Moon

The moon (月) is giant celestial body, a rock-composed sphere, that orbits the world. It is often seen during night scenes.

Details
The moon appears throughout the series, but the first time the moon was seen when Kuro used it as a signal to carry out his three years 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 six 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.

Traveling to the moon seems to be considerably easier than in real life, as Enel managed to go there using the Ark Maxim, and the four spaceys managed to go there traveling in balloons.

Ancient Civilization
There is an ancient city on the moon named Birka (ビルカ), where technology and architecture were highly advanced. The city's name is 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 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 spaceys.

The descendants of the citizens who left the moon, who came to be citizens of the sky islands, appear to have forgotten their origins, although the moon, called "Fairy Vearth" and believed to be a place of dreams, remained 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 the 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, has been uninhabited for a long time, and has turned ruins. Despite this, the city remains quite advanced looking, leaving the Spaceys in capsules with inactive electric generators and machinery. The Space Pirates have been digging in the ruins while colliding with the four Spaceys that were created by Dr. Tsukimi.

During his adventures on the moon, after defeating the Space Pirates and gaining the loyalty of the four spaceys, Enel eventually finds the ruins of Birka, and after reading the murals on the walls, recharges the ancient spaceys, and decides to settle here.

Moons Progressions
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 phrase 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.


 * 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
 * Chapter 213 - waning gibbous (heading onto half-moon)
 * Chapter 232 - half moon
 * Chapter 253 - half moon
 * Chapter 334 - half moon (heading to last quarter)

Trivia

 * Luffy and his group begin the story in the equivalent of the northern hemisphere of their world according to the phases of earth's moon. However, phases of the moon are relative to where the sun is positioned in relation to the planet, meaning that this probably just means that the sun and moon follow different orbits in the One Piece world.
 * The ancient lunar city and sky island may have been named after the real-life Birka, a Viking-age trading center in Sweden.