Moons

The planet Britannia is orbited by two moons, Trammel and Felucca. They play a significant role in the course of the Ultima series, notably in the mechanics of blue moongates, magic portals whose functioning is typically determined in some fashion by interactions between the phases of each moon and the eight moonstones. Trammel and Felucca appear or are referenced in Ultima III: Exodus, IV, V, VI, VII, IX, and Ultima Online.

Description
Trammel and Felucca are both thought to be devoid of life and, according to the Moonglow observatory's orrery in Ultima VII: The Black Gate, follow a clockwise orbit around Britannia (unlike Earth's moon, which orbits anticlockwise). However, each moon is distinct both visually and in the influence it has exerted upon the planet throughout history.

Trammel
Trammel is the larger of the two moons as observed on Britannia, with a synodic period (one complete phase cycle) of 9 days. In Ultima III: Exodus and Ultima IV: Quest of the Avatar, it is the phase of this moon that determines which moongate is currently open. In later Ultima installments beginning with Ultima V: Warriors of Destiny, Trammel's phase determines the destination gate for the first half of the night, until and including when both moons are at equal altitude in the sky.

Trammel is generally depicted as white and grey throughout the series. However, in the introduction sequence of Ultima VI: The False Prophet it exhibits a slight blue tint, while during gameplay in Ultima IX: Ascension it appears reddish-brown.

Felucca
Felucca is the smaller of the two moons as observed on Britannia, with a synodic period of 14 days. In the moongate systems of and, it is the phase of this moon that determines a gate's destination. In later installments beginning with Ultima V, Felucca's phase determines the destination gate for the second half of the night, as soon as both moons are no longer at equal altitude in the sky.

Felucca most frequently appears white and grey throughout the series, save for gameplay in, where it is cyan.

Appearances
Although not in the game itself, which takes place on Earth, the first appearance of the moons is on the rear cover of the Ultima II: The Revenge of the Enchantress box, the artwork for which was later reused both for Escape from Mt. Drash and the Sierra On-Line re-release of Ultima I: The First Age of Darkness.

In Ultima III: Exodus, Trammel and Felucca are only represented with numbers, and in Ultima IV: Quest of the Avatar and Ultima V: Warriors of Destiny with small moon phase icons. Their first detailed in-game appearance comes in the introduction sequence of Ultima VI: The False Prophet, during the gargoyles' attempted sacrifice of the Avatar; here, they are shown as wet moons. Throughout gameplay, the traditional moon phase icons make their final appearance.

In Ultima VII: The Black Gate, Trammel and Felucca are only visually represented by their models mounted upon Moonglow's orrery, and play no part in the operation of moongates.

In Ultima IX: Ascension, the moons are shown in-game and in one cutscene. However, there are discrepancies between these appearances: in the cutscene, Trammel and Felucca are both white, as opposed to the aforementioned hues observed during gameplay. They are of particular importance in the events of the game, gradually being pulled from their orbits by the Guardian's columns and threatening to collide with Britannia in what would be an unprecedented catastrophe. Although this is averted, the moons again do not influence moongates in any traditional sense; rather, the gates are operated by the Summon Moongate spell in conjunction with an Orb of the Moons and spoken mantra.

Outside the main series, one of the moons appears in the original introduction sequence of Ultima Online as the Stranger approaches Mondain's stronghold. Trammel and Felucca also lend their names to an official Ultima Online server's two parallel "facets", which, first introduced in the Renaissance expansion, are each governed by different rules concerning player-to-player combat.

Synodic periods
The cycle of phases for each moon over a calendar month are as follows:

Trivia

 * The cloth map of Sosaria included with Ultima III: Exodus indicates Trammel's synodic period as 48 days and Felucca's 16.
 * In Drennal's The Light Until Dawn, a study of Trammel and Felucca found in Ultima VII: The Black Gate, the author speculates as to the possibility of extraterrestrial civilizations dwelling on the twin moons.
 * Both moons can be seen in a cutscene that was ultimately removed from Ultima IX: Ascension, where they appear low in the sky over the environs of the Shrine of Justice.