Referring to unique items

There are several things which are said to be unique in that only one example of them (or one set of them) exists. This is an obvious case for using the definite article.

And we're one of the most generous nations in the world.

After a while the sun gets warmer.

...when the stars fall from the sky.

...with the moon shining in darkness.

Here are some words which belong to this group.

the devil the earth

the equator the moon

the north pole the planets

the Pope the sky

the solar system the south pole

the stars the sun

the universe the weather

the world

In some ways unique nouns are like proper nouns which also typically refer to only one item or set of items. And there is a tendency to use a capital letter with some of them (especially 'devil', 'earth', 'equator', 'north pole', 'south pole' and 'pope'), as with proper nouns.

...a human being possessed by the Devil.

...on the surface of the Earth.

However, it is not true to say that all these nouns only occur with the definite article. You can use most of them (but not 'earth' or' weather') with the indefinite article or as plurals. If you talk of 'a sun' or 'a moon', you may be talking about another sun or moon elsewhere in the universe, or you may be trying to give a particular impression of ours.

...under a still-warm October sun.

The moons, too, rapidly grew larger.

And 'earth' is very often used without an article, especially after 'on'.

... the smallest nation on earth.

In reality, unique nouns are an extreme case of the situational uses described above in sections 6 and 7. Here the 'situation' is so large that it includes almost all of our experience; but it is still possible to go beyond it in space and time. There is only one pope at the moment but in the past there have been many popes. When we say 'the sun', the situation is our solar system, but of course there are other suns elsewhere in the universe. Other nouns which can be considered as referring to unique items are nouns referring to directions, and nouns referring to specific periods of time.