05:05 PM
Alternatives, NF? I thought the simulation guys couldn't get anything else to work.

They can't.
As far as I am aware- there are no viable alternatives.

But it can still be fun to discuss them:)

John Mendenhall
2008-Sep-15, 05:19 PM
Hm, yes, just read through the Wiki article.

Confirmation might be available when enough craters on Mercury and Mars have been examined in situ and dated. The impact must have blasted large pieces to everywhere else in inner solar system.

toothdust
2008-Sep-15, 05:56 PM
Wow. So quick you guys are to throw ATM at me.

I was looking for an answer to an orbital question, not about the impact theory. Let me clarify:

My question was pertaining as to if the moon has an orbital inclination that is stable relative to Earth equator (which observations don't seem to imply), or if its incline is stable relative to the ecliptic plane.

I would draw a diagram of what I am getting at, but I have to get going to work, so it will have to wait.

Neverfly: Yes, I have noted that the sun changes position in the sky during the season change. The sun, however, is not a satellite body of the Earth as the moon is. Therefore the apparent change in motion of the moon is a bit more of a quandry.

I guess my question could be better stated as: Why doesn't the moon follow a stable orbit relative to Earth's equator?

grant hutchison
2008-Sep-15, 05:58 PM
The moon's orbit does indeed precess at a five-degree inclination to the ecliptic, not the equator. Which means the full moon occurs roughly opposite the sun all the time, putting it high in winter and low in summer. (The five degrees tilt can tweak that up or down a little, depending on the position of the lunar orbital nodes.)
The reason it precesses in the ecliptic is because it's so far from the Earth: all distant satellites have the same behaviour, and it's driven by orbital mechanics, not the origin of the satellite.

Orbital precession is caused by orbiting within a non-spherically-symmetrical gravitational field. There are two big sources of such asymmetry: the equatorial bulge of the parent planet, and the presence of the Sun. The equatorial bulge acts like an extra (equatorial) ring of mass inside the satellite's orbit; the Sun, over the course of a year, acts like an extra (ecliptic) ring of mass outside the satellite's orbit. For close-in satellites, the equatorial component strongly dominates, and precession takes place in the equatorial plane. For far-out satellites, the ecliptic component strongly dominates, and precession takes place in the ecliptic plane. Between the extremes, precession takes place in a compromise plane, which is called the Laplacian or invariant plane.

Grant Hutchison

grant hutchison
2008-Sep-15, 06:23 PM
Wow. So quick you guys are to throw ATM at me. Yeah, I thought that, too. :cry:
You raised a good point. People missed it.

Grant Hutchison

Jeff Root
2008-Sep-15, 07:05 PM
The Moon's motion in Earth's sky is probably the most complex motion
of any known celestial body.

-- Jeff, in Minneapolis

slang
2008-Sep-15, 07:32 PM
Wow. So quick you guys are to throw ATM at me.

Just one. Not "us guys". Let's not judge the entire forum population, or even the participants in this thread, by the posting style of one member.

mugaliens
2008-Sep-15, 08:05 PM
My question was pertaining as to if the moon has an orbital inclination that is stable relative to Earth equator (which observations don't seem to imply), or if its incline is stable relative to the ecliptic plane.

Have you looked up the stats on Wikipedia, which readily provides all of this information?

Neverfly
2008-Sep-15, 09:16 PM
Just one. Not "us guys". Let's not judge the entire forum population, or even the participants in this thread, by the posting style of one member.

Well... I kinda did. But in all fairness- Toothdust kinda worded it that way.
So it's kinda a stalemate.

tony873004
2008-Sep-15, 09:28 PM
Here's an animation of the Moon's orbit precessing from a view within the ecliptic.
http://www.orbitsimulator.com/gravity/saros.GIF

frankuitaalst
2008-Sep-15, 09:35 PM
The Moon's motion in Earth's sky is probably the most complex motion
of any known celestial body.

-- Jeff, in Minneapolis
Well , the moons orbit is indeed a very complex one , tilted to the plane of the ecliptic , excentric to Earth ( or the barycenter of Earth-Moon system)....
Moreover , it is known that the inclination will change over a long period of time ( as also the inclination of Earth's orbit will change over the range of millions of years by some degrees ) ....
Generally also Nasa provides some good information about the moons orbit .
Different pictures exist
Here for instance is an Anaglyph animation ( 3D red/cyan) of how Plutonians perceive the moons orbit over a time-span of about 3 years . In this animation Earth is kept centered .
Animation was done using JPLs data system and can be watched here :
http://www.orbitsimulator.com/cgi-bin/yabb/YaBB.pl?num=1210608962

Edit : while typing I saw another animation came in . Toothdust is being served well...

toothdust
2008-Sep-15, 09:36 PM
Slang: Just one. Not "us guys".

Your right. My apologies.

Mugaliens: Have you looked up the stats on Wikipedia, which readily provides all of this information?

Yes, but felt a lacking of a good diagram/animation to put it in visual perspective.

Tony: Thanks for the animation.

There is no doubt in my mind that the moon did form from the Earth. As to how it did, well, I think there are some good theories out there, but none have entirely captured me yet.

slang
2008-Sep-15, 09:45 PM
Well... I kinda did. But in all fairness- Toothdust kinda worded it that way.
So it's kinda a stalemate.

I disagree.. He asked, you responded that he might be wrong. That's not the same as suggesting that a move to ATM might be necessary, without any obvious intent of causing havoc. There is such a thing as being too paranoid, except in computer security :)


Slang: Just one. Not "us guys".

Your right. My apologies.

None necessary, just keeping the record straight for those round-up posts where some people suggest that new posters are treated horribly by 'everyone'.

FWIW I liked your question, and enjoy reading the multiple answers you get.

toothdust
2008-Sep-16, 05:09 PM
Tony:

Its hard to tell how many years are elapsing from the counter at the bottom of the animation. The lowest and highest numbers I can catch are 06-54? Is this a future predicted path of from the last century?

Jeff Root
2008-Sep-16, 06:07 PM
I suspect that the left-side group of numbers is the month, date and year.
The numbers on the right end of the left-side group range from zero to 18.
The saros cycle is 18 years long. http://en.wikipedia.org/wiki/Saros_cycle

-- Jeff, in Minneapolis

mugaliens
2008-Sep-16, 06:26 PM
Mugaliens: Have you looked up the stats on Wikipedia, which readily provides all of this information?

Yes, but felt a lacking of a good diagram/animation to put it in visual perspective.

A picture can indeed be worth a thousand words!

And a short video is just that much better...

tony873004
2008-Sep-16, 07:09 PM
Jeff R is right. The 3rd number from the left is the year. In America, its common to express dates as: MM-DD-YY, which of course makes little sense. It makes it difficult to sort a list by date when the most significant number is hiding in the middle of the string. So the animation is showing 1 complete Saros cycle (slightly over 18 years), and looping it over and over again. I forget the dates that were actually used, as I reset the date to January 1, 0 to treat it more as a counter than a calendar date. Its probably from this decade, but it doesn't really matter since the Saros cycle has been happening for a long time.

toothdust
2008-Sep-17, 04:20 AM
I had no idea that the moon had such a wobbly orbit. Are there any theories as to how and why it got this way? Is it due to tidal interactions with a precessing Earth rotation?

tony873004
2008-Sep-17, 05:11 AM
Grant explains it nicely in post #8 of this thread.

Tim Thompson
2008-Sep-17, 05:12 AM
Are there any theories as to how and why it got this way?

That question is answered on the Wikipedia Saros Cycle webpage (http://en.wikipedia.org/wiki/Saros_cycle#Description) you were already pointed to. It's because the simple lunar orbit is a two-body problem, Earth and Moon. But the real Moon is subject to the gravitational pull of the Sun, and beyond that the other bodies of the solar system. So its orbit (and all orbits of everything in the solar system) is more complex than simplicity implies. All of this is very independent of the origin of the moon; these kinds of orbital adventures will happen relatively quickly to the Moon, regardless of how it got there.

grant hutchison
2008-Sep-17, 12:02 PM
Grant explains it nicely in post #8 of this thread.Thanks. :)
I'm glad someone read that post.

Grant Hutchison

Fiery Phoenix
2008-Sep-17, 04:47 PM
So, is Luna's orbit "moving" or what? I mean, looking at the animations posted above, it seems to rock back and forth.

frankuitaalst
2008-Sep-17, 06:42 PM
yes , it is moving and "rocking" , but there's nothing to worry about . Our moon still obeys the known laws of gravity , as Grant explained above .

Fiery Phoenix
2008-Sep-17, 07:41 PM
yes , it is moving and "rocking" , but there's nothing to worry about . Our moon still obeys the known laws of gravity , as Grant explained above .

I assume that's the reason its position in the sky is changed every month?