The Gliss and Poseidon's "sun" are two sides of the same object (which I'll call the Gliss), and you can pass "into" the Gliss and come "out of" Poseidon's sun, like a black hole and a white hole. Poseidon's sun acts as a source of the Weave and the Gliss acts as a sink of the Weave. The Weave is a vector field similar to the electromagnetic field that imparts a force onto objects, and its corresponding scalar potential is a "magic potential" (maybe the party will name this something else). The Gliss and Poseidon's sun are situated such that they act as a magic charge / image charge pair that create an equi-magic-potential surface at the surface of Hestia. Poseidon's sun is not at the center of the whole structure (otherwise the Gliss would be at infinity to be the image of Poseidon's sun). When Poseidon's sun moves for whatever reason, the Gliss moves to be its image and preserve the equi-magic-potential surface.
The Weave is a very powerful force, and therefore it must stay at a relatively small level for anyone to be born with the capability for magic. This is the function of the equi-magic-potential surface — it places Hestia's local Weave at the right level for everyone to be born with magic and pursue it if they wish.