In a 5th-grade exam (for 10-11-year-old pupils in Finland), there was a question, "What is a moon?" The model answer was: "A satellite of a planet that doesn't produce light itself but reflects it." Most pupils answered simply "A satellite of a planet" and received half the points, which is also what I would have answered!

To my understanding, planets don't have satellites that are stars (i.e., ones that produce light themselves) or ones that don't reflect any light (like black holes?). Therefore, I don't see why the second part of the answer is relevant.

I am looking for arguments to correct the evaluation.

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    $\begingroup$ As with any exam question, context of the teaching style in the classroom is very relevant. Although you ask for arguments to correct the evaluation, the grading might not be an astronomy issue at all. Perhaps the teacher has kept emphasizing in class to the students that explaining the property of an object is crucial, and has done previous repeated quizzes that emphasize this property of a moon. Also, you wrote "most pupils", so there are pupils who include the 2nd? In short, the second part may very well be redundant astronomy-wise, but may still be required to get the full point exam-wise. $\endgroup$
    – justhalf
    Commented Oct 31, 2023 at 4:53
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    $\begingroup$ This sounds like a test format that just expects kids to regurgitate whatever definition they got during class more than anything actually about science. $\endgroup$
    – eps
    Commented Oct 31, 2023 at 16:31
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    $\begingroup$ @justhalf both the exam and its model answers came from the organization which provides also the study material. So some pupils did include the emphasized part as well, as it was mentioned in the material. However I don't think this is something that the exam should test - you should be able to get perfect score with the correct knowledge without ever seeings the material. $\endgroup$
    – tputkonen
    Commented Oct 31, 2023 at 19:39
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    $\begingroup$ It seems to me like it's partly what the user eps said in the comment above, and partly because there is an assumption that a number of kids will think that the moon shines its own light, so the definition tries to preempt that (how common that misconception actually is today I'm not sure). $\endgroup$ Commented Nov 1, 2023 at 12:57
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    $\begingroup$ The additional bit is certainly owed to the fact that the Earth's Moon (and likely many others) is the second-brightest object in the sky by a large margin, and one of the essential learning goals of that educational segment was to understand that its birghtness is only from reflected sunlight. It's a bit like answering "what is a free-falling body in vacuum" with "an unpropelled object which changes its velocity only through gravitational forces". The second part is fully implied by the first, but it is unintuitive to many (who think a cannonball continues acceleration) and must be learned. $\endgroup$ Commented Nov 1, 2023 at 14:14

6 Answers 6


I'd ask the test author to provide an example of an object that would be improperly defined as a moon in the typical answer ("A satellite of a planet") that would properly be defined as not a moon in the model answer ("A satellite of a planet that doesn't produce light itself but reflects it"). If they can't produce an example, then the emphasized portion of the model answer is irrelevant (adds no value).

The only examples I can think of are the rare artificial satellites that use laser communication links. There are no perfect blackbody satellites that don't reflect any light.

The model answer includes all the other artificial satellites in planetary orbits. These certainly aren't moons, so the model answer is very flawed.

From the Keck Observatory page: New Aurorae Detected On Jupiter’s Four Largest Moons

Astronomers using W. M. Keck Observatory on Maunakea in Hawaiʻi have discovered that aurorae at visible wavelengths appear on all 4 major moons of Jupiter: Io, Europa, Ganymede, and Callisto.

enter image description here

Here is an artist's rendition from the same website (Credit Julie Inglis). Hence, the model answer excludes Jupiter's major moons since they do produce light.

This can easily be fixed by adding one word to the typical answer: "A natural satellite of a planet", which I think is deserving of full marks. I wouldn't give full marks for "A satellite of a planet" since it doesn't exclude artificial satellites.


  1. My definition above unfortunately includes planetary rings, but there isn't really a lower boundary for the size of an object to call it a moon. Objects in planetary rings are sometimes called moonlets.

  2. We could also argue about asteroid satellites (sometimes called moons and sometimes called moonlets). And what would we call a satellite around a blanet (which is a planetary equivalent orbiting a black hole)? These questions might be too advanced for the 5th grade.

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    $\begingroup$ Since moon just means satellite, albeit usually in the context of a rocky body orbiting a planet in a solar system, you could even say that the addition makes the answer false since you can have stars which are satellites of other stars and galaxies which are satellites of other galaxies. Granted, you wouldn't call either moons, but they are satellites so if moon is defined as satellite, then the requirement that it not generate its own light is even more wrong. $\endgroup$
    – terdon
    Commented Oct 31, 2023 at 12:57
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    $\begingroup$ It would probably be good to also mention that there can be moons of "minor planets", given that there's an official definition for "planet" which excludes bodies in our solar system which have natural satellites which we call moons. $\endgroup$
    – Makyen
    Commented Oct 31, 2023 at 19:10
  • $\begingroup$ a star satellite of a star and a galaxy satellite of a galaxy both fail both definitions "a satellite of a planet that doesn't produce light itself but reflects it" and "a satellite of a planet". So the extra portion is still irrelevant. Likewise, the minor planet thing is irrelevant since the "correct" answer also specifies a planet. $\endgroup$
    – cowlinator
    Commented Oct 31, 2023 at 22:53
  • $\begingroup$ @Makyen While your comment is true, I think it might get too far in the weeds for the 5th grade. I'd also give full marks to "A natural satellite in orbit around something other than a star or black hole." $\endgroup$
    – Connor Garcia
    Commented Nov 1, 2023 at 0:22
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    $\begingroup$ @terdon: You're playing fast and loose with your definitions here. (1) Moon does not mean satellite, they're not synonyms. All moons are satellites, but not all satellites are moons. In the same sense, you wouldn't say "apple means fruit", even though all apples are fruit. (2) The question is not asking what a satellite is (which would include all the examples you give), the question is what a moon is (which your examples are unrelated to, as they are not moons). It seems that your comment is based on a false equivalence that treats "A ∈ B" and "B ∈ A" interchangeably. $\endgroup$
    – Flater
    Commented Nov 1, 2023 at 22:50

Is this a test of science or of Suomi (or English)?

In English, a moon is a natural satellite of a planet, dwarf planet, or asteroid.

In Suomi, Yleisnimenä kuu on planeetan, kääpiöplaneetan tai pienkappaleen luonnollinen kiertolainen, eli satelliitti

And the IAU has not published a definition of Moon to mirror the definition of "Planet". So there is no technical definition.

In general scientific usage, the model answer is wrong to limit "moon" only to satellites of planets, and it is wrong not to limit the meaning of moon to natural bodies. It is odd to mention the thing about "not giving off light". It is true for every moon (at least in the visible part of the spectrum, the Moon does glow gently in the infrared), but it is probably wrong at least in theory. It is possible to imagine a young planet with a moon that is still red hot with the heat of its formation (not a common scenario but conceivable).

But hey! It's a test for 10-year-olds...This isn't most badly worded question that I've seen on a test for elementary school pupils.

Here is how it happens. The teacher is focused on misconceptions. In contrast to the sun, there are two differences that may give rise to a misconception. 1) That the moon and sun both go round the Earth. 2) The moon and sun both shine with their own light. So the teacher has taught that these are the differences between "sun" and "moon", and has tried to write a question which will enable the pupils to demonstrate that they understand the differences between moon and sun. Unfortunately the teacher has misphrased the question.

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    $\begingroup$ Or a moon made of phosphorous. :) $\endgroup$
    – Barmar
    Commented Oct 31, 2023 at 14:34
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    $\begingroup$ A moon with no atmosphere made of phosphorous would not glow. If it did have an oxygen atmosphere... it wouldn't for long. $\endgroup$
    – cowlinator
    Commented Oct 31, 2023 at 22:56
  • $\begingroup$ Can hot juipiters have moons? If so they could be hot enough to stay molten and glowing indefinitely. $\endgroup$ Commented Nov 2, 2023 at 5:53
  • $\begingroup$ @cowlinator: Perhaps he meant phosphorescence? $\endgroup$ Commented Nov 2, 2023 at 21:13
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    $\begingroup$ The two misconceptions don't read as misconceptions; they read as fact. Perhaps rephrase? $\endgroup$ Commented Nov 2, 2023 at 21:24

I, unlike the other answerers, read the model answer to be saying that the planet rather than the satellite does not produce light. I think the point of this is to avoid having the planets be moons because they are satellites of the sun.

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    $\begingroup$ Could be. But then planets produce light: my desk lamp next to the window is ruining it for Earth, Jupiter's got lightning storms. It makes more sense to define a moon as not orbiting a star. $\endgroup$
    – user71659
    Commented Nov 1, 2023 at 5:24
  • $\begingroup$ Presumably in this case the model answer should read "A satellite of a body that doesn't produce light itself ..." $\endgroup$
    – Allure
    Commented Nov 2, 2023 at 0:50
  • $\begingroup$ Thanks for pointing this out. I used ChatGPT for translating the answer from Finnish to English, and I didn't notice this before posting the question. However, the Finnish version was exact. $\endgroup$
    – tputkonen
    Commented Nov 2, 2023 at 17:17
  • $\begingroup$ I see that the planet, not the satellite being the body which doesn't produce light itself wouldn't be an unreasonable conclusion in ordinary English but how little idea of astronomy would be needed to negate that? If you're working on that level of detail, don't you think the idea should be rather that the body 'doesn't itself produce light'? Isn't it clear that 'doesn't itself produce…' refers to the body while 'doesn't produce light itself…' refers to the stuff (here, light itself) produced? $\endgroup$ Commented Nov 3, 2023 at 18:59

I think you are correct, because there are practically no moons that emit light (in the visible spectra), because in order for light to be emitted, electron excitation has to occur (when atom electron jumps to higher level), regardless of its radiation type, unless it is conversion of energy.

In space, naturally only a few types of radiation can occur, black-body/thermal radiation (like, in stars or hot Jupiters), or sometimes when an electric current passes through the atmosphere which excites the electrons.

Natural satellites are usually not that excited and active, because they don't have sufficient mass and core size for black-body radiation, and only cosmic rays cause electron excitations otherwise. Overall, most of the moons are not giving off light in visible wavelength; the only difference this can make is probably between a protomoon and a moon. When it is a protomoon it is usually sort of a little hot, having sufficient heat to emit visible light.

According to Wien's law: At maximum the wavelength that is in the visible wavelength (emphasis because moons, like our moon, themselves emit in the lower spectra), it requires a minimum temperature of 4140 kelvins (with exceptions. Note: This is peak wavelength), which a moon can only have if it is either to close to its planet that tidal heat is generated, or its host planet is too close to the host star, or it is newborn, or a protomoon.

So, although the definition is not such important because it is in most of the bodies by default, but it can differentiate between emitted light and reflected light, and mostly because this is the agreed and popular definition by Anaxagoras, a Greek philosopher in 500 BC, which he later proved by studying lunar eclipses.

  • $\begingroup$ I don't see any reason it couldn't be produced from a chemical or nuclear reaction. $\endgroup$ Commented Nov 1, 2023 at 12:56

I'm not 100% sure, but according to me, some moons of Jupiter get so "massaged" by Jupiter's gravity that this adds enormous amounts of energy to those moons, causing phenomena which might cause emitting electromagnetic radiation. In particular Io: that moon has so much volcanic eruptions that I can't believe none of the electromagnetic radiation being visible light.

So, as I believe having found at least one counter-example (without even needing to get out of our own solar system), I believe that extra condition on that "definition" being wrong.


The second part (distinction about reflecting light) is vital to our understanding of our moon and therefore all moons.

Students should already be familiar with other forms of objects that orbit Earth such as artificial satellites, these they would be aware of, but not be able to see or recognize in the sky with their naked eyes.

  • At that age level, because these satellites look no different to stars in most cases, if/when they see them, they will assume that they are stars, reinforcing that you cannot see these artificial satellites.

So they are aware that there are many satellites, but not all satellites are moons. Students can only visually identify one of Earth's satellites (our moon), and would reasonably offer that the only reason we can see it is because it is reflecting the light of the sun.

Students at this level will be aware that the Sun (Sol) is itself a Star, and that the only reason the sun looks so big to us compared to other stars is due to our close proximity to it.

The reason that emission vs reflection of light is important is that of the visible bodies in our sky, only stars (including our Sun) produce or emit light and only moons reflect that light.

  • Reflection of light is perhaps the most important aspect of the moon that they would have covered in the syllabus so far, it is the requisite understanding to explain the phases of the moon as well as the most intriguing aspect, eclipses of the moon and sun. It is reasonable to assume that there would have been a lot of discussion about reflection versus emission of light, especially when trying to explain the difference between a solar and lunar eclipse.

If such discussions have not been held prior to the exam, I would suggest that the delivery of the curriculum prior to the exam was not sufficient, especially as the general model and broad definitions should have been clearly outlined to educators before the syllabus was prepared.

For lower levels I would still expect them to be aware of this but might not put it in the exam. For 5th grade, this is a great entrance exam question because it would properly gauge the depth of understanding at the start of the year, by the end I would absolutely expect students to understand the concept of the moon reflecting light and use it when defining what a moon is. Think about it, if you ignore the gravitational effects on the Earth, before 4th/fifth grade, all our moon does is reflect light.

In other answers there is talk about a distinction between natural and artificial. For the year level, it is acceptable to omit natural from the definition of satellite as apart from Earth and our influence on the Solar system, so far all identified satellites are naturally occurring.

It is also acceptable to use the phrase satellite of a planet in place of the more technically correct astronomical body that orbits a planet as those concepts are probably a bit too abstract at that level.

  • $\begingroup$ But why would light be in any way relevant? At best, it is misleading since it gives importance to something that isn't in any way relevant to the definition of moon or satellite, and at worst it is flat out wrong since satellite (albeit, not moon) can and is used to describe both stars orbiting other stars and galaxies orbiting other galaxies. $\endgroup$
    – terdon
    Commented Nov 1, 2023 at 14:14
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    $\begingroup$ "Students can only visually identify one of Earth's satellites (our moon)" This statement is not correct, many elementary students can identify artificial satellites such as the international space station. They appear to glow like stars but move quickly across the sky. $\endgroup$
    – Connor Garcia
    Commented Nov 1, 2023 at 15:49

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