I am one of the very few flat earthers that advocates a firmament. In fact its probably my fault this thread was made as I bang on about it from time to time. But this is because it so eloquently explains many of the quirks of flat earth theory. Whilst I know the round earthers will not believe anything I write, I will elaborate and hopefully win over a few flat earthers.
So, the firmament.
It does pop up in classical FET quite a lot. Below you can see a guy ( I understand he is a monk) who reached the edge of the earth, lifted the curtain of the sky (firmament) and was able to see the inners workings of the universe. This ties in with the theories on celestial gears but that's another topic.
The dictionary definition is
firmament (ˈfɜːməmənt)
— n
the expanse of the sky; heavens
[C13: from Late Latin firmāmentum sky (considered as fixed above the earth).
I like the description below
The firmament was a great tent-like ceiling made of solid crystalline-like material
So that's what it is.
What is does or rather the questions it answers.
It explains why the air doesn't all spill over the edge of a flat earth. The firmament keeps it all in.
It explains light distortion in the atmosphere. Long derided as 'bendy light' by round earthers, I suspect the distortions of sun light to be due to refraction of sunlight through this solid crystalline-like material. the light is being bent by this crystal bowl over earth.
It explains how high frequency radio signals can be bounced off a ceiling and back down to other ground based receivers.
Now the picture above shows the sun, moon and stars inside the firmament. I suspect this not to be the case. I suspect all celestial bodies to be outside the firmament as below.
I believe the firmament to be about 700 miles up and suspect that was the height Rowbotham calculated as the sun's by mistake. The sun actually being the 3000 miles up that Voliva calculated.
Does it move? Why would it?
What colour is it? As I said, its a clear crystalline substance.
Could you ascend to it? Erm, I guess so with the right technology. Its certainly possible to penetrate it temporarily as the larger meteors are able to push their way in, but most are vaporised on contact.
The x-15 flew at 108km for the world record. The ISS is allegedly only between 330km and 410km. I know the ISS is a hoax but even still its some way off 700 miles up. It would be well inside.
What shape is it? Looks a bit like the artists impression below.