The Moon is a giant celestial body, a rock-composed sphere, that orbits the world. For the world's inhabitants, it is typically the main (if not only) source of natural light at nighttime.
The moon appears throughout the series, but the first time the moon was seen was when Zoro fought Kuina in his childhood flashback. Later Kuro used the moon as a signal to carry out his three-year plan; the sight of a crescent moon makes him uncontrollably murderous. During the sequences that influence Kuro, the moon was shown to be a red-orange crescent. In subsequent appearances the moon begins to be hard-sketched.
During Nico Robin's flashback of Ohara, a model of the world with seven satellites (which includes their world's moon) can be seen inside the library of the Tree of Knowledge. The model was destroyed when the Buster Call attack caused the tree to collapse.
During nights in which the full moon is visible, members of the Mink Tribe can look at it to transform into their Sulong form, which they can maintain as long as their sight to the moon remains unobstructed.
There is an ancient city on the moon named Birka (ビルカ Biruka?), where technology and architecture were highly advanced. The city's name is the same as Birka, a Sky Island far to the southeast of Skypiea.
From paintings on the walls, some of the city's history is revealed: it was home to "Moon People" (月の人 Tsuki no Hito?, Viz: "Mooninites")—the ancestors of Skypieans, Shandorians, and Birkans, but one day they left Birka and headed to the main planet due to lack of resources on their homeland. They left their creations on the moon: the robot-like creatures named automata. It is unknown when their emigration took place, but it was at least 1,100 years ago, since it was at that time that the city of Shandora was prosperous.
The descendants of the citizens who left the moon appear to have forgotten their origins, although the moon, called "Fairy Vearth" and believed to be a place of dreams, remains a folk tale amongst the Birkans. Enel, after studying history within the Upper Yard, came to the belief that the moon is the legendary land that he sought: an endless land of sacred dirt, also known as "Vearth". This is the reason that he constructed the Ark Maxim.
The city, which was in an underground basement within the moon's surface, has been uninhabited for a long time, and has long since fell into ruin. Despite this, the city retains its advanced look, with the automata being in capsules with inactive electric generators and machinery. The Space Pirates had been digging into the ruins while also warring with the four automata that were created by Dr. Tsukimi.
During his adventures on the moon, Enel eventually finds the ruins of Birka after defeating the Space Pirates and gaining the loyalty of the four automata he had rescued from certain death. After reading the murals on the walls, he recharged the ancient automata and decided to settle there.
During the early chapters, it was possible to use the moon to create an estimated time span for Luffy's adventures, presuming the One Piece world's time span is equal to that of Earth's. In some cases, the phase of the moon even gave the chapter its title. Over time, however, this idea seemed to have been retired, and it is no longer possible to use the moon as a time scale. The full moon later plays a part as the minks' Sulong form was introduced.
- Chapter 28 - waxing crescent (moon stage noted in chapter's title)
- Chapter 95 - first quarter
- Chapter 107 - waxing gibbous
- Chapter 152 - full moon (moon stage noted in chapter's title)
- Chapter 160 - waning gibbous (still nearly full)
- Chapter 162 - waning gibbous
- Chapter 213 - waning gibbous (heading onto half-moon)
- Chapter 232 - half moon
- Chapter 253 - half moon
- Chapter 334 - half moon (heading to last quarter)
- Chapter 811 - full moon
- Chapter 888 - full moon
- Chapter 973 - waning crescent
- Chapter 987 - full moon
- The ancient lunar city, Birka, may have been named after the real-life Birka, a Viking-age trading center in Sweden.
- The minks's Sulong transformation is similar to the myth about Werewolves, who would also transform from humans into a ferocious bipedal animal (wolves) under the light of the full moon.