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To quote:
"In Shinto cosmology, the square stands for the Sun God Amaterasu, the circle for the Moon God Tsukuyomi no Mikoto, and the triangle for the God of the Stars, Susanoo-no-Mikoto."
...
"The Sun (Amaterasu, represented by the square), symbolizes our inner light, and rules the day, the state of Divine Presence. When presence is absent, the moon gives light, and rules the night, the state of sleep , in which man spends most of his days. The moon (Tsukuyomi no Mikoto, represented by the circle) symbolizes our ruling faculty or the mind, which Gurdjieff called the steward. It tries to bring us back to the state of presence. Gurdjieff spoke about creating moon in oneself, which means creating a steward, or a faculty in oneself that keeps reminding one to not sleep, but to be present."
This would 'explain' why Susano'o is able to counter Tsukuyomi. Also of interest is that the symbol on Naruto's hand is that of the full moon.
Anyway - I'm not quite sure what to make of Jashinism. While it is possible that Jashinism is related to the Sage's brother (and I suspect that Jashinism will return to play a role, one way or another)... I doubt it has much to do with the Uzumaki.
The Uzumaki were, most likely, the original tenants of the God Tree. The inspiration for the Uzumaki likely came from a particular island chain known as the Izu islands, where an ancient tree-worshiping religion existed. Even without tree worshiping per se - Japanese mythology speaks of the Kodama - a tree spirit, of sorts, that is often vengeful toward anyone who takes of its produce.
The Uzumaki, from what little we have seen of their traditions, would appear to resemble the more shamanistic practices involving chants, rituals, sacred objects, etc. While Jashinism certainly has some rituals behind it... I suspect it and the Uzumaki are not the same.
Perhaps at best, Jashinism would be an off-shoot of Uzumaki who went from worshiping trees to worshiping Jashin.
It's an interesting idea, though.