And indeed, where is it?

source of quote above: the Imperial Library

All the Aedric planets are named after their attendant deity. So there is a planet Mara, a Dibella and so forth. Usually in TES we call these planets plane(t)s, because they double as a physical planet and a spiritual plane.

In a very real sense there is no difference between Akatosh the plane(t) and Akatosh the deity.

These plane(t)s are infinite in nature and the true form of the deity and only appear to mortal eyes as finite planets due to sensory stress, the limits of mortal consciousness unable to comprehend such concepts and limited mortal senses translating the image of the divine into something that makes sense to it.

When Mannimarco achieved godhood he became a celestial body, a moon in orbit of Arkay, the celestial mundus and the divine are very much the same thing.

The http://www.uesp.net/wiki/Oblivion:Orrery from Oblivion shows that Nirn is at the heart of a geocentric universe, all things orbit the planet Nirn, including the sun, which is a hole to Aetherius from which magic pours into the world.

Creation is called the Mundus and it is made up of the planet Nirn on which Tamriel lies and its attendant celestial bodies. So Akatosh orbits Nirn.

The TES universe is called the Aurbis and it consists of Aetherius, the plane of magic, Oblivion, the planes of the Deadra and the Mundus, all as concentric circles with Tamriel at the very centre.

The Temple Zero tract http://www.imperial-library.info/content/cosmology provides further insight.

Merari's right - Akatosh is his planet, so it is simply called Akatosh.http://www.gamer.ru/system/attached_images/images/000/582/712/original/cosmology_of_nirn.jpg?1354730182pretty good model of the structure of Mundus.

so the simple answer would be that his plane(t) would be named either Akatosh or Auriel? But which I wonder

makes me wonder if the Jills are moons or asteroids

never looked at the orrery as I was stuck in Morrowind and never visited Cyrodiil

The elves would call the planet Auri-El and the Empire would call it Akatosh.

Makes me wonder how that applies to the moons. To some, it's Lorkhan('s plane(t)). But to the Aldmer and Khajiit, the moons are actually something completely different; Masser and Secunda are two other gods, Jone and Jode, not Lorkhan split in two.

Ah, interesting because I have no idea how to answer that.

A new idea to mull over, yay

At first glance I would suggest the moons are both, at the same time, depending on the viewpoint of the onlooker.

The way I look at it, the body of a god is not that god. Without his divine spark, Lorkhan ceased to inhabit that body. Jone and Jode are probably akin to Lorkhan in the way that a reanimated corpse is kin to the person who died to leave the corpse behind. It's not really him anymore; his Heart-divinity is now trapped on Nirn, which in many respects can be considered his new body (made up of other gods' parts like a divine Frankenstein). So the moons might well have been possessed by other divinities now that Lorkhan is absent from them.

Either that, or The Lunar Lorkhan and the Khajiiti understanding of the universe are mutually exclusive. I keep forgetting that The Lunar Lorkhan is still, technically, just a theory...

Smaller question: What is the connection between Mara, Dibella, and Zenithar? Considering the Nordic pantheon, I would have expected to see Mara orbiting Kynareth, or at least nearer to it, as opposed to orbiting a seemingly-unrelated body like Zenithar on the opposite side of the known universe.