Avoid using annotations within sentences. Just write "A moon is already shining."

+1

No response for more than a year.
Corrected.

the English doesn't match the Esperanto...

*The* Moon is *still* shining = *La* luno ankoraŭ brilas = Noch scheint der Mond

The moon?

@NNC

Supposedly, this sentence could make sense on Jupiter or Saturn, or millions of multi-moons exoplanets...

Selena might have different moons in her chariot.

On Earth it wouldn’t make sense unless referring to other planets, so it depends on the perspective of the speaker

The sentence is correct and theoretically natural

Could the sentence be used anywhere but in a sci-fi novel?

Sure.
Next time I see "the" moon I say to my partner "Hey, a moon is already shining".

"It's the same moon as every day" - "How can you know?"

Maybe it’s a random moon everyday. In these conspirational times, who knows what ?

<Maybe it’s a random moon everyday.
It was an ancient belief.

It’s probable that ancients were conspiracists already...and certainly anti-vaxx