That's the thing. I've always assumed so because they're in a pair.
The FFWiki and
this website conflate them with the months... I don't have Encyclopaedia Eorzea to corroborate :x but at face-value, and borrowing "Astral" as the excited state of the element vs a passive "umbral"...
Azeyma is Astral bc she's the goddess of the Sun and Justice, so she's the goddess that brings light. Whereas the twins are Umbral since they're the god of Life\Death, the lighting of pyres and stuff like that.
Byregot is the builder who uses his power to power up machines and cause progress, whereas Rhalgr is the destroyer who in a moment reduces all to inactivity.
Same for Nophica who is Astral because she is the goddess of the harvest and growth, while Althyk represents the slow crawl of time and the eventual death of things.
I've actually wanted to work on a program for roleplaying where I developped the Deck of Sixty more than what EE says on it (there's a page for it online :x). Especially since I have experience with stuff like tarot cards and runes. That way people could draw cards, read their meanings and read someone's fortune. Y'know, bc we don't have a feature like that in the game |D
And I envisioned doing it like that. With the upright meanings of the Major Arcana being the Astral gods, and the reversed ones being the Umbral gods.
I just haven't bc a) roleplaying in FF14 is kind of dead, and b) I can't make an alt on Balmung.