# Are moon counts really meaningless? **URL:** \ **Category:** Factual Questions\ **Created:** [March 8, 2003, 4:43am UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856 "2003-03-08T04:43:38Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Biotop](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/biotop/32/321_2.png) [@Biotop](https://boards.straightdope.com/u/Biotop)\ **Post date:** [March 8, 2003, 4:43am UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856/1 "2003-03-08T04:43:38Z")
I was watching CNN tonight and there was something about several new moons being discovered around Jupiter, bringing the total number to 47. I have tried searching the the SDMB archives and elsewhere on the internet trying to get a definition of how big an orbiting object has to be in order to be called a “moon”. From everything I gather, a moon could be as small as a pebble, as long as someone in authority defines it as a moon. If any little rink-a-dink piece of orbiting rock is technically a moon, isn’t it likely Jupiter has thousands? Millions? And what about Earth? BTW, my Webster’s New Universal Unabridged Dictionary 2nd Edition defines **moon** as “any planetary satellite” which doesn’t really help. And if there is no size limitation for calling an object a moon…_shouldn’t there be?_ ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [March 8, 2003, 4:57am UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856/2 "2003-03-08T04:57:54Z")
Astronomy is rife with similar size demarcation problems. When is a planet a planet and when is a planet a star? Some astronomers say Pluto isn’t really a planet, but rather a large Kuiper Belt object. Some astronomers say the Earth-Moon systrem is really a double-planet. These lines have always been somewhat grey and fuzzy, and I think the debate will continue for some time. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [March 8, 2003, 5:30am UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856/3 "2003-03-08T05:30:24Z")
Jupiter’s satellite count is [up to 52](http://www.ifa.hawaii.edu/~sheppard/satellites/jup2003.html) ! The [The Jupiter Satellite Page](http://www.ifa.hawaii.edu/~sheppard/satellites/) has the dope on all of them. ---
**Author:** ![Biotop](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/biotop/32/321_2.png) [@Biotop](https://boards.straightdope.com/u/Biotop)\ **Post date:** [March 8, 2003, 5:43am UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856/4 "2003-03-08T05:43:17Z")
Thanks, **Squink**. I enjoyed that Jupiter link. It seems that the answer is indeed “Yes” —moon counts are completely subjective. And to think thousands of elementary school kids are still forced to memorize the current number of official moons orbiting this or that planet as if it means something. Seems silly. Also seems silly for CNN to cover this “news”. ---
**Author:** ![Trigonal\_Planar](https://avatars.discourse-cdn.com/v4/letter/t/0ea827/32.png) [@Trigonal\_Planar](https://boards.straightdope.com/u/Trigonal_Planar)\ **Post date:** [March 8, 2003, 7:44am UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856/5 "2003-03-08T07:44:26Z")
Not to skirt the question, but I’ve never understood why there need be such precise size demarcations. As long as everything is labeled in a consistent way, ie, so everybody knows what particular body “Pluto” is, why worry about whether its a small moon or large asteroid? ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [March 8, 2003, 8:03am UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856/6 "2003-03-08T08:03:50Z")
**Trigonal** : Precisely. The only friction we could have is if we discover, say, a body in the Oort Cloud (that mass of rocks orbiting out beyond Neptune, which may or may not include Pluto and Charon) exactly half the size of Pluto. Is it the tenth planet to be actually named? Is it just another hunk of rock to be assigned a serial code? Different astronomical bodies could differ, causing just the inconsistencies you fear. And I do agree that having schoolkids remember the minor moons of any body is silly. Should they know Io, Europa, and such? Sure. At least until the next test. 😉 But small moons are more or less meaningless unless someone decides to major in it or somehow take a more active interest in the subject. Just mention that there’s a lot of crap floating around Uranus and that we aren’t sure how much of it meets our rather arbitrary standards for classification into the moon category. ---
**Author:** ![Big\_Jeff](https://avatars.discourse-cdn.com/v4/letter/b/e274bd/32.png) [@Big\_Jeff](https://boards.straightdope.com/u/Big_Jeff)\ **Post date:** [March 8, 2003, 8:52am UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856/7 "2003-03-08T08:52:57Z")
Derleth, I’m sure you meant the Kuiper Belt in your post rather than the Oort Cloud. 😉 [Source.](http://seds.lpl.arizona.edu/nineplanets/nineplanets/kboc.html) ---
**Author:** ![DreadCthulhu](https://avatars.discourse-cdn.com/v4/letter/d/db5fbb/32.png) [@DreadCthulhu](https://boards.straightdope.com/u/DreadCthulhu)\ **Post date:** [March 8, 2003, 9:10am UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856/8 "2003-03-08T09:10:25Z")
> [@](#): > > \*Originally posted by Derleth \* > \*\ ***Trigonal** : > Just mention that there’s a lot of crap floating around Uranus and that we aren’t sure how much of it meets our rather arbitrary standards for classification into the moon category. \*\* Did anyone else catch the humor of a bunch of crap floating around Uranus? 🙂 ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [March 8, 2003, 9:19am UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856/9 "2003-03-08T09:19:54Z")
**Big Jeff** , thanks for the catch. **Dread** : Ya know the best thing about a cheap joke? Don’t gotta spend a lot of brains coming up with it. 😉 ---
**Author:** ![pkbites](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pkbites/32/204_2.png) [@pkbites](https://boards.straightdope.com/u/pkbites)\ **Post date:** [March 8, 2003, 10:58am UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856/10 "2003-03-08T10:58:14Z")
The other night I heard on coast to Coast (which I’ll admit may not be a credible source) that Jupiter, all in all, is only slightly larger than the planet Earth, and almost everything we’re looking at is storms/clouds/dust/etc… So if what we call a “planet” is really mostly a bunch of gas, I assume then the definition of a moon could really be skewed. Is it possible that, in actual solid matter, some of Jupiters moons are actually larger than the solid matter of the planet itself? ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [March 8, 2003, 10:59am UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856/11 "2003-03-08T10:59:16Z")
> [@](#): > > \*Originally posted by Derleth \* > \*\*. The only friction we could have is if we discover, say, a body in the Oort Cloud (that mass of rocks orbiting out beyond Neptune, which may or may not include Pluto and Charon) exactly half the size of Pluto. Is it the tenth planet to be actually named? Is it just another hunk of rock to be assigned a serial code? Different astronomical bodies could differ, causing just the inconsistencies you fear. > > \*\* Quaoar might be 1/2 the size of Pluto– > [@](#): > > Pluto 2320 km > Charon 1270 km > Quaoar 1200+/-200 km > Ixion 1065+/-165 km > Varuna 900+/-140 km [http://www.ifa.hawaii.edu/faculty/jewitt/kb/big\_kbo.html](http://www.ifa.hawaii.edu/faculty/jewitt/kb/big_kbo.html) they are all much of a muchness… ---
**Author:** ![SPOOFE](https://avatars.discourse-cdn.com/v4/letter/s/c5a1d2/32.png) [@SPOOFE](https://boards.straightdope.com/u/SPOOFE)\ **Post date:** [March 8, 2003, 12:01pm UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856/12 "2003-03-08T12:01:33Z")
Just standardize the system using gravity as the basis, setting 1 G as the standard. Have it so that anything that has at least .1 G be classified as a “planet”. Anything between that and, say, .03 G or .01 G (or whatever) be classified as a “moon”. Anything less than that be classified as “debris”. Of course, I just came up with random numbers… feel free to adjust them appropriately… ---
**Author:** ![Pablito](https://avatars.discourse-cdn.com/v4/letter/p/ed655f/32.png) [@Pablito](https://boards.straightdope.com/u/Pablito)\ **Post date:** [March 8, 2003, 12:25pm UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856/13 "2003-03-08T12:25:21Z")
> [@](#): > > \*Originally posted by pkbites \* > \*\*The other night I heard on coast to Coast (which I’ll admit may not be a credible source) that Jupiter, all in all, is only slightly larger than the planet Earth, and almost everything we’re looking at is storms/clouds/dust/etc… > > So if what we call a “planet” is really mostly a bunch of gas, I assume then the definition of a moon could really be skewed. > Is it possible that, in actual solid matter, some of Jupiters moons are actually larger than the solid matter of the planet itself? \*\* Interesting idea, but those moons would have to grow quite a bit, as that minuscule rocky core of [jupiter](http://www.nineplanets.org/jupiter.html) is pretty big by Earth standards: > [@](#): > > Jupiter probably has a core of rocky material amounting to something like 10 to 15 Earth-masses. Everything is relative. ---
**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:** [March 8, 2003, 1:56pm UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856/14 "2003-03-08T13:56:08Z")
And their is another weird “thing” orbiting the sun: Between Saturn and Uranus, their is the Centaur Chiron. ---
**Author:** ![TeleTronOne](https://avatars.discourse-cdn.com/v4/letter/t/4af34b/32.png) [@TeleTronOne](https://boards.straightdope.com/u/TeleTronOne)\ **Post date:** [March 8, 2003, 2:22pm UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856/15 "2003-03-08T14:22:05Z")
FWIW, I think the biggest reason why Pluto isn’t really a planet it lots of people’s minds is that it has a highly eccentric orbit, and actually crosses the orbit of Uranus and is closer to the sun at some points - but it’s still sort of an arbitrary classification I guess. I think the only strict requirement for calling something a ‘moon’ is that it orbits both around the sun with its planet, and around the planet itself in some fashion. But somehow I just don’t see myself getting excited when I look up in the sky and say: "Ah! Look at that! That miniscule patch of black sky where you can’t see anything - that’s RQM-773X9Q - the pebble moon! There’s going to be some wackos out tonight? ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [March 8, 2003, 3:38pm UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856/16 "2003-03-08T15:38:38Z")
Pluto will never be a moon, as it doesnt orbit anything(except, technically, the Pluto/Charon common barycentre 🙂 but it could be reclassified as a planetoid or even a Kuiper belt object- A moon has to be orbiting a planet- small moons could be referred to as moonlets if necessary. There aren’t any (known) cases of moons having stable satellites of their own, so we could have a three part classification system- planets round stars, moons round planets- and by size planets\>planetoids\>asteroids\>meteoroids moons\>moonlets\>meteoroids/ ring particles Surely there’s no need to learn the names of the moonlets. ---
**Author:** ![Crowbar\_of\_Irony\_3](https://avatars.discourse-cdn.com/v4/letter/c/f08c70/32.png) [@Crowbar\_of\_Irony\_3](https://boards.straightdope.com/u/Crowbar_of_Irony_3)\ **Post date:** [March 8, 2003, 4:59pm UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856/17 "2003-03-08T16:59:26Z")
> [@](#): > > \*Originally posted by Mjollnir \* > \*\*And their is another weird “thing” orbiting the sun: Between Saturn and Uranus, their is the Centaur Chiron. \*\* [Hijack] What is this Centaur Chiron? I am terribly interested. [/Hijack?] ---
**Author:** ![Trigonal\_Planar](https://avatars.discourse-cdn.com/v4/letter/t/0ea827/32.png) [@Trigonal\_Planar](https://boards.straightdope.com/u/Trigonal_Planar)\ **Post date:** [March 8, 2003, 6:13pm UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856/18 "2003-03-08T18:13:10Z")
About Jupiter: Its believe the core of the planet is the size of Earth. The rest of its size does indeed come not from solid matter, but a very dense atmosphere. ---
**Author:** ![mcbiggins](https://avatars.discourse-cdn.com/v4/letter/m/7c8e57/32.png) [@mcbiggins](https://boards.straightdope.com/u/mcbiggins)\ **Post date:** [March 8, 2003, 6:30pm UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856/19 "2003-03-08T18:30:18Z")
What is the Centaur Chiron? > [@](#): > > An object discovered in 1977 by Charles Kowal which orbits mainly between Saturn and Uranus, although it passes slightly inside Saturn’s orbit at perihelion. Originally classified as a “Centaur object” asteroid and given minor planet number 2060, it has shown comet-like activity since 1988. It is believed to have a diameter between 150 and 200 km. It is in a 51 year orbit, and reached perihelion on Feb. 14, 1996. I got that from [http://scienceworld.wolfram.com](http://scienceworld.wolfram.com). ---
**Author:** ![mcbiggins](https://avatars.discourse-cdn.com/v4/letter/m/7c8e57/32.png) [@mcbiggins](https://boards.straightdope.com/u/mcbiggins)\ **Post date:** [March 8, 2003, 6:32pm UTC](https://boards.straightdope.com/t/are-moon-counts-really-meaningless/159856/20 "2003-03-08T18:32:37Z")
And while the size of Jupiter’s solid core may be close to that of Earth’s, it’s mass is much greater than that of Earth. When it comes to celestial bodies, mass is much more impartant than diameter.