Would 'w niebu' also work for 'in the sky'?
"w niebu" does not exist. "w" is followed by accusative or locative, so "w niebo", or "w niebie".
but stars are "na niebie".
Is the word 'niebieski' (blue) derived from 'niebo' (sky)?
Yes, only that Nominative is 'niebo'.
Oh! Double thanks, then :D
what about "wspaniałe"?
Well, it's not "beautiful", rather "wonderful".
Added. All formal pronouns should work now.
"Widzisz tamte piękne gwiazdy na niebie?" is not accepted, 05-apr-2020.
Is it wrong?
These can't be translated with tamte, only te.
Why is ładne not accepted?
We decided that beautiful is a really strong word that doesn't match ładne in intensity. It's more like "pretty".
Never thought i would be the polish Shakespeare:D