# Moon, or Planet? **URL:** \ **Category:** Factual Questions\ **Created:** [February 10, 2000, 3:09am UTC](https://boards.straightdope.com/t/moon-or-planet/7046 "2000-02-10T03:09:00Z")\ **Posts on this page:** 20\ **Page:** 1
**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:** [February 10, 2000, 3:09am UTC](https://boards.straightdope.com/t/moon-or-planet/7046/1 "2000-02-10T03:09:00Z")
The Earth’s natural satellite is very large in proportion to the planet (What? About 1/4 the size?). Nowhere else in our solar system is a moon so large in relation to its planet. Is the Moon really a moon? Or are we in a binary planet system? ---
**Author:** ![Commander\_Fortune](https://avatars.discourse-cdn.com/v4/letter/c/5f8ce5/32.png) [@Commander\_Fortune](https://boards.straightdope.com/u/Commander_Fortune)\ **Post date:** [February 10, 2000, 3:39am UTC](https://boards.straightdope.com/t/moon-or-planet/7046/2 "2000-02-10T03:39:00Z")
The way I understand it, for us to be in a binary planet situation, the Earth and Moon would both have to simultaneously be revolving around each other as well as revolving around the Sun. Like you’ve probably heard elsewhere, “Size doesn’t matter” ---
**Author:** ![Llardball](https://avatars.discourse-cdn.com/v4/letter/l/f6c823/32.png) [@Llardball](https://boards.straightdope.com/u/Llardball)\ **Post date:** [February 10, 2000, 3:57am UTC](https://boards.straightdope.com/t/moon-or-planet/7046/3 "2000-02-10T03:57:00Z")
“Nowhere else in our solar system is a moon so large in relation to its planet.” Not true. Pluto’s only satellite, Charon, is half it’s size. * * * If at first you don’t succeed, skydiving is not for you. ---
**Author:** ![whc.03grady](https://avatars.discourse-cdn.com/v4/letter/w/a6a055/32.png) [@whc.03grady](https://boards.straightdope.com/u/whc.03grady)\ **Post date:** [February 10, 2000, 4:05am UTC](https://boards.straightdope.com/t/moon-or-planet/7046/4 "2000-02-10T04:05:00Z")
Actually, Charon is about 1/6.7th of Pluto’s mass, and the Moon clocks in at around 1/81st of Earth’s. These are still the only planets in this solar system with moons even close to the planet’s size, and I think both are considered double-planet systems. ---
**Author:** ![hightechburrito](https://avatars.discourse-cdn.com/v4/letter/h/f6c823/32.png) [@hightechburrito](https://boards.straightdope.com/u/hightechburrito)\ **Post date:** [February 10, 2000, 5:56am UTC](https://boards.straightdope.com/t/moon-or-planet/7046/5 "2000-02-10T05:56:00Z")
If I remember correctly, a planet revolves around a star, and a moon revolves around a planet. So, regarless of size, our moon is still a moon. * * * We must blame them and cause a fuss before somebody thinks of blaming us. Sheila Broflofski ---
**Author:** ![neuro-trash\_grrrl](https://avatars.discourse-cdn.com/v4/letter/n/3ab097/32.png) [@neuro-trash\_grrrl](https://boards.straightdope.com/u/neuro-trash_grrrl)\ **Post date:** [February 10, 2000, 6:07am UTC](https://boards.straightdope.com/t/moon-or-planet/7046/6 "2000-02-10T06:07:00Z")
I have to think waaaaay back to the explanation my ninth grade physics teacher had for this subject, but if I remember it correctly, in a planet-moon system, the moon will sometimes come to a complete stop or even go backwards when viewed in relation to the star. If you traced the moon’s orbit, it would look like a bunch of curlicues adorning the planet’s orbit. A double planet system, on the other hand, has both members constantly moving in a forward direction. The orbits of the earth and moon will look like two strings braided together, instead of one string wrapped around the other. The Earth and Moon fulfill the latter criteria, and hence are double planets. Unless I’m totally wrong, of course. * * * An infinite number of rednecks in an infinite number of pickup trucks shooting an infinite number of shotguns at an infinite number of road signs will eventually produce all the world’s great works of literature in Braille. ---
**Author:** ![Triskadecamus](https://avatars.discourse-cdn.com/v4/letter/t/b19c9b/32.png) [@Triskadecamus](https://boards.straightdope.com/u/Triskadecamus)\ **Post date:** [February 10, 2000, 7:47am UTC](https://boards.straightdope.com/t/moon-or-planet/7046/7 "2000-02-10T07:47:00Z")
The Earth, and the Moon both orbit around their mutual center of gravity, as do any two objects, without regard for which is a planet, or which is not. Deimos, Phobos, and Mars orbit around their mutual center of gravity. The fact that it is close to the center of Mars is a mathematical resultant of the masses, not the designations we give them. ## There was a recent furor in popular astronomy over the designation of Pluto as a “Kuiper belt Object” rather than a planet. The matter was settled more by consensus of emotional preference than any hard differentiation of characteristics of the planet itself. It’s official, though, Pluto is a planet, Charon is a moon. Earth is a planet, Luna is THE MOON. \

          **Tris** \ “For every complex question, there is a simple answer-- and it’s wrong.” --\*\* H.L. Mencken \*\* ---

**Author:** ![Earl\_Snake-Hips\_Tucker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/earl_snake-hips_tucker/32/3269_2.png) [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)\ **Post date:** [February 10, 2000, 2:30pm UTC](https://boards.straightdope.com/t/moon-or-planet/7046/8 "2000-02-10T14:30:00Z")
Our current models on the moon’s formation say that it was formed early in the solar system’s history after earth was really smacked hard by a large (I think I’ve read Mars-sized) object. It blew the earth apart, with the resultant goo forming into two separate bodies. The other gas giants are large enough to have possibly to have possibly caused heavy matter to coalesce into distinct satellites, although some might be captured asteroids of KBO’s. Mars’ satellites are probably captured asteroids, and Pluto/Charon might possibly be a large KBO system that formed similarly to earth and moon (cosmic cue-balling). ---
**Author:** ![AWB](https://avatars.discourse-cdn.com/v4/letter/a/bb73d2/32.png) [@AWB](https://boards.straightdope.com/u/AWB)\ **Post date:** [February 10, 2000, 5:10pm UTC](https://boards.straightdope.com/t/moon-or-planet/7046/9 "2000-02-10T17:10:00Z")
IMO: If the mutual center of gravity is located _outside_ of the two bodies concerned, then it’s a dual planet system. But since the Earth/Moon center is still inside the Earth, the Moon (Luna) is a moon. * * * What would Brian Boitano do / If he was here right now / He’d make a plan and he’d follow through / That’s what Brian Boitano would do. ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [February 10, 2000, 5:28pm UTC](https://boards.straightdope.com/t/moon-or-planet/7046/10 "2000-02-10T17:28:00Z")
> [@](#): > > Originally posted by neuro-trash grrrl: **…the moon will sometimes come to a complete stop or even go backwards when viewed in relation to the star.** You’re trying to describe retrograde! This only exhibited by planets whose orbits lay outside of yours. The very term, planet, means “wanderer” as retrograde motion was first described. ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [February 10, 2000, 5:42pm UTC](https://boards.straightdope.com/t/moon-or-planet/7046/11 "2000-02-10T17:42:00Z")
> [@](#): > > Originally posted by Jinx: > \*\* …planets whose orbits lay outside of yours…\*\* Translation: For clarity, I should say that I am referring to the superior planets whose orbits, by defintion, lay outside the earth’s. So, from our perspective, the relative motion between earth and a superior planet can create the retrograde effect as seen from our perspective here on earth. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [February 10, 2000, 5:55pm UTC](https://boards.straightdope.com/t/moon-or-planet/7046/12 "2000-02-10T17:55:00Z")
The last I knew, and it’s probably still true, there is no official criteria for how big something orbiting the Sun has to be before it’s a planet. Nor is there some cut-off between a moon being a moon or part of a double planet. It’s basically done by consensus and historical usage. * * * _It is too clear, and so it is hard to see._ ---
**Author:** ![mrblue92](https://avatars.discourse-cdn.com/v4/letter/m/5daacb/32.png) [@mrblue92](https://boards.straightdope.com/u/mrblue92)\ **Post date:** [February 11, 2000, 6:03am UTC](https://boards.straightdope.com/t/moon-or-planet/7046/13 "2000-02-11T06:03:00Z")
You people are so earth-centric, you have forgotten the great contributions of lunatics throughout history! Hmm… Perhaps I’ve been reading too many America-bashing threads. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [February 10, 2000, 10:25pm UTC](https://boards.straightdope.com/t/moon-or-planet/7046/14 "2000-02-10T22:25:00Z")
Johnny L.A. asked: > [@](#): > > The Earth’s natural satellite is very large in proportion to the planet (What? About 1/4 the size?) Earth’s orbit: 149,600,000 km (1.00 AU) from Sun diameter: 12,756.3 km mass: 5.9736e24 kg Moon’s orbit: 384,400 km from Earth diameter: 3476 km mass: 7.35e22 kg By size, the moon to earth ratio is 3476/12,756.3 = 0.2725 or roughly 27%. By mass, the ratio is about 0.0123, or 1.23%. Commander Fortune said: > [@](#): > > The way I understand it, for us to be in a binary planet situation, the Earth and Moon would both have to simultaneously be revolving around each other as well as revolving around the Sun. As Triskadecamus said, that is the technical description regardless of the size of the bodies involved. A dust speck orbiting the Earth _technically_ is orbiting the center of gravity of the Earth and the dust speck, and the Earth is doing the same. Jinx said: > [@](#): > > You’re trying to describe retrograde! This only exhibited by planets whose orbits lay **outside** of yours. [emphasis added] That should be _inside_. Venus and Mercury exhibit retrograde motion, not Saturn, Jupiter, and Mars et al. AWB said: > [@](#): > > IMO: If the mutual center of gravity is located outside of the two bodies concerned, then it’s a dual planet system. But since the Earth/Moon center is still inside the Earth, the Moon (Luna) is a moon. I almost missed the IMO part. I guess that’s as reasonable distinction as any, though are you sure the Earth/Moon center is inside Earth? I tried a few sources, but couldn’t find anything quickly. There really isn’t a systematic distinction between planets, moons, asteroids, and comets. The “Big” objects are planets, anything that is not the biggest at that orbit is a moon (i.e it orbits the bigger object at that distance). Asteroids are “small”. Comets start with lots of volitiles, but can go “dead”. This lack of definition gained attention over the recent discussion of Pluto’s status as Kuiper Belt Object vs. Planet (or major planet vs. minor planet - i.e. asteroids and comets). It’s all custom and usage, or the historical creation of the terms. Without a driver for figuring a logical system, it will stay custom and usage. ---
**Author:** ![RM\_Mentock](https://avatars.discourse-cdn.com/v4/letter/r/e274bd/32.png) [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)\ **Post date:** [February 11, 2000, 2:10am UTC](https://boards.straightdope.com/t/moon-or-planet/7046/15 "2000-02-11T02:10:00Z")
> [@](#): > > Originally posted by **\Irishman\** : > \*\*That should be inside. Venus and Mercury exhibit retrograde motion, not Saturn, Jupiter, and Mars et al. \*\* Just out of curiousity, what’s _your_ definition of retrograde motion? > [@](#): > > Originally posted by **\Irishman\** : > \*\*I almost missed the IMO part. I guess that’s as reasonable distinction as any, though are you sure the Earth/Moon center is inside Earth? I tried a few sources, but couldn’t find anything quickly. \*\* How’s this for quick: the distance to the moon from the earth is only about 60 times the radius of the earth–but you calculated that the mass of the moon was 1/80. So, the center of mass of the earth-moon system is inside the earth. \---------------- rocks\ \Sure would be nice to have a preview feature\ ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [February 11, 2000, 2:38am UTC](https://boards.straightdope.com/t/moon-or-planet/7046/16 "2000-02-11T02:38:00Z")
Cdr. Fortune is correct in his definition of a double planet. What is not commonly realized is that we (Earth-Moon system) **are** one. In any three- (or more) body situation, the orbit of the least massive of the three is influenced by the other two, following Newton’s and Kepler’s laws. (Scholarly physics types can post the details if they like.) For the typical satellite, the adjacent much-more-massive planet influences it to a far greater degree than the distant though humongously-more-massive Sun. On a guess, Titan’s orbit is influenced 99% by Saturn, 1% by the Sun, and vanishingly small percentages by the other satellites. Similar figures prevail for the satellites of Mars, Jupiter, Uranus, and Neptune and of course Saturn’s other satellites. In the case of the Moon, however, 52% of the influence comes from the Sun, 48% from the Earth. The Moon orbits the Sun in a lace-doily-shaped scalloped orbit that is always concave towards the Sun but moves in lockstep with Earth, so that it is apparently moving around the Earth (which it does in fact do, as a result of Earth’s movement through and across that scalloped orbit – draw a circle of the radius that averages the scallops out, and give it scallops of its own that are so shallow as to be nearly imperceptible). ---
**Author:** ![mangeorge](https://avatars.discourse-cdn.com/v4/letter/m/d78d45/32.png) [@mangeorge](https://boards.straightdope.com/u/mangeorge)\ **Post date:** [February 11, 2000, 4:11am UTC](https://boards.straightdope.com/t/moon-or-planet/7046/17 "2000-02-11T04:11:00Z")
The gravity on the moon is 1/6th that of the earth, right? Peace, mangeorge ---
**Author:** ![APB9999](https://avatars.discourse-cdn.com/v4/letter/a/8e8cbc/32.png) [@APB9999](https://boards.straightdope.com/u/APB9999)\ **Post date:** [February 11, 2000, 4:12am UTC](https://boards.straightdope.com/t/moon-or-planet/7046/18 "2000-02-11T04:12:00Z")
What are you talkling about Polycarp? When you say “52% of the influence” what do you mean? Gravitational Force? How did you get that number? Also, the moon orbits the sun in what you call a scallop-shape only because it is orbiting the earth as the earth orbits the sun. Maybe you know this; it isn’t clear from your post. Obviously, if earth weren’t there, the moon - assuming it were still in orbit about the sun - would follow a smooth elipse. Quite simply: the earth and moon revolve about their common center of gravity. Do a little thought experiment: Two bodies of equal mass will revolve about a point halfway between them. Label them body A and body B. Now start shrinking the mass of B. As you do, the point about which they revolve will move toward the center of body A. When B becomes massless, it will actually be orbiting about the exact center of A. The earth is sixteen times more massive than the moon, so the center of gravity of the earth/moon pair will be one sixteenth of the way from the center of the earth to a point halfway between the center of the earth and the center of the moon. In other words, 1/32 of the distance to the moon. I’m certain this is less than the radius of the earth, but I don’t have the actual numbers. The center of gravity of the earth/moon pair itself orbits the common center of gravity it has with the sun. Of course the sun is so massive that this center of gravity is not only within the sun, but very close to its center. ---
**Author:** ![NickyLarson](https://avatars.discourse-cdn.com/v4/letter/n/b2d939/32.png) [@NickyLarson](https://boards.straightdope.com/u/NickyLarson)\ **Post date:** [February 11, 2000, 4:13am UTC](https://boards.straightdope.com/t/moon-or-planet/7046/19 "2000-02-11T04:13:00Z")
Yup, 1/6th. ---
**Author:** ![APB9999](https://avatars.discourse-cdn.com/v4/letter/a/8e8cbc/32.png) [@APB9999](https://boards.straightdope.com/u/APB9999)\ **Post date:** [February 11, 2000, 4:16am UTC](https://boards.straightdope.com/t/moon-or-planet/7046/20 "2000-02-11T04:16:00Z")
[Hijack] There are bodies that orbit the sun, i.e. planets. There are bodies that orbit the planets, i.e. moons. Are there, anywhere in the solar system, instances of moons with moons of their own? [/Hijack] [Next page](https://boards.straightdope.com/t/moon-or-planet/7046.md?page=2)