I believe that Earth means planet and earth means soil. But I've seen earth used in published works to mean the planet earth. But no one writes jupiter.

Similarly, Moon should mean the name of Earth's moon and lower case moon means the general category. This one is more complicated because I've read that the name of Earth's moon is Luna. Except no one calls it that. We don't say "Neil Armstrong was the first man on Luna."

Am I correct or is this a matter of convention?


3 Answers 3


According to Garner's Modern American Usage, when referring to the planet we live on, when used with the definite article, it is "the earth", lower case. However, when it is used as a name for the planet without an article, like "Mars is smaller than Earth", you capitalize it. It says sun and moon are treated the same way.

  • 1
    'Earth is bigger than Moon'? You're having us on. Commented Feb 18, 2014 at 22:55
  • Hmmmm . . . right. That's what I get copying things out of book without thinking. Commented Feb 19, 2014 at 3:41

The bottom line is that there is little consensus in all usage cases, but guidelines exist.

Typically, we capitalize the names of astronomical bodies, except when there's a preceding article ("Earth" or "the earth"). Earth is never capitalized when referring to soil or land—only when referring to the planet as a whole.

NASA and ESA (among others) prefer to capitalize Moon when referring to Earth's moon as a special case. My guess is that nobody says "Moon is out tonight!", as they would with Jupiter or Venus. We always add "the" to the Moon. It's as if we give it special consideration because doesn't have its own, standalone name. (Some may disagree on this point, preferring to always refer to it always as "the moon", following the rule mentioned above.)

  • Correct: "The sun is far from the earth."
  • Correct: "The sun is far from Earth."
  • Correct: "Jupiter has several moons."
  • Correct: "The Moon is right next to the horizon."
  • Incorrect: "The Earth is far from the Sun."
  • Incorrect: "The Sun gives light to our moon."
  • Incorrect (debatable): "The moon is bright tonight."

There are a number of excellent examples on the MLA page here: https://style.mla.org/2016/11/07/capitalizing-earth-sun-moon/

Note also that the name of our host star, the sun, is "Sol". We can say "Sol is larger than Earth", or "The Sun is larger than Earth" or "The sun is out today", but we would say "Sol is larger than Earth" rather than "Sun is larger than Earth".


Astronomers and others involved in space travel and exploration typically capitalize "the Earth" (or alternatively just "Earth") and "the Moon," although there are many counterexamples. When moons in general are discussed, the M is not capitalized.

edit: Actually, this turns out to be more complicated than I expected. The European Space Agency prefers to capitalize the M in "Moon", but NASA tends to favor a small m. (Ties should go to Neil DeGrasse Tyson, in my opinion, and he seems to favor a capital M.)

I believe Luna (along with its sidekick, Terra) is a convention from science fiction and is not used by actual scientists.

  • 1
    Luna and Terra are the Latin names for our moon and our planet, hence words in English like lunar and terrain. This of course predates science fiction by some millennia.
    – TylerH
    Commented Feb 18, 2014 at 20:08
  • Yes, they are Latin words. Use of them in English as proper names for the Moon and the Earth remains largely confined to science fiction.
    – phenry
    Commented Feb 18, 2014 at 20:11
  • I commented because I read "convention" as including "origin" somewhat. The term Lunarian was popular in the earth 18th century as an inhabitant of the moon. Today it might be popular to call the moon Luna in science fiction (I don't know, I don't read the genre), but all sorts of words are in use today outside of sci-fi that relate to Terra and Luna, if not those forms of the words, specifically, while the last line of your answer seems to imply otherwise.
    – TylerH
    Commented Feb 18, 2014 at 20:15
  • 1
    You can prove me wrong by finding the word "Luna" in use at nasa.gov to refer to Earth's moon in English. Have at it.
    – phenry
    Commented Feb 18, 2014 at 20:46
  • 2
    @phenry: According to NASA's historical and style guide, and the website you linked, NASA prefers "Moon" over "moon".
    – jvriesem
    Commented Feb 2, 2017 at 15:32

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