Earth

Globally, the 20th century was marked by: (a) two devastating world wars; (b) the Great Depression of the 1930s; (c) the end of vast colonial empires; (d) rapid advances in science and technology, from the first airplane flight at Kitty Hawk, North Carolina (US) to the landing on the moon; (e) the Cold War between the Western alliance and the Warsaw Pact nations; (f) a sharp rise in living standards in North America, Europe, and Japan; (g) increased concerns about environmental degradation including deforestation, energy and water shortages, declining biological diversity, and air pollution; (h) the onset of the AIDS epidemic; and (i) the ultimate emergence of the US as the only world superpower. The planet's population continues to explode: from 1 billion in 1820 to 2 billion in 1930, 3 billion in 1960, 4 billion in 1974, 5 billion in 1987, 6 billion in 1999, and 7 billion in 2012. For the 21st century, the continued exponential growth in science and technology raises both hopes (e.g., advances in medicine and agriculture) and fears (e.g., development of even more lethal weapons of war).

Geography

The surface of the Earth is approximately 70.9% water and 29.1% land. The former portion is divided into large bodies termed oceans. The World Factbook recognizes and describes five oceans, which are in decreasing order of size: the Pacific Ocean, Atlantic Ocean, Indian Ocean, Southern Ocean, and Arctic Ocean. Because of their immense size, the Pacific and Atlantic Oceans are generally divided at the equator into the North and South Pacific Oceans and the North and South Atlantic Oceans, thus creating seven major water bodies - the so-called "Seven Seas."

Some 97.5% of the Earth's water is saltwater. Of the 2.5% that is fresh, about two-thirds is frozen mostly locked up in the Antarctic ice sheets and mountain glaciers worldwide. If all the surface ice on earth fully melted, the sea level would rise about 70 m (230 ft).

In a 100-year period, a water molecule spends 98 years in the ocean, 20 months as ice, about two weeks in lakes and rivers, and less than a week in the atmosphere. Groundwater can take 50 years to just traverse 1 km (0.6 mi).

Earth's land portion is generally divided into several, large, discrete landmasses termed continents. Depending on the convention used, the number of continents can vary from five to seven. The most common classification recognizes seven, which are (from largest to smallest): Asia, Africa, North America, South America, Antarctica, Europe, and Australia. Asia and Europe are sometimes lumped together into a Eurasian continent resulting in six continents. Alternatively, North and South America are sometimes grouped as simply the Americas, resulting in a continent total of six (or five, if the Eurasia designation is used).

North America is commonly understood to include the island of Greenland, the isles of the Caribbean, and to extend south all the way to the Isthmus of Panama. The easternmost extent of Europe is generally defined as being the Ural Mountains and the Ural River; on the southeast the Caspian Sea; and on the south the Caucasus Mountains, the Black Sea, and the Mediterranean. Portions of five countries - Azerbaijan, Georgia, Kazakhstan, Russia, and Turkey - fall within both Europe and Asia, but in every instance the larger section is in Asia. These countries are considered part of both continents. Armenia and Cyprus, which lie completely in Western Asia, are geopolitically European countries.

Asia usually incorporates all the islands of the Philippines, Malaysia, and Indonesia. The islands of the Pacific are often lumped with Australia into a "land mass" termed Oceania or Australasia. Africa's northeast extremity is frequently delimited at the Isthmus of Suez, but for geopolitical purposes, the Egyptian Sinai Peninsula is often included as part of Africa.

Although the above groupings are the most common, different continental dispositions are recognized or taught in certain parts of the world, with some arrangements more heavily based on cultural spheres rather than physical geographic considerations.

Based on the seven-continent model, and grouping islands with adjacent continents, Africa has the most countries with 54. Europe contains 49 countries and Asia 48, but these two continents share five countries: Azerbaijan, Georgia, Kazakhstan, Russia, and Turkey. North America consists of 23 sovereign states, Oceania has 14, and South America 12.

countries by continent: Africa (54): Algeria, Angola, Benin, Botswana, Burkina Faso, Burundi, Cabo Verde, Cameroon, Central African Republic, Chad, Comoros, Democratic Republic of the Congo, Republic of the Congo, Cote d'Ivoire, Djibouti, Egypt, Equatorial Guinea, Eritrea, Eswatini, Ethiopia, Gabon, The Gambia, Ghana, Guinea, Guinea-Bissau, Kenya, Lesotho, Liberia, Libya, Madagascar, Malawi, Mali, Mauritania, Mauritius, Morocco, Mozambique, Namibia, Niger, Nigeria, Rwanda, Sao Tome and Principe, Senegal, Seychelles, Sierra Leone, Somalia, South Africa, South Sudan, Sudan, Tanzania, Togo, Tunisia, Uganda, Zambia, Zimbabwe;

Europe (49): Albania, Andorra, Austria, Azerbaijan*, Belarus, Belgium, Bosnia and Herzegovina, Bulgaria, Croatia, Czech Republic, Denmark, Estonia, Finland, France, Georgia*, Germany, Greece, Holy See (Vatican City), Hungary, Iceland, Ireland, Italy, Kazakhstan*, Kosovo, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Moldova, Monaco, Montenegro, Netherlands, North Macedonia, Norway, Poland, Portugal, Romania, Russia*, San Marino, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey*, Ukraine, United Kingdom (* indicates part of the country is also in Asia);

Asia (48): Afghanistan, Armenia, Azerbaijan*, Bahrain, Bangladesh, Bhutan, Brunei, Burma, Cambodia, China, Cyprus, Georgia*, India, Indonesia, Iran, Iraq, Israel, Japan, Jordan, Kazakhstan*, North Korea, South Korea, Kuwait, Kyrgyzstan, Laos, Lebanon, Malaysia, Maldives, Mongolia, Nepal, Oman, Pakistan, Philippines, Qatar, Russia*, Saudi Arabia, Singapore, Sri Lanka, Syria, Tajikistan, Thailand, Timor-Leste, Turkey*, Turkmenistan, United Arab Emirates, Uzbekistan, Vietnam, Yemen (* indicates part of the country is also in Europe);

North America (23): Antigua and Barbuda, The Bahamas, Barbados, Belize, Canada, Costa Rica, Cuba, Dominica, Dominican Republic, El Salvador, Grenada, Guatemala, Haiti, Honduras, Jamaica, Mexico, Nicaragua, Panama, Saint Kitts and Nevis, Saint Lucia, Saint Vincent and the Grenadines, Trinidad and Tobago, United States;

Oceania (14): Australia, Fiji, Kiribati, Marshall Islands, Federated States of Micronesia, Nauru, New Zealand, Palau, Papua New Guinea, Samoa, Solomon Islands, Tonga, Tuvalu, Vanuatu;

South America (12): Argentina, Bolivia, Brazil, Chile, Colombia, Ecuador, Guyana, Paraguay, Peru, Suriname, Uruguay, Venezuela

the world from space: Earth is the only planet in the Solar System to have water in its three states of matter: liquid (oceans, lakes, and rivers), solid (ice), and gas (water vapor in clouds); from a distance, Earth would be the brightest of the eight planets in the Solar System; this luminous effect would be because of the sunlight reflected by the planet's water

Earth is also the only planet in the Solar System known to be active with earthquakes and volcanoes; these events form the landscape, replenish carbon dioxide into the atmosphere, and erase impact craters caused by meteors.

Earth

Earth