A photograph of a portion of the Earth's surface taken from an aircraft or from a satellite. Most often, these photographs are taken sequentially and overlap each other, to give complete coverage of the area of interest. Thus they may be viewed stereoscopically to give a three-dimensional view of the Earth's surface (Fig. 1). Although the camera in the aircraft may be pointed obliquely to the side of the line of flight, by far the most common type of aerial photograph is taken with the camera pointed vertically downward beneath the plane. See also: Stereoscopy

A continent that straddles the Equator, extending between 37°N and 35°S. It is the second largest continent, exceeded only by Asia (see illustration). The political geography of the continent continues to evolve as the traditional states face ongoing threats of instability. The total area of the continent, is 11,700,000 mi² (30,300,000 km²), comprising approximately 20% of the world's land area. Despite its large area, the continent has a simple geological structure, a compact shape with a smooth outline, and a symmetrical distribution of climate and vegetation.

The coldest, windiest, and driest continent. The lowest temperature ever measured on Earth, −89.2°C (−128.5°F), was recorded at the Russian Antarctic station of Vostok in July 1983. Katabatic (cold, gravitational) winds with velocities up to 50 km/h (30 mi/h) sweep down to the coast and occasionally turn into blizzards with 150 km/h (nearly 100 mi/h) wind velocities. Antarctica's interior is a cold desert averaging annually only a few centimeters of water-equivalent precipitation (Fig. 1), while the coastal areas average 30 cm (12 in.). See also: Continent; Earth; Polar meteorology; Precipitation (meteorology); Temperature; Wind

The Antarctic Circle is an imaginary line that delimits the northern boundary of Antarctica. It is a distinctive parallel of latitude at approximately 66°30′ south. Thus it is located about 4590 mi (7345 km) south of the Equator and about 1630 mi (2620 km) north of the south geographic pole.

Defined primarily by climatic rather than latitudinal criteria, arctic islands are those in the Northern Hemisphere where the mean temperature of the warmest month does not exceed 50°F (10°C) and that of the coldest is not above 32°F (0°C). Subarctic islands are those in the Northern Hemisphere where the mean temperature of the warmest month is over 50°F (10°C) for less than 4 months and that of the coldest is less than 32°F (0°C).

The parallel of latitude approximately 66°30′ north of the Equator, or 23.5° from the North Pole. Named for the northern constellation Bear, the Arctic Circle has the same angular distance from the Equator as the inclination of the Earth's axis from the plane of the ecliptic. Thus, when the Earth in its orbit is at the Northern Hemisphere summer solstice, June 21, and the North Pole is tilted 23.5° toward the Sun, the Sun's rays extend beyond the pole 23.5° to the Arctic Circle, giving that parallel 24 h of sunlight. On this same date the Sun's rays at noon will just reach the horizon at the Antarctic Circle, 66°30′ south. The highest altitude of the noon Sun at the Arctic Circle is on June 21, when it is 47° above the horizon.

The largest of the world's continents. With its peninsular extension, commonly called the continent of Europe, it is the major portion of the broad east-west extent of the Northern Hemisphere landmasses. In many ways Asia is more a cultural concept than a physical entity. There is no logical physical separation between Asia and Europe, and even Africa is separated from Asia merely by the width of the Suez Canal. For convenience, however, the Eurasian landmass is considered to be divided by the Ural Mountains into Europe in the west and Asia in the east (Fig. 1). Thus restricted Asia has an area of about 17,700,000 mi² (45,800,000 km²), about one-third of the land area of the Earth. In the north, mainland Siberia reaches past the 77th parallel. Southward, India and Sri Lanka reach nearer than 10° north of the Equator, while the Indonesian islands extend more than 10° south of the Equator. The continental heart of Asia is more than 2000 mi (3200 km) from the nearest ocean. These vast distances have great significance in the climates and vegetation of Asia and thus in the erosional and depositional patterns of the land. See also: Continent; Europe

Earth's oldest desert and the most arid place outside of Antarctica's Dry Valleys. Located along the edge of the Pacific Ocean and to the west of the Andes in Chile's Antofagasta Province, the Atacama Desert (Fig. 1) has long been recognized as among the Earth's driest locations, with some locations having never reported precipitation. Intense solar radiation, broad temperature fluctuations, and a thin atmosphere contribute to the Atacama being characterized as close to, or perhaps beyond the dry limits of life for living organisms. Though they are often depicted as hot and barren landscapes, the common element of all arid regions is low precipitation. In classifying aridity, the index p/PET is often applied using a sliding scale based on temperature and moisture, where p = precipitation and PET = potential evapotranspiration, as determined by solar radiation, atmospheric humidity, and wind. Hyperarid (extremely dry) regions such as the Atacama receive less than 200 mm of annual rainfall and have a p/PET of less than 0.05. In comparison, southern Africa's Kalahari Desert receives about 250 mm of precipitation each year, and the Mojave Desert, located in the U.S. Southwest, receives about 350 mm of annual precipitation. Another distinguishing feature of the Atacama is its temperature. In comparison to hot deserts, such as North Africa's Sahara or the Kalahari, the Atacama is relatively cold, with temperatures ranging from 0 to 25°C. See also: Antarctica; Desert; Kalahari Desert; Pacific Ocean; South America

The large body of seawater separating the continents of North and South America in the west from Europe and Africa in the east and extending south from the Arctic Ocean to the continent of Antarctica. The Atlantic is the second-largest ocean water body, and in area it covers nearly one-fifth of the Earth's surface. It receives the freshwater runoff from a continental drainage area approximately four times larger than that draining into either the Pacific or Indian oceans. The two major divisions, North and South Atlantic oceans, have the Equator as the common boundary. The North Atlantic, because of projecting land areas and island arcs, has numerous subdivisions. These include three large mediterranean-type seas, the Mediterranean Sea, the Gulf of Mexico plus Caribbean Sea, and the Arctic Ocean; two small mediterranean-type seas, the Baltic Sea and Hudson Bay; and four marginal seas, the North Sea, English Channel, Irish Sea, and Gulf of St. Lawrence. See also: Arctic Ocean; Baltic Sea; Equator; Gulf of Mexico

An island continent situated in the Southern Hemisphere and extending from 10 to 44°S and from 113 to 153°E. Australia's total area (2,941,526 mi² or 7,618,517 km² for the Australian mainland and 26,383 mi² or 68,332 km² for the island of Tasmania) is somewhat less than that of the United States. Bounded on the west by the Indian Ocean and on the east by the Pacific Ocean, Australia straddles the Tropic of Capricorn (Fig. 1). Originally part of the ancient supercontinent of Gondwanaland, which began to disintegrate 200 million years ago, Australia broke away from what is now Antarctica about 70 million years ago and began its long drift northward to its present position. Australia's long-term isolation has contributed to the uniqueness of its flora and fauna. Australia possesses about 650 species of birds and 400 species of reptiles, a large proportion of which are endemic. Most interesting are its 255 species of mammals, in particular the marsupials (lacking placenta), which include the kangaroo, wallaby, and koala; and the monotremes (egg-laying), which include the platypus and spiny anteater. These animals appear to represent stages along the evolutionary path toward fully developed placental mammals. See also: Continental drift; Tropic of Capricorn