By Aquiantus & Nigedo

Among the many sights offered within the Dwemeri Observatory on Stros M'Kai, there is what may be the only still functioning Dwemer Orrery in existence today.

The Dwemer race's great understanding of the planets is well documented and was developed through their detailed observations of the heavens using such excellent observatories. The Stros M'Kai Orrery presents a rare opportunity to learn about the Dwemeri model of the planets and their movements.

Stros M'Kai Orrery
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According to the 'Elder Scrolls Cosmology' there are eight other planets besides Nirn, the mortal planet. This is how mortals literally perceive the Eight immortals (et'Ada) who, according to myth, created the mortal planet in the Dawn Era.
"The eight planets correspond to the Eight Divines. They are all present on the Dwarven Orrery, along with the mortal planet, Nirn."
We also understand from the 'Cosmology' that the three Guardian Constellations are 'governed' by the Dominion Planets, "Akatosh (eye of the Warrior), Julianos (eye of the Sage), and Arkay (eye of the Thief)".

As they are perceived from Nirn, the constellations follow a predictable, annual pattern of rotation across the sky. This allows mortals to attribute fixed annual seasons to the constellations, when each occupies the area of the sky where the sun rises for the same period each year.

In order for mortals to perceive three of the planets as forming fixed points in the constellations, these planets must also follow the same predictable, annual path in the sky and appear to orbit Nirn. So we would reasonably expect the Dwemer to have modelled a Nirn-centric planetary system, which the Stros M'Kai Orrery confirms.

Dominion Planets

Since the three Dominion Planets do maintain fixed positions within the Guardian Constellations and relative to each other, we would also expect the three Orrery spheres that represent them to have constant relative positions throughout their orbits.
Examining the Orrery more closely (see the diagram to the right), we can see that the three spheres p1, p2 and p3 are the only ones that maintain constant relative positions throughout their orbits and they must represent the Dominion Planets.

These are the three spheres that are connected by the triangular, dragon-like frame of the Orrery and it seems likely that the Dragon's head, p2, represents Akatosh, the Dragon Planet.

Comparing the Orrery with the Observatory constellation chart, we can see that, of these three, the sphere p1, with the smallest orbit and closest position to Nirn (p0), is likely to be Arkay, the Dominion Planet of the Thief constellation, which is mapped in the lowest latitude (latitude 1).
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Julianos, Dominion Planet of the Mage constellation which the constellation chart places in the middle latitude, is therefore likely to be the sphere at the Dragon's tail, p3.

Planetary Orbits

As we have seen, the three Dominion Planet spheres have synchronous orbits. They also share the same curious wobble effect in their orbital plane where the common axis of their orbits shifts several degrees from the true Nirn axis, X, to the tilted position, Y, and back again. This wobble occurs twice during each complete orbit, each time causing the Dragon Planet (p2) to narrowly avoid colliding with the sphere p6.

Of the five remaining planets represented on the Orrery, only two, p5 and p6, orbit Nirn directly. The other three orbit larger planets. These might be thought of as moons, but the 'Elder Scrolls Cosmology' points out that moons are not (significant enough to be) represented on the Dwemer Orrery.

Nevertheless, the 'Cosmology' explains that smaller planets orbiting larger ones can be regarded as "minor gods", attendant to their god planets. This might raise questions about which of the Eight Divines could be regarded as 'attendants' to which of the others.

The orbit of sphere p5 appears quite complicated due to the rotation of its tilted orbital plane. The angle of tilt is constant, but the sphere's orbital plane performs one full rotation, about Nirn's true axis, for every complete cycle of the Orrery. The sphere itself performs four complete orbits of Nirn in the same time.

It is also worth noting that sphere p6 is the only one that directly orbits Nirn in the opposite direction to the others. Whether this observation, or any made here, could hold deeper significance are matters for wider debate, but the Orrery certainly stands as a further testament to the ingenuity of the Dwemer race.