# Moons' origin **URL:** \ **Category:** Factual Questions\ **Created:** [July 25, 2009, 5:23pm UTC](https://boards.straightdope.com/t/moons-origin/504231 "2009-07-25T17:23:07Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Jormungandr](https://avatars.discourse-cdn.com/v4/letter/j/a6a055/32.png) [@Jormungandr](https://boards.straightdope.com/u/Jormungandr)\ **Post date:** [July 25, 2009, 5:23pm UTC](https://boards.straightdope.com/t/moons-origin/504231/1 "2009-07-25T17:23:07Z")
Ok, I was out last night looking at the stars with the neighbors. His son asked how the moon was formed. I mentioned the “Theia” moon theory, where a Mar-sized body supposedly hit Earth and the blast-offed material formed the Moon. Right off the bat, that didn’t seemed right to me. If that were true, how did the other moons, such as Titan and Io, form if they revolve around gas giants. The recent impact, as well as Shoemaker-Levy 9, on Jupiter shows a “Theia” type event unlikely there. So, is there a prevailing theory on moon evolution for the solar system overall? ---
**Author:** ![RickJay](https://avatars.discourse-cdn.com/v4/letter/r/bb73d2/32.png) [@RickJay](https://boards.straightdope.com/u/RickJay)\ **Post date:** [July 25, 2009, 6:21pm UTC](https://boards.straightdope.com/t/moons-origin/504231/2 "2009-07-25T18:21:56Z")
> [@Jormungandr](#): > > Ok, I was out last night looking at the stars with the neighbors. His son asked how the moon was formed. I mentioned the “Theia” moon theory, where a Mar-sized body supposedly hit Earth and the blast-offed material formed the Moon. Right off the bat, that didn’t seemed right to me. If that were true, how did the other moons, such as Titan and Io, form if they revolve around gas giants. The recent impact, as well as Shoemaker-Levy 9, on Jupiter shows a “Theia” type event unlikely there. So, is there a prevailing theory on moon evolution for the solar system overall? The likely explanation is simply chance; some blobs of matter were consumed into the gravity well of the growing big planets, and some were not. They were formed the same way the planets were, just in their own little planetary system. Jupiter, Saturn, Uranus and Neptune become very, very big, and have so much gravity, they captured more matter in the right orbital velocity to make moons. The theory behind Earth’s moon is the one that’s different. The reason they theorize our moon is different is that, when you think about it, our moon is weird. For one thing, it’s absurdly big as compared to the size of its planet. For another, it’s the only thing like it in the inner solar system. Inside the asteroid belt there is nothing else like it; just it and four planets. Mercury and Venus don’t have moons, and Mars has two tiny little things that wouldn’t be a pimple on our moon. So a logical conclusion is that the inner planets were not big enough to capture a lot of moons, while the outer ones were. So why did we get a big honkin’ moon unlike any other non-planetary object in the innter solar system? A collision seems like a possible reason. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 25, 2009, 6:49pm UTC](https://boards.straightdope.com/t/moons-origin/504231/3 "2009-07-25T18:49:40Z")
> [@](#): > > The reason they theorize our moon is different is that, when you think about it, our moon is weird. In addition to the oddities **RickJay** mentioned, the Moon’s orbit is also weird in a few ways. First off, it’s close(ish) to the plane of the ecliptic, not in the plane of its planet’s rotation (as almost all other moons are). Second, it’s actually far enough away from the Earth that it feels a larger gravitational force from the Sun than from the Earth-- It can be considered to be orbiting around the Sun, just with an orbit that’s highly perturbed by the Earth. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [July 25, 2009, 7:03pm UTC](https://boards.straightdope.com/t/moons-origin/504231/4 "2009-07-25T19:03:00Z")
> [@Chronos](#): > > It can be considered to be orbiting around the Sun, just with an orbit that’s highly perturbed by the Earth. The Moon also exerts a gravitational force on the Earth; so the Earth’s orbit should be perturbed. Question: Where is the CG of the Earth-Moon system? i.e., How far from the Center of the Earth is the balance point; or, how far below the surface of the Earth is it? ---
**Author:** ![gazpacho](https://avatars.discourse-cdn.com/v4/letter/g/6f9a4e/32.png) [@gazpacho](https://boards.straightdope.com/u/gazpacho)\ **Post date:** [July 25, 2009, 7:24pm UTC](https://boards.straightdope.com/t/moons-origin/504231/5 "2009-07-25T19:24:15Z")
> [@Johnny\_L.A](#): > > The Moon also exerts a gravitational force on the Earth; so the Earth’s orbit should be perturbed. Question: Where is the CG of the Earth-Moon system? i.e., How far from the Center of the Earth is the balance point; or, how far below the surface of the Earth is it? The moon in 7.3477e22 Kg. The earth is 5.9736 e24 Kg. The distance between earth and moon varies from 363,104 Km to 405,696 Km. The means the center of mass is at max 4990 Km from earths center. Earth radius is 6371 Km. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [July 25, 2009, 7:28pm UTC](https://boards.straightdope.com/t/moons-origin/504231/6 "2009-07-25T19:28:31Z")
So the center of mass is approximately 80% of the way from the center to the surface, or about 20% of the way from the surface to the center. Thank you. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [July 25, 2009, 8:51pm UTC](https://boards.straightdope.com/t/moons-origin/504231/7 "2009-07-25T20:51:32Z")
> [@Chronos](#): > > In addition to the oddities **RickJay** mentioned, the Moon’s orbit is also weird in a few ways. First off, it’s close(ish) to the plane of the ecliptic, not in the plane of its planet’s rotation (as almost all other moons are). Second, it’s actually far enough away from the Earth that it feels a larger gravitational force from the Sun than from the Earth-- It can be considered to be orbiting around the Sun, just with an orbit that’s highly perturbed by the Earth. Not perturbed all that much. While most people think of the Moon’s orbit around the Earth to be Spirograph-like, it’s actually convex the whole way. Think of a (nearly) 12 side polygon with the corners rounded a little and the edges bulging very slightly. ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [July 25, 2009, 9:36pm UTC](https://boards.straightdope.com/t/moons-origin/504231/8 "2009-07-25T21:36:54Z")
If I remember correctly, one of the biggest sources of evidence is from the rocks on the moon itself. They’re different enough to be sure the moon didn’t just split off the from the Earth, similar enough to see that the moon is very different from most other space objects. For example, the moon has very little iron compared to the Earth or most asteroids which could be explained, since collision debris would mostly be from the outer, non-metallic part of the Earth. The moon is much drier than would normally be expected, which could be explained by being formed out of debris that had been superheated. ---
**Author:** ![Gbro](https://avatars.discourse-cdn.com/v4/letter/g/c6cbf5/32.png) [@Gbro](https://boards.straightdope.com/u/Gbro)\ **Post date:** [July 25, 2009, 11:43pm UTC](https://boards.straightdope.com/t/moons-origin/504231/9 "2009-07-25T23:43:31Z")
> [@Jormungandr](#): > > Ok, I was out last night looking at the stars with the neighbors. His son asked how the moon was formed. I mentioned the “Theia” moon theory, where a Mar-sized body supposedly hit Earth and the blast-offed material formed the Moon. Right off the bat, that didn’t seemed right to me. If that were true, how did the other moons, such as Titan and Io, form if they revolve around gas giants. The recent impact, as well as Shoemaker-Levy 9, on Jupiter shows a “Theia” type event unlikely there. So, is there a prevailing theory on moon evolution for the solar system overall? You should have opened the Bible. It clearly spells out how and why we have a Moon in Genesis Chapter 1 verse 16 > [@](#): > > God made two great lights, the sun and the moon-the larger one to govern the day,and the smaller one (the moon) to govern the nights. > [@Johnny\_L.A](#): > > The Moon also exerts a gravitational force on the Earth; so the Earth’s orbit should be perturbed. Question: Where is the CG of the Earth-Moon system? i.e., How far from the Center of the Earth is the balance point; or, how far below the surface of the Earth is it? That is kind of like a pile of BS my friend. Remember the hammer feather drop experiment done in 1968? The moon is a large mass and the earths gravity is(trying to) pull it in, well you know all that right. well its the earths gravitational pull (forces) cause the tides, sand shifts, etc.not the moons pull/gravity. Of course you are only remembering the BS from 6th grade science. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [July 26, 2009, 12:05am UTC](https://boards.straightdope.com/t/moons-origin/504231/10 "2009-07-26T00:05:31Z")
> [@Gbro](#): > > That is kind of like a pile of BS my friend… Of course you are only remembering the BS from 6th grade science. I assume you have a cite? [Here’s mine](http://en.wikipedia.org/wiki/Center_of_mass#Barycenter_in_astrophysics_and_astronomy), after doing a quick search after reading your post. (Emphasis mine.) > [@](#): > > The barycenter (or barycentre; from the Greek βαρύκεντρον) is the point between two objects where they balance each other. For example, it is the center of mass where two or more celestial bodies orbit each other.\*\* When a moon orbits a planet, or a planet orbits a star, both bodies are actually orbiting around a point that lies outside the center of the primary (the larger body).\*\* For example, the moon does not orbit the exact center of the earth, but a point on a line between the Earth and the Moon approximately 1710 km below the surface of the Earth, where their respective masses balance. This is the point about which the Earth and Moon orbit as they travel around the Sun. > > The barycenter is one of the foci of the elliptical orbit of each body. This is an important concept in the fields of astronomy, astrophysics, and the like (see two-body problem). [Here are some animations](http://en.wikipedia.org/wiki/Center_of_mass#Animations). Also, distant planets have been found using the [wobble method](http://www.nasa.gov/topics/universe/features/C-VB10b-20090528.html). > [@](#): > > In the astrometry planet-hunting technique, researchers attempt to measure very tiny wobbles in stars – the result of planets yanking them around. ---
**Author:** ![Gbro](https://avatars.discourse-cdn.com/v4/letter/g/c6cbf5/32.png) [@Gbro](https://boards.straightdope.com/u/Gbro)\ **Post date:** [July 26, 2009, 12:11am UTC](https://boards.straightdope.com/t/moons-origin/504231/11 "2009-07-26T00:11:11Z")
> [@](#): > > I assume you have a cite? Of course not, I just don’t believe everything I read. But before anyone pounces on that, I do believe what I read in the Bible.🙂 ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [July 26, 2009, 12:16am UTC](https://boards.straightdope.com/t/moons-origin/504231/12 "2009-07-26T00:16:38Z")
The Bible is hardly a scientific reference. If you think that Center of Mass is BS, try bolting a kilo of lead on one rotor blade and see if the helicopter doesn’t wobble. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [July 26, 2009, 12:58am UTC](https://boards.straightdope.com/t/moons-origin/504231/13 "2009-07-26T00:58:01Z")
> [@Gbro](#): > > You should have opened the Bible. It clearly spells out how and why we have a Moon in Genesis Chapter 1 verse 16 > > That is kind of like a pile of BS my friend. Remember the hammer feather drop experiment done in 1968? > > The moon is a large mass and the earths gravity is(trying to) pull it in, well you know all that right. well its the earths gravitational pull (forces) cause the tides, sand shifts, etc.not the moons pull/gravity. Of course you are only remembering the BS from 6th grade science. …the fuck? ---
**Author:** ![silk1976](https://avatars.discourse-cdn.com/v4/letter/s/8edcca/32.png) [@silk1976](https://boards.straightdope.com/u/silk1976)\ **Post date:** [July 26, 2009, 1:56am UTC](https://boards.straightdope.com/t/moons-origin/504231/14 "2009-07-26T01:56:17Z")
> [@Gbro](#): > > well its the earths gravitational pull (forces) cause the tides, sand shifts, etc.not the moons pull/gravity. Of course you are only remembering the BS from 6th grade science. Riiiiiiight… a body such as the earth that is spherical will exert a different gravitational pull on one spot on the surface than another, which causes tides. And it’ll do that on a basis that is cyclical and happens to follow the appearance of a full moon! How coincidental! > [@Gbro](#): > > Of course not, I just don’t believe everything I read. > But before anyone pounces on that, I do believe what I read in the Bible.🙂 I’d like you to provide a list of studies, experiments, etc that provides the basis for the bible’s version of the moons origin. Go ahead. I’ll wait. In the meantime, [here’s another cite](http://news.nationalgeographic.com/news/2007/12/071219-moon-collision.html) that tells of studies regarding the earth-moon-collision idea. ---
**Author:** ![Silophant](https://avatars.discourse-cdn.com/v4/letter/s/a698b9/32.png) [@Silophant](https://boards.straightdope.com/u/Silophant)\ **Post date:** [July 26, 2009, 3:17am UTC](https://boards.straightdope.com/t/moons-origin/504231/15 "2009-07-26T03:17:02Z")
> [@Gbro](#): > > Remember the hammer feather drop experiment done in 1968? Wait, they dropped a hammer and a feather on the moon a year before they actually made it there? Whoa. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [July 26, 2009, 4:25am UTC](https://boards.straightdope.com/t/moons-origin/504231/16 "2009-07-26T04:25:58Z")
As I understand it, the Moon is not the only satellite expected to have a collision as its origin. A number of asteroids and TNOs (including Pluto) have satellites and pretty much all of them are thought to have collision origins. As far as Mars’ satellites, I believe they are thought to be captured asteroids. As are many (all?) of the outer Jovian satellites, including all the ones that orbit retrograde. There may be some objects that cycle back and forth from being Jovian satellites and Trojan asteroids. This kind of thing has happened with Earth recently. There’ve been a couple cases of objects captured into temporary Earth orbit for a year or so. One was a probably a bit of space junk (rocket stage from one of the Apollo missions) but the other was a natural object. ---
**Author:** ![DSYoungEsq](https://avatars.discourse-cdn.com/v4/letter/d/c6cbf5/32.png) [@DSYoungEsq](https://boards.straightdope.com/u/DSYoungEsq)\ **Post date:** [July 26, 2009, 4:40am UTC](https://boards.straightdope.com/t/moons-origin/504231/17 "2009-07-26T04:40:36Z")
If God made the moon to rule the night, why is the moon often not seen during the night? :rolleyes: ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [July 26, 2009, 5:04am UTC](https://boards.straightdope.com/t/moons-origin/504231/18 "2009-07-26T05:04:30Z")
> [@Gbro](#): > > You should have opened the Bible. It clearly spells out how and why we have a Moon in Genesis Chapter 1 verse 16. > [@Gbro](#): > > Of course not, I just don’t believe everything I read. > But before anyone pounces on that, I do believe what I read in the Bible.🙂 [Moderator note] Witnessing is only permitted in Great Debates. Please keep it out of GQ. Colibri General Questions Moderator ---
**Author:** ![Gorsnak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gorsnak/32/7587_2.png) [@Gorsnak](https://boards.straightdope.com/u/Gorsnak)\ **Post date:** [July 26, 2009, 5:10am UTC](https://boards.straightdope.com/t/moons-origin/504231/19 "2009-07-26T05:10:46Z")
> [@Silophant](#): > > Wait, they dropped a hammer and a feather on the moon a year before they actually made it there? > > Whoa. Well of course. Editing and other post-production stuff takes time, you know? Those fake moon landing videos weren’t shown live. Showing them live would have been crazy! The home audience would have seen all the bloopers, like Armstrong getting hit in the head with a boom mic on his way down the lander steps. Plus the timing would have gotten messed up, since they had to show all the hopping footage in slow motion to give the appearance of low gravity. So yeah, the hammer and feather thing was done in '68. 😛 ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [July 26, 2009, 6:41am UTC](https://boards.straightdope.com/t/moons-origin/504231/20 "2009-07-26T06:41:24Z")
Additions to my post above: Triton, a moon of Neptune, is also thought to be captured. It’s a very big moon (roughly the same size as our moon) and it revolves retrograde around Neptune. The retrograde orbit is a clear sign that it didn’t form in place. Hyperion, a moon of Saturn, is another oddity. It’s a very strange object and rotates about its axis in a chaotic manner, although its orbit is not unusual. [Wikipedia](http://en.wikipedia.org/wiki/Hyperion_(moon)) says > [@](#): > > A possible explanation for the irregular morphology is that Hyperion is a fragment of a larger body that was broken by a large impact in the distant past, an event which has been linked to the enigmatic darkening of Iapetus. [Next page](https://boards.straightdope.com/t/moons-origin/504231.md?page=2)