While doing research for my next video lesson on articles, I stumbled over the use of the definite article with the name of our planet. When do we refer to it as “the Earth” and when do we simply say “Earth”? For that matter, do we always have to capitalize the name or is it optional? Example: Why on earth is this word so problematic? Should “earth” in that example be capitalized?
I think the first guideline is the one we apply to the names of the planets in our solar system. We use the zero article, and we capitalize each name: Mercury, Venus, Mars, etc. Following this guideline, we can say we live on planet Earth. We’re simply stating the name of our planet.
Also, when talking in the context of outer space and science, we tend to capitalize “Earth” and the definite article is usually needed to make a specific reference:
the Earth’s rotation
orbit the Earth
the Earth’s atmosphere
the surface of the Earth/ the Earth’s surface
However, “earth” can also be used as a synonym for “world”, right? As in why on earth did I choose such a tricky topic? = Why in the world did I choose such a tricky topic? In this case and in other set expressions, we tend to use a lowercase “e” and the zero article:
Desserts at that restaurant are heaven on earth.
She’s very down-to-earth.
What on earth made you say that?
Of course, every guideline has its exceptions: He’d follow you to the ends of the earth. The phrase is idiomatic, but it uses the structure “the [noun 1] + of the [noun 2]”.
What are your thoughts? What guidelines would you share with students to help them decide when to use the definite article with “earth” and when to capitalize the name?