Defining moons

[KerikBalm]

When I consider the debate over the definition of a planet, I can't help but think there is a similar debate to be had over what constitutes a moon.

Certainly when we launch satellites, we would call them artificial satellites, but I've never heard them called artificial moons...

Then consider the Jupiter and Saturn system for example:

http://en.wikipedia.org/wiki/Moons_of_Jupiter

http://en.wikipedia.org/wiki/Moons_of_Saturn

"The planet Jupiter has 67 confirmed moons. This gives it the largest retinue of moons with "reasonably secure" orbits of any planet in the Solar System. The most massive of them, the four Galilean moons,

...

From the end of the 19th century, dozens of much smaller Jovian moons have been discovered

...

The Galilean moons are by far the largest and most massive objects in orbit around Jupiter, with the remaining 63 moons and the rings together comprising just 0.003 percent of the total orbiting mass.

Eight of Jupiter's moons are regular satellites, with prograde and nearly circular orbits that are not greatly inclined with respect to Jupiter's equatorial plane. The Galilean satellites are ellipsoidal in shape, due to having planetary mass, and so would be considered (dwarf) planets if they were in direct orbit about the Sun. The other four regular satellites are much smaller and closer to Jupiter; these serve as sources of the dust that makes up Jupiter's rings.

The remainder of Jupiter's moons are irregular satellites, whose prograde and retrograde orbits are much farther from Jupiter and have high inclinations and eccentricities. These moons were probably captured by Jupiter from solar orbits. There are 17 recently discovered irregular satellites that have not yet been named."

"Saturn has 62 moons with confirmed orbits, 53 of which have names and only 13 of which have diameters larger than 50 kilometers.[1][2][3] Seven Saturnian moons are large enough to be ellipsoidal in shape, though only two of those, Titan and Rhea, are currently in hydrostatic equilibrium

...

Twenty-four of Saturn's moons are regular satellites; they have prograde orbits not greatly inclined to Saturn's equatorial plane. [6] They include the seven major satellites, four small moons that exist in a trojan orbit with larger moons, two mutually co-orbital moons and two moons that act as shepherds of Saturn's F Ring. Two other known regular satellites orbit within gaps in Saturn's rings. The relatively large Hyperion is locked in a resonance with Titan.

...

The rings of Saturn are made up of objects ranging in size from microscopic to moonlets hundreds of meters across, each in its own orbit around Saturn.[7] Thus a precise number of Saturnian moons cannot be given, because there is no objective boundary between the countless small anonymous objects that form Saturn's ring system and the larger objects that have been named as moons. Over 150 moonlets embedded in the rings have been detected by the disturbance they create in the surrounding ring material, though this is thought to be only a small sample of the total population of such objects."

Where do we draw the line at what is a Moon, vs just a satellite. Should Saturn be considered to have "the largest retinue of moons with "reasonably secure" orbits of any planet in the Solar System" if we start counting the ring "moonlets" or even individual particles and rocks?

Could we apply the same parameters that we use to define planets around a star, to define moons around a planet?

Would the two moons of Mars still be given "Moon" status, or would they be demoted to mere "satellites"

And then of course, there is the debate to be had over when a system stops being a planet+moon system, and starts being a double planet system.

Thoughts?

How should we define what constitutes what a moon is?

[Dunbaratu]

There was an old episode of QI in which Rich Hall was deducted points for saying the Earth has one moon, and he famously raised a stink about it and made reference to it again and again in later episodes (to the point where the show's makers, predicting he'd do it again, made the phrase "Which moon are you talking about?" into another forfeiture).

And Rich was right to be annoyed by it because QI was wrong.

They claimed the Earth has 2 moons because they counted Cruithne as a moon. ( http://en.wikipedia.org/wiki/3753_Cruithne ).

Cruithne is emphatically NOT a moon of Earth.

To be a moon of a body and object needs at minimum the following two conditions:

(A) It is a satellite of the body, and

( It is not man-made.

Questions of whether there's a minimum size or not never really enter into it in this case because Cruithne fails (A). It's NOT a satellite of Earth because to be a satellite it needs these to be true:

(A) It goes around the body in a body-centric frame of reference, and

( The gravitational pull between it and the body it goes around, due to their masses, must be the cause of (A).

Cruithne fails (.

It only "goes around" Earth in an Earth-centric frame of reference because of they way they are both affected by the SUN's gravity on them, Cruithne doesn't go around Earth due to the EARTH's gravity pulling on it. Earth's gravitational pull on it is too weak to account for its motion "around" Earth. Therefore it's not a satellite of Earth, therefore not a Moon of Earth.

Therefore Rich Hall needs to be given his 10 points back.

Sorry to derail, but that always annoyed me about QI. They take points away on the premise that someone "fell for" a common "incorrect" naive claim, when often it's the shallow research of the QI team that's being naive, as in the case of calling Cruithne a moon.

[magnemoe]

Good question, appears that the problem with planets did not came up for moons, probably as the numbers was to high pretty early.

Guess its the same as asteroids who also have no minimum size i know about.

Junk in the ring would not be moons however an car sized asteroid who got an mission to it would be named.

[Bill Phil]

When Sputnik was launched, it was called a Red Moon.

The real debate is if moons are orbiting planets at all. They have what I like to call Flower Petal orbits. Their trajectory around the sun is altered by the planetary body, so it looks a lot different than an ellipse. So let's count the Moon as a dwarf planet, okay?

Edited November 5, 2014 by Bill Phil

[Camacha]

Certainly when we launch satellites, we would call them artificial satellites, but I've never heard them called artificial moons...

I have. In some other languages satellites are called this too.

[Armchair Rocket Scientist]

The real debate is if moons are orbiting planets at all. They have what I like to call Flower Petal orbits. Their trajectory around the sun is altered by the planetary body, so it looks a lot different than an ellipse. So let's count the Moon as a dwarf planet, okay?

By the same logic, the definition of a planet is meaningless because they're all orbiting the galactic center, the star just alters their trajectory. Therefore, planets should simply be called sub-brown-dwarf objects.

Some researchers have proposed classifying the earth-moon system as a binary/double planet, but that's due to the very high relative mass of the moon compared to Earth, and does not say anything about other moons in the solar system (apart from Charon).

[Bill Phil]

By the same logic, the definition of a planet is meaningless because they're all orbiting the galactic center, the star just alters their trajectory. Therefore, planets should simply be called sub-brown-dwarf objects.

Some researchers have proposed classifying the earth-moon system as a binary/double planet, but that's due to the very high relative mass of the moon compared to Earth, and does not say anything about other moons in the solar system (apart from Charon).

Yes, that's true. But if you look at the trajectory, it's altered by such a small margin compared to the galactic center. Plus, they would still be planets, as the major gravitational body acting in them is their parent star.

[metaphor]

Earth's Moon is the only moon/satellite in the solar system whose orbit around the Sun is convex (convex means that if you take any two points on the orbit, the straight line between them will be entirely inside the orbit). That's because it's so far out in the Earth's SOI and therefore its orbital period around the Sun is a very small multiple of its orbital period around Earth. That's one reason for some people calling Earth and Moon a double planet instead of a planet and moon.

Maybe "moon" could only apply to round objects, like "dwarf planet" does. In that case, Earth would have 1 moon, Mars would have none, Jupiter would have 4, Saturn would have 7, Uranus would have 5, and Neptune would have 1.

[Kryten]

Maybe "moon" could only apply to round objects, like "dwarf planet" does. In that case, Earth would have 1 moon, Mars would have none, Jupiter would have 4, Saturn would have 7, Uranus would have 5, and Neptune would have 1.

A problem would be that can be harder to determine than you might expect. For example, here's an image, at about the best quality we have available, of Jupiter's fifth-largest moon Himalia;

round enough or not?

[Camacha]

A problem would be that can be harder to determine than you might expect. For example, here's an image, at about the best quality we have available, of Jupiter's fifth-largest moon Himalia;

http://upload.wikimedia.org/wikipedia/commons/d/df/Cassini-Huygens_Himalia.jpg

round enough or not?

We don't know is an acceptable answer in science, though preferably temporary.

However, it also might be the case that this is that moon that we see in Minecraft and actually looks like the picture.

[KerikBalm]

Maybe I should have put up a poll.

Should Jupiter be considered to have 4-5 "proper moons", and then scores of ther satellites or "dwarf moons"?

I think we'll all agree that the constituents of Saturn's rings shouldn't be considered moons.

Does mars have any "proper moons"?

Do we just use the hydrostatic equilibirum test (used for dwarf planets?)

Or the "dominating its orbit"/"clearing its orbit" test (which a number of Saturns moons would seem to satisfy, ie the "shepard moons" that result in gaps in the rings)?

Or do we set an arbitrary size/mass limit?

[psyper]

A problem would be that can be harder to determine than you might expect. For example, here's an image, at about the best quality we have available, of Jupiter's fifth-largest moon Himalia;

http://upload.wikimedia.org/wikipedia/commons/d/df/Cassini-Huygens_Himalia.jpg

round enough or not?

I want a future where we define a moon to a certain size or shape and a small probe approaches Himalia to categorise it - as it approaches one of the scientists state - we are approaching the moon Himalia. Another scientist after doing the calculations stand up and with a very serious look on his face he states "Thats no moon"

[maltesh]

Earth's Moon is the only moon/satellite in the solar system whose orbit around the Sun is convex (convex means that if you take any two points on the orbit, the straight line between them will be entirely inside the orbit). That's because it's so far out in the Earth's SOI and therefore its orbital period around the Sun is a very small multiple of its orbital period around Earth. That's one reason for some people calling Earth and Moon a double planet instead of a planet and moon..

For every one of Earth's orbits, the Moon makes about 13 orbits around Earth, that's true.

For every one of Neptune's orbits, Neptune's moon Neso only makes a little over 6 orbits around the planet. That said, Neso's orbit is also retrograde and elliptical, and I'd guess it probably doesn't wind up entirely convex for those reasons.

[KSK]

Maybe I should have put up a poll.

Should Jupiter be considered to have 4-5 "proper moons", and then scores of ther satellites or "dwarf moons"?

I think we'll all agree that the constituents of Saturn's rings shouldn't be considered moons.

Does mars have any "proper moons"?

Do we just use the hydrostatic equilibirum test (used for dwarf planets?)

Or the "dominating its orbit"/"clearing its orbit" test (which a number of Saturns moons would seem to satisfy, ie the "shepard moons" that result in gaps in the rings)?

Or do we set an arbitrary size/mass limit?

I like the 'clearing it's orbit' definition personally, for the same reason that you pointed out regarding Saturn's moons. Formally, I think that only applies to planets though?

[KerikBalm]

Formally, yes, it only applies to planets.

Formally, there is no definition for moons. I think we should have one, hence this thread.

[Skyler4856]

Maybe "moon" could only apply to round objects, like "dwarf planet" does. In that case, Earth would have 1 moon, Mars would have none, Jupiter would have 4, Saturn would have 7, Uranus would have 5, and Neptune would have 1.

And what shall the scientific community do with Phobos?

Or Deimos

[DarkStar64]

And what shall the scientific community do with Phobos?

http://i.imgur.com/htpJvAY.jpg

Or Deimos

http://i.imgur.com/uUw48pT.jpg

Phobos and Deimos are dwarf moons or asteroid moons.

[Camacha]

I think we'll all agree that the constituents of Saturn's rings shouldn't be considered moons.

Why not? You can't exclude or include them without proper definitions and if space rocks can be moons, ring objects can too.

[YNM]

Maybe something can be called a moon if it's larger than the average particles around them... ?

[KerikBalm]

Why not? You can't exclude or include them without proper definitions and if space rocks can be moons, ring objects can too.

That is why I'm proposing that there should be proper definitions.

As the wikipedia article notes, there is no objective boundry... yet when it comes to saying which planet has the most moons, nobody ever estimates the number of particles in the rings (if they did, I'm pretty sure Saturn would win by a large margin)

[hugix]

Defining a moon is very simple.

This is a moon :

This is not a moon :

Notice the absence of lasers on the moon.

[magnemoe]

And what shall the scientific community do with Phobos?

http://i.imgur.com/htpJvAY.jpg

Or Deimos

http://i.imgur.com/uUw48pT.jpg

obviously moons, no discussion about it. And yes if earth had an asteroid less than an km in diameter orbiting it, it would also be an moon.

Distance and number counts. Mars only have two, and the moons are interesting for future missions.

[Camacha]

I would not mind applying the requirement of hydrostatic equilibrium also to moons. That would (unfortunately) mean that Mars becomes moonless, but draws a pretty clear line and prevents every speck of spacedust from becoming a moon. As I said before, I feel that hierarchy is also important.

Roughly you would get:

Round things: star(s) > planets > moons

Rest: asteroids, comets, assorted rocks

[KerikBalm]

Whereas I would not mind applying the "orbit clearing" requirement, which would allow for smaller than hydrostatic equilibirum moons

- the smaller the orbit, the smaller the mass needed to clear the orbit of "dust" of arbitrarily low mass.

this would also exclude the constituents of the rings of saturn from being classified as moons.

Most of saturn's named moons are not in hydrostatic equlibrium. I think this includes some that have formed the gaps in the rings.

These moons have cleared their orbit, but they haven't reached the mass needed to be in hydrostatic equlibrium.

Phobos and Deimos are in clear orbits, but I'm not sure theire was anything to clear. As with planets, I think you can basically take the mass and orbital parameters, and determine its "orbit clearing power".

I would be interested to know if Phobos and deimos would pass that test (ie treat them like planet candidates, and mars like the sun, for the purposes of the equations)

[metaphor]

More things to consider:

Trojan moons - Saturn's moons Tethys and Dione each have two small moons 60 degrees ahead and behind them in their L4/L5 points, much like Jupiter's Trojan asteroids.

Moons switching orbits - Saturn's moons Janus and Epimetheus are almost in the same orbit, but every few hundred orbital periods, the moon in the inner orbit catches up to the moon in the outer orbit. At that time, the gravitational pull between them pulls up the inner moon to the outer orbit and the outer moon to the inner orbit, which means they switch orbits. Afterwards, they drift apart until they meet again from the other side a few hundred orbital periods later.

As far as moons clearing their orbits, I guess the Stern-Levison parameter can apply to moons too. But you would have to account for the different mass of each primary.