The definite article with unique objects

The definite article is used before a noun which refers to a specific, unique object:

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 determine the location of his ship.

Other examples are:

the sky
the equator
the North Pole
the internet
the Matrix
the world

We should bear in mind, however, that there are other suns and moons in the universe, and when we refer to these they might not be considered as unique:

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