The Planet Britannia is circled by the twin moons of Trammel and Felucca. Trammel needs 9 days for one complete orbit and Felucca 14 days. While barren and lifeless, they play a big part in the working of the blue Moongates -- the destinations of which are entirely dependent on the moons' phases.
Trammel is the bigger of the two moons. In the gate systems of Ultima III and Ultima IV, it is the phase of this moon that determines the gate that is currently open. In later Ultima installments, Trammel is the moon whose phase is the destination gate for the first half of the night.
Felucca is the smaller of the two moons. In the gate systems of Ultima III and IV, it is the phase of this moon that determines the gate that is the destination. In later installments, Felucca is the moon whose phase is the destination gate for the second half of the night.
The moons played a part in the story of Ultima IX.
It is interesting to note that the phases of Trammel and Felucca are the exact opposite of Earth's moon; while our moon's orbit is counter-clockwise, Trammel and Felucca both orbit Britannia in a clockwise orbit (also seen on the Orrery).
Appearance in the games
Considering that these two moons play a big role in the games, their graphic representation was once strangely lacking. In Ultima III, they were only represented with numbers, and in Ultima IV and Ultima V with small moon phase icons. The first "real" look at them was in the intro of Ultima VI, in the sacrifice scene. However, their phases were totally off, with both moons appearing as if rotated 90 degrees. In-game, the moon phase icons were again used. In Ultima VII, the only hint of them was the moon models on the Orrery in Moonglow.
In Ultima IX, the moons were shown in-game and in a cutscene; however, they didn't look alike. While in the cutscene, Trammel and Felucca where both white, in-game Trammel had a reddish hue.
Cycles during the Month
The cycles of the moon during the month are as follows:
Under the auspices of Lord British, studies of the properties of the Moon gates are beginning to yield a clearer understanding of the Gates' mystical workings. People forecast that future citizens will use these gates as a normal means of distant travel. The destinations of the gates appear to be rigidly bound to the phases of the twin moons Trammel and Felucca. The appearance and disappearance of the gates are represented on most maps of the realm as phases of the moons. The gate active is indicated by the phase of the moon Trammel. Once a gate is entered, thy destination is indicated by the phase of the moon Felucca.
In the heaven beyond our skies, dwell stars whose numbers are beyond our reckoning. Within our heaven, circling our world, sweeps our bright sun. Ten other orbs trace tight paths around our world. The twin moons Trammel and Felucca are desolate, their light only a reflection of the sun's brilliance.
There exists an extraordinary cosmic association between the gravity of the moons and the delicate fabric of time and space. As though pulling the strings of some cosmic marionette, the moons cause radiant moongates to appear on the land below.