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moon, sun, earth

are these proper nouns,

Please explain it.

When they refer to the specific planet on which we live, the specific satellite which orbits it, and the specific star which warms it, they are proper nouns: Earth, the Moon, the Sun. There are of course uncountable suns and moons throughout the Universe, though to our knowledge no other Earths. There is plenty of earth beneath our feet, however, to grow potatoes in. Also in non-scientific contexts, for instance in poetry, you will often find neither sun nor moon capitalized, unless it is personified.