When to Capitalize 'Earth'

Figuring out when to capitalize the word "earth" isn't always straightforward.

Mignon Fogarty
3-minute read
The Quick And Dirty

Different style guides make different recommendations about when to capitalize the word "earth," but in general, you capitalize it when you're using it in a scientific context as the name of our planet and keep it lowercase when you're using it metaphorically and in idioms.

It's the name of our planet, and we capitalize names because they're proper nouns, right? Well, the names of other planet names, like Mars and Venus, are always capitalized because they're names of specific places, but people treat “earth” differently and don't always capitalize it.

You can tell this one is tricky because the AP Stylebook Q&A section has a lot of questions about whether to capitalize the word "earth" in different situations. In other words, even though they have an entry explaining their rules, people still wonder if they are doing it right.

Here's the deal. 

'Earth' as dirt

First, when you are using the word “earth” to refer to dirt, it's lowercase, of course.

'Earth' as the proper name of a planet

Second, when "Earth" is listed along with other planet names, it's always capitalized. For example, "Earth" and "Mars" are capitalized in this recent teaser from the New York Times:

NASA’s Ingenuity, a small robotic helicopter, took its initial flight over Mars on Monday making history as the first powered aircraft from Earth to fly on another planet.

Both The Chicago Manual of Style and the AP Stylebook say to capitalize "Earth" when it's being used as the proper name of our planet in this way and in this kind of scientific context, whether it's listed alongside other planets or not. 

'Earth' in other uses

But finally … they both say to keep it lowercase in "other uses," such as when it's being used "in nontechnical contexts," "casual contexts," and "in idioms."

For example, they both say you'd keep it lowercase in examples such as moving "heaven and earth" to make something happen, describing someone as "down-to-earth," or asking "where on earth" someone has been, presumably because you're using "earth" as more of a metaphor than a reference to the actual planet.

Examples of idioms in which "earth" is lowercase: "move heaven and earth," "down-to-earth," "earth-shattering," "salt of the earth," and "where on earth."

But even that isn't straightforward. For example, Chicago says to keep "earth" lowercase when you put the word "the" in front of it, as in "The earth is definitely round," while the AP Stylebook has examples of keeping it capitalized when used in a scientific context even when it has the word "the" in front of it, as in "The most abundant minerals in the Earth's crust are feldspars."

To sum up, capitalize "earth" when it's clearly a reference to the planet, especially when it's listed along with other planet names, which are always capitalized. Keep it lowercase when it's being used more generally and in idioms when it's being used more metaphorically. If you have to follow a style guide, know what that style guide recommends. And finally, "earth" is also a good word to add to your personal or house style guide, with examples of when you prefer to capitalize it and when you prefer to keep it lowercase because it can be hard to remember and in some situations, it could go either way.

Image courtesy of Shutterstock.

About the Author

Mignon Fogarty

Mignon Fogarty is the founder of Quick and Dirty Tips and the author of seven books on language, including the New York Times bestseller "Grammar Girl's Quick and Dirty Tips for Better Writing." She is an inductee in the Podcasting Hall of Fame, and the show is a five-time winner of Best Education Podcast in the Podcast Awards. She has appeared as a guest expert on the Oprah Winfrey Show and the Today Show. Her popular LinkedIn Learning courses help people write better to communicate better.