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So I had an argument with a friend, which was about whether the Moon is a planet or a satellite.

IAU 2006 Resolution B5 gives definitions on what it is to be a planet, but there is a vagueness, as I perceived, is that it does not define satellites, while using it to define dwarf planets.

There is a supplementary Q&A sheet, and it uses the barycenter to define a satellite, but it is implied, not explicitly defined.

My questions follow:

  1. Should the supplementary Q&A sheet considered as legitimate official definitions, or should it be considered as a one way of interpreting the Resolution B5?
  2. If Q&A is legitimate, then why is the definition of the satellite is not included in the Resolution B5? If not, is the official definition of the satellite nonexistent currently?
  3. Are there any source or archive of the official or widely accepted definitions of glossaries in the fields of astronomy?
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  • $\begingroup$ I am actually against the Pluto being a planet. If my question seemed to imply that, I am terribly sorry and I hereby should make clear that it is none of my intentions. What bugged me was the lack of rigorous definitions of satellites, making 2006 def. of planets seem to be a bit..dangerous. $\endgroup$ – Hojin Cho Jul 14 '15 at 5:59
  • $\begingroup$ Besides, if what you worried is that this question would cause a new battlefield in this site regardless of my intentions: 1. Forbidding discussion about scientific matters from such reasons is not appropriate. 2. I don't think any of the answers to my question could support Pluto being planet; it is anyway is not a planet in current definition, and re-defining satellite cannot do anything. $\endgroup$ – Hojin Cho Jul 14 '15 at 6:14
  • $\begingroup$ I really do not find this question has "Rhetoric". It's just asking if there is an official definition of a "satellite". Saying "Get over it" is inflammatory. To the original question - the Moon is definitely not a planet, doesn't that answer your first-line question? As far as I'm aware there is no formal definition of satellite, because it is thought to be obvious and non-controversial. $\endgroup$ – Andy Jul 14 '15 at 7:10
  • $\begingroup$ I edited the question to change "logical fallacy" to "vagueness". "Logical fallacy" has a very specific meaning, dating back to ancient Greece. A vague definition does not constitute a logical fallacy. It's just not quite precise. $\endgroup$ – David Hammen Jul 15 '15 at 11:43
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In fact there is no official definition of a satellite.

Issac Asimov makes a good argument that the Moon is a planet since its orbit is convex around the Sun for its entire orbit, unlike the other moons of the solar system which are concave when in opposition and convex (to the Sun) when in conjunction.

Additionally, the idea that a body is a satellite of another body based on the barycenter being inside one body is flawed. Consider moving the Moon further or closer from/to the Earth. In which direction would you move it to make the two a "binary system" as opposed to a primary/moon system. In fact, though naively one would consider two closer bodies to be a binary system, moving the Moon closer puts the barycenter closer to the center of the Earth. Yet moving the moon further away moves the barycenter outside the Earth. Would you consider the Earth/Moon system as a binary system if the Moon were 30% further from the Earth?

Thus, we see that all proposed definitions for a satellite are flawed and have corner cases which make a precise definition contentious. This is the same problem as we had when trying to define a planet. No definition will nicely fit all the corner cases that Nature will give us.

Note that this answer was translated into English by my father, who is the one typing it. I'm eight years old and don't use SE unsupervised. A good portion of this information I learned just now because of this question, so thank you to the OP.

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    $\begingroup$ That's good! I hope the IAU doesn't come up with the idea to define satellites. $\endgroup$ – LocalFluff Jul 14 '15 at 22:53
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    $\begingroup$ qz.com/550296/… is another, more recent, attempt to define the Moon as a planet, based on observations of exoplanets. $\endgroup$ – barrycarter Dec 28 '15 at 7:01
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Your final three questions:

Should the supplementary Q&A sheet considered as legimate official definitions, or should it be considered as a one way of interpreting the Resolution B5?

That supplementary Q&A absolutely should not be considered as legitimate official definitions. That Q&A pertains to the original draft of the resolution. The final, voted-upon version represents a significant departure from the original. The final version added the dynamicists' concept of "clearing the neighborhood" to the definition of a planet, and the concept of double planets is gone. The IAU currently deems Pluto to be a dwarf planet, but not Charon (but that may change). From https://www.iau.org/public/themes/pluto/, "For now, Charon is considered just to be Pluto's satellite. The idea that Charon might qualify to be called a dwarf planet in its own right may be considered later."


If the Q&A is legitimate, then why is the definition of the satellite is not included in the Resolution B5? If not, is the official definition of the satellite nonexistent currently?

This question is a bit moot as that Q&A is not legitimate. There is no official definition of what constitutes a satellite. By any sane definition of the concept, Pluto is not a satellite of Charon, so Pluto's status as a dwarf planet is secure. Whether or not Charon is a satellite of Pluto is a matter of debate, and I suspect the IAU has had it's fill with rhetorical debates given the heated objections to their definition of the term "planet".


Are there any source or archive of the official or widely accepted definitions of glossaries in the fields of astronomy?

There is one word that does have an official meaning, and that is "planet". "Satellite"? No. "Star"? No. "Galaxy"? No.


Regarding the debate with your friend that made you ask this question, the only way in which one could view the Moon as a planet is by the fact that its orbit about the Sun is convex. That is a consequence of the gravitational force exerted on the Moon by the Sun is always greater in magnitude than that exerted by the Earth on the Moon. That's not a good definition; the IAU certainly doesn't use it. Along with the Earth's Moon, there are several outer moons of the giant planets that have this same characteristic, and they're all listed as satellites.

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