Talk:Orbit of the Moon

From Wikipedia, the free encyclopedia

	Space Portal This article is within the scope of WikiProject Space.
B	This article has been rated as B-Class on the assessment scale.
Related projects:
	WikiProject Solar System
	WikiProject Moon	Importance to Moon: High
Help with this template This article has been rated but has no comments. If appropriate, please review the article and leave comments here to identify the strengths and weaknesses of the article and what work it will need.

To-do list for Orbit of the Moon:	edit · history · watch · refresh
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 10 CCW? 11 Secular acceleration of the Moon 12 Image, The Earth and Moon's path 13 There is no reference point! 14 Spin orbit resonance

[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)

The Moon orbits the Sun? No, it doesn't. Not even close. You can see my comments on this notion at http://en.wikipedia.org/wiki/Talk:Moon/Archive_3. mdf 16:13, 30 March 2007 (UTC)

Asimov's proposal of considering the Earth-Moon system a double-planet system is based, yes, on Moon's orbit around the Sun, Moon's size and mass in comparison to Earth's (only the Pluto-Charon system come close in proportions - that one fully recognised as a double-planet) and also (I believe this is the most important point) the "tug of war" (Asimov's name): Sun's gravitational pull on Earth is stronger than Earth's. This is true only for Moon among all other big satellites in Solar System (some of the outer, tyniest moons of Jupiter and Saturn also have this property), false even for Charon. It also makes me doubt that, if Sun would dissappear suddenly, Moon's orbit around Earth would continue as if nothing had happened. Since Sun's gravitational pull on the Moon is stronger than Earth's, I'd expect Moon decaying into a lower orbit in case the Sun dissappears. Can anyone support the claim that "nothing would happen" with some calculations, or providing an animation using "Gravity" or any other simulation program? Thanks —Preceding unsigned comment added by 148.244.69.177 (talk) 22:54, 16 October 2007 (UTC)

Interesting thoughts, but I'm sure "nothing would happen" to the moon's orbit if the sun disappeared. A simple two-body conservation of energy disallows the orbit to change its semimajor axis length. Tom Ruen 21:04, 17 October 2007 (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)

A quick scan of the DE-406 follows. The ten closest are:

356337.064 -2338 Nov 09 03:22:22
356349.827 -2683 Nov 13 02:33:02
356352.945 -1055 Nov 13 21:40:37
356354.171 -2665 Nov 23 13:31:54
356356.491 -0851 Dec 08 02:26:27
356356.657 -2356 Oct 28 16:22:36
356360.917 -1400 Nov 17 21:11:04
356365.136 -2320 Nov 19 14:21:41
356365.621   796 Dec 19 05:44:44
356366.204 -2869 Oct 30 08:25:51

The ten furthest are:

370389.858 -0367 Nov 19 13:58:27
370390.249 -2915 Nov 20 20:59:18
370391.003 -2588 Nov 05 10:27:44
370392.976 -0256 Dec 06 21:55:54
370393.790 -2699 Oct 18 19:22:03
370394.088    90 Dec 03 02:19:14
370395.483 -2933 Nov 08 21:26:07
370397.683 -1212 Nov 16 20:04:09
370404.210 -2088 Nov 11 23:55:44
370407.525 -1650 Nov 15 01:16:01

When you plot the ranked data (79532 points) one obtains a roughly straight line, so the distribution is about as uniform. At least on a 6000 year time-scale, where dynamics are being smeared. On a month-to-month basis, things are more correlated. Graphics can be uploaded etc on request. mdf 16:51, 30 March 2007 (UTC)

Right, if you draw it out for that long it'll look like a line. What's neat is that there are cycles upon cycles upon cycles. Where apogee and perigee repeat like a sine wave every month, which then has a ~yearly sine wave superimposed on it, and then higher order cycles on top of that and so on.

A series of graphs of distance vs time showing this would be nice. (you might have to show the max-apo, min-apo, max-peri, and min-peri top to bottom on different scales, otherwise a few kilometers out of 50,000 wouldn't be seen)

The article could also use a illustration showing the movement of the nodes and apsides. Sagittarian Milky Way 08:09, 1 April 2007 (UTC)

[edit] CCW?

I see from the article that the ellipse of the lunar orbit rotates counterclockwise, and that the precession of its orbital plane is clockwise. How about the orbit itself? The animation of the Moon as it cycles through its phases allows one to infer that the orbit is counterclockwise (same as the rotation of the Earth, and the Earth's orbit around the Sun) but this really should be stated explicitly both here and at Moon. --Wfaxon 22:01, 28 July 2007 (UTC)

The physical Moon itself moves counterclockwise. (directions are always measured from above the north pole). Sagittarian Milky Way 21:18, 29 August 2007 (UTC)

[edit] Secular acceleration of the Moon

Someone looking for info on the "secular acceleration of the Moon" in Wikipedia will search long and hard. Perhaps it is here but searching on this familiar term yields nothing.Cutler 09:51, 24 August 2007 (UTC)

That is already discussed in this article under Orbit of the Moon#Tidal evolution of the lunar orbit as well as the main article Tidal acceleration already listed under that heading. But I've added your term, secular acceleration of the Moon, as a redirect to tidal acceleration. — Joe Kress 23:58, 24 August 2007 (UTC)

[edit] Image, The Earth and Moon's path

The equations which are said to be the basis of the image are not clear. In the first place the given equations of the earth's orbit would describe a circle, not an ellipse? This is easily corrected and should be shown as precisely as possible in an encyclopedia article. In the second place, the origin of the symbol " p:(synodic months+1)=14 " is not clear. If "p" is a constant(14), then how can it be an integer? Should it be more precisely the number of synodic months in a sidereal year ~12.368? And why plus one? Will the contributor of this image and these equations please clear this up? PSpace —Preceding unsigned comment added by Alexselkirk1704 (talk • contribs)

[edit] There is no reference point!

I've searched the internet, and can only find recorded dates for one of the lunar cycles, the synodic. This is a problem due to the importance of the draconic (nodical) cycle in predicting eclipses. Someone should do some research. 68.144.80.168 (talk) 08:00, 30 March 2008 (UTC)

[edit] Spin orbit resonance

According to this article, cited by a source I can't access, the Earth and the Moon will achieve spin orbit resonance in 2 billion years. This seems a remarkably short time. I've heard estimates as high as 50 billion years. Serendi^podous 16:08, 8 June 2008 (UTC)