Talk:Orbit of the Moon

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Find image demonstrating what a Cassini state is. Find image demonstrating the different types of precessional periods.

Contents 1 Rename article 2 distance 3 Query re Axial tilt 4 Merge results 5 Suggested Merge 6 double planet? 7 Proposed rename results 8 Proposed rename 9 Perigee distance should be fixed

[edit] Rename article

If things like "synchronous rotation" are to be kept, then it makes sense to rename this article to "The Moon (motion)" from the current "The Moon (orbit)". mdf 15:03, 11 July 2006 (UTC)

[edit] distance

I had an argument with a friend who said that the distance between the earth and the moon was increcing. From what I know about physics, this is impossible. Orbital energy must be lost due to tides etc. However, he argued so convincingly that I came here to know for sure. If he is correct, I think it deserves a section in the article, if he is wrong, the decay rate of the orbit would be an interesting factoid to add to this article.

the Moon is indead receding from the earth. "Measurements show that the Moon is receding from Earth at a rate of about 3.8 centimeters per year" http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/ApolloLaser.html. But the is also a corrsponding change in the rotation which keep the engergy conserved. Roguebfl 11:01, 25 August 2006 (UTC)

Predictions suggest that the range will increase until the Earth and Moon become double synchronised, that is, both are tidally locked to one another. (So the Earth's day length would match the Moon's future orbital period of about 47 days, and the Earth-Moon distance would be about 550000km, compared to today's figure of 400000km). This won't occur for something like 50 billion years, by which point the Sun will be a white dwarf and will have passed through a red giant stage, which may result in the destruction of the Earth. [1]

 :Roguebfl 11:05, 25 August 2006 (UTC)

[edit] Query re Axial tilt

In the heading Inclination of the rotation axis, this article explains the axial tilt as 6.69° to ecliptic (my emphasis).

However, in the table lower down Other properties of the Moon's orbit Mean inclination of lunar equator to ecliptic is listed at 1° 32'Roo60 12:50, 15 July 2006 (UTC)

[edit] Merge results

[edit] double planet?

Recently the statement that Earth+Moon form a double planet has been reversed. That apparently has been done on the grounds that the COM lies within the Earth. That is only one possible criterium. IMNSHO it also is a poor one: if the Moon were twice as small but four times more distant, the COM would lie outside of the Earth, and the smaller Moon would be part of a double planet anyway? Asimov's proposal, based on the fact that the Moon orbits the Sun rather than the Earth (also looking at the actual shape of its orbit in space) makes more sense. Anyway, with even the concept of "planet" in confusion, I don't believe we have a solid base to securely classify E&M as a double planet or not. Tom Peters 10:45, 4 December 2006 (UTC)

I agree this is a bad definition. The Moon is evolving outwards as a result of tidal interactions and will someday become a double planet. I'll find the official IAU definition and reference for this later today. Nevertheless, I find all proposals at defining what a double planet are to be arbitrary at best. The Moon only appears to orbit the Sun from a Sun fixed perspective; If the Sun disappeared, the Moon would still continue to orbit about the Earth as if nothing happened. The fact that the trajectory of the Moon looks like a closed loop (instead of sometimes going backwards) about the Sun is (in my opinion) an illusion based on the fact that its orbital velocity (about the Earth) is small compared to the Earths orbital velocity about the Sun. I've been thinking of coming up with a definition based on angular momentum, but this would suffer some of the same problems as with the barcycenter definition, even though (again, imho) it would adress the manner in which planets and moons form.Lunokhod 11:14, 4 December 2006 (UTC)

After looking into this, it appears that the barycenter definition is only an informal one. It was considered by the IAU at the last general assembly, but was dropped. See 2006_redefinition_of_planet. Lunokhod 19:14, 4 December 2006 (UTC)

[edit] Proposed rename results

[edit] Perigee distance should be fixed

The perigee listed in the article may be good for an "average" month but the moon has been known to come as close as 356,300 km. Someone should find a better source. Sagittarian Milky Way 05:00, 23 March 2007 (UTC)

Yes, but the question is, where does it end? We could always find a closer perigee. The distance you cited was very, very rare. The closest perigee within the range of A.D. 1500 to A.D. 2500 is 356371 km; the Moon approaches within 356425 km 14 times within that range. This comes from Meeus' Astronomical Algorithms. He used the ELP-2000/82 lunar theory. Solex90, an excellent numerical intergration program, could only find one as close as 356,313 km going bace to -15,000. (15001 B.C.) Although the predictions get pretty uncertain that far back and one could have easily been less than 356,000 km.

It is a good question in general-should a record or typical perigee be used. Saros136 07:01, 23 March 2007 (UTC)

Right, I suppose 364,000 km is the elliptical perigee, which is perturbed to as much as ~356,000 / ~370,000 km by the other two orbs. The Moon's orbit is amazing, like an elastic thing continously being stretched and played with. Sagittarian Milky Way 01:54, 24 March 2007 (UTC)

My bad, [2] states the extremes for 1750-2125 as 356,375 / 406,720, I remembered it wrong, thought it was 356,325, and rounded it down just to make sure I wasn't overstating accuracy!

When were the top ten closest of -3000 to 3000 AD? Sagittarian Milky Way 02:22, 24 March 2007 (UTC)