Moon

The Moon is natural white satellite in Sonic the Hedgehog games and the only satellite of the Earth, the main setting of games. All the time, it is rotates around the earth along with the sun, making it the only glowing thing in the sky during the night time on earth. It isn't shown to have much role at first in video games as only being featured at the background of several zones until it became notorious, minor plot element in Sonic Adventure 2 and even becoming setting of titular extra zone in Sonic Advance.

Game Appearances
The moon is first time seen briefly in the background of Chrome Gadget, which is third zone at the competition mode of Sonic the Hedgehog 3. The moon is also seen at the night background of all race tracks in Sonic R.

The moon hasn't been featured until in Sonic Adventure 2, where Doctor Eggman have gotten six of seven Chaos Emeralds being set on to the super weapon of Space Colony ARK called Eclipse Cannon at the mid-point of the game. Eggman then manages to get a signal to all networks all around the world, giving threat declare and showing the power of Space Colony. He then activates the yet not fully loaded Eclipse Cannon, firing its large, energy wave to the moon, which shatters the moon half. This leads the entire mankind looking the blast at the sky, while Eggman has given less than one day time before the next strike, this time earth being his target. Along with all humans, Sonic, Tails and Amy notices this event as well, making them wonder where Eggman have fire all the energy from.

The moon is next seen full again (see the next section) in Sonic Advance, where it can be seen right corner at the background of X-Zone after destroying Eggaman's last vehicle. Eventually it is shown after the battle, that Eggman is escaping to the moon. Sonic then transforms to Super Sonic and goes after Eggman, who yet got one card on his sleeve. The moon becomes the setting to titular extra zone in the game, where only Sonic only can enter after gaining all seven Chaos Emeralds from Special Stages. After that the battle and Eggman getting defeated, Super Sonic travels back to the earth a few days later, while Tails notices him during his flight of Tornado.

The moon is then seen once again at the background of Hang Castle in Sonic Heroes, small and huge one at Chaos Angel in Sonic Advance 3 and at the beginning of Sonic's story and the ending of Sonic the Hedgehog (2006) during the ceremony of Soleanna. In Sonic Unleashed, it is shown that, when the sun goes down and the moon rises up, Sonic later than that transforms to the Werehog. Similarly the moon is shown the same way at one scene in Sonic: Night of the Werehog. In the near end of Sonic Colors, when Eggman starts firing his giant Mind Control Ray to the earth, the cannon manages to get destroyed due the energy leakage, which begins from the beginning of the game. As many mind control rays starts flying headlong around the space as one of the rays makes the impact to the moon. The moon is once again seen at the background during the rematch on Space Colony ARK in Sonic Generations.

Inconsistency
During Sonic Adventure 2, Doctor Eggman is seen blowing the moon half in by using the Eclipse Cannon, but it is however shown to be full again at night backgrounds of levels in later games. It has lead to common confusion in community, whatever happened to the moon in between two games. One theory has been, that half blowed part of the moon has kept hidden while cleaner part of the moon has been from the earth at the time. No good explanation has never given for this, although reason could be that the twist event from Sonic Adventure 2 haven't been then known or remember precisely afterwards and has got its full form again

Sonic Advance 3 also shows that Sonic's world also has got another larger moon at the background of Chaos Angel, which is yet replicated later on the background of Crimson Crater race track in Sonic Riders: Zero Gravity. It is implied that, it would be Little Planet or another planet. Like with the case of half moon, there haven't been no good explanation has never given for this.

Trivia

 * It is speculated, that one mind control ray that impacted to the moon in Sonic Colors might possibly had led to the birth of Time Eater on its original form, since both Sonic Colors and Sonic Generations share heavy continuity connection and Eggman seem to found this being very quickly from outer-space after the destruction of his interstellar amusement park.