• 4-minute read
  • 29th July 2019

How to Capitalize the Earth, Sun, and Moon (Plus Other Celestial Objects)

The night’s sky can inspire many thoughts. A scientist might dream of finding a new planet. An artist, on the other hand, may find the beauty of the stars captivating. But as proofreaders, our minds immediately drift to capitalization. So join us for a look at when to capitalize celestial objects.

Proper Nouns in Space

When we say “celestial objects,” we basically mean “naturally occurring space stuff.” This includes planets, stars, moons, galaxies, comets, and pretty much anything else that you might see in space.

Nebulae and stars seen through a telescope.
Everything in this picture, then. You just have to zoom in a lot to pick anything out.

We capitalize the name of a celestial object when it is a proper noun. Or in other words, we only start a word with a capital letter if it names a specific celestial body, not just a type. So, for instance, the word “planet” is a common noun (i.e., a type of celestial body). “Uranus,” meanwhile, is a proper noun (i.e., a specific planet). As such, we don’t need to capitalize “planet,” but we do use a capital letter at the start of “Uranus.” Other examples include:

Common Noun

Proper Noun


Mars, Venus, Saturn


Europa, Titan, Callisto


Polaris, Rigel, Sirius


Milky Way, Andromeda


Halley’s Comet, Hale-Bopp


Orion Nebula, Crab Nebula

The key in most cases, then, is to think about whether you’re naming something generic or specific. However, there are a few words that can cause confusion, including “earth,” “sun,” and “moon.”

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How to Capitalize Earth, Sun and Moon

Certain “space” words can be either common nouns or proper nouns. When we say “the moon,” for example, we’re usually referring to the round object we see in the night’s sky. However, other planets have their own moons, so how do you capitalize this term in those cases? Or others like it?

Sadly, there is no single “correct” approach here. NASA, for instance, say:

Capitalize “Moon” when referring to Earth’s Moon; otherwise, lowercase “moon” (e.g., “The Moon orbits Earth,” “Jupiter’s moons”).

But other style guides, including MLA and Chicago, suggest using “sun,” “moon,” and “earth” except when:

  • Used with other planet names (e.g., “Mercury is larger than the Moon”).
  • When “Earth” isn’t preceded by “the” (e.g., “The shuttle will return to Earth”), except in idiomatic expressions such as “what on earth.”

Ultimately, this may come down to which style guide you are using (or personal preference). Generally, though, we favor only capitalizing “earth,” “sun,” and “moon” when you are both:

  1. Referring to the Earth, its Moon or its Sun (not other moons or suns).
  2. Using “Earth,” “Sun,”or “Moon” with other capitalized celestial objects, such as planet names, and/or when “Earth” isn’t preceded by “the.”

So, following these rules, we would capitalise these terms as follows:

The sun is shining brightly today.

What on earth are you talking about?

The moon is almost full tonight.

The Earth is much closer to Mars than the Sun.

You should not, of course, capitalize “earth” when referring to dirt or the ground. That kind of “earth” is always a common noun.

Summary: How to Capitalize Celestial Objects

As a rule, you will only need to capitalize celestial objects when they are proper nouns. In practice, this means:

  • Capitalizing the names of specific objects (e.g., Saturn, Mars).
  • Not capitalizing generic objects (e.g., planet, star, galaxy).

This becomes a little more complicated with the words “earth,” “sun,” and “moon.” The rules here vary between different style guides and institutions, but we tend to capitalize these terms only when:

  1. Referring to the Earth, its Moon or its Sun (not other moons or suns).
  2. Using the term alongside other capitalized celestial objects and/or when “Earth” isn’t preceded by “the” (e.g., “Mars is Earth’s nearest neighbor”).

The most important factor, though, is applying a consistent capitalization style throughout your work. And if you’d like a professional to check your writing, submit a document for proofreading today.

Comments (2)
C. C. Writer
4th December 2020 at 16:32
I see in one example, "The Earth is much closer to Mars than the Sun," you chose not to capitalize "the" which precedes "Sun." Some style guides state to capitalize "the" when it precedes a capitalized "Sun", "Moon", or "Earth". Such as the following examples: The moon orbiting Earth is called The Moon. Are all suns as hot as The Sun? The reasoning being is that despite there being an astronomical number of moons in the universe, we only address our moon as "The Moon", likewise "The Sun" only refers to our sun. Therefore, the proper noun for our sun is "The Sun". Similar to a when a name starts with "The," it should be written with a capital letter (e.g., The Rolling Stones, The Sex Pistols, The Beatles). Although one source of this rule does state that there is a lot of leniency on this ruling, particularly with regard to "The Sun" and "The Moon." So I am curious as to what say you.
    4th December 2020 at 17:31
    Hi there. As you note, style guides vary on this (and in some cases it depends on the type of proper noun in question). In this post, it was just because we don't think capitalizing the "the" before words like "Sun" and "Moon" adds anything (the capitalization of the main word makes it clear that they are being used as proper nouns in the contexts that you mention), and it looks neater not to vary between lowercase and capital letters more than necessary. However, as long as you apply the capitalization consistently (and follow the guidance in whichever style guide you are using), either approach would be fine.

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