Earth

"It's beautiful." "Yes, it is."

- Samantha Carter and Jack O'Neill

Earth (Terra in Ancient, Midgard to the humans settled by the Asgard, Tau'ri in Goa'uld, and designated P2X-3YZ by the Tau'ri themselves), is the home planet of the Tau'ri and the cradle of human civilization in the Milky Way. With a population of over 7 billion, Earth is one of the most populated known human worlds in both the Pegasus and Milky way Galaxies. This is due to a long period of isolation from any alien contact, as well as their success in thwarting various alien attacks.

History
Earth was the second planet that the Ancients colonized after they left their home galaxy (the first planet being Dakara). Known to them as Terra, it was the center of their empire in the Milky Way and was from where they launched Destiny, a ship designed to explore the universe. Earth was also home to the city of Atlantis situated in Antarctica which was then much closer to the equator. Between thirty and ten million years ago, the Ancients were forced to leave in Atlantis for Pegasus due to a plague, that was very similar to the Prior plague that was used by the Ori during the crusade. They eventually returned after the war with the Wraith where some chose to join the native humans and integrate into human society, passing on the ATA gene. They also influenced societies such as the Greeks and Romans, inspiring the myth of Atlantis and passing on their language.

When the Ancients came back to Earth c. 8,000 BC, they found the humans in a primitive state of society and knowledge, and under control of the Goa'uld known as Ra. The Ancients then, seeing that there was no hope of rebuilding their societies, spread out to many lands (integrating into the primitive societies, ascending, making their way to the Stargate in the Southern Pole or dying, leaving the remaining humans in a primitive and defenseless state). Ra remained, controlling Earth until circa 3000 BC, when an alternate SG-1 traveled back in time and helped the ancient humans to start the rebellion. After that, Ra abandoned the Earth.

Thanks to the rediscovery of the Stargate, the Tau'ri have come to find out that Humans live on other planets in addition to Earth, each with their own unique cultures. As the Stargate program progressed, the term Tau'ri eventually came to be used to specifically identify humans from Earth. Earth has one of the largest human populations of any planet in the known galaxies, possibly the largest. Major General Henry Landry believed Earth to be the most densely populated planet in the Milky Way Galaxy.

In 2004, Earth became the focus of the System Lord Anubis. In what became known as the Battle of Antarctica, Anubis came with his mothership and a massive fleet of Ha'taks, which were destroyed by Drone weapons launched by SG-1 from the Ancient outpost in Antarctica, Anubis was left without a physical form in orbit.

In 2009, Earth came under attack from the ZPM powered Hive Ship. After waiting behind the Moon for its systems to mature, the ship moved into Earth's orbit and sent out a wave of Wraith Darts that was able to destroy the Control chair, one of Earth's greatest defenses, during the Battle of the ZPM powered Hive Ship. All other Earth attempts to defend against the upgraded Hive Ship also failed. However, Atlantis utilized its Wormhole drive and was able to confront the Hive Ship, and through the work of Lt. Colonel John Sheppard's team was able to detonate a Nuclear warhead on board the ship. After the battle, Atlantis remained on Earth in San Francisco Bay.

Military
The combined Earth military is divided between roughly 190 independent governments, most of which are not privy to the knowledge of life beyond the stars. Only five countries possess interstellar military capability. Thanks to its long period of isolation, Earth has managed to develop beyond the pre-enlightenment period most societies are forced into by either the Wraith or Goa'uld. Earth is still primitive in comparison to some alien cultures but possesses technology arguably more advanced than any other in existence, particularly militarily. Tau'ri military technology relies on metallic projectile weapons, chemical explosives, and short to intercontinental ballistic and nuclear missiles. The more powerful Tau'ri nations possess Nuclear warheads.

The members of the 5 United Nations Security Council have access to an array of extra-terrestrial technology and weapons. These nations have reverse-engineered various starship and weapons designs, particularly those of the Asgard and the Ancients. The United States of America in particular has risen in little more than a decade from a planetary-based power to an intergalactic power, with several capital ships and offworld bases.

Continents

 * Africa
 * America
 * Antarctica
 * Asia
 * Australia
 * Europe

Alternate realities

 * In an alternate reality Earth was attacked by Apophis in 1998 and most major cities were destroyed. Efforts to combat the invasion were headed in Cheyenne Mountain by the SGA. Dr. Samantha Carter stated that Washington D.C. and Philadelphia had been lost. Cities that, based on the SGA's map, were almost certainly destroyed include the following: Cairo, Berlin, Rome, Helsinki, London, Moscow, New York City, and Paris. Colorado Springs was presumably destroyed during the attack on Cheyenne Mountain. The Goa'uld attack started in Europe, Asia and Africa before spreading to the United States of America. Since the destruction started on the East Coast and moved west and Dr. Daniel Jackson left the reality when it reached Colorado, the annihilation of the West Coast was unseen.
 * In an alternate reality, Earth was attacked by Apophis and Stargate Command was invaded in 1999. Earth was saved when Dr. Samantha Carter successfully contacted the Asgard.
 * In an alternate reality, Earth was attacked by the Ori, but saved by the Lt. Colonel Samantha Carter of our reality in 2007. This was achieved by using the United States power grid, the Control chair, and Arthur's Mantle to take the planet out of phase. 
 * In an alternate reality, Earth came under attack by a Hive Ship, but it was destroyed by the Antarctic outpost. A single Wraith survived and was later killed, but not before sending a message that was picked up by the ZPM powered Hive Ship in our universe, telling it the location of Earth and about the weapons chair.

Alternate timeline

 * In the alternate timeline created by Ba'al in 1939, Earth was attacked by a Goa'uld fleet in 2009.

Behind the scenes
While never given a planetary designation during any of the on-screen material, in the Secret Files of the SGC bonus features on the Stargate SG-1 DVDs (in which the viewer is said to be watching top secret SGC files), Earth is identified as P2X-3YZ.