Earth or earth?

[keannu]

As I know, unique things should be capitalized, but I can see a bunch of examples of " earth" for "Earth". As the Earth is only one, it should be capitalized, but why is it used as "earth" as well? Just due to a habituation? Moon also seems a same one.

ex)The earth revolves around the sun.
I must be the happiest person on earth.

[tedtmc]

As I know, unique things should be capitalized, but I can see a bunch of examples of " earth" for "Earth". As the Earth is only one, it should be capitalized, but why is it used as "earth" as well? Just due to a habituation? Moon also seems a same one.

ex)The earth revolves around the sun.
I must be the happiest person on earth.

Some words are used both as a normal noun and a special noun. E.g. we can call the earth by its name, planet Earth (as distinct from Mars, Jupiter, etc), though this is less common.
However, when you use 'the', then 'earth' is not capitalized.

not a teacher