In biblical cosmology, the firmament is the vast solid dome created by God on the second day to divide the primal sea (called tehom) into upper and lower portions so that the dry land could appear.
Then God said, “Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.” Thus God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament; and it was so. And God called the firmament Heaven. So the evening and the morning were the second day.
In English, the word "firmament" is first recorded in the Middle English Story of Genesis and Exodus dated 1250. [a] It later appeared in the King James Bible. The same word is found in French and German Bible translations, all from Latin firmamentum (a firm object), used in the Vulgate (4th century). This in turn is a calque of the Greek στερέωμᾰ (steréōma), also meaning a solid or firm structure (Greek στερεός = rigid), which appears in the Septuagint, the Greek translation made by Jewish scholars around 200 BCE.
These words all translate the Biblical Hebrew word rāqīaʿ (רָקִ֫יעַ), used for example in Genesis 1.6, which can be contrasted with shamayim (שָׁמַיִם), translated as "heaven" in Genesis 1.8. Rāqīaʿ derives from the root rqʿ (רָקַע), meaning "to beat or spread out thinly". Gerhard von Rad explains:
Rāqīaʿ means that which is firmly hammered, stamped (a word of the same root in Phoenecian means "tin dish"!). The meaning of the verb rqʿ concerns the hammering of the vault of heaven into firmness (Isa. 42.5; Ps.136.6). The Vulgate translates rāqīaʿ with firmamentum, and that remains the best rendering.— Gerhard von Rad 
The Hebrews regarded the earth as a plain or a hill figured like a hemisphere, swimming on water. Over this is arched the solid vault of heaven. To this vault are fastened the lights, the stars. So slight is this elevation that birds may rise to it and fly along its expanse.
The 6th-century Egyptian traveller Cosmas Indicopleustes formulated a detailed Christian view of the universe, based on various Biblical texts and on earlier theories by Theophilus of Antioch (2nd century CE) and by Clement of Alexandria (c. 150 – c. 215). Cosmas described a flat rectangular world surrounded by four seas; at the far edges of the seas, four immense vertical walls supported a vaulted roof, the firmament, above which in a further vaulted space lived angels who moved the heavenly bodies and controlled rainfall from a vast cistern. Augustine (354-430) considered that too much learning had been expended on the nature of the firmament. "We may understand this name as given to indicate not it is motionless but that it is solid", he wrote. Saint Basil (330-379) argued for a fluid firmament. According to Saint Thomas Aquinas (1225-1274) the firmament had a "solid nature" and stood above a "region of fire, wherein all vapor must be consumed".
The Copernican Revolution of the 16th century led to reconsideration of these matters. In 1554 John Calvin proposed interpreting the "firmament" as clouds. "He who would learn astronomy and other recondite arts, let him go elsewhere", wrote Calvin. "As it became a theologian, [Moses] had to respect us rather than the stars", Calvin wrote. Such a doctrine of accommodation allowed Christians to accept the findings of science without rejecting the authority of scripture.
According to Stefan Wild, in many Quranic verses, "heaven seems to be less the place where God resides than the physical firmament"- God will ‘roll up heaven like a scroll’ (Q21:104) and sends down plagues (Q2:59) as portents of the Day of Judgement (55:37). The Quran provides imagery of God’s throne (23:86), its position in heaven with angels surrounding it (39:75). 
The Greeks and Stoics adopted a model of celestial spheres after the discovery of the spherical Earth in the 4th to 3rd centuries BCE. The Medieval Scholastics adopted a cosmology that fused the ideas of the Greek philosophers Aristotle and Ptolemy. This cosmology involved celestial orbs, nested concentrically inside one another, with the earth at the center. The outermost orb contained the stars and the term firmament was then transferred to this orb. There were seven inner orbs for the seven wanderers (known planets) of the sky, and their ordering is preserved in the naming of the days of the week.
Even Copernicus's heliocentric model included an outer sphere that held the stars (and by having the earth rotate daily on its axis it allowed the firmament to be completely stationary). Tycho Brahe's studies of the nova of 1572 and the Comet of 1577 were the first major challenges to the idea that orbs existed as solid, incorruptible, material objects.
In 1584, Giordano Bruno proposed a cosmology without a firmament: an infinite universe in which the stars are actually suns with their own planetary systems. By 1630, the concept of solid orbs was no longer dominant.
On an oðer dai ðis middel-erd,
was al luken and a-buten sperd;
ðo god bad ben ðe firmament,
Al abuten ðis walkne sent,
Of waters froren, of yses wal,
ðis middel werld it luket al;—
May no fir get melten ðat ys;
He ðe it made is migtful and wis,—
It mai ben hoten heuene-Rof;
It hiled al ðis werldes drof,
And fier, and walkne, and water, and lond,
Al is bi-luken in godes hond,
Til domes-dai ne sal it troken.
The second day, this Middle Earth
Was all locked and barred;
Then God commanded the firmament to exist,
All around this he dispatched clouds
Of frozen water, walls of ice,
Which locked in this middle World completely
So that no fire should melt the ice.
He who did it is mighty and wise.
It was to be called “Heaven’s Roof”
It healed all the troubles of this world,
And fire and clouds and water and land,
All are locked up in God’s hand,
Till Doomsday it will not fail.
—Story of Genesis and Exodus, verses 93–105
- Pennington 2007, p. 42.
- Ringgren 1990, p. 92.
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