Does heat come from the sun or a different source? If so wouldn't this mean Aetherius is super hot?

Yes. Numerous books speak of the heat of the sun, such as Boethiah's Proving: "She came arrayed in ebony darker than a moonless night, wielding a blade burning hotter than the surface of the sun."

We've seen numerous trips to Aetherius that didn't result in the character being burnt to a crisp, so it's not all like that, but the portal the sun purportedly represents is certainly hot.

An extension on the initial query, if I may; What of the seasons?