The Moon (月) is a celestial object revolving around Earth in the Dragon Ball universe just like its real life counterpart. Its significance is that it has caused some Saiyans to transform into their Great Ape form and has been destroyed two times and revived one time.
Role of the Moon in Dragon Ball series
Prior to Dragon Ball
21st World Tournament
22nd World Tournament
Man-Wolf, a fighter in the 22nd World Martial Arts Tournament, could not turn back to a human because Roshi destroyed the moon. Master Roshi used a Hypnosis Technique on Man-Wolf to convince him that Krillin's head is the moon. The Hypnosis Technique worked and Man-Wolf turned back to a human.
During the 23rd World Martial Arts Tournament, Goku mentions that Kami removed his tail during his training on Kami's Lookout so that the moon could be fixed. The moon is later seen, in Dragon Ball Z, indicating that it has indeed been restored.
Dragon Ball Z
Gohan first transformed into a Great Ape on seeing the moon during his training with Piccolo. To prevent it, Piccolo destroys the moon. After this, the Earth's moon was never shown again until the Trunks Saga.
Goku's Space pod projected the image of a moon which again made Gohan transform into a Great Ape. When Piccolo destroyed the Space pod the projector was destroyed.
Fake moon during the battle against the Saiyans
This happened when Vegeta made a Power Ball that transformed him into a Great Ape. Yajirobe cut off his tail so Vegeta could not transform anymore. Later, Gohan transformed after looking at the Power Ball. Vegeta cuts Gohan's tail with a Destructo Disk after a short struggle, but Gohan landed on him and Vegeta retreated.
In Dragon Ball Z episode 118, "Frieza's Counterattack", the moon can be seen while the Earth is briefly shown from King Cold's spaceship. The moon also appears in the sky in one scene of the movie Dragon Ball Z: Wrath of the Dragon. It is possible the moon was restored when it became clear that there would no longer be threats from evil Saiyans/Great Apes.
Dragon Ball GT
During Dragon Ball GT, Goku was fighting for the humans of Earth in a death-lock battle with the Tuffle King's descendant, Baby, on New Planet Plant. Although, the moon was not present, Goku was able to transform into a Golden Great Ape due the reflection of Blutz Waves by the Earth. Although it was the Earth that caused the transformation, all of the people present for the battle questioned where the moon was, including a brainwashed Bulma and Baby himself, giving an elusion to the moon whereabouts.
Moons of other Planets
Most moons or satellites of other planets have been shown when the planet was shown, and without much significance. However, the moons of Planet Vegeta and of Planet Kanassa were important, as Saiyans had used them to transform. The moon played a significant role during the Saiyan-Tuffle war. It allowed the Saiyans to transform into Great Apes and gave them the victory over the Tuffles after a 10 years war on planet Plant.
The sky of Arlia holds two moons. During the destruction of the planet, it shows one moon being destroyed with the planet.
Appearances in Video Games
The Tail&Moon card allows Saiyan characters such as Gohan to transform into Great Ape in the Gokuden games.
In Dragon Ball Z: Budokai Tenkaichi 3, the moon is seen in battle stages at night, allowing Saiyans with tail to transform into Great Apes. The moon can also be seen in the computer game Dragon Ball Online.
In Dragon Ball Heroes, it is possible to create a moon using special cards in order to transform into Great Apes or Golden Great Apes. The moon is shown in the sky when Gohan turns into a Great Ape in the story mode of Dragon Ball Heroes: Ultimate Mission.
- The moon is also destroyed in Akira Toriyama's previous manga series, Dr. Slump. Arale Norimaki breaks it by throwing a rock at it in chapter 23. It is shown repaired in chapter 27.