The firmament is an ancient word for the sky, especially when you think of it as a vast dome or vault stretching overhead. Picture yourself lying in a field at night, staring up at what looks like a huge curved ceiling scattered with stars. That's the feeling firmament captures.

The word comes from the Latin firmamentum, meaning “support” or “strengthening,” because ancient people imagined the sky as a solid dome that held back waters above the earth. In old cosmologies and religious texts, the firmament was thought to be a physical structure, like an enormous upside-down bowl, that separated the heavens from the earth below.

Today we know the sky isn't actually a solid dome, but we still use firmament in poetic or literary writing when we want to make the sky sound grand and mysterious. You might read about “stars glittering in the firmament” or “the sun crossing the firmament.” The word gives the heavens a timeless, majestic quality that plain old “sky” doesn't quite capture. It's the kind of word that appears in epic poems, creation stories, and any writing that treats the cosmos with wonder and awe.