The Eclipse Cannon (エクリプスキャノン,Ekuripusu Kyanon?) is the ultimate weapon of mass destruction on the Space Colony ARK created by Professor Gerald Robotnik, able to destroy planets and pierce stars. The Chaos Emeralds are needed to power the cannon, as it takes large amounts of energy to power it. The cannon can be seen on the outside of the ARK as it forms the "nose" of the research center.
Despite its frightening power, the intended purpose of the Eclipse Cannon was to destroy the Black Comet in order to save the Earth from being overrun by the evil alien forces residing there. Around fifty years later, in Sonic Adventure 2, the Eclipse Cannon was used by Dr. Eggman in an attempt to assume control over the world by threatening to fire it at the planet, but the Eclipse Cannon was stopped due to the efforts of Sonic the Hedgehog and his friends, and the late Gerald Robotnik's doomsday program. In Shadow the Hedgehog, the Eclipse Cannon finally carried out its purpose when Shadow the Hedgehog used it to destroy the Black Comet.
Using only five Emeralds, it is capable of destroying a large city and producing a large crater in a planet; with six Emeralds, it is able to destroy half of a moon. However, with either case, the cannon would require twenty-four hours to recharge due to the large energy consumption.
Sonic Adventure 2
In Sonic Adventure 2, the Eclipse Cannon was first introduced to Eggman by Shadow to grant his one wish of world conquest. Shadow explained that all seven Chaos Emeralds were needed to fully power the weapon. The two of them, with the government spy Rouge, who was sent there to gather information about Shadow, gathered a total of six Emeralds (one Eggman already had, one Rouge offered to join up, the one that Shadow stole, and three stolen from Prison Island). Eggman then used the Cannon to destroy half of the moon to terrorize Earth into submission. However the demonstration proved to have flaws, such as the cannon requiring a full day to recharge due to not possessing all of the Chaos Emeralds. Eggman also made a video conference call with the President, where he supplied the latter with the ultimatum of either surrendering the United Federation or else he will ensure that every single major city in the United Federation, if not the country itself, will be wiped out by the Eclipse Cannon. He also planned on giving the President twenty-four hours to decide whether to submit to Eggman, although Sonic's arrival at the President's limousine a second after this announcement, as well as Sonic bluntly refusing, forced Eggman to withdraw the ultimatum. With the cannon requiring twenty-four hours to charge due to not having all the Emeralds, Sonic and his friends were able to penetrate the ARK and stop the Cannon before it fired again.
Eggman, who was apparently unaware of what happened, installed the last Emerald he took from Tails. A hologram of Eggman's grandfather Gerald Robotnik appeared, which revealed that he secretly downloaded a doomsday device that activated when the seven Chaos Emeralds were brought together. With the immense energy being moved to the Cannon's Core, the ARK was sent on a collision course to Earth. When Knuckles and Sonic bring the Master Emerald to the core, it turns out to be a shrine similar to the one on Angel Island, designed to maximize the emeralds' power. Shadow defeated the Biolizard, and they managed to control the Chaos Emeralds. Despite this, the ARK was still crashing to Earth. Eggman revealed that the prototype had used Chaos Control to attach itself to the exterior of the ARK, pulling it towards the planet.
Shadow and Sonic used the Chaos Emeralds to transform into their Super States. They fought and defeated the "Finalhazard," as it fell to Earth. Though, in order to stop the ARK from falling, the two used Chaos Control to bring it away from the planet's atmosphere. Due to exhausting his powers by teleporting the space colony, Shadow crashed to Earth. Sonic returned to the control room with his ring and told everyone what happened. As they all left the ARK, Sonic looked back and said, "Sayonara, Shadow the Hedgehog.". At this point, Sonic believed this was Shadow's death.
Shadow the Hedgehog
In Shadow the Hedgehog, after the events of Sonic Adventure 2, the Eclipse Cannon was apparently repaired by G.U.N (Presumably, they also removed the Gerald Robotnik doomsday program from the Eclipse Cannon's main control system so as to not allow history to repeat itself, as evidenced by Shadow later using the seven Chaos Emeralds to destroy the Black Comet shortly after teleporting it out of Earth's range). The military also deployed robots to guard the control room, including Gun Beetles and BigFoots. During a possible Dark Side storyline, the Eclipse Cannon and the ARK were later to fall under control of Black Doom and the Black Arms (with the assistance of Shadow), who wanted to use the cannon to take control of Earth. With five Emeralds, the cannon destroyed the White House and Central City as a demonstration to Black Doom's ultimatum of exterminating the world leaders who refuse to submit to the Black Arms. During the Last Story when Shadow was fighting Black Doom, a hologram of Gerald revealed to Shadow that the Cannon was developed to destroy the Black Comet. The Cannon's purpose was fulfilled after Black Doom was defeated by Super Shadow. Shadow used Chaos Control to teleport the comet into outer space and fired the Cannon to destroy the comet once and for all.
Even though the ARK was shut down, G.U.N. soldiers were still guarding the room of the Eclipse Cannon as seen in Shadow the Hedgehog in the fight against Blue Falcon.
The color of the beam fired from the Eclipse Cannon varies in color in the games and other media. In Sonic Adventure 2 and Sonic X, the cannon's beam was blue, while in Shadow the Hedgehog, the cannon's beam was changed into green.
When Shadow introduced the Eclipse Cannon in in the Japanese version of Sonic Adventure 2, he identified it as a weapon capable of piercing stars (a trait later mentioned in Shadow the Hedgehog). In the English version though, he identified it as a weapon capable of destroying an entire planet.
Curiously, the cannon's use on the moon is never referenced again in any games or other media. The moon appears unscathed whenever it does appear in the games, such as Sonic Heroes and Shadow the Hedgehog, and it is assumed that the damage the moon took had no effect on its orbit around Earth, the tides, etc. At the 2013 Sonic Boom event Q&A, when asked about the moon's unscathed status, Takashi Iizuka explained that the dark side of the moon was blown up and remains so, even when it's unseen in the games.
The Eclipse Cannon destroying the White House with its blast in Shadow the Hedgehog might be a possible reference to the movie Independence Day.