Earth

Earth is the main planet in the series and third planet from the Sun. It is also referred to as the Earth, Planet Earth, the World, and Terra. It is the setting of all canonical Sonic games according to the original Japanese continuity. According to the old American continuity however, all games prior to Sonic Adventure were set on the fictional planet Mobius.

Home to millions of species, including humans which now has a population of over 6.8 billion as of 2008, Earth is the birth place of several principal characters. Scientific evidence indicates that the planet formed atb the same time as Mobius and they were one planet however split at an unnamed point seperating into different dimensions also.Since then, Earth's biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks harmful radiation, permitting life on land.

Earth's outer surface is divided into several rigid segments, or tectonic plates, that gradually migrate across the surface over periods of many hundreds of years. About 71% of the surface is covered with salt-water oceans, the remainder consisting of continents and islands; liquid water, necessary for all known life, is not known to exist on any other planet's surface. Earth's interior remains active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.

Earth interacts with other objects in outer space, including the Sun and the Moon. At present, Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis. This length of time is a sidereal year, which is equal to 365.26 days. The Earth's axis of rotation is tilted 23.4° away from the perpendicular to its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year. Earth's only known natural satellite, the Moon, provides ocean tides, stabilizes the axial tilt and gradually slows the planet's rotation. A cometary bombardment during the early history of the planet played a role in the formation of the oceans. Later, asteroid impacts caused significant changes to the surface environment.

Concept
Sonic the Hedgehog was created as a mascot for Sega, replacing its former mascot Alex Kidd. In order to maximize the appeal of the new character, his background (and the planet on which he lived) was given no detailed information in any of the initial games of the series (from the first game through to Sonic 3D Blast). This allowed the separate branches of the company (Sega of Japan, Sega of America and Sega of Europe) to each create its own stories to fit third-party sponsors such as comics and TV series.

Whereas Sega of Japan considered the setting of the games to be Earth, the English-speaking versions of the games considered the setting to be a fictional planet. The name Mobius was adopted as canon by Western cartoons and comics. However, since the release of Sonic Adventure, which was obviously set on Earth and featured more human interaction than previous games, the Japanese backstory has risen to prominence in the rest of the world as well. The name "Mobius" is sometimes believed to have originated from a mistranslation of a 1992 Yuji Naka interview, but Sega of America had been using the name since at least 1991.

Overview
Humans, regular Animals, Plants and so called "Mobians" live on Earth. Mobians is the term sometimes used by fans of the series to describe Sonic the Hedgehog and other humanoid animals as they hail from Mobius in several other continuities such as comics and cartoons (According to Sonic X they are aliens from an unknown planet that if often called "Mobius" by fans). Humans and Mobians co-exist peacefully with each other and are affirmated with each other. Examples of this include Rouge the Bat and Shadow the Hedgehog both being part of the human military force G.U.N.

In the games, it seems like most of the Planet is under the rule of one single, giant country; The United Federation. In Central City lies the White House with the the President as the head of state. It's main military is the Guardian Units of Nations (G.U.N) which prove as a brutal "shoot now ask questions later" organisation that aim for good to triumph but are often clouded in judgement). Earth is very advanced and futuristic, robots and super technology are commonplace especially to G.U.N who create weapons primaraly to combat Dr. Eggman. Full future cities are seen in Sonic Heroes, Sonic Riders and Sonic Riders: Zero Gravity holding robots, flying cars, Extreme Gear and tubes similar to those in Futurama.

History
Over 4000 years ago the sentient weapon Gizoid was created and destroyed the "Fourth Great Civilisation". About 3000 years ago there existed a Maya-like civilization of Echidnas called the "Knuckles tribe",. This civilisation was destroyed by the the water monster Chaos. The alien Black Doom seems to have been observing the planet and warped the Glyphic Canyon to it over 2000 years ago.

50 years ago aboard Space Colony ARK, Professor Gerald Robotnik was forced by the President of that time to create the Ultimate Life Form, he was aided by Black Doom and they created Shadow the Hedgehog. Gerald also studied the Gizoid, Chaos and the Chaos Emeralds. G.U.N invaded ARK and either imprisoned or killed all those related with Project: Shadow. Gerald's granddaughter Maria Robotnik who had become very close to Shadow was killed and Shadow was sent to Earth. Gerald was later executed on Prison Island.

Recently in the planet's history, it has been shocked by the monster Chaos, threatened with destruction by the falling ARK, invaded by Black Doom and his Black Arms soliders and threatened to being sucked into a giant black hole. All disasters have been stopped by Sonic, Shadow and their allies who use the powers of the Chaos Emeralds to protect the world time and again, something the President has admiration to them for.

Christmas Island
According to a Japanese manual for the original Sonic the Hedgehog game, this is Sonic's birthplace (this is possibly the only instance in which a specific real-world location has been referred to in the Sonic video game continuity).

Green Hill Zone
see main article for details

Angel Island
see main article for details

Station Square
A large seaside city that was home to many humans and Amy Rose, famous for it's train station, hotel and amusement park. It was defended by Sonic the Hedgehog and Miles "Tails" Prower from Eggman, but it was still destroyed by Perfect Chaos prior to his battle and reformation at the invincible hands of Super Sonic.

Stages
 * Emerald Coast
 * Casinolopis
 * Twinkle Park
 * Speed Highway

Mystic Ruin
A group of ancient Echidnean ruins located in a forest near Station Square.

Stages:
 * Windy Valley
 * Hot Shelter
 * Final Egg

Prison Island
G.U.N's high security island where Gerald, Shadow and Sonic were all prisoned on at separate times. Eggman and Shadow manage to blow it up using an "Eggman Bomb Bomb". Although destroyed, the island remains as a toxic waste dump where Black Arms were held.

Stages:
 * Iron Gate
 * Weapons Bed
 * Security Hall
 * White Jungle
 * Green Forest
 * Prison Lane
 * Metal Habour

G.U.N. Fortress
The G.U.N. Federation's headquarters appears in Shadow the Hegehog and is considered the most secure shelter on Earth. The President is taken there for safety when the Black Arms attack Central City. The underground fortress is heavily guarded by three mother computers, armed security cameras, and an immeasurable number of G.U.N. troops and mechs. It is located in a mountain near the sea, as seen on the platform where the goal ring is located.

Central City
Seemingly the capital city of the entire planet, home to the President.

Stages:
 * City Escape
 * Mission Lane
 * Aquatic Mine
 * Route 101
 * Radical Highway
 * Route 280
 * Central City
 * Highway
 * Office

Soleana
The city of water, a beautiful and elegant city resembleing Venice, Italy. It was led by Duke Soleanna until his death but is currentley ruled by Princess Elise the Third. The events of Sonic the Hedgehog  (2006) took place there.