The Moon is a natural white satellite in Sonic the Hedgehog games and the only satellite of the Earth, the main setting of the games. All the time, it rotates around the earth along with the sun, making it the only glowing thing in the sky during the night time on earth along with the stars. It was not shown to have much of a role in Sonic games at first, only being featured in the background of several zones until it became a notorious, minor plot element in Sonic Adventure 2 and even becoming a setting of the titular extra zone in Sonic Advance.
The moon is first time seen at the night background of Special Stages in 8-bit version of Sonic the Hedgehog, where it is shown to be yellow crescent. It is later seen briefly in the background of Chrome Gadget Zone, 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 appeared once again in Sonic Adventure 2, where Doctor Eggman has gotten six of the seven Chaos Emeralds and set them on to the super weapon of Space Colony ARK called the 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 not yet 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's 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 Eggman'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 defeating Eggman once again, Super Sonic travels back to the earth a few days later, while Tails notices him during his flight in the Tornado.
The moon is then seen once again at the background of Hang Castle in Sonic Heroes, two moons are seen in Chaos Angel in Sonic Advance 3, in the intro of Shadow the Hedgehog and at the ending of Sonic the Hedgehog (2006) during the festival 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 then shown again fully at the background during the rematch on Space Colony ARK in Sonic Generations. The moon can be recognized on the night background of Silent Forest Zone 2 in Wii U version of Sonic Lost World.
During Sonic Adventure 2, Doctor Eggman is seen blowing the moon half by using the Eclipse Cannon, but it is however shown to be full again in later games. It has led to common confusion in community, leaving questions and theories about whatever happened to the moon in between two games. One popular theory suggests that the half-blown up part of the moon was kept hidden while remainder faces the planet's rotation, thus being the visible half. Another potential reason could be that the twist event from Sonic Adventure 2 hasn't been recalled by the current creators due the different developer teams in later installments. At Sonic Boom 2013, Takashi Iizuka confirmed the former explanation.
Sonic Advance 3 depicted the moon with a second, smaller one at the background of Chaos Angel, which is yet replicated later on the background of Crimson Crater race track in Sonic Riders: Zero Gravity. As Little Planet is also in an irregular orbit, it could be that or a similar planetoid.