From Word Genius
Here’s a throwback to elementary school English: Common nouns are lowercase, and proper nouns are capitalized. Simple, right? But what about those tricky nouns that can be both? When are they capitalized? What turns a common noun into a proper noun?
Earth and earth
Aren’t they the same thing? Sort of. When referencing the soil or ground, “earth” is lowercase. However, when referring to “Earth” as a celestial body or the particular planet in the solar system, capitalize it. In its proper noun form, “the” is rarely in front of “Earth.”
Bonus: There’s a town in Texas named Earth — of course, capitalized.
North and north (goes for all cardinal directions)
These guidelines apply to all cardinal directions, but the distinction can definitely get confusing, especially in writing. As a general rule, lowercase both cardinal and intermediate directions when they’re being used as compass directions (i.e. “The storm is moving west.” “The interstate runs northeast.”) These words are capitalized, however, when identifying specific regions. For example, “She had a preppy, East Coast fashion sensibility.” “Southern folk have charming accents.”)
Cancer and cancer
The medical noun is not capitalized, even when referring to a specific type, such as lung cancer. But when asking “What’s your sign?,” “Cancer” is a proper noun that’s always capitalized, as are all the other zodiac signs.
God and god
The lowercase version is a common noun that refers to any deity, such as “the Egyptian gods” or “Apollo was the god of archery, music, and dance.” The big “G” is commonly reserved for the Abrahamic deity of religions like Christinaity, Judaism, and Islam. For example, “In Genesis, God created the Earth in six days.”
Governor and governor
The governor of a state is a common lowercased noun — unless that governor is being addressed directly or the word is used as a title or in place of the official’s name. (i.e. “Governor McMaster will see you now,” or “Governor, the attorney general is here.”) There are other definitions of this word that don’t have to do with politics. A truck’s governor is used to limit its top speed, and semis often require them by law. That kind of governor is also a common, lowercase noun.
Apple and apple
When Forrest Gump told that stranger on the bench that Lieutenant Dan invested his money in “some kind of fruit company,” he meant Apple, Inc., the Silicon Valley powerhouse that makes sleek computers, watches, and smartphones. The company is capitalized; the fruit is not. Several companies share this common-proper noun duality, including Subway and Delta, but Amazon the company and Amazon the rainforest are both proper nouns.