Student or Learner
I was reading Conan Doyle's Sherlock Holmes and noticed that nouns like sun, heaven are all countable in his books.
with a bright sun and a few fleecy clouds in the heavens.
with a skin which had been burned by tropical suns,
the slanting rays of a low sun
Examples like these abound. Anyone call tell me why are they countable? I remember back in high school, my English teacher (not a native speaker) told us "sun", "sky", "heaven" are all uncountable because there's only ONE in the universe.
As an NES but not a teacher, all I can say is "heavens above!".
A potential reason for them being treated as countable nouns may be that they are not as unique as your teacher claimed.
There may be a "tropical sun", a "hazy sun" or a "wintery sun" - all describing different "types" of sun.
Equally well we may say the "heavens opened", when there is a very heavy rain shower - here, "heaven" is refering to the "physical" heavens, rather than any possibly "religious" concept of a single, and possibly exclusive, heaven.
Beaten to the bell by Bhaisahab!
The fact that there's only one of something does not make it uncountable; after all how else have you managed to calculate how many there are? Knowing that there is one suggests the opposite to me- countability. We only have one moon, but many planets have several.
I think it's better to think of countability and unaccountability as characteristics rather than the fixed nature of a nouns- many nouns can be both, so if the dictionary suggests something is uncountable, it means it is most of the time.
Additionally, 'a skin burnt by tropical suns' means it's been exposed to the same sun but over the course of many days, weeks and years.
It's also worth pointing out that professional writers like Conan Doyle have skills not available to most of us. They are able to employ uncommon usages for dramatic or literary effect. And if they push the boundaries, as they sometimes do, we feel obliged to forgive them because their writing is so good, in other words because their unusual usage was so effective in communication. Skin burned by tropical suns : one heck of a phrase.