We use the definiteRelating to a specific or determined person, thing, idea or time. E.g.: I know the guy you are talking about. ("the guy you are talking about") articleAny of the words "a", "an" and "the", used to limit or give definiteness to a noun. "A" and "an" are indefinite articles, while "the" is a definite article. E.g.: A man was standing on the platform. ("a", "the") before a nounA part of speech that names a person, place or thing. E.g.: The koala is a marsupial. ("koala", "marsupial") which refers to an object of which there is only one.

The moon rises at 7.30 tomorrow.
The earth revolves around the sun.
I’d love to live by the sea.
The captain checked the position of the Northern Star to see where they were.

Also:

the sky
the equator
the North Pole
the Internet
the Matrix
the world

Bear in mind, however, that there are other suns and moons (or even worlds!) in the universe.

The planet Mars has two moons.
Every solar system has a sun.

The definite article with titles of books, newspapers etc.
The definite article (THE) The definite article with certain nouns