I propose, just pure speculation mind you, that the world within the shards does not include the moons. That they are outside of the captured plane that the world within exists on.
If that were the case, then what we see in the sky is from the prime world, but perhaps changed. There would be some sort of etherial membrane between the shard world and the prime world outside, and that membrane may cause distortions. The time variations can be explained by this. So may the moongate's functions, as they are made of moonlight, and passing through this membrane that separates realities could have almost any affect, like light passing through a prism.
This is actually how I have chosen to look at the shards vs Sosaria Prime. The shards are simply portals to the other versions of Sosaria that exist in the same space as Sosaria Prime. (i.e. they frame drag along with the original world.)
The shattering of the Gem didn't create a multiverse, per se, just echoes of the planet. We (UO I mean) are the echoes.
The moons being outside them shards works for me, and since we already accept the wonkiness of time this is as good an explanation as any.
Re: Mars
Mars is roughly half the size of Earth and it's moons are considerably smaller than Luna - hence their ability to be speedy.
In Ultima IX, the only game to truly show the moons, they appeared analogous to Luna in size.
Now, we know that Sosaria was considerably larger before the shattering. Four entire continents disappeared. We have a few of them as facets now, but they aren't accessible from the water.
What if the other continents are still here, just the planet's dimensions folded in on themselves...thus making the planet smaller from the perspective of any outside observer.
This could account for two things:
1) How the moons are known to exist over the other facets.
2) How the moons are able to zoom around the planet at what would be an obscene speed for satellites of their probable size.
In essence, the moons are orbiting a very small planet whose perception of time is being warped by the dimensional folds.
Hmm, Martyna, that gives me a couple of thoughts. So the Shattering caused what we might think of as a "reverberation" that was captured in the Shards? Only instead of thinking sound, convert that thought into light, captured visions, or parts of, or distorted, visions of Sosaria. Reflections, distorted by the shard's size and shape, captured within.
The idea that the smaller worlds within shards cause the speed up of the moons is interesting. But that would mean that the moons are physically part of the Shard's plane of existence. So I like my "etherial membrane" time distortion theory better. But still, yours is worth considering.
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One final thought. The light of the moons create moongates. Could it be that this same power, reflecting the worlds as the prime world was shattered, are how each is created as a shard?
Prime world shatters-explosion of etherial nature, and the light of which travels to the moons, reflects back, and is captured by shards. The light sort of "settles" back where it came from, but since that point of origin is no longer what it was, but rather many shards of various sizes and shapes, gets distorted as it's captured. Thus are born many changed worlds of Sosaria, visions carried by the light of the moons, and captured by their changed origins in the shards.
This, in turn, makes me think of when Nystul created Trammel.
Nystul:
"I stood at the largest of the stone structures in Britain as I opened the tome and started. All over the land I could sense the other structures ignite with life and burst to the heavens above. Pure energy as I have never felt in all of my years of arcane studies tore through me; cradled me in its grasp. I pulsed with the magic, drew it into me. Then released it into the night sky. As the incantation neared completion, a moonstone fell from a pouch on my belt and landed on the ground. After a few seconds, the moonstone buried itself in the ground and an unusual looking moongate sprung into being in its place. I felt myself being drawn into the strange moongate. What occurred next is possibly beyond even the bards to put into words. The air split and shifted. In one breath I was staring into an endless torrent of light, the next I was looking over a pristine landscape. The world I had always dreamed of. Our world. With a silent roar of thunder my consciousness slid from my grasp."
Casting the Spell