The idea of a firmament is definitely a thing and has been for thousands of years. en.m.wikipedia.org/wiki/Firmament

I’ve heard about it all my life from mainstream Christian churches too.

So. the firmament is a thing.

Mostly these days, it’s considered to be allegory, as apposed to firm fact. but people- typically Young Earth Creationist types- will insist the english-translated bible is the absolute word of god meant to be taken absolutely literally. (and will reject things like aging and dino bones because it was made to look that way. for some reason.)

there are some middle-ground type people who espoused a belief that there was a solid shell of ice- the firmament- and that it melted to create the cataclysm written of Noah’s flood to explain why it wasn’t there, but was there before.

They, uh, also tend to go in for a flat earth.

You can’t reason anyone out of a position they didn’t reason themselves into. She believes this because it’s a follow-up on the “the bible is literally true” position she has taken as an apparently core part of her personality.

Unless you’re looking for a fight, consider just ignoring this person. Otherwise, call her out and make her call you a liar in front a crowd.

I can’t find anything about this dome in a google search

I gotta question your methodology here a bit, because just googling “firmament” will get you quite a lot of results and you don’t have to read very far into the actual Bible to come across the term (depending on the version of the Bible you read, different translations use different terms, sometimes I’ve seen “vault” used instead) its probably going to be right there on the first page of Genesis.

Now the Bible doesn’t exactly do a great job of describing the firmament. Coming up with an actual model for for the heavens and the earth kind of fell to various scholars over the years trying to make sense of it.

In general, if you take it literally, the firmament is normally interpreted as a sort of solid structure somewhere up above the sky that separates heaven and a whole bunch of water from the rest of creation and probably more or less resembles a dome above a flat earth. The exact structure of all of that will vary quite a bit depending on which denomination and scholars you listen to.

Even the word firmament should kind of clue you in that it’s supposed to be kind of solid, you can really go down a linguistic rabbit hole through all manner of English, Latin, Greek, Hebrew, etc. words with this but for the most part you’ll get the impression that you’re supposed to believe there is some sort of physical barrier there

Most of the mainstream sects have adopted somewhat less-literal interpretations over the years because obviously the science doesn’t support that interpretation at all.

But of course there are a whole lot of absolutely wacky Christian denominations (all of them are at least a bit wacky, but some really go the extra mile into the heart of crazytown) and even in the less-insane denominations some individuals pick and choose and come up with their own batshit interpretations of things.

Off the top of my head, I don’t know which sects stick to a literal interpretation of it being a solid dome, but I’m absolutely sure they’re out there, but most of them are probably not very mainstream, most of them are probably independent churches not affiliated or only loosely affiliated with any larger organization. And a lot of people who believe it are probably picking it up outside of any organized church and either coming up with their own interpretations or getting it from some wackadoo on the internet.

Trying to talk sense into her is probably an exercise in futility and I would not have even the faintest idea of how to go about doing that. If you do want to learn more about her specific beliefs though, a good place to start is probably going right to the source and figuring out which church she goes to and where she’s getting her info- what websites, books, YouTube channels, etc. and the best way to do that is probably just to ask her. Don’t try to argue with her and convince her she’s wrong, you’re not going to get anywhere, just get some links and recommendations, she’s probably going to view it as a chance to convert you and born again types are all about that, she’ll probably dump a whole bunch on you.

This was roughly the early cosmology of Judaism, but even by Jesus’s time was being abandoned. For example, the Greeks and Romans were familiar enough with both lunar eclipses and the Earth being round that the generally accepted explanation for lunar eclipses was that the Earth was eclipsing the sun and casting a shadow on the moon, which we know was popular because in De Rerum Natura Lucretius appeals to keep more of an open mind as that might not be the only explanation (meaning it was commonly enough endorsed that it was nearly considered the sole explanation in Lucretius’s circles).

This may even connect to the description of the “crucifixion darkness” in the earliest copies of Luke where it is explained as being caused by the sun being eclipsed. That language is changed in later versions, and the language of ‘eclipse’ was criticized by early church commentators given that solar eclipses were known to be impossible on a full moon (such as Passover) and only last around 8 minutes.

But what’s often overlooked was that being written after 50 CE, visible nighttime lunar eclipses whose previous Saros cycle eclipses were during the daytime in the 30s CE would have been able to be trivially calculated by astronomers of the time.

Lunar eclipses take 3 hours, and have a 1 in 6 chance of occurring on Passover. We even know there was a daytime lunar eclipse on Passover of 33 CE, whose subsequent Saros cycle eclipse was visible in both Judea and Greece before any of the Synoptic gospels were written.

So not only would some of the anti-firmament cosmology known by the era of the New Testament, it’s quite possible that there was even originally text reflecting both knowledge the earth was round and that lunar eclipses are caused by the earth eclipsing the sun, but it may have been subsequently removed because later editors failed to realize the event was not an eyewitness testimony but a calculated celestial event and thus dismissed it as erroneously describing an impossible solar eclipse.

TL;DR: Your family member is nearly going pre-NT with the commitment to that cosmology there.

Like others have said, this is flat earther stuff. A lot of Christian evangelical types question things like the Big Bang and how old the universe/Earth really are, but afaik there isn’t an entire religion with this as a belief.

I used to work at a space museum and we would get Christan folks who would sometimes argue with us over the number that was shown on the sign telling them the age of our Moon rock, but never that the earth was flat. If that is a thing, it must be new.