In English, the word “Earth” can either be written with a capital “E” or a lowercase “e”, depending on the context. It is possible to find all kinds of variants in written texts. This issue is not a big deal and many people write the word however they like – especially when chatting, the capital letters might take too much time… but let’s look at some general rules!
The word “Earth” is the proper name of this planet, so we should capitalize the “E” when mentioning the name of this whole planet. For example: “The Earth is smaller than the Sun”. Also notice, that when mentioning the proper name, the “the” is usually omitted – it’s usually not written as “the Earth”, although you can find it for example on Wikipedia here and it might feel natural in some contexts. And actually, some sources say that writing “the earth” (“the” + lowercase “earth”) meaning “the planet we live on” is okay too.
If “earth” is used to mean “the world”, like in the idiom “What an earth are you talking about?”, it’s usually written in lower-case, even though some sources say both variants are possible… Also if we use this word with the meaning of “a dirt”, “a ground” or “a soil”, we should just use “the earth”. For example “He dug into the earth to plant the tree”. Although he did dig into the planet Earth as well. We’re just not focusing on the whole planet, so it’s only “earth”.
More formally put: When it is a proper noun (a name of the planet), it gets capitalized and when it is a common noun it’s not capitalized.
In the context of the flat Earth conspiracy, we’re talking about the shape of the whole planet, named Earth, so it is appropriate to use capital E. Even though the flat Earthers usually only look at the earth that they stand on and since it’s locally flat, they conclude the whole Earth must be flat too.
Links:
- Dictionary.com article: When To Capitalize “Earth”
- YouTube video: What on earth…?! and other conversational expressions in English
- Wikipedia: Earth etymology – says it actually used to be commonly written as “the earth” before the Middle English