Determine whether the boldface prepositional phrase is adjectival or adverbial

The stars are twinkling in the night sky
1 adjectival phrase
2 adverbial phrase

1. adjectival phrase - "in the night sky" describes the stars and tells us where they are twinkling.

2. adverbial phrase - "in the night sky" describes the action of twinkling and tells us where the stars are twinkling.

The boldface prepositional phrase in the sentence "The stars are twinkling in the night sky" is an adverbial phrase. It modifies the verb "twinkling," giving information about where the action is taking place.