In what cases do we have to put the definite article the before each of these words:

  • Sun
  • Moon
  • Earth

and in what cases do we not need to?

  • 2
    Excellent question! One of those cases where many of us are consistently following "rules" we're not conciously aware of. Jul 8, 2011 at 3:36
  • related; english.stackexchange.com/q/11577/8019 (definitely not duplicate) Jul 8, 2011 at 14:00
  • The question has an undefined aspect. Not needing "the" could mean either no article or a different article. "Sun" is often used to refer to the star in the center of any solar system, and "moon" is often used to refer to any moon. So they can take "a" instead of "the". "Earth" is typically used to refer just to our planet, but it can sometimes be used to refer to any earth-like planet, so it can also take "a".
    – fixer1234
    Jun 7, 2017 at 23:40

10 Answers 10


For Sun, you always need the definite article when referring to the star itself. The only time you don't need it is when you're referring to the Sun's light/heat output...

"I like sun" is just about valid, but sun there just functions as shorthand for sunshine. Certainly that's what it means in the more common form "I like the sun" (note lack of capitalisation).

For Moon I can't come up with any context where you don't need the article...

"We put a man on the Moon", but you couldn't do anything on Moon.

Earth can take it or leave it (unlike world, which always needs the article)...

"The Earth is flat".

"Climate change threatens the Earth"

"The astronauts returned to Earth".

"It's like nothing on earth".

"Where on earth have you been?" (contrast with "What in the world was that?")

When the Earth is spoken of as a physical body, occupying space-time, it's normally preceded by the article, and often capitalised. As are Sun and Moon, but there's no universality about either convention, nor is capitalisation necessarily governed by whether the article is present or not. I've capitalised every usage after the article (as do most people), but there are exceptions.

When Earth is used more "metaphorically" to mean our whole environment (really, just the thin skin of biosphere on the surface of the planetary body, where nearly all things that concern us take place) it's more normal to omit the article, and I wouldn't normally capitalise either.

  • Wouldn't be "more normal" to capitalise those nouns when you refer to the single object? For example "What on earth have you been doing???" here it's an exclamation but "There's nothing on Earth like that" here you're talking about the only one planet we know with this name no?... What I'm trying to say that, in my opinion but I want to bring this out, capitalisation depends on single definite object VS mass noun/other, and not on the article usage... Am I wrong regarding the English rules?
    – Alenanno
    Jul 8, 2011 at 8:36
  • @Alenanno: Umm. Not sure I understand that. Apart from the capitalisation which you yourself introduced there, in what sense is the "earth" of your first example sentence any different to the second? A cursory scan of Google Books suggests a marked tendency not to capitalise unless the article is present, so your second example seems somewhat less than standard - particularly when juxtaposed with the first, which I think is effectively the same usage. Jul 8, 2011 at 13:36
  • ...as to the 'rules' (which I've said either don't exist are aren't observed with any consistency), consider a sci-fi tale where a 'space cowboy' returns to the home planet after a tour of duty in the asteroid belt. He says "Man! It's good to be back on Earth!". I suggest that usage would always be capitalised, but would never have an article. In direct contravention of any rule you might think exists. Jul 8, 2011 at 13:39
  • 2
    Once in a blue moon, I have to something worthwhile to add. However, in this use of moon, it's not the proper noun to which @LordCover referred. Additionally, I believe these names used in headlines may be an exception to the rules of needing a definite article.
    – Spare Oom
    Jul 18, 2011 at 12:02
  • 1
    Can't you lie under the clear night sky and soak up some moon? Call me a lunatic, but I guess that's, at least technically, correct.
    – Færd
    Jan 14, 2017 at 7:09

"Sun" and "moon" can be countable nouns. "Earth" is the name of this planet (a proper noun). For instance, you wouldn't say "The Jack and the Jill rolled down the hill".

Moon is demonstrated as a countable noun with this sentence: Jupiter has many moons.

It used to be common to refer to any star as a sun, and in this context it makes sense to say "the" before it to make sure we are talking about our sun.

However, "star" is now used much more commonly to denote stars other than the Sun, so it would make sense to drop the "the" before mention of the Sun. Maybe it will happen, maybe it won't.

"Earth" as a proper noun is no different than talking about Jupiter or Mars. We never say "The Jupiter" or "The Mars".

But earth is also used as an uncountable mass noun, and like sun, could also have been used to refer to other planets. It is in a nice situation where it can be referred to as "the Earth" OR "Earth". A lucky little planet it is.

  • 3
    I don't think the countable/uncountable noun business has much to do with it. OP capitalised all three bodies, so I think we can take it he means the three named proper nouns, for which countability isn't really an issue. The problem is because of its unique importance in so many ways "earth" has some usages and capitalisation issues the other two don't. Jul 9, 2011 at 2:40
  • "A layer of earth."
    – apaderno
    Jul 9, 2011 at 17:14

OK, one more attempt to synthesize an answer (with particular thanks to Ham and pavium). The word 'sun' may refer to sunshine (a patch of sun) and 'earth' to dried mud; that's no more relevant than 'moon' meaning point your backside at. "Sun" and "moon" have astronomical meanings, in which they take neither capital letter nor article: Mars has two small moons. ('Earth' in this sense has been superseded by planet; 'sun' here is a synonym of 'star') All three words may also refer specifically to the body in our solar system, in which case, as names, they require "the". "Earth" is a special case; as well as the name of this planet (with article), it may refer to the place where humans live (without): coterminous but not synonymous. If an asteroid passed close, as in the H.G. Wells story, the Earth would not be affected from an astronomer's viewpoint, but Earth as we know it would be devastated.


I think you can see things roughly as follows:

  • "earth", "moon" and "sun" define types of bodies: you could use them just like any boring old noun-- countably, with an article, possessive etc ("Jupiter has over sixty moons", "our earth may simply be one of many", "we are discovering that many stars are themselves suns with planets orbiting around them", "we may discover another earth within our lifetime")...
  • ...but, in practice, all three are used with "the", and often capitalised, when they mean "the most local [sun/moon/earth] in question"-- in practice, this tends to mean 'our' sun/moon/Earth, but if you said e.g. "Klax lived on the planet Blingon and one day decided to visit the Moon", that this could imply that he visited the moon of Blingon, not the moon of Earth;
  • simultaneously, Earth is the name of a celestial body, just like Venus, Jupiter, Titan, Europa etc. So in contexts where you might use the name of any other planet/moon/asteriod/other heavenly body, you would generally not use the article: "Of the eight planets orbiting our sun, Earth is the thirdmost inner planet and Mars the fourth"; "When will the robot land on Venus/Earth/Mars/Titan?"; "aliens visiting Earth (/Mars/Europa) may fail to find life intelligent enough to be worth conversing with".

(As other posters have mentioned, there is also a secondary use of "sun" to mean "heat/light from the sun", in which case it behaves more like a mass noun such as "water".)


According to the International Astronomical Union and famous physicist Johnny Wheeler, you should omit the definite article and capitalize Sun, Moon, and Earth.

From Wheeler's book Spacetime Physics on page 32:

Note: Neither astronomers nor newspapers say "the Venus" or "the Mars." All say simply "Venus" or "Mars." Astronomers follow the same snappy practice for Earth, Moon, and Sun. More and more of the rest of the world now follows -- as we do in this book -- the recommendations of the International Astronomical Union.

The ocean's rise and fall in a never-ending rhythmic cycle bears witness to the tide-driving power of Moon and Sun. In principle those influences are no different from those that cause relative motion of free particles in the vicinity of Earth.

  • 2
    That is a bad example. No, you do not need articles there. But normally you do. "What is that ball of fire in the sky?" It's the sun, or it's our sun, or even it's a sun. But not just it's sun.
    – tchrist
    Dec 21, 2012 at 22:06

For Sun, Moon, and Earth, a determiner is always needed, if the word is being used to denote single countable nouns.

That is: Sun, Moon, and Earth.

What about the expression "What on earth?"

"earth" in this case is being used in a different way:

the inhabitants of this planet, especially the human inhabitants:

Thus, "earth" in this expression is being used as a single uncountable noun, which means that it doesn't need the determiner "the".

  • I live on planet the Earth. OK. Why is Earth different from Mars in this regard?
    – GEdgar
    Jul 8, 2011 at 14:11
  • @GEdgar: I think that's a red herring. The planet Earth is third from the Sun: it would be strange to say Planet Earth. But "Planet Earth is threatened by global warming" doesn't take an article, precisely because it doesn't mean the astronomical body, but the biosphere; 'Spaceship Earth' is another way of putting it. Jul 8, 2011 at 15:22

I think you should use 'the' before any of those words when capitalised like that. There is only one of each.

I've been trying to think of an example where you wouldn't, but I can't think of one.

This is not the same as using, for example, 'moon' in a generic sense "Jupiter's moons" or "the sky was full of suns"

  • 1
    My, my, this Question has attracted a lot of attention since I answered first (12 hours ago) and still no-one's upvoted an answer. But I can see my answer was too simplistic. I can try to synthesise a more complete definition, if it's not too late. As @FumbleFingers said "we're using rules we're not consciously aware of"
    – pavium
    Jul 8, 2011 at 14:21
  • I certainly don't claim I've got a definitive answer, but I hope I'm following the trail. If you can find any more to 'synthesise' into it, please do. Or copy the lot, modify it, and post it as your own revised answer - I don't mind. This is one of those questions I'd really like to see nailed as firmly as we can (i.e. - more firmly than jelly on the ceiling! :) Jul 9, 2011 at 2:47
  • Thanks FumbleFingers. What attracted me to this question was my interest in astronomy, coupled with the fact that my wife is Polish and doesn't use articles. It has always seemed that native speakers of English seem to know when to use articles without knowing why. If I could explain it, my wife would speak more like a native, after 20+ years, but I'm still trying to explain. I hope the liguists here can explain better than I.
    – pavium
    Jul 9, 2011 at 4:00

You can use a or an when you are...

  1. Referring to representations of the real thing.

    I added a sun and a moon to our logo.

  2. Talking about a hypothetical situation.

    I can't imagine a sun with no sunspots.
    An Earth with no humans? It may happen.

  3. Generalizing.

    There is at least a Sun in every galaxy.

  4. Using these words as adjectives.

    We have yet to find an Earth-like planet.

  • 3
    I asked for definite article, not for indefinite one.
    – Ken D
    Jul 8, 2011 at 2:32
  • Sorry, :O. Well I hope my answer would fit the "in what cases we don't" portion?
    – json
    Jul 8, 2011 at 2:37
  • Hmmm this makes me think. "How long does it take light from the Sun to reach Earth?" and "Will a comet hit Earth one day?" all talk about the Earth but does not use the, maybe because here, Earth is treated like a place and not as an object or thing. However the same is not true with the Moon or the Sun... you can't say "Light from the Sun reaches Moon in..." but you can definitely say "Light reaches Earth..." as well as "Light reaches the Earth..."
    – json
    Jul 8, 2011 at 3:06
  • Moon, sun? Somebody has copied my idea.
    – apaderno
    Jul 9, 2011 at 17:08
  • You do not capitalize sun.
    – tchrist
    Dec 21, 2012 at 22:07

You can't ever use those words (with the definitions you're using) alone in a full sentence without a definitive article or a determiner.

That is, you can never say "I like Sun." You can say:

  • I like the Sun.
  • This is their Sun.
  • This is the people of Alpha Centauri's Sun.
  • 4
    So what on Earth do you make of this sentence? Jul 8, 2011 at 3:25
  • 6
    Here on Earth, we sometimes omit the definite article.
    – MT_Head
    Jul 8, 2011 at 4:52
  • 6
    I like sun, but I don't like rain.
    – psmears
    Jul 8, 2011 at 7:21

Moon is Earth's satellite. Titan is a satellite, not a moon. No one says the Titan. But everyone says Saturn has many moons.

The name of Earth's satellite is simply Moon. We should call it Moon as if it were someone's name. If you want to use the, use it as the satellite: Saturn has many satellites, not moons. Moon is our satellite's proper name. Same goes for our star, Sun, and our planet, Earth. All stars, planets, and satellites have their own name, so use them.

You may think you sound silly, but the correct way to say it is:

It takes 365 days for Earth to rotate around Sun, while it takes Moon only 24 hours to rotate along with Earth.

Listen to this using the:

It takes 365 days for the planet to rotate around the star, while it takes the satellite only 24 hours to rotate along with the planet.

Mix it all together:

It takes 365 days for the planet Earth to rotate around the star Sun, while it takes the satellite Moon only 24 hours to rotate along with the planet Earth.

  • No, it is not right to say that Sun is our star's proper name or that Moon is our satellite's proper name. If they were proper names, like Titan or Mars, they would take no definite article. But they do, which means those are not proper names like (say for example) Sol or Luna might conceivably be.
    – tchrist
    Dec 21, 2012 at 21:35
  • 3
    Are you describing your own personal language, or the English that everyone else speaks? If the former then good for you and your logical language, you'd be interested in Ithkuil, a constructed language. If the latter, it is not an accurate portrayal of how the great majority of native English speakers speak (i.e. you're just plain wrong).
    – Mitch
    Dec 21, 2012 at 22:07

Not the answer you're looking for? Browse other questions tagged or ask your own question.