Firmament

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Firmament is a name for the sky or the heavens, generally used in the context of Christianity, Judaism and Islam. In the Vulgate, the word firmamentum is used, which means in classical Latin a strengthening or support. For Jewish and Christian astronomers familiar with Greek astronomy, the firmament was the eighth sphere carrying the fixed stars, which surrounded the seven spheres of the planets in the geocentric model.

The word is mentioned in the Bible, in the course of the creation story of (Genesis 1:6–8): Bereishit (Genesis) Perek (chapter) 1, Pasuk (sentence) 6:

God said, “Let there be a firmament in the midst of the waters, and let it separate between water and water.” So God made the firmament, and separated the waters which were beneath the firmament and the waters which were above the firmament. And it was so. God called to the firmament: “Heaven.” And there was evening and there was morning, a second day.

It appears that the ancients believed that there were bodies of water, one in the sky and one on earth, that would have a tendency to fall into each other. A firmament would thus be necessary to separate these bodies of water so that the water of the heavens could remain separate from the body of earthly water. They may have believed that the sky was essentially an elliptical dome, atop a flat Earth, upon which the stars were affixed. Once medieval Christians had taken up the Ptolemaic system, the idea of the firmament had to be accommodated to a spherical Earth and celestial spheres.