Words define the reality in which people exist, as they call the phenomena and objects of this world. In fact, the word is a description of the rest of the world. Thus, depending on what words describe the world, the perception of this world in the minds of many people is determined.
Flat earth
As some scholars point out, "firmament" is a word that came about by understanding the shape and structure of the planet. The earth was previously considered flat and covered with a kind of spherical dome, that is, a vault. This is where the name came from, which actually points to some construction that covers heaven from above.
Many may recall the absence of a significant spread of the concept of the plane of the earth, even in ancient times. Indeed, in ancient Greece, people did not always imagine the geocentric model of the world, and even less did not consider the surface of the planet flat. Nevertheless, the meaning of the word "firmament" still indicated the most convenient description of the world.
Visible part of the sky
Even if the Earth is not considered flat, then from the point of view of a human observer, the sky still looks like a kind of sphere that covers the planet from above. Therefore, in a more figurative understanding, the firmament is simply the visible part of the sky that is accessible to man for contemplation. Here, the word “arch” also remains relevant, in the meaning of “dome” or “visible sphere of the sky”.
In the most general sense, the words "firmament" and "sky" have the same meaning, but the former is considered more poetic and figurative. Therefore, it is often used by poets in poetry, as well as in fiction.
Depending on where a person is observing, the firmament is different. In different parts of the Earth, the visible part of the sky includes various stars, and the Moon can have a different tilt and shape. Thus, the narrower meaning of this word is defined as “part of the sky visible in a separate territory”. Just as the Earth’s surface is different in different countries, the sky is different.