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