Moon

The Moon is giant rocky white sphere that orbits the world of One Piece. It was often seen during night scenes. The moon was labeled Fairy Vearth by Enel. It appears throughout the series, but the first time the moon was seen was Chapter 28 when Kuro use it as a signal to carry out his three years plan. The moon first appeared to look red/orange fancy cartoonish crescent, after that the moon begins to be hard sketched.

On Chapter 392, a Geocentric model of the One Piece world with 6 satellites (which includes their world's moon) can be seen at Ohara's Tree of Knowledge.

The moon seems to be considerable more accessible in One Piece world. Enel managed to go there using the Maxim, and the four Spaceys managed to go there traveling in balloons.

Ancient civilization
There is an ancient city on the moon named Birka where technology and architecture were highly advanced. From paintings on the walls we know some of its history: it was home to the ancestors of Skypieans, Shandorians, and Bilkans, but one day they left Birka and headed to the main planet. They left on the moon their creations - the robot-type creatures named Spaceys. Later, Enel arrived at the ruins of the ancient city, recharged the Spaceys, and decided to settle here.

Fairy Vearth
Current sky peoples appear to "forget" their origins, although the moon, called the "Fairy Vearth" and believed to be a place of dreams, remained a folk tale amongst the Bilkans.

Moons Progression

 * Chapter 28 - Waxing Crescent (moon stage noted in title of chapter)
 * Chapter 95 - First Quarter
 * Chapter 107 - Waxing Gibbous
 * Chapter 152 - Full Moon (Chapters 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)
 * Chapter 514 - Waning Crescent

One can use the moon's progression of time as a indication for the Straw Hats journey. However, this makes it from the Kuro's attack on Merry in the Kuro arc to Luffy's arrival on Amazon Lily just 21 days. Considering the progression of time has often been argued in One Piece, this seems hard to accept considering at least two weeks passed between the events of Water 7 and the Red Line. The moon seen when Luffy is flying through the air can explained by one of three methods:
 * The phrases of the moon simply do not follow the real worlds own phases, meaning that the lengths are much greater. However the only problem with this particular theory is from Whiskey peak to Drum, the timespan mention in the storyline matches perfectly the real worlds own moon phrases.
 * The Waning Crescent seen in Chapter 514 is actually a southern hemisphere Waxing moon (in the southern hemisphere, phases of the moon are reversed).
 * The moon was on its second cycle (or more), making it 42 days a much closer match to the minimum estimated canon storyline timespan.

Trivia

 * Luffy and his group are in the Northern Hemisphere of their world according to the phases of the moon.