Talk:Moon/Archive 10

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Other natural satellites?

The article The Great Daylight 1972 Fireball#All known Earth-grazing fireballs cites an old paper about a 1913 series of fireballs, which believed them to have resulted from the breaking up of a (very small) natural satellite. Is this still regarded as plausible, and if so, could there be others? This article says the Moon is the Earth's only natural satellite - should that be modified to the only known natural satellite? Though given the recent news on 2008 TC3 I suppose any unknown satellites would have to be utterly miniscule. Wnt (talk) 00:07, 8 October 2008 (UTC)

We track chunks of junk the size of bolts in Earth orbit, so I suspect that there are no other natural satellites. kwami (talk) 01:40, 8 October 2008 (UTC)
Thanks - I'd been thinking that only good radar reflectors could be tracked, but now I read the ESA Space Debris Telescope can detect 15-cm objects by sight at the geostationary orbit. Maybe there could still be a chunk of black rock the size of a Volkswagen circling the L5 point? On the other hand: can someone explain why a system like the Earth and Moon can't manage to capture so much as a stray meteoroid? —Preceding unsigned comment added by Wnt (talkcontribs) 19:15, 9 October 2008 (UTC)

Natural occurring orbits are statistically extremely rare and even harder to maintain over long periods of time. (talk) 17:43, 22 October 2008 (UTC)

However the earth has a naturally occurring 'temporary satellite'-- a Sun orbiting asteroid (only 6 meters across) that periodically enters Earths orbit (usually just about four Earth orbits) before being thrown back out into its solar orbit. (talk) 17:43, 22 October 2008 (UTC)

Luna should be boldface in lead

According to Wikipedia:Boldface "proper names and common terms for the article topic, including any synonyms and acronyms" should be in bold typeface. According to the article, "The Moon is occasionally referred to by its Latin name, Luna." Luna is a synonym for the Moon, which is why it is included in the lead in the first place, and therefore should be in bold. Rreagan007 (talk) 16:19, 15 November 2008 (UTC)

Reverted (until discussion completes) as this could also affect Sun. As per what has already been stated, "Luna" is neither the proper name (in English) or a common term; it tends to appear in science fiction rather than in science or day-to-day use. This has been discussed before, which is why the bold was removed quite some time ago by other editors. (There were actually several occasions where "Luna" was removed from the lead altogether, which speaks to the occasional text you quoted and also to the fact it is not common.) --Ckatzchatspy 22:20, 15 November 2008 (UTC)
If it's not common enough then it should be removed altogether. I do admit that I've mainly heard it used in science fiction, particularly star trek, but even if it is primarily a science fiction/pop culture reference then I think that alone could make it common enough to be listed as an alternate term in the lead. Rreagan007 (talk) 22:24, 15 November 2008 (UTC)
I'd have to dig through the talk page and the edit history to be sure, but I seem to recall that "Luna" was left in the lead in part for historical purposes. The debate surfaces every so often because of questions over the name "Moon"; per the IAU, there is no formal English name for our satellite other than "Moon", but the use of "Luna" in science fiction causes confusion. (The same issue arises with "Sun" and "Sol" as well.) Obviously, to bold or not to bold is a minor issue, but with regards to this particular word it is a bit more involved. Let's give this some time on the talk page so that the other regular editors can add their thoughts. --Ckatzchatspy 22:33, 15 November 2008 (UTC)
Fair enough. Rreagan007 (talk) 23:02, 15 November 2008 (UTC)

It's been used in poetry since the 16th century. From the OED:

1529 Whan Luna, full of mutabylyte, As emperes the dyademe hath worne Of our pole artyke.
1588 (Shakespeare) Dul.: What is dictima? Nath.: A title to Phebe, to Luna, to the Moone.
1592 And Luna hides her selfe to pleasure vs.
1836 Luna shone bright in the blue arch above.

I don't get similar results from 'Selene'. You could argue this is really the Roman goddess, the Moon personified, but one of the Webster's defines 'Luna' simply as 'the moon'.

BTW, the OED has a similar entry for Sol as 'the Sun (personified)':

c1450 Sol is hote & dry but not as mars is.
1592 Ere Sol had slept three nights in Thetis lap.
1593 More beautiful ... Than Sol himself amid the Planets seven.
1609 His smile is like the Meridian Sol Discern'd a dauncing in the burbling brook.
1670 The Vines ... doth shelter them from the scorching beams of Sols fiery influence.
1712–4 (Pope) Sol thro' white curtains shot a tim'rous ray.
1791 (Cowper) Till Sol, declining in the west, Shall call to supper.
1820 In bright Sol's diurnal round, No such delightful place was found.
1837 Clytie, inconsolable for the loss of the affections of Sol, ... is represented as brooding over her griefs in silence and in solitude.

kwami (talk) 00:49, 16 November 2008 (UTC)

Webster's Revised Unabridged Dictionary defines Luna as simply "the moon." Rreagan007 (talk) 04:53, 16 November 2008 (UTC)

Per the same page link, defines it as the Roman goddess, silver, and a religious item, while the American Heritage Dictionary and Wordnet both define it exclusively as the goddess. (None of those three works describe it as the Moon; additionally, the Webster's entry lists silver.) Furthermore, the Merriam-Webster Collegiate Dictionary doesn't even have an entry for "luna". --Ckatzchatspy 05:08, 16 November 2008 (UTC)
Even one major dictionary defining Luna as the Moon demonstrates that the term is in use in English. I did a quick google search and found multiple uses of Luna as the moon.
This demonstrates to me that Luna is accepted in English as a synonym for the Moon. Whether this usage originated with modern science fiction or classical English literature, Luna is an alternative name for the Moon and the lead should reflect that with proper MoS formatting. Rreagan007 (talk) 05:58, 16 November 2008 (UTC)
Again, it does not meet the "common term" requirement, nor is it recognized as a scientific or proper term for the Moon. Furthermore, many of the Google hits you've listed are not relevant; Urban Dictionary, for one, is completely unreliable. (I could sign up tonight, enter a definition saying "Luna" means "cheese", and it would have equal listing with what is there now.) lists Luna as the Latin name, not the English name. Astronomy for Kids is riddled with inconsistencies (Pluto is a planet and a dwarf planet; it has one and three moons.) does not describe the Moon's name as "Luna"; it only uses the term in a title. Both the Apollo Society and Excelsis do not appear to meet any "reliable source" definitions. Look, I'm not saying the term isn't used occasionally, only that we cannot justify claiming it is a "common term" for Moon. The number of reliable sources - dictionaries, scientific resources, and so on - that do not endorse such a claim appear to far outweigh any that do. --Ckatzchatspy 06:17, 16 November 2008 (UTC)
Further to this, remember that the "Luna" in the lead sentence is there as a Latin translation, not as a synonym. --Ckatzchatspy 06:22, 16 November 2008 (UTC)
I was not using those links as "reliable sources" per se. I was merely using them to show that the term is in common use in English (though your criticism of Urban Dictionary sounds a lot like a criticism of Wikipedia that I hear all the time). I think your definition of what constitutes a "common use" is a little narrow. Certainly Luna is not the most common term used for the moon, but it is certainly not an obscure use if many mainstream websites and a major published dictionary use it. Indeed, I'm sure you would agree that "Luna" is the most common alternative to "the Moon" in English. However, if it is not a common enough use to be recognized, then there is no need to have the Latin translation in the lead at all. Why not put the Latin translation on the Earth article? Because it would be stupid and pointless to just stick the Latin translation there. The only reason the Latin translation is in the lead in the Moon article is that it is the most common synonym for the Moon in English. It should therefore be in bold. Rreagan007 (talk) 17:07, 16 November 2008 (UTC)people often thought that there were canals on the moons surface, but they were wrong.

Sigh, In English, the Moon is called "the Moon". In Spanish, Italian and Portuguese, the Moon is called "Luna", and in French, the Moon is called Lune. Therefore, "Luna" really does not belong here! In the respective language version of Wikipedia, "Luna" may be properly applied. Bottom line, "Luna" is not the English name for the Moon! (talk) 00:30, 23 June 2009 (UTC)

Gif image reduction

I am concerned with the size of the Image:Lunar_libration_with_phase_Oct_2007.gif; while it is spectacular I'm surprised and taken aback at the 9874.97 KB it requires to download. Initially I wanted to remove the gif immediately, but after further consideration I would like to request that a downgraded version be done for article use. The goal being to reduce load time for users and mitigate long term costs to Wikipedia, as this article has heavy traffic. This makes sense given it is thumb-nailed anyway. Thoughts? - RoyBoy 02:02, 3 December 2008 (UTC)

Early tides

It seems worth mentioning in this article that in the geologic past, when the moon was closer to the earth, the tides were much larger as well (100s of meters, I believe) -- which has a lot of relevance to Earth geology and probably evolution. Ethan Mitchell (talk) 16:57, 4 December 2008 (UTC)

is this statement correct?

"although several countries have either sent or announced plans to send people and/or robotic spacecraft to the Moon."

Which countries since 1972 have sent people to the moon?Tigershoot (talk) 20:20, 12 December 2008 (UTC)

It is correct: there have been multiple countries to send probes (or orbiters) to the moon and many countries have plans to go to it. Twintop (talk) 22:22, 12 December 2008 (UTC)

It reads though as if people have been sent. I know that probes have, and that there have been announcements of sending people, but no human has been sent since 1972. Would it would read better as "although several countries have sent robotic spacecraft to the Moon, or announced plans to send people as well." Tigershoot (talk) 08:41, 13 December 2008 (UTC)

Moon Formation: Volcanic Eruption Theory

I just had this idea and it seems very valid, especially based on the silica and other contents of the moon. Whenever the moon was formed the Earth core had to be much much hotter, thus larger, more frequent volcanic eruptions. Perhaps an eruption might have been so enormous that it was able to propel 2% of the Earth's contents beyond its atmosphere. There in space it was floating out in a straight line but liquids in space tend to form into spherical shapes, thus it formed into a sphere and was captured into orbit. Feedback please! —Preceding unsigned comment added by Absolutekos (talkcontribs) 08:54, 13 December 2008 (UTC)

May I first recommend reading Giant impact hypothesis? Moon is (AFAIK) formed from Proto-Earth's crust, not the core. With some shoe-horning, the Giant impact hypothesis counterparts fairly well to your reasoning, except the cause was a planetary impact, not a giant volcanic eruption. But, if you make your own theories, you have to publish them outside Wikipedia, and if they seems to explain as many facts and more than the current theories, we may consider making an article of it. Said: Rursus () 16:43, 17 December 2008 (UTC)

This is not such a ridiculus idea as it might seem to most, though instead of a volcanic eruption a nuclear explosion at the core/mantle boundary is sugested. Though this theory is still under construction and modeling as has been done for the giant impact hypothesis has still to be carried out.Grissini (talk) 11:29, 10 February 2009 (UTC)


I was watching a documentary on channel four called catastrophe a few weeks ago. it was saying about the giant impact theory and someother things that i cant remember, what i can remember is that it was said that the moon was around ten times closer to us when the earth was young than it is today. im just wondering if this is true and if it is would it be worth putting into the article.

Mr Deathbat 14:04, 16 December 2008 (UTC)

It's already here: Giant impact hypothesis. If you consider there's something missing in that article, you may add it into there, if you provide suitable external sources. Otherwise proofreading and spell correction is always welcome. Regarding it's truth: the theory is accepted by the majority of astronomers that have an opinion in the topic, but it is far from proven. Said: Rursus () 16:50, 17 December 2008 (UTC)
What i was actually Enquiring about was what i said about the distance being ten times closer to the earth not long after the moons formation. do you think this could be true? my bad should of written it a bit clearer
Mr Deathbat talk 10:07, 5 January 2009 (UTC)

Moon as a planet

Just out of curiosity (and boredom), would the moon be classified as a planet if it didnt orbit earth? —Preceding unsigned comment added by (talk) 03:13, 9 January 2009 (UTC)

It could be along with some of the other bigger moons that orbit other planets, if they had their own orbit around the sun.
the moon is bigger than Pluto and Pluto is classed as a dwarf planet so maybe. Mr Deathbat (talk) 11:49, 9 January 2009 (UTC)
Actually that is a very good question. I believe if Pluto is treated as a real planet than the 7 largest moons of the solar system should also be real planets based on their large sizes. The problem with the moon being a planet (if it did not orbit the Earth) is it would have a weak Stern-Levinson parameter, and I doubt it would have a stable orbit (of 5 Billion years) if it had an orbit between say Earth and Venus. See Giant impact hypothesis. -- Kheider (talk) 12:53, 9 January 2009 (UTC)
It could, but it depends on the circumstances. The first two IAU definitions of planet are that it needs to be in orbit around the Sun and that it needs to be of sufficient mass to assume a nearly round shape. The moon is round by its own gravity and could have been in a direct orbit around the Sun. But there is one third requirement which is where Pluto among others fail; it needs to be the dominating body in it's vicinity, having cleared any other larger objects from its orbit. This is basically why Mercury, the only sizable object in the innermost region of the solar system, is a planet while Pluto, located in the Kuiper belt, is not. Replace Mercury with the moon and it would classify as a planet, replace Pluto with the moon and it would not classify as a planet. Finally I might suggest asking questions like this one at Wikipedia:Reference desk/Science instead as the talk pages are really meant for discussing work on the article, also you are more likely to receive a quick response at the help desk. Njaelkies Lea (talk) 23:28, 23 January 2009 (UTC)

Incorrect use of "thus" in second paragraph

The second paragraph has a sentence "Thus, the Moon's volume is about 2 percent that of Earth;" However, the volume of the Earth is not given (there's not even a Wikipedia link to Earth!). "Thus" implies a conclusion from previous information, which is not available here.

In short: replace "Thus" with "As such", and it will be fine. And a wiki-link to Earth would be appreciated too.

Yes, minor nitpick, would've edited myself, but for some reason I couldn't find, the page is protected. —Preceding unsigned comment added by (talk) 09:44, 26 January 2009 (UTC)


what color is the moon really? —Preceding unsigned comment added by (talk) 22:47, 28 January 2009 (UTC)

Judging from the color photos of the Moon that I've seen taken outside the effects of the Earth's atmosphere, the moon appears to be a dull, somewhat darkish grey. The moon appears much lighter on Earth because of the effects of reflected sunlight (and earthshine), as well as the effects of the Earth's atmosphere. However, not having been to the Moon myself, I'll simply have to trust that the color photographs (already pushing 40 years old, and who knows what quality film was used) are more or less accurate. --Ericdn (talk) 19:07, 15 March 2009 (UTC)
Apollo 8 astronauts reported the Moon as having no color other than grey. In a post-Apollo 11 Life magazine article, Neil Armstrong said the Moon appears usually grey but sometimes tan, depending on the lighting angle. Apparently (good word here), the surface reflects different visible light wavelenghts at different lighting angles, the combined wavelngths sometimes providing the illusion of a tawny-brown surface. Indeed, the Moon looks brown in the article's Deep Impact photo.
One scientist remarked that Apollo lunar samples looked like "burnt potatoes," (From the Earth to the Moon, Chaikin) so, up close, the Moon is the color of say asphalt. (talk) 06:26, 17 March 2009 (UTC) 68Kustom (talk) 22:19, 17 March 2009 (UTC)

Moon landings on other bodies?

The third paragraph states: "The Moon is the only celestial body to which humans have travelled and upon which humans have performed a manned moon landing". If we were to land on another celestial body, wouldn't it be a "manned landing" and not a "manned moon landing"? Moon landings only occur on Earth's moon.--Rosattin (talk) 06:56, 2 February 2009 (UTC)

Cood catch. Saros136 (talk) 19:46, 10 February 2009 (UTC)
Not necessarily. As I stated elsewhere on this talk page, "Moon" is Luna, the Earth's satellite, whereas "moon" is any natural satellite. Therefore, any landing on a planet's satellite can properly be described as a "moon landing". However, since common useage of the word "moon" (technically as "Moon") refers to Earth's satellite, I can see where some confusion would arise. However, it would be perfectly acceptable to land on Mars' moon Phobos, for example, and call it a manned moon landing. "Lunar" landing, however, would be incorrect, because "Luna" refers only to Earth's moon. --Ericdn (talk) 19:10, 15 March 2009 (UTC)

Bettering the Article

This website seems to explain better Moon rotation and libation. The slow Moon rotation on it's slanted axis, plus the monthly rotation around the Earth, is the reason for only one side to be visible always to Earth. But that the Moon does rotate around itself as a spinning top, similar to Earth. What it would seem is that it is not rotating on its own axis, and stationary, and only rotating around the Earth. [1] -- IS this correct or right...??? (GeorgeFThomson (talk) 00:52, 6 February 2009 (UTC)).

A very interesting number is 1.0022km/s = 3,679.2km/h. The "rotation" speed around it's axis. And it does not have "magnetic field", only the "gravitational" one. —Preceding unsigned comment added by GeorgeFThomson (talkcontribs) 01:15, 6 February 2009 (UTC)

Earth's only natural Satellite?

Aren't there also other small rocks that orbit earth? —Preceding unsigned comment added by (talk) 21:55, 6 February 2009 (UTC)

Notice on talk page about Cruithne

I think it would be helpful to add a notice near the top of this talk page summarizing why Cruithne is not considered a moon of the Earth, and thus not suitable for more mention in the article than it already has. The topic has come up many times, at least twice since the last archive, and a notice might reduce this. My reasoning is similar to the reasoning behind the notice on the article for Earth, mentioning that references to 'Mostly Harmless' aren't suitable for the article. I'd add such a notice myself, but I am inexperienced at Wikipedia technical wizardry of that sort. :P --Patteroast (talk) 19:05, 10 February 2009 (UTC)

Actually, you could do it easily. Go to the Earth talk page, click the edit tab, and copy and paste the relevant part to this page. Saros136 (talk) 00:37, 12 February 2009 (UTC)

Moon or moon

Should we use initial caps on the word Moon? (This question is related to the question above about the name of our moon.) Is Moon the proper name for our moon, or is it just a generic term identifying the kind of heavenly body nearest Earth? In other words, is Moon to our moon as Earth is to our planet? John (talk) 00:45, 12 February 2009 (UTC)

We capitalize it , as the Moon is the proper name. Both are considered acceptable in English, and both get done here at times. Saros136 (talk) 00:54, 12 February 2009 (UTC)
It doesn't seem to be used that way; we often say "the moon". I couldn't convince myself that moon is a proper name. How do you know? John (talk) 21:51, 25 February 2009 (UTC)
"Moon" is a proper name, whereas "moon" isn't. (See my other comments in this section, as well as Dfoofnik's.) This is a case where the regular noun "moon" simply became the proper name "Moon" out of common useage. --Ericdn (talk) 19:05, 15 March 2009 (UTC)
A slight correction to this... from what I recall, "Moon" originally referred exclusively to our satellite, and the term was then extended to other natural satellites as they were discovered. I'll try to find the reference for this; it was either from the IAU or NASA. --Ckatzchatspy 19:23, 15 March 2009 (UTC)
Both "Moon" and "moon" are in common useage to refer to Earth's satellite, but, as it's the proper name of an object, the capitalized version is correct. The lowercase version technically refers to any natural satellite. --Ericdn (talk) 19:05, 15 March 2009 (UTC)

"Moon" is a capitalized proper name, as is "Luna". The use of the word moon for other natural satellites is not capitalized, nor usually its combination with contemporary phrases that imply Luna, as in "moon landings" and "moon rocks" Dfoofnik (talk)

Dfoofnik, how am I supposed to give "official" answers as an English teacher when you already do all the work for me? :) And, of course, you're absolutely correct. --Ericdn (talk) 19:05, 15 March 2009 (UTC)

Need more images from the southern-hemisphere perspective

Most of the images in the article are almost all images of the Moon from the perspective of the Northern Hemisphere. (The exceptions are Fir0002's Belt of Venus shot where the moon image is rather small, and the eclipse image is ambiguous.) There are no large images of the moon that show what the moon looks like as seen from the Southern Hemisphere. This is a violation of the Manual of Style (limited geographic scope). Adding some additional southern-hemisphere content in the form of a large image of the southern-hemisphere view of the moon would improve the article by providing more balanced information on the topic. -- B.D.Mills  (T, C) 23:04, 24 February 2009 (UTC)

Just flip one of the images upside-down. We could display them side-by-side, to contrast the difference. And of course we need them rotated 90° so we get the view from the equator. kwami (talk) 23:42, 24 February 2009 (UTC)
That can work. The only difficulty with doing this is how to integrate four more large images of the moon into the article while writing some good text to accompany them. The equatorial description may need a little creativity, as the two equatorial views would be the rising and setting moon. Any thoughts? -- B.D.Mills  (T, C) 02:39, 25 February 2009 (UTC)
I don't think it's a violation of MOS, because we're not specifically excluding Southern Hemisphere images of the Moon. The fact simply is that the vast majority of images made available to us in this article come from the Northern Hemisphere. Perhaps there are some Wikipedians in the Southern Hemisphere reading this who would be willing to step outside one night, snap a few pictures, upload them, and put them in the article? Of course, we must also be careful so that the article doesn't become too picture-heavy (as beautiful as the Moon is to look at, I'll admit), and, if a Northern Hemisphere picture is going to be replaced with a Southern Hemisphere picture, then a legitimate reason for the switch must be made... something better than just the "different hemisphere" and "not a picture gallery" reasons. --Ericdn (talk) 19:01, 15 March 2009 (UTC)


the moon doesnt give itself light —Preceding unsigned comment added by (talk) 17:07, 26 February 2009 (UTC)

Yes, you're right. Most of the light we see from the Moon is reflected sunlight, and the rest is light reflected from the Earth, which, itself, is also reflected sunlight. I'll now pass around some milk and cookies for everyone to enjoy, and then we can all take a short nap. --Ericdn (talk) 18:57, 15 March 2009 (UTC)

Double planet 1

I added the following information to the section about why some people consider the Earth and Moon to be a double planet system:

the sun actually has a stronger pull on the Moon than the Earth does. As a result of that, when the Moon is between the Sun and the Earth, it does not move away from the sun and towards the Earth, as most satellites do. It keeps moving towards the Sun. It just slows down, which allows the Earth to pass the Moon, which gives the appearance that the Moon is circling the Earth.

Someone reverted my edit, claiming it was utterly incoherent. I restored it. If you think it's incoherent, feel free to make it more coherent. Or explain why you think it's incoherent, and I'll address it myself. But don't just delete good information because you don't like how it's phrased. And by the way, my phrasing isn't incoherent at all. The guy probably just rejected it because he'd never considered it before, and didn't know what he was talking about. - Shaheenjim (talk) 00:23, 2 March 2009 (UTC)

Umm, OK, if the Moon "keeps moving towards the Sun", why doesn't it fall into it? Presumably the Moon moves away from the Sun at some point? You appear to just be saying that the Moon orbits the Sun (which it does) and its orbit is perturbed by Earth gravity (which it is). But so what? Earth-Moon are a gravitationally bound system, Earth is bigger. You can state that Asimov thinks it's a double-planet system, but yes, your explanation is incoherent. Please do rephrase, or we can just take it out if you can't do better. Franamax (talk) 06:01, 8 March 2009 (UTC)
Hello again, Franamax! A quick look at any astronomy guide to the planets will show that true "planets" always fall toward their star. All this means is that the planet's orbital momentum around the star counteracts the planet's tendency to actually accelerate into the star. The fact is: The Moon always falls toward the Sun, just like the "official" eight planets in our Solar System. Only satellites spend part of their time falling toward the Sun and the other part falling away from the Sun. Our Moon only moves away from the Sun in the same manner as the other planets, that is, the slight moving away and moving closer due to their Solar orbits being elliptical rather than circular.  .`^) Painediss`cuss (^`.  16:48, 8 March 2009 (UTC)
No, the Moon goes around the other side of the Earth, so at some point it has to move away from the Sun. And it's "falling" toward the Earth all the time, like you say, that's what an orbit is. I'll accept your proposition that the Moon doesn't show retrograde motion, but I suspect that is just an artifact of solar system geometry.
Now if you feel the need to tell me here that I need to read an astronomy guide in order to understand your addition to the article, then quite clearly your wording has failed its purpose. This is an encyclopedia for average folk, meant to provide clear explanations. Please consider rewording to make more clear what you are trying to convey. Franamax (talk) 21:05, 8 March 2009 (UTC)
Ahh. I see your point. You may be right. The edit has been rephrased, and hopefully it's correct now. It was probably my error, rather than Asimov's. - Shaheenjim (talk) 00:44, 11 March 2009 (UTC)
(Dfoofnik) Yes, the information here (re:Asimov) is both scientifically incorrect and misleading, as the solar gravitational effect on the Moon is only greater in absolute magnitude, not orbital magnitude, or the Moon would not remain with the Earth. This should be removed immediately, and referred as a possible putative revert.
Not sure what you mean by "absolute magnitude" vs. "orbital magnitude", but what was said is simply that the Sun's "pull" on the Moon is twice that of Earth's gravitational effect upon the Moon. And this is an indisputable fact. The only change I would make is to replace "pull" with "gravitational effect".  .`^) Painediss`cuss (^`.  16:48, 8 March 2009 (UTC)
The absolute value of Sun's gravitational force is irrelevant here. You are trying to compare apples with oranges. Only gradient of the Sun's gravitational field really matters. In other words the difference between the Sun's pull on the Moon, when the latter is the furthest from the Sun, and the pull, when the moon is closest to the Sun, is what matters. However this difference is much smaller than the pull of the Earth on the Moon. Ruslik (talk) 16:49, 9 March 2009 (UTC)

I was reading the article (which is very good btw), when I stumbled upon this section. Now I won't comment on the relationship of the earth's and the sun's gravitational pull on the moon. But the last part: "which allows the Earth to pass the Moon, which gives the appearance that the Moon is circling the Earth" is in fact, saying that the moon does not actually orbit the earth, or that the orbital movement is simply an illusion. Now 'im no expert, but isn't this just downright wrong? If it really is wrong I'd suggest the bit be removed asap. (talk) 11:47, 19 March 2009 (UTC)

The article is right. But it is very uncommon knowledge. For more information, see this link. For a rough picture I drew to show how they move over time, see this link. The black line is the Moon's orbit, the red line is the Earth's orbit, the black circle is the Moon, and the red circle is the Earth. It shows their positions at five different times, ending up in the same relative positions in which they started. - Shaheenjim (talk) 21:51, 19 March 2009 (UTC)
OK, time to revisit the article then and prune whatever is confusing to the reader. We don't represent "uncommon knowledge" here as being truth. We only represent various views with appropriate weight. Asimov spotted an artefact of solar geometry - the Moon has prograde motion wrt the Sun. Doesn't change the laws of orbital dynamics though - right? Franamax (talk) 22:46, 19 March 2009 (UTC)
Of course we represent uncommon knowledge here as being truth, if it is truth. Doesn't change what laws of orbital dynamics? - Shaheenjim (talk) 23:21, 19 March 2009 (UTC)
The text that you added to the article, as I've already mentioned, implies that the Moon does not orbit the Earth. That goes against pretty much everybody's most basic understanding of how the Solar System works. It's quite an assertion, and all you're providing to back it up are a series of drawings made by yourself. As things presently stand you haven't even been able to demonstrate that your "moon-orbits-sun" model is significant even as a minority held beleif (such as the whole flat Earth non-sense), let alone as a credible and widely accepted fact, as the article implies. I took a few minutes to Google this, and as it turns out there are a few astronomical blogs with posts devoted to debunking this assertion. Bottom line, I think it needs to be removed, or at least seriously re-edit to convey a clearer picture, in case my reading of this section was not actually the one you intended (talk) 01:47, 20 March 2009 (UTC)

Comment Given the level of discussion regarding this text, I have removed it to the talk page while consensus is reached. The text removed is as follows:

"Some people refer to the Earth–Moon system as a double planet system rather than a planet–moon system.[1] Isaac Asimov proposed such a description, in part because the Sun's gravitational pull on the Moon is approximately twice as strong as the Earth's pull on the Moon. Because of that, the Moon never travels in a retrograde direction in relation to the Sun. Instead, when it is ahead of the Earth in their orbit of the Sun, the Moon keeps moving forward, slowing enough to allow the Earth to pass by and creating the appearance that the Moon is circling the Earth.[citation needed]"

--Ckatzchatspy 02:33, 20 March 2009 (UTC)

I think it's a credible fact that is widely accepted by people who know what they're talking about. Now, most people, like you, don't know what they're talking about. But that doesn't make it any less true. I didn't make this up myself. In fact, it was already mentioned on Wikipedia's article for Orbit of the Moon#Path of Earth and Moon around Sun. That cites this source. If you find something that says otherwise, you should cite the source. But I searched for the words "moon" "concave" and "sun" on Google, and got 185,000 results. A quick scan of the first bunch of them seemed to indicate that all of them agree that the Moon's orbit around the sun is concave. - Shaheenjim (talk) 02:35, 20 March 2009 (UTC)
The only credible facts I see are that Asimov used the tug-of-war definition for double-planets in an essay; the Sun's pull is greater than the Earth's; and the Moon has prograde motion. I don't see any causation between gravity and motion, rather the motion is an artefact of geometry as pointed out in Orbit of the Moon. I don't see any confirmation that the Moon only "appears" to circle the Earth. The source you cite above for a concave orbit goes on to state that Earth is a planet, not a double-planet system. I don't see which "some people" refer to a double-planet, I see one person, Asimov, who was quite good at teasing out interesting facts and speculating based on them. What evidence do you have that the idea has been adopted by scholarly sources? It seems to be a fringe view, not even a minority one. Franamax (talk) 17:05, 20 March 2009 (UTC)
You don't see any causation between gravity and motion? Seriously? Do you know what gravity is? It's something that causes motion. That's the whole point. The motion may be a result of geometry, but the geometry is driven by gravity.
And if the Moon has prograde motion, then that means it only "appears" to circle the Earth. Those two statements are one and the same. So your failure to recognize a link between them is curious.
Asimov himself is one of the "some people" who refer to the Earth and Moon as a double-planet system, as you noted.
And when you ask about whether "the idea" has been adopted by scholarly sources, which idea do you mean? I mean that the idea that the Moon has prograde motion has been adopted by scholarly sources. I am not claiming that scholarly sources agree that the Earth and Moon are a double-planet system. In fact, I explicitly acknowledge that scholarly sources do not agree that the Earth and Moon are a double-planet system. And it's also acknowledged by the part of that article to which people are objecting. - Shaheenjim (talk) 23:45, 20 March 2009 (UTC)
Moving beyond your evident scorn for people who don't "understand" with the acuity you yourself possess, and repairing my non-clarity in my previous comment: I see no causation between the ratio of relative gravitational influence of the Earth and Sun upon the Moon and the artefact of geometry which conveys upon the Moon continuous prograde motion. You are defending the wording "Because of that..." as a causal relationship - so yeah, what about the relative gravitational forces causes the prograde motion? If it's so obvious, I'm sure you can explain to a dullard such as myself, and more importantly explain to our worldwide readership exactly what "because" means. Do you have a particular equation?
And when we discuss "the idea", again let me clarify: I mean the idea that Earth-Moon is a double planet system. The geometrical facts are irrefutable, the conclusion is not. The proposed wording includes the wording "some people" [who?] - we can only identify one person at this point, Asimov, a prolific author known for his innovative writing. So again, where is the evidence this is anything othr than a fringe viewpoint, and does it deserve weight in this article?
And I notice that you fail to address the objection to the wording which implies that the Moon only "appears" to circle the Earth, whereas many people would likely agree that you are objectively wrong on that. Franamax (talk) 00:48, 21 March 2009 (UTC)
1. Am I to understand that you agree that the Sun has the strongest pull on the Moon, and you agree that the Moon's motion is prograde with respect to the sun, but you don't understand the relationship between those two facts? It seems self apparent to me. Now, those two facts are uncommon knowledge. They're directly contradicted by a very popular misconception. So I expect that most people wouldn't agree with one or the other, or probably both, of those two facts. But if someone does agree with both of those facts, then I'd expect that person to understand the relationship between the two facts as self apparent. I don't know what the equations are, but I expect they'd be the equations that determine how gravity affects motion in general.
2. It sounds like you asked me what evidence I have that scholarly sources have adopted the idea that the Earth and Moon are a double-planet system. That seems like a strange question to me, since I never said I had any evidence of that. You should probably reread the last paragraph of my previous response. But I'll expand on it:
3. First of all, Asimov might be the only famous person who referred to the Earth and Moon as a double-planet system (although I don't know, either way). But even if that's true, he isn't the only one who referred to them as a double-planet system. There were other people who aren't famous. For example, the Wikipedia user Paine_Ellsworth who started this whole thing. He got it from Asimov.
4. I'm not disputing that the claim that the Earth and Moon are a double-planet system is a fringe viewpoint. I acknowledge that it's a fringe viewpoint. Actually, you could say it's even worse than a fringe viewpoint. When people talk about a fringe viewpoint, they're usually talking about something that some people think, and most people don't, and which probably isn't true, but which might be true (since it hasn't been definitively disproven). I'd say that this is something that definitely isn't true. The article even says that it isn't true. I don't think we should give this position any weight in the article at all. But I still think the viewpoint should be mentioned in the article, as a point of historical interest. The link you gave to Wikipedia's policy on Weight gives an example of what I'm describing. It says that when talking about whether or not the Earth is flat, you're allowed to mention that people used to think the Earth was flat, but now we know it isn't. It's the same thing here. I'm suggesting that we mention that some people used to think the Earth and Moon are a double-planet system, but now we know it isn't. If you want to edit my addition to the article to make that more clear, that's fine with me. But there's no need to remove my addition to the article altogether.
5. Finally, you're trying to assess the accuracy of the statement that the Moon only appears to circle the Earth. You seem to assume that we're using the same definition of the word "circle", then you're focusing on the meaning of the word "appears." I suggest that you have that backwards. We're using the same definition of the word "appears." You should focus on the meaning of the word "circle." That's the part I think you don't understand. In order to circle the Earth, the Moon would need to have retrograde motion with respect to the Sun. The Moon doesn't have retrograde motion with respect to the Sun, therefore the Moon doesn't circle the Earth. But the Moon appears to have retrograde motion with respect to the Sun, therefore the Moon appears to circle the Earth. - Shaheenjim (talk) 03:22, 21 March 2009 (UTC)
(undent) In order:
  1. No, sorry, Shaheen, it's not apparent to me. It doesn't seem especially apparent to many other editors here either. I can only reiterate the peculiar fact of geometry that makes it so. As evidenced in statements in other articles we've both linked above, if the Earth-Moon system was much more distant from the Sun, the Moon would have retrograde motion. It seems curious that planet/double-planet definitions depend solely on their distance from the primary body (the Sun) - but not to worry, all I asked you for was an equation showing the causal relationship between the gravitational ratio and the prograde motion. The proposed wording notes the ratio and proceeds to say "Because of that..." - so yes, please provide the equation. "because" denotes a causal relationship, which should be easy enough to prove. I for one do not believe that Asimov ever said "because of" and I see no other source supporting the putative statement.
  2. Yes, exactly. But let's see what's at your point 3.
  3. There's my question! So we now have Asimov and a Wikipedia editor. But that still boils down to one single person making the assertion (us editors being cannon-fodder doncha'know - we all add up to exactly zero, me included :) Looking at the reliable sources we have only Asimov, and while I have at least 30 of his works littering my house, science and SF, the guy embodied "speculative" or perhaps defined the term. So yes, other than the single person, who else? Keeping in mind that apparently Asimov's essay was first published in MFSF.
  4. Weight. Again, you're talking about one single person's view. We have no evidence this view has had any serious consideration beyond those people of the opinion that if Asimov said it, it will eventually come true. Yes, I maintain that it is beyond fringe and approaches single viewpoint - though I hate to say it, pushing to the Velikovsky and von Daniken edges - but only because I think Asimov shared the same spirit of open thought and speculation at the time, not based on the scientific merits. So anyway, I asked you for more sourceable evidence and apparently you have none forthcoming.
  5. Appears to circle. Hmm, what to say? Why is the "appears to" wording in there? The plain fact is tha the Moon does circle the Earth - should we enlist 'pedians to join reputable news-gathering organizations to report that they witnessed the moon passing over points all about the earth in the plane described by the zodiac? Do you wish Druids to confirm this? Alternatively, simple enough, what's the equation for not-circling? (Oh lest we forget - what is the equation that describes how the two bodies orbit their common barycentre, which barycentre is dominated by Earth's gravity and is located within one radius of the Earth CG - but the body of lesser mass fails to complete a circular orbit around the barycentre located within the planet?)
Picking through all these nits (which is certainly fun, I'm sure you'll agree) - I don't see any particular wording appropriate to a major article such as Moon. The proposed wording is not satisfactory on several fronts; reduced, it at present comes down to one man's views. I'd perhaps support a drastically reduced wording as in one sourced sentence - but I'll defer to others on whether even that is appropriate weight. Recall that we're not bound to describe every view that any person on the planet has ever espoused. Franamax (talk) 04:54, 21 March 2009 (UTC)
To anyone who says the Moon doesn't circle the Earth, because there is no retrograde motion from the Sun: Should we judge our own motion using geocentric coordinates? Should we describe the planets' motions, and whether they orbit the Sun, by their galactic coordinates? Shifting the reference frame is bound to change the shape. Saros136 (talk) 05:22, 21 March 2009 (UTC)
1. I think the other editors are confused about the two facts that you acknowledged. Once people acknowledge those two facts, I think it's not all that confusing to understand that they're related. But I'll try again to convince you anyway. You keep describing it the Moon's prograde motion as a "peculiar" fact of geometry. I'm not sure what you think determines that geometry. Again, the geometry of astronomical objects is determined by gravity (and momentum). You seem to be confused about the fact that the Moon would go retrograde if the Earth and Moon were further from the Sun. That fact actually supports my argument. If the Moon would go retrograde if the Earth and Moon were further from the Sun, it's only because being further from the Sun would cause the Sun's gravity to have less of a pull on the Moon. That's how gravity works. Check out this link to Newton's law of universal gravitation. The formula for gravity is F=G*m1*m2/r^2 where:
F is the magnitude of the gravitational force between the two point masses,
G is the gravitational constant,
m1 is the mass of the first point mass,
m2 is the mass of the second point mass, and
r is the distance between the two point masses.
So when r increases, F decreases.
2. You implied that it's wrong, and I agreed, and it seems like you're still arguing with me. That makes no sense.
3. Sometimes cannons need to be fed. The typical Wikipedia editor might not count for much, but I maintain that he still counts for something. And the bigger point is that if you can see one random person who believes some obscure incorrect thing, there are probably more random people who believe the same obscure incorrect thing, but whom you can't see. Sort of like insects in a house.
4. You said it shouldn't be given weight, then I agreed, and now you're still arguing with me. That makes no sense.
5. Again, the "appears" is there because the Moon "appears" to go retrograde, when it actually doesn't. If it doesn't go retrograde, then it's not a circle. A circle is where the center stays in one place, and a line goes around that one place. The equation is x^2+y^2=c. Any other equation is the equation for not-circling. Circling would require retrograde motion on the part of the circler. But that's not what happens with the Earth and the Moon. The Earth doesn't stay in one place and have the Moon go around it. The Moon speeds up and passes the Earth on the outside of their orbit with the Sun. Then the Moon passes in front of the Earth. If the Moon were circling the Earth, its next move would be to go retrograde and pass the Earth on the inside of their orbit with the Sun. But that's not what happens. It appears to happen from our perspective, but it's not what actually happens. Instead, the next move if for the Moon to merely slow down, which allows the Earth to pass by the Moon while the Moon is on the inside of their orbit with the Sun. That's not a circle.
6. Again, I'm not suggesting we give his views any weight. Just mention them as a point of historical interest. Now, you're right that his views are less historically interesting, since they were always a fringe viewpoint (as opposed to a flat Earth, which was once a common belief). But I want to include his views anyway, because it gives people a way of understanding just how unusual it is that the Moon maintains prograde motion. That's the important point. Not that someone once proposed that the Earth and Moon are a double planet system. The important point is the Moon's prograde motion, and I don't think you're disputing that point. But I think that mentioning that Asimov thought of the Earth and Moon as a double planet system will help people to understand that the Moon has prograde motion.
7 (To Saros136). I'd judge the Earth's motion using the Sun as a basis. I'd judge Mars's motion using the Sun as a basis. I'd judge Mars's moons' motion using Mars as a basis. But I'd judge Earth's Moon's motion using the Sun as a basis. And I'd judge Jupiter's motion using both Jupiter and the Sun as a basis. - Shaheenjim (talk) 11:00, 21 March 2009 (UTC)
Franamax, I readded the part of the article to which you were objecting, but I clarified it to address your objections. Hopefully it should be ok now. - Shaheenjim (talk) 00:44, 22 March 2009 (UTC)
I've removed it, until Franamax has a chance to reply, and until others have a chance to comment as well. The revised version read as:

"The Moon is exceptionally large relative to the Earth, being a quarter the diameter of the planet and 1/81 its mass. The Sun's gravitational pull on the Moon is approximately twice as strong as the Earth's pull on the Moon. From the Earth's perspective, the Moon appears to circle the Earth by moving in a retrograde direction in relation to the Sun. However in reality, the Moon always moves forward along its orbit of the Sun. The appearance of retrograde motion is a result of the fact that when the Moon is ahead of the Earth in their orbit of the Sun, the Moon keeps moving forward, but it slows enough to allow the Earth to pass it. Isaac Asimov considered the Earth–Moon system to be a double planet system rather than a planet–moon system,[2], although it is officially still considered a planet-moon system, since the common centre of mass of the system (the barycentre) is located about 1,700 km beneath the surface of the Earth (about a quarter of the Earth's radius). The surface of the Moon is less than one-tenth that of the Earth, and only about a quarter the size of the Earth's land area (or about as large as Russia, Canada, and the U.S. combined)."

--Ckatzchatspy 16:19, 24 March 2009 (UTC)
Heh, I was waiting for other people to comment. :) I think it's still too wordy, and the general view seems to be that it is still giving undue weight. Also note that Earth-Moon is not "officially" considered a planet-satellite system, as far as I know the IAU bailed out on adopting the barycentre method of definition last time they met. More comments below. Franamax (talk) 17:56, 24 March 2009 (UTC)

Double planet 2

A question for Shaheenjim. Jupiter's moon Callisto moves around Jupiter with the speed of about 8 km/s. Jupiter revolves around the Sun with the average speed of about 13 km/s. It is obvious that Callisto is always in the prograde motion relative to the Sun. Would you support calling Jupiter a double planet (Jupiter-Callisto double planet!)? Or would you support a claim that Callisto does not orbit Jupiter? Ruslik (talk) 12:48, 21 March 2009 (UTC)

First of all, your question seems to assume that Jupiter orbits the Sun. I don't grant that premise. And that's an interesting topic in and of itself, but I don't want to get into it here. Click on this link for a good place to discuss that.
But for the purposes of this discussion, I'll assume that Jupiter does orbit the Sun. Now, it may be true that Callisto's movement around Jupiter is slower than Jupiter's movement around the Sun. But I don't think that necessarily means that Callisto is always in prograde motion relative to the Sun. I suggest you check that. Although I'm not sure.
If it does move retro to the Sun, then obviously it'd orbit Jupiter, and it'd be a moon. If Callisto is always in prograde motion relative to the Sun, then I'd think of it just like I think of Earth's Moon.
So how do I think of Earth's Moon, you ask? Good question. I wouldn't say it's a planet, or that the Earth and Moon are a double planet system. But I wouldn't say it circles the Earth either, as I mentioned in the section above. So what would I say? Again, good question. Currently we divide things into two categories: planets that orbit stars, and moons that orbit planets. I actually think we need a third category (maybe more if you consider the fact that Jupiter doesn't orbit the Sun, but that's a separate issue). The first category would be planets, and it would be defined as things with a barycenter inside the Sun. The third category would be moons, and it would be described as things with a barycenter inside a planet, who circle that planet (which requires them to have retrograde motion with respect to the sun). And I'd have another category in between those two things. It'd consist of things with a barycenter inside a planet, but which don't circle that planet (since they have prograde motion with respect to the sun). I'd put Earth's Moon in that category.
This "third category" idea of mine isn't very widespread. But I think that's because most people aren't aware of the relevant facts. They aren't aware that Earth's Moon maintains prograde motion. If they were aware of that, I think the third category idea would gain more support. - Shaheenjim (talk) 13:34, 21 March 2009 (UTC)
This is an encyclopedia - not a forum for discussing what 'should' be. We are not in the position to make up new categories of objects, but rather stick to the definitions that have been established. The definitions of planet and moon have already been settled. As an encyclopedia, we must adhere to definitions as they stand, and defer to peer-reviewed sources and references from experts in the field. PhySusie (talk) 21:58, 21 March 2009 (UTC)
And here I thought that talk pages would be used as a forum for discussing what 'should' be included and what shouldn't be included based upon known facts and references. It should be no surprise to anyone that such a process would sometimes be controversial, because some people give more "weight" to certain facts than others do. I think Franamax below has the right idea. The only thing I can think of there is that rather than thinking "geometry" which seems to introduce a relativistic viewpoint, you might think in terms of something related to "geography". I'm not sure if there's even a word for it; however, it's the location of the Earth/Moon system, it's closeness to the Sun that makes it different from, say, a planet and satellite farther from the Sun. Charon for example goes retrograde with respect to the Sun and has loops in it's Solar orbit. Also, if I may note that the definitions of planet and moon/satellite are by no means "settled". There are many astronomers ("experts") who say they refuse to accept the IAU definition from the 2006 meetings. So the 2009 meetings can be expected to be heavy with debate on this subject. Lastly, I would like to add that if one goes strictly by the 2006 definition, which says absolutely nothing about barycenter locations, one could very easily "officially" categorize the Moon as a planet and the Earth/Moon system as a binary-planet system.  .`^) Painediss`cuss (^`.  05:53, 25 March 2009 (UTC)
Shaheenjim has readded the removed text, once again claiming that the moon only "appears to circle the Earth". Can someone please handle this? I can't edit the article (talk) 09:34, 24 March 2009 (UTC)
It's been handled. You must've missed it. Look up, to the Double Planet 1 section. - Shaheenjim (talk) 10:49, 24 March 2009 (UTC)
I have just removed the text. I think maybe what Shaheenjim is getting at is that the moon's absolute path through space is always curved toward the sun--more curved when outside the earth and less when inside, but never curved away from the sun. This is actually a stronger statement than that the moon never moves "backward" relative to the sun--in order to do that, earth's gravitational pull on the moon would have to be several times larger than that of the sun. I think something along these lines actually belongs in the "orbit" section of the article, but certainly not the current draft. Rracecarr (talk) 16:48, 24 March 2009 (UTC) (well, I thought I removed it--looks like Ckatz actually beat me to it). Rracecarr (talk) 16:52, 24 March 2009 (UTC)
If you want to add something to my text, you're welcome to do so. But I don't see any reason to just delete my text altogether. You certainly didn't give any reason to delete it. And by the way, its curvature isn't what I was getting at. I really was getting at the fact that the moon never moves backward relative to the sun. - Shaheenjim (talk) 17:08, 24 March 2009 (UTC)
The article as it stands notes one thing that makes the Moon unusual among moons in this solar system, its relative size. There are two more unusual features, the gravitation ratio (Sun/Earth) and the prograde motioncontinuously-convex orbit. These should probably be noted in this article, though not in the wording Shaheenjim is trying to introduce, i.e. more briefly. I would do it in a second with bullet points, but we're supposed to use prose.
These three anomalies all support the double-planet notion, so I wouldn't mind seeing some wording where the Asimov ref could be used (but not necessarily Asimov's name).
I'm thinking something like "The moon is unusual in several respects: size; gravity; progradeconvex. This has given rise to suggestions that the Earth-Moon system is actually a double planet [ref] however the Moon is commonly considered a satellite since the barycentre definition..." I may give it a try later today. Franamax (talk) 17:50, 24 March 2009 (UTC)
Whoa. Prograde motion is not unusual. It's usual. All the moons of the inner planets (that is, Earth's Moon plus two Mars moons) move strictly prograde. Even Jupiter, the heaviest planet and therefore the one whose moons orbit fastest, has only 6 moons, of 63 total, that orbit their planet faster than they orbit the sun. Also, "gravity" and "prograde" are not two independent items: it is impossible for any moon of any planet around any sun to move retrograde if the gravity of its sun attracts it more strongly than that of its planet. Rracecarr (talk) 18:19, 24 March 2009 (UTC)
See now, that's why I like to discuss things on talk pages first. :) I set up a spreadsheet and took a look at 20 various moons of 5 planets, looking at inmost/outmost/major satellites (ignoring moons with retrograde orbits, seems to me they will always have looping orbits wrt the Sun). I used the definitions given here for d and p and classified them as having "looping" (i.e. partly retrograde, d<p), "wavy", or "convex" (d>p2) orbits from the heliocentric reference. (Copy of ss on request, send me an email) Of the moons I checked, 68 have wavy orbits, 1311 have looping orbits (including Deimos and Phobos, which seems contrary to your statement above), one had a continuously convex orbit viewed from the heliocentric reference (guess which one?).
I'm not clear on the math which gives the simple definition of orbit their planet faster than they orbit the sun as being the determinant of the heliocentric motion, the source cited seems to trifurcate the motions. I'm always happy though to see derivations that result in simple equations, so I'd be happy to see it.
And I'm certainly interested in the gravity ratio and the necessity for prograde motion, again I'd love to see the equations. These could be a useful addition to our Orbit of the Moon article.
Ultimately, I think this comes down to the Moon being a cheater, since it's the closest one to the Sun. R-squared comes into play in gravitation, Kepler tells us about the geometry. Nevertheless, we should report the salient facts. My spreadsheet showing the Moon as the only satellite with a continuously-convex orbit is definitely WP:OR and as I've said all along, I think it's an artefact of geometry - however if it can be sourced, it's notable. Franamax (talk) 22:40, 24 March 2009 (UTC)
OH! Rracecarr, I just figured out what your problem is. I think it was Ruslik's problem too, although it took me several days to realize it. You guys were talking about whether or not a moon orbits its planet faster than it orbits the sun. You were talking about it as if it were relevant to whether or not a moon has prograde motion. And at first I couldn't figure out why you thought it was relevant, but I just did. When you guys talk about moons having prograde motion, you're talking about them having prograde motion in relation to their planets. When I said that the Earth's moon has prograde motion, I meant that it's prograde in relation to the Sun. That really is unusual.
Franamax: You continue to describe the Moon's prograde motion (in relation to the Sun!) as an "artefact of geometry." You say it as if that's an alternative to being a result of gravity. I have told you now on a number of occasions that the geometry is a result of gravity. And yet you keep repeating your claim. So since you don't respond to statements, I'll try a question: What do you think causes the geometry to work this way? Do you understand that it's a result of gravity? Or do you think it's something else? And if it's something else, what do you think it is? Do you think it's just determined randomly? - Shaheenjim (talk) 01:11, 25 March 2009 (UTC)
And as I've said before, it is geometry. If the Earth-Moon system were much farther away from the Sun, the Moon would exhibit retrograde heliocentric movement. The masses would be the same, gravity would still obey the universal law, but the perceived motion would be different. All that would change is the value of RE - try it with a value of 1.28E11 km or so, magically, the Moon now shows retrograde motion. Yes, the existence of Earth and Moon in their respective orbital positions was determined randomly, I don't believe in an intelligent creator getting into the details of placing the Moon just so. You can maybe counter with the double-planet hypothesis of Earth-Moon formation, but that hypothesis doesn't seem well supported by modelling. The impact hypothesis is much more well-supported. Obviously gravity would determine where the ejected material would stabilize in orbit, but no, I don't think God shot a Mars-size body at just the right place with just the right speed to put the Moon where it is, having first placed Earth in just the right orbit. If you can produce a reliable source showing that the inevitable consequence of an impact event ejecting mass into orbit is a convex orbit of the resulting satellite, please do supply it. I love reading that stuff. Franamax (talk) 22:33, 25 March 2009 (UTC)
Wow. I don't know where you're getting this stuff. I never said anything remotely like what you seem to be implying that I've said. I've never said an intelligent creator placed the Moon just so. I've never said the Earth and Moon formed through the double-planet hypothesis. I've never said that a moon captured through impact inevitably results in a convex orbit. Perhaps I should remind you what has been said so far, since you seem to have forgotten. In your post above at 00:48, 21 March 2009 (UTC) you said, "I see no causation between the ratio of relative gravitational influence of the Earth and Sun upon the Moon and the artefact of geometry which conveys upon the Moon continuous prograde motion." All I said is that there is causation between the ratio of relative gravitational influence of the Earth and Sun upon the Moon and the Moon's continuous prograde motion.
Now, it may be true that moving the Earth and Moon further away from the Sun would cause the Moon to have retrograde motion. But that fact doesn't disprove a causation between the ratio of relative gravitational influence of the Earth and Sun upon the Moon and the Moon's continuous prograde motion. Actually, that fact supports a causation between the ratio of relative gravitational influence of the Earth and Sun upon the Moon and the Moon's continuous prograde motion. Because if you move the Earth and Moon further away from the Sun, you would change the ratio of relative gravitational influence of the Earth and Sun upon the Moon. It's true that the masses would be the same, and gravity would still obey the universal law. But it would still change the ratio of relative gravitational influence of the Earth and Sun upon the Moon. That's because the ratio of relative gravitational influence of the Earth and Sun upon the Moon depends on the Earth's and Moon's distance from the Sun. - Shaheenjim (talk) 22:57, 25 March 2009 (UTC)
Please keep in mind that our purpose here is to improve the wording in the article, not to have a pissing-match on who knows more about what.
You directly challenged my statement that the particular facts of placement of Earth-Moon have anything to do with the Moon's heliocentric proper motion. ("An artefact of geometry") I tried to refute your challenge by pointing out that varying one single parameter changes the proper motion, i.e. the motion can become retrograde merely by changing the geometry. You say that "may be true" - but I gave you the parameter, did you actually check the equations to see that it in fact is true? You further asked whether I thought the Earth-Moon system is a random occurence, and I replied that indeed I do. There are two alternatives: an intelligent creator, or a rational explanation based on physics as to why the system has to be the way it is. I outlined my objections to both alternatives and invited you to provide sources for the physical explanation. You ignored that invitation and instead have responded with what appears to be back-pedalling, saying "all I said..."
And that's fine too, but you started out by introducing wording saying that the fact that the Sun gravity pull on the Moon exceeds that of Earth (true) and the Moon shows heliocentric prograde motion (true) adds up to the double-planet theory. But the second fact is not unique to the Moon, many satellites show helio-prograde motion. The distinction is continuous-convex motion wrt the Sun. Your wording has consistently confused the issue and has been twice rejected by consensus. While I understand that you may feel that you have superior understanding on this issue, I would suggest that you adopt a more conciliatory approach.
Referring to the latest below, it seems that Rracecarr and I at least may agree that the "cut-point" is at relative gravitational influence, which is equal to the transition between "wavy" prograde and convex prograde motion. Again, if you can show the equations to support your contention that grav-ratio determines the difference between retrograde and prograde motion, please do. The ratio does not seem to be 1:1, but I'll check that later today - will you do the same? I've run through all the equations and articles, looked for sources, set up a spreadsheet, I'm trying to do more than just argue on a talk page. I want to get good consensus wording into the article, appropriately sourced. Franamax (talk) 00:18, 26 March 2009 (UTC)
I never disputed that the Earth's and Moon's distance from the Sun affect the Moon's heliocentric motion. I merely said that the Moon's heliocentric motion was a result of gravity. If the Earth's and Moon's distance from the Sun affect the Moon's heliocentric motion, that's only because the Earth's and Moon's distance from the Sun affect gravity. So my statement remains true.
I never asked if you thought the Earth-Moon system is a random occurrence. Or, never intended to. I was asking if you thought the Earth-Moon orbit is independent of gravity. That's what I meant by "random." I wasn't intending to get into a debate about an intelligent creator.
You (and Rracecarr) may be right that I'm emphasizing the difference between prograde and retrograde motion too much, and not emphasizing the difference between wavy prograde and convex prograde enough. Which would be ironic, since you're the one who introduced the concept of retrograde motion, in your edit above at 21:05, 8 March 2009 (UTC). In any case, I'm not opposed to a rewrite of my addition that places the emphasis more on convexity.
To clarify, I'm suggesting that if the Sun's gravity affects a Moon more than its planet's gravity, that means the Moon will have prograde motion. And I think you agree with that, if only lately. I am not suggesting that the converse is true. I think you might have incorrectly inferred that. - Shaheenjim (talk) 00:44, 26 March 2009 (UTC)
Well, the irony is lost on me, since my edit stated that I accepted the idea of prograde motion [2]. Now the fact that you just previously stated that the Moon's motion is "prograde in relation to the Sun" and that "really is unusual" - whereas now you are saying maybe you didn't emphasize the differences enough, that may indeed qualify as irony, being said all in the same paragraph. Whatever though, I'm happy to state that you were right all along and it was all my fault. If we get good wording out of it, I'll be quite happy.
And per your clarification, yes indeed in the case you state, the moon will have prograde motion, but that is not the only case where the moon will have prograde motion, so it's not a useful fact for this article. The useful fact is that the Moon is the only moon to show heliocentric continuous-convex motion in this solar system. This is a necessary property of the fact that the Sun grav-pull on the Moon is greater than the Earth-grav. Do we agree on that? (We also agree that the Moon heliocentric motion is prograde, but that is not unique) If you agree with those, we may be able to proceed to a good wording. Franamax (talk) 01:12, 26 March 2009 (UTC)
The irony is that you brought it up at all, then went on to say it's not a useful fact.
I don't follow your second sentence. I didn't say I didn't emphasize the prograde motion enough. In fact, I said I may have emphasized it too much. And my statement that it was unusual was in a separate paragraph from my statement about how much I emphasized it.
I agree that moons can have prograde motion even if the Sun's gravity doesn't affect them more than their planet's gravity.
I propose this alternative wording for the article:
The Moon is exceptionally large relative to the Earth, being a quarter the diameter of the planet and 1/81 its mass. The Sun's gravitational pull on the Moon is approximately twice as strong as the Earth's pull on the Moon. From the Earth's perspective, the Moon appears to circle the Earth by moving in a retrograde direction in relation to the Sun. However in reality, the Moon always moves forward in its yearly path around the Sun, and its path around the Sun is convex. The appearance of retrograde motion is a result of the fact that when the Moon is ahead of the Earth in their orbit of the Sun, the Moon keeps moving forward, but it slows enough to allow the Earth to pass it. Some people consider the Earth–Moon system to be a double planet system rather than a planet–moon system[3], although most astronomers still considered it to be a planet-moon system, since the common centre of mass of the system (the barycentre) is located about 1,700 km beneath the surface of the Earth (about a quarter of the Earth's radius). The surface of the Moon is less than one-tenth that of the Earth, and only about a quarter the size of the Earth's land area (or about as large as Russia, Canada, and the U.S. combined).
It's a bit wordy, but anything less and we run the risk of the average person not understanding it. We've certainly had enough trouble understanding it, and we know a lot more about this than most people. - Shaheenjim (talk) 01:45, 26 March 2009 (UTC)
Either I don't understand what prograde/retrograde means (which is possible) or you all are making things more complicated than they need to be. Looking at the solar system from the "north" (above the north pole of the earth) I'm assuming motion around the sun in a counterclockwise direction is prograde motion, clockwise is retrograde. If that's not what the words mean, please straighten me out. If those definitions are correct, then all you have to do to decide if a moon ever exhibits retrograde motion is to compare the speed of its orbit around its planet with the speed of the planet's orbit around the sun. Obviously, if the moon is moving along with its planet faster than it orbits the planet, the motion around the planet can never overcome the motion with the planet, and it can never move backward, or "loop". This is the case with Deimos and Phobos, whose speeds around Mars are less than a tenth of Mars' speed around the sun. For noncircular orbits, these speeds are not precisely constant, but the eccentricity of the orbits of planets and moons is small enough that the variation in speed can safely be neglected.
The reason that retrograde motion cannot result if the sun pulls more strongly on a moon than its planet does is that the moon is closer to its planet than to the sun. Assuming a circular orbit (a good enough approximation), the gravitational force is balanced by centripetal acceleration: v^2/r. Since v^2/r is bigger for the orbit around the sun, and since r is bigger for the sun, v^2, and therefore v must certainly be bigger, meaning retrograde motion is impossible. Rracecarr (talk) 14:38, 25 March 2009 (UTC)
A more intuitive argument gives an even stronger result (that not only must the orbit be prograde, it must be convex): if the sun pulls on the moon harder than does the planet, the moon must always accelerate toward the sun, even when the planet is pulling it the other way. That is, the moons orbit is convex. Rracecarr (talk) 15:04, 25 March 2009 (UTC)
Yes indeed, if you move Earth out to almost Mars orbit (where the Moon gravitation is balanced between Earth and Sun), it is also the transition point between a wavy and continuously convex orbit, so the math appears to back up your intuitive argument. So the two things are related, grav-ratio and convexity. I'd still prefer to see both mentioned in any brief wording discussing the double-planet hypothesis.
And in ref to your comments just above, I believe that we are talking here solely about moons in prograde orbit about their planets, but we are discussing whether these moons exhibit retrograde motion with respect to the Sun. This does depend on whether the moon orbital velocity exceeds the planet orbital velocity, defined as a "looping" orbit in the ref I link above. When the moon velocity is less than the planet, the heliocentric motion is continuously prograde (by my reading of it) but is "wavy", i.e. varying between convex and concave wrt the Sun. The sole exception is the Moon, which is continuously convex. (And I found my error with Phobos and Deimos, they're wavy, not looping as I'd previously said - you were right there, I forgot to, uhh, carry the one :). Franamax (talk) 22:07, 25 March 2009 (UTC)

Formation theories

I propose to revamp the formation theories section. It goes through the antiquated theories first, and rejects them, before spending a little bit of space (not much) on the current theory. I would like to see the current theory expanded, and the others put later, in a subsection, or at least downsized in importance, as they currently take up the majority of that section of the article. I'll wait a day or two for objections or (hopefully) suggestions, and then get to work. Awickert (talk) 07:03, 8 March 2009 (UTC)

Perhaps it is time to do this, and perhaps not. Keep in mind that the current theory is still just a "theory", and that there are still problems with it. For example, the current theory still cannot explain fully why the Moon has a near-circular orbit, nor why the Moon orbits so close to the ecliptic. Until these and other questions are answered, the current theory is still just a better refinement of older theories that may or may not bring us a little closer to a clear picture of physical reality. Just a thought, though, so go with your conscience and "be bold"  .`^) Painediss`cuss (^`.  17:21, 8 March 2009 (UTC)
Wow, sorry for not replying for so long; I must have missed your reply when I was checking my talk page. I actually don't know much about the orbit of the moon, though what you say seems to make sense for the "spinning off" theory. The current theory, however, does a great job of explaining its composition and, as far as I know, the large combined angular momentum. I think I'll read up on this, and then edit once I'm better-informed. Awickert (talk) 18:49, 15 March 2009 (UTC)

May 31, 2009: —Preceding unsigned comment added by (talk) 23:03, 31 May 2009 (UTC)

I feel that there is another problem with the "Formations Theories" section -

All of the "discounted" theories are mentioned first, with a statement discounting each one. The theories themselves are well-referenced. Fine. But the statements discounting them are *not* referenced at all. Considering that the intent is obviously to persuade that the final theory mentioned is true, it seems that this is an important ommission. In order to persuasively build the case, there should be references for each statement discounting the supposedly "disproved" theories.

  1. ^ Asimov, Isaac (1975). Just Mooning Around, In: Of time and space, and other things. Avon.
  2. ^ Asimov, Isaac (1975). Just Mooning Around, In: Of time and space, and other things. Avon.
  3. ^ Asimov, Isaac (1975). Just Mooning Around, In: Of time and space, and other things. Avon.