When you look up at the night sky, you’ll see the soft light of distant stars twinkling in the distance. And some of these stars have proper names: Sirius, Rigel, Betelguse. But when you look up at the daytime sky, there’s just one star: the sun. Or is it “the Sun”? Our sun is typically the only star we refer to as the sun, but “sun” isn’t a name– it’s an astronomical term. So is sun capitalized? Let’s take a quick trip to space and learn more about when we should capitalize the term “sun.”

Oh, and stop looking up at the sun. You’ll hurt your eyes!

Is ‘Sun’ Capitalized?

Have you ever noticed that we capitalize the names of the planets –Earth, Saturn, Mars, Jupiter, and the rest – but not “the sun,” ‘the moon,” and other similar terms? There’s a reason for this.

“The sun” is not a name. Sun, moon, planet, asteroid– these are all generic terms for types of heavenly bodies. 

If you want to call our sun by its proper name, use the word “Sol.”

“Sol” is capitalized (and so is “Luna,” a proper name used for our moon). But because we only have one sun and one moon, it’s highly unusual to use their proper names, unless maybe you’re referring to them in an allusion.

And this is true for technical writing, too! “Sol” is a poetic proper name, not an official name.

Naming Conventions by the International Astronomical Union

Even astronomers just call it the sun … although to make things even more confusing, astronomers do capitalize the word. Confused yet?

Celestial figures are given their names by the International Astronomical Union. They name everything from galaxies to asteroids, and they do not have an official name for our sun or our moon. However, this organization encourages people to capitalize the terms, and they often use “Sun” and “Moon” to identify ours – the reason being that capitalization sets them apart from similar astronomical bodies.

So if you’re reading a scientific paper about space, you will likely see “Sun” capitalized.

Capitalization in Non-Scientific Writing

But what about non-scientific writing? If you’re reading a book or a casual article, sun will probably not be capitalized. In fact, most style guides state that “sun” should not be capitalized. This is true for most other space terms. Even the name of our own planet, Earth, isn’t capitalized all the time. When you put “the” in front of it, it loses its capitalization to form “the earth.” 

MLA and AP Style Guidelines for Referencing the Sun

When it comes to referencing the sun in writing, both the Modern Language Association and the Associated Press have specific guidelines. According to MLA style, “sun” is not capitalized unless it appears at the beginning of a sentence.

Similarly, the AP Stylebook advises writers to use lowercase for “sun,” “moon,” and “earth” (when referring to soil or the ground), but capitalizes “Earth” when it is used as the proper name of our planet.

Is ‘Sun’ a Proper Noun?

So, is sun capitalized all of the time in scientific writing? Sun is only capitalized when it refers specifically to the central star in our solar system. This is because the word “sun” is not a proper noun. Technically speaking, any star with planets orbiting it is a sun. However, these stars are typically referred to by their proper names or reference numbers if they don’t have a name. While they might be referred to as the sun of their solar system, only our sun is the Sun.

Examples of How to Capitalize ‘the Sun’

Use these examples to guide you when learning how to capitalize “sun.”

When “Sun” is capitalized

In scientific writing:

• “The Sun is approximately 93 million miles away from Earth.”
• “Scientists study solar flares to understand the behavior of the Sun.”

As a proper noun specifically referencing Earth’s star:

• “The Sun provides the energy necessary for life on Earth.”
• “Photosynthesis in plants depends on light from the Sun.”

When “sun” is not capitalized

In general usage referring to any star:

• “The nearby sun hosts a planetary system with several Earth-like planets.”
• “Astronomers discovered a distant sun with a habitable zone similar to ours.”

When used generically:

• “The sun sets in the west and rises in the east.”
• “Basking in the warm sun, the cat fell asleep on the porch.”

Shine Bright With Proper Grammar

Capitalization rules for astronomical terms can be challenging, but following these tips can help. What’s the best way you remember the rules for capitalizing “sun”?

Do you want more information about the right way to use capitalization, punctuation, and other technical parts of writing? Check out our Grammar Tips articles for the answers you need!