The firmament means “expanse” and refers to the “expanse” of man’s hearing to the words of God (“And God said…). The expanse in the hearing is in the midst of the “waters” of men. The waters are then divided over the hearing of His words. It depends on how man responds to His words. Some will hear and others will refuse to hear.
Genesis 1:6 JPS
(6) And God said: 'Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.'
The firmament is the “expanse” of hearing. For example in Jeremiah 13: 15: “give ear” = to expand.
Jeremiah 13:15-16 JPS
(15) Hear ye, and give ear, be not proud; for the LORD hath spoken.
Some will hear and give glory and some will not.
(16) Give glory to the LORD your God, before it grow dark, and before your feet stumble upon the mountains of twilight, and, while ye look for light, He turn it into the shadow of death, and make it gross darkness.
God sent His servants the prophets…sending them many times…to speak His words.
Jeremiah 44:4-6 JPS
(4) Howbeit I sent unto you all My servants the prophets, sending them betimes and often, saying: Oh, do not this abominable thing that I hate.
Yet, they did not hear nor “inclined their ear” ….that is, to stretch or spread out. There was no “expanse” as they did not hear and turn from their wickedness.
(5) But they hearkened not, nor inclined their ear to turn from their wickedness, to forbear offering unto other gods.
(6) Wherefore My fury and Mine anger was poured forth, and was kindled in the cities of Judah and in the streets of Jerusalem; and they are wasted and desolate, as at this day.
The creation in Genesis 1 is metaphors for the renewing of mankind as the earth became without form (without righteousness) and void of understanding (“they know Me not). Man’s understanding was darkened and there was no one to teach them (the heavens had no light).
Jeremiah 4:22-23 JPS
(22) For My people is foolish, they know Me not; they are sottish children, and they have no understanding; they are wise to do evil, but to do good they have no knowledge.
(23) I beheld the earth, and, lo, it was waste and void; and the heavens, and they had no light.
In order to reconcile man’s relationship with God, He sends people to speak His words (the knowledge of God is light) as they are moved by Spirit (the Spirit of God moved upon the face of the waters). David spoke with the Spirit of the LORD. God’s word was upon his tongue.
2 Samuel 23:1-4 JPS
(1) Now these are the last words of David: The saying of David the son of Jesse, and the saying of the man raised on high, the anointed of the God of Jacob, and the sweet singer of Israel:
(2) The spirit of the LORD spoke by me, and His word was upon my tongue.
(3) The God of Israel said, The Rock of Israel spoke to me: 'Ruler over men shall be the righteous, even he that ruleth in the fear of God,
When men are moved to speak by the Spirit of God (such as David), those who hear shall have the light shine within them…as the light of the morning when the sun rises. The light shines out of the darkness within (the “evening and the morning”). When men have the light of His knowledge within, they are expected to respond to the light with light…with righteousness. Righteousness is supposed to spring up out of the heart after the “rain” of teaching.
(4) And as the light of the morning, when the sun riseth, a morning without clouds; when through clear shining after rain, the tender grass springeth out of the earth.'
The “expanse” of the hearing is called “Heaven” because what man hears is straight from Heaven.
Genesis 1:8 JPS
(8) And God called the firmament Heaven. And there was evening and there was morning, a second day.
The lights in the firmament (the expanded hearing ear) of the heaven are the commandments of God.
Genesis 1:14 JPS
(14) And God said: 'Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days and years;
The “greater light” is set in the hearing of heaven (Hear, O Israel….)
Deuteronomy 6:4-6 JPS
(4) HEAR, O ISRAEL: THE LORD OUR GOD, THE LORD IS ONE.
(5) And thou shalt love the LORD thy God with all thy heart, and with all thy soul, and with all thy might.
(6) And these words, which I command thee this day, shall be upon thy heart;
They are for “signs”:
Deuteronomy 6:8 JPS
(8) And thou shalt bind them for a sign upon thy hand, and they shall be for frontlets between thine eyes.
If we know the Rishonim had it wrong, how can we trust their other interpretationsThe Rishonim worked with the science of their times. The vast majority of what they wrote has nothing to do with science, so distrusting their science is no reason to distrust them.