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(For the purpose of this question, I will refer to Earth's Moon as Luna to keep it separate from the general term of 'moon')

I know the IAU has been hedgey about giving an official definition of what a moon is, but generally it seems to be something like: A natural satellite, a solid object in orbit around a planet, a dwarf planet, a minor planet, or a transneptunian object, where "orbit" is defined as the host body having gravitational dominance over that of the sun (as in the moon stays circling the host body and does not randomly leave and come back).

Also, the official definition of planet is: a celestial body that 1) is in orbit around the Sun, 2) has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and 3) has "cleared the neighbourhood" around its orbit.

My question is that Luna does not meet the commonly agreed definition for a moon.

(Please view the website: https://en.m.wikipedia.org/wiki/Double_planet for reference to some of my further paragraphs)

Isaac Asimov calculated a "Tug of war" value for planets in regard to their moons as a way of expressing how strong the gravitaional hold a planet had on their moon in comparison with the hold the sun had on the moon, with a value of anything over 1.0 being that the gravity of the planet held dominance over the moon. This explains why and how a moon moves in opposition to the rest of the celestial disk that moves counterclockwise around the sun. A moon, in it's orbit around a planet always, at some point in its orbit of its planet, moves from the right to the left while everything else moves from the left to the right.

This being said, Luna fails both of those. Luna's Tug of War value is 0.46, so the sun has twice the influence as the Earth. Additionally, Luna does not truly orbit Earth. The path of it's movement in relation to the sun is always left to right, the same as planets and in contrary to almost all moons (excepting moons in backwards orbits).

Meaning that, theoretically, if Earth (for some reason) abruptly disappeared, Luna's orbit would not drastically change since it's placement and movement is not predominently governed by Earth.

So... can Luna be truly considered a moon if it does not move like a moon (direction-wise), does not truly orbit in a circle around a host planet, and is instead dominantly bound to the sun's gravity? The main idea of a moon is that a moon orbits their planet and Luna... does not.

While from Earth it might appear like it does, but when viewed from above, Luna's orbit is somewhat reminiscent of the orbits of Epimetheus and Janus in the way they trade places, though with Luna permanently having a wider swinging arc and the trade being much more frequent. It is proven that Luna is always only ever going counterclockwise and falling in towards the sun in a solor-dominated orbit.

(Please refer to web page https://en.m.wikipedia.org/wiki/File:Moon_trajectory1.svg for the diagram of the Luna/Earth orbital movement.)

Now, with the question in mind that Luna might not qualify as a moon... what is it?

Additionally, this brings up the question on what Earth would then be. Afterall, if Earth has this relatively huge neighbor constantly within its "orbital neighborhood" then Earth has failed the third condition of a planet to "clear the neighbourhood around its orbit." This has also been used as the reason why Luna itself cannot be considered a planet, as it would not have "cleared it's neighborhood" because of Earth's presence.

So then... Earth is not a planet for the same reason that Pluto is not a planet.

However, despite two celestial bodies orbiting the sun at the same pace and sharing the same orbital area at the same time, it also does not meet all the IAU prerequisites for a binary planet, specifically that the barycenter is within Earth's crust (although it is calculated that this barycenter will eventually be outside of Earth in a few billion years).

So, if Luna does not qualify as a moon, and Earth does not qualify as a planet, and the two together do not qualify as a binary system... what are they?

And, as the IAU is strict on following approved guidelines and not common conventions, I expect an answer beyond a historical one (such as how we've based all moons off of Luna and all planets off of Earth and this makes them the measure of the rules and so cannot fail them).

Of course, if "orbital neighborhood" is defined as small as the space of the minimum distance between Earth and Luna, which would make Earth a planet, then bear in mind that it would also automatically make Luna a planet and Pluto might pass that requirement as well.

According to the IAU definitions... Luna and Earth do not fit in any category...

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  • $\begingroup$ speaking of luna, Does the moon have a name? $\endgroup$
    – uhoh
    Commented Dec 12, 2023 at 14:53
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    $\begingroup$ -1 for bad-faith question to relitigate 2006 planet definition. $\endgroup$
    – notovny
    Commented Dec 12, 2023 at 15:55
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    $\begingroup$ The IAU definition of a planet is pretty rediculous, but most people are able to just shrug it off and move on. $\endgroup$ Commented Dec 12, 2023 at 16:33
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    $\begingroup$ What is the question? $\endgroup$
    – ProfRob
    Commented Dec 12, 2023 at 20:01
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    $\begingroup$ "My question is that Luna does not meet the commonly agreed definition for a moon." That is a statement, not a question. Please edit your post to include an actual question. $\endgroup$ Commented Dec 12, 2023 at 20:06

1 Answer 1

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The moon "Luna" is a moon.

There is no "IAU" definition, so you have to fall back to general scientific usage. You have a lot of irrelevant stuff about "Tug of War" values. This is an irrelevant observation that the path of the moon is convex relative to the sun. These things are not part of any definition of "moon".

You make the claim that the moon does not orbit the Earth. This is False. The moon moves around the Earth under the gravitational influence of the Earth. Hence in common scientific language it "orbits". The trajectory of the moon is wholly within the Hill Sphere of the Earth. The motion of the moon relative to the Earth is almost entirely due to the gravitational influence of the Earth.

The barycentre of the Earth-Moon system is well within the Earth. The motion of the Earth relative to the moon is not due to the mass of the moon. There is a clear imbalance. The Earth is much bigger.

The moon (Luna) is a moon. It is the archetype of a "moon", it is the standard by which other bodies are judged to be moons. Indeed the definition of "moon" in general scientific language is a body which has a relationship to a planet which is analogous to that which the moon (Luna) has to Earth.

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  • $\begingroup$ In your last paragraph you should have written "The Moon (Luna) ..." (and elsewhere). With an initial capital letter, there is only one Moon in the entire universe, which is the natural satellite that orbits the Earth. But as you mentioned, our Moon is the archetype of all other moons (lower case), so +1 overall. $\endgroup$ Commented Dec 14, 2023 at 13:44
  • $\begingroup$ you're probably right, but I'm suspicion of semantic distinctions being made by unpronounceable orthographic changes. It should be possible to read this answer and understand it. $\endgroup$
    – James K
    Commented Dec 15, 2023 at 22:40

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