1. The earth moves round the sun.

Question

2. The Earth moves round the sun.
3. Earth moves round the sun.
4. The earth moves round the Sun.
[Which ones are okay? Do we have to use 'the' in #2 or #3? What about #4? "the Sun?"]

Answer 1

All are acceptable, with or without "the" Earth. Capitalization is inconsistent, though. Earth and Sun are always capitalized when referring to the planet and the star. They are proper nouns. Note that generic uses, as in sunshine or digging in the earth (dirt, soil) they are not capitalized, not proper nouns as in the names of the astronomical bodies.