Tùy câu mà dùng in hay on.
Look up on the sky. Nhìn lên trên bầu trời.
source http://www.nightskyinfo.com/The size of a planet on the sky in a sense "averages out" the turbulent effects of the atmosphere, and the total effect is one of steady light.
The bird soars in the sky. Con chim bay cao trên bầu trời.