firmamentum

Firmamentum is the Latin term that denotes the firm, the vault, or the expanse above the earth, commonly rendered in English as firmament. In classical and medieval Latin, firmamentum refers to the celestial realm—the sky considered as a protective vault enclosing the earth. The word derives from Latin firmare "to make firm," via the suffix -mentum, and was used in translations of biblical texts to render the Hebrew raqia, meaning an expanse or vault.

In biblical exegesis and scholastic cosmology, the firmamentum was imagined as a solid or semi-solid sphere, often described as transparent, with stars attached to its inner surface. It formed part of a geocentric cosmos with concentric celestial spheres and a crystalline outer firmament that separated the heavens from the waters above from the earth below, though details varied by author.

With the scientific revolution, the notion of a physical outer vault fell out of scientific use, and firmamentum became mainly a historical or liturgical term. In modern English translations of many religious texts, firmamentum is used to denote the biblical vault rather than a physical object in contemporary astronomy.

Today, the term is primarily encountered in philology, theology, and the history of science; it is not a technical term in contemporary astronomy.