Plural of firmament; the visible sky or heavens; the vault or arch of the heavens.
From Latin 'firmamentum' (support, strengthening) pluralized with English '-s' suffix. In medieval cosmology, the firmament was thought to be a crystalline vault holding the stars.
When the Bible says 'the firmament showeth his handiwork,' it's referring to what people believed was literally a solid dome—and our modern word keeps that ancient misconception alive, even though we now know the sky isn't solid at all.
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