Originally Posted by Adrianna Danere

The real Britannia (formerly known as Sosaria) is the planet where most Ultima games happen. Felucca and Trammel are the two moons which orbit that planet.

In UO things are slightly different though, because the shards aren't the real Britannia. Shards have facets, which essentially are alternate dimensions/realities. When the initial new facet was created, it was just dubbed Trammel and the old land dubbed Felucca after the two moons.

It's unclear whether or not the shards actually have Trammel and Felucca (the moons) though, since it's only said that each shard holds a likeness of Britannia... no mention of any nearby stars, moons or planets.