Q1. Proper nouns typically are not preceded by 'the'. Some,
however, are - either optionally or obligatorily. Some writers
prefer to distinguish between the sun (i.e. central star) of any
planetary system and that of our own by writing the latter with
a capital. Thus, we might have  Every star in the sky is a
potential sun.  All life on Earth depends on the Sun. This is,
however, not a rule, but a tendency, generally restricted to
contexts where the sun is being considered from a scientific
point of view. In  The sun was shining that day. it would be
unusual to use a capital, even though we are referring to the
same celestial object! Much the same thing applies to 'moon', so
that we have 'a moon' (common noun, any celestial body orbiting
a planet), 'the Moon' (our moon, in a scientific sense) and 'the
moon' in more everyday contexts. 'Earth' is a little different
since, as mentioned, it may stand as a typical proper noun (i.e.
without any article), but the acceptability/naturalness of this
tends to depend on