Use the word empyrean when you're talking about the heavens or the sky. You might describe the empyrean curve of the night sky, scattered with stars, particularly if you wanted to sound poetic.
The adjective empyrean, pronounced "em-PEER-ee-an," can describe a religious idea of heaven. It can also describe the sky itself, or something that is awe-inspiring, like the empyrean beauty of the Himalayas. The phrase the empyrean means "the heavens" or "the sky," or in Greek cosmology, the highest, fiery sphere of heaven, empyros in Greek, and the root of empyrean.