We all know what the equator line on Earth is. Basically, every planet must have one since every sphere can be divided in half.
What about Nirn? is there any source that talk about this?
Is tamriel in the North or South hemisphere? Or maybe both and the "equator" line is maybe at the height of Cyrodill?
Don′t forget that Pyandonea is the southern continent, smaller than Tamriel but it is said that Pyandonea is tropical so it can't be that far in the south hemisphere.

I never have seen anything about its equator but I would have to guess that it is near Pyandonea. Aldmeris is undoubtedly in the Southern Hemisphere.

Nirn is more of a 'plane' than a planet. I'm not sure the usual 'rules' apply. If it's a plane with the sky being some kind of 'blanket' that covers the world, then there may not be an equator at all.

Magic can alter temperature. So maybe somewhere like Skyrim lacks/ is in abundance of magic?