Great Circle

A GREAT CIRCLE IS defined as a direction line that forms a full circle. This is also called a true azimuth. A great circle divides the Earth into equal halves and on the idealized surface of the Earth, the shortest distance between two points lies along a great circle. The equator is a great circle, as are all meridians of longitude. A great circle has a number of considerable properties. It is the largest possible circle on the globe, and every great circle has its center in the center of the Earth and its circumference is the circumference of the Earth.

One of the most important uses of great circle distances is by navigators of ships and aircraft. Measuring and determining direction is different between that on the spherical earth and that of flat surface direction finding. Traveling along a great circle (called the great circle route) is the shortest distance between two places on the Earth. Air and sea transportation lines tend to use great circle routes as much as possible to save on travel time and costs. Finding distances between points on a great circle is accomplished using the known latitude and longitude of the two points and using trigonometric tables.

Users of the great circle route need to select the proper map PROJECTION to display properly a great circle route. Maps distort distances so the great circle route may appear longer than a straight line on a flat map. The only map projection that portrays great circles as straight lines is the gnomonic projection. The drawback to using this projection, however, is that they cannot illustrate a whole hemisphere in one map. 

Other azimuthal projections are sometimes useful for navigators looking at great circle routes because of their symmetrical nature, making all great circle routes passing through their centers appear as straight lines. Another challenge appears when plotting a course using a great circle route. Compass direction along a great circle route varies constantly. To avoid constantly having to check and recalculate compass direction, followers of great circle routes can lighten their burden by following what is called a constant azimuth path. The difference between a true azimuth path and a constant azimuth path is that the true azimuth path is the shortest distance between two points on the surface of a globe and the constant azimuth path is the path followed by using a constant compass heading.

Great Barrier Reef

THE GREAT BARRIER REEF, located on the northeast coast of AUSTRALIA, is comprised of the world’s largest cluster of coral reefs. It is 1,250 mi (2,012 km) long; running along 10 mi (16 km) of the eastern Australia shore in the north to 100 mi (161 km) in the south. Its length is comparable to that of the entire West Coast of the continental UNITED STATES.

The Great Barrier Reef is an example of the key stages in the Earth’s continuing ecological and biological evolution of natural phenomena and habitats. The Great Barrier Reef’s diversity shows the development of an ECOSYSTEM that has evolved over hundreds of thousands of years. It is the world’s most extensive coral reef system and is one of the world’s richest areas in terms of plant and animal uniqueness. There is more than just coral to the Great Barrier Reef. It also contains widespread areas of sea grass, mangrove, soft bottom, and island communities. The reef is not a continuous barrier, but a broken maze of coral reefs and cays (ledges). There are approximately 2,800 individual reefs, of which 760 are fringing reefs that form against or near an island or continental coast and grows seaward, sloping sharply toward the seafloor.

The Great Barrier Reef provides habitats for abundant forms of marine life. The reef is the largest structure ever built by living creatures, and consists of thousands of separate reefs, islets, and islands. The reef contains approximately 1,500 species of fish, 400 species of sponges, 4,000 different types of mollusks, and approximately 400 species of hard, reef-building corals. The coral is a growth of calcareous (calcium or calcium carbonate) remains accumulated over a period of some 25 million years. The marine life of the warm, clear waters of the reef is plentiful and varied, consisting of anemones, marine worms, gastropods, crayfish, lobsters, prawns, crabs, starfish, sea urchins, and a great variety of fish and seabirds. Among the better known species are the crown-of-thorns starfish (Acanthaster), the giant clam (Tridacna gigas) the deadly box jelly, sea wasps, and many brightly colored fishes.

The reef also supports a wide variety of fleshy algae that are eaten by the resident turtles, fish, sea urchins, and mollusks. The reef is the habitat of such endangered species as the dugong sea cow and the large green loggerhead turtle. The reef sea-grass beds are an important feeding ground for the dugong and are a nesting ground for the loggerhead turtles. It is also a breeding area for humpback whales, which come from the Antarctic to give birth to their young in the warm waters.

The islands and cays support several hundred bird species, many of which have breeding colonies there. Reef herons, osprey, pelicans, frigate birds, sea eagles, and shearwaters are among the numerous sea birds that have been recorded.

The Great Barrier Reef Marine Park was acknowledged in 1975 with the purpose of preserving the area’s outstanding biodiversity while allowing reasonable use for local economy through zoning laws. The environmental risks related to shipping within the Great Barrier Reef are operational pollutants and accidental pollutants, the major threat being from oil, either carried as fuel or cargo.

There are over 30 historic shipwrecks in the area, as well as a scattering of islands ruins, operating and historically significant lighthouses, and World War II sites. Captain James Cook is believed to have been the first European to encounter the reef when his ship the Endeavour ran aground on it in 1770 near what is now Cooktown. Human involvement with the Great Barrier Reef began thousands of years before Captain James Cook encountered the reef. Aboriginal peoples and Torres Strait Islander communities have fished, hunted, and gathered from its waters, islands, and adjacent coastal areas for thousands of years. These people have significant traditional cultural connections to the Great Barrier Reef. The many important cultural sites that are evidence of past and in some cases contemporary activity are one aspect of their living cultural heritage that exists across the islands and reefs in the Great Barrier Reef Region. The hunting of marine animals such as dugongs and turtles has long been part of Aboriginal culture, and these activities continue as part of the cultural heritage. Chinese fisherman and Japanese pearl divers also frequented the waters of the reef in the 19th and early 20th centuries.

Grasslands

MANY DIFFERENT TYPES of ecosystems throughout the world are described as grasslands because they are dominated by species of grass with a range of other plant types as subordinates within the community. Grasslands occur at most latitudes and altitudes reflecting the wide range of environmental tolerances that characterize this huge taxonomic group of plants known as the Gramineae.

There are approximately 9,000 species within this group. They are described as monocotyledons because there is only one leaf protecting the seed, in contrast to most flowering plants, which have two seed leaves and are known as dicotyledons. Grasses reproduce by generating and dispersing seed; many species spread through the production of underground stems called tillers. These two characteristics have been widely exploited in agricultural systems. First, many species producing particularly carbohydrate-rich seeds have been domesticated, a process begun more than 10,000 years ago, and today they are major cereal crops of arable agriculture, notably rice, maize (corn), wheat, barley, oats, rye, and sorghum. Second, the ability of grasses to produce a turf ground cover through tillering and the placement of the growing points close to the ground facilitates grazing by herbivorous animals and thus underpins pastoral agriculture worldwide.

Broadly, grasslands may be classified as natural, seminatural, and artificial. Each of these groups can be further subdivided. Natural grasslands are widespread and tend to occur where climatic or soil conditions prohibit the growth of shrubs and trees; annual precipitation regimes are especially influential, as a pronounced dry season or seasons is characteristic of grasslands, along with a low annual precipitation. The occurrence of natural fire may have also played a role in the formation of continental grasslands. Other factors may give rise to grasslands locally, such as persistently cold, high-speed or salt-loaded winds in coastal areas.

GRASSLAND CATEGORIES

The main categories of natural grasslands are alpine or high mountain grasslands; high-latitude TUNDRA; temperate grasslands such as the prairies of North America; the PAMPAS of South America; the STEPPES of Eurasia; the grassveld of the high PLATEAUs of SOUTH AFRICA; and tropical grasslands such as those of savanna regions. Most are in continental interiors with moist continental or dry subtropical climates. All of these grasslands can be further subdivided. The prairies, for example, comprise the eastern and western prairies that are separated by the ROCKY MOUNTAINS.

Moreover, the eastern prairies comprise three communities: tall-grass, mixed-grass, and short-grass prairies, which reflect decreasing precipitation along an east to west gradient. A similar gradation exists in the steppes, where tall-grass communities are known as meadow and short-grass communities as typical steppe. Savanna, by definition, is a vegetation type in which grasses form a continuous ground cover.

However, there may also be trees and shrubs present in various degrees from sparse to abundant. All of these grasslands have been considerably modified by human activity, especially for arable and pastoral agriculture. Indeed, the prairies and steppes are considered to be the “bread basket” of the world because of their production of wheat. Alpine grasslands are exploited for summer feed for animals, for example, yak production in MONGOLIA and cattle production in SWITZERLAND, in a practice known as TRANSHUMANCE.

Seminatural grasslands occur in areas once occupied by trees and shrubs and where some type of human activity has favored the spread of grasses. Grazing and forestry are the most significant causes of this shift. Grazing prohibits the growth of trees and shrubs, as they are eaten by animals, while favoring grasses that can withstand grazing pressure. Forestry, once land has been cleared of trees either in small patches or through clear felling, generates unshaded conditions favored by grasses. Controlled firing may be a significant component of such management strategies. Consequently, seminatural grasslands exist throughout the world in areas that could and once did support a forest cover. They vary in size from small glades to many hectares and the grass species present reflect local climate and soils as well as the plant community composition of the source areas from which the invaders derived.

Artificial grasslands are so called because they occur in areas that could support trees and shrubs but these are prevented from growing through management. Their floristic composition is manipulated through seeding and dressing with fertilizers and herbicides that favor species with high nutritional value for grazing animals. Such grasslands are common in countries where high-technology agriculture is practiced, especially in the developed world.

Grand Canyon

WITH DIMENSIONS of 277 mi (446 km) long and 18 mi (29 km) wide, the Grand Canyon is one of the most spectacular natural wonders of the world. Crafted by the Colorado River, it is an enduring and unchallenged natural symbol of North America. At its present size of 1,904 square mi (4,911 square km), the Grand Canyon is the fourth-largest park in the lower 48 states of the UNITED STATES.

Formation of the canyon is a relatively recent event, in geologic time. The Grand Canyon is a vast landscape composed of layers upon layers of different kinds of rock that are distinct in color and texture. Basement rocks of black Vishnu schist, hard metamorphic rock, and granite form the inner gorge of the canyon. The Kaibab limestone found on the canyon crest is estimated to be 225 million years old.

River runoffs from the rims have cut hundreds of side canyons, which spread and gradually isolated the buttes and mesas that now tower thousands of feet above the canyon floor. Half of the Earth’s history is represented in the rocks at the Grand Canyon. On its walls, four geologic eras are visible in the sedimentary rocks. The fossils in its layers illustrate the evolution of life: Nowhere else in the world has been found a clearer view of geologic processes.

Climate at the canyon varies with the season and altitude. The North Rim, which is 8,000 ft (2,438 m) above sea level, receives so much snow that access by road is cut off to tourists all winter (late October to early May). Between June and August, normal daytime temperatures at the rim rise above 70 degrees F (21 degrees C), while nights remain somewhat cool. The South Rim, at 7,000 ft (2,134 m) above sea level, is open year-round. At night, the temperature still drops well below freezing between late October and April.

Upper portions of hiking trails get dangerously icy. Between May and September daytime temperatures will have highs above 70 degrees F (21 degrees C) at the rim; no matter what the season, it is always much warmer at the bottom of the canyon. The temperatures in the Inner Canyon, at river level—almost 5,000 feet (1,524 m) below the South Rim—are often in excess of 100 degrees F (38 degrees C) on most days between late May and early September.

The temperature at the river is unlikely to drop below 70 degrees F (21 degree C) even at night. Little direct sun reaches the bottom of the canyon, but the bottom of the canyon seldom freezes, and December highs remain well over 50 degrees F (10 degrees C). Precipitation in the canyon is fairly rare, except for some snow in winter and rain in August, when afternoon thunderstorms sweep in and blow over quite rapidly.

The canyon receives 15 in (38 cm) of rainfall in an average year, and at any time during the year the canyon can be invisible beneath a layer of cloud or fog. Despite the fact that Grand Canyon boasts some of the cleanest air in the United States, air pollution from faraway cities, smelters, and neighboring power plants continues to reduce visibility at the Grand Canyon by 30 percent below natural levels. In September 2001, the owners of the Navajo Generating Station made an agreement with the Grand Canyon Trust to reduce sulfur emissions by 90 percent.

The Grand Canyon receives nearly 5 million visitors annually. Thirty percent are foreign tourists. Most come to the South Rim, stay only for a few hours, and use the many observation points to look across and down into the canyon. But to truly experience the Grand Canyon, one must go down in it. To get inside the canyon, tourists can hike down preserved trails or take a mule ride to the bottom of the canyon. Rafting whitewater rapids through the Colorado River is another way to experience the massiveness of the Grand Canyon from within. There are also several campsites inside the canyon for adventurers seeking to enjoy more than a few hours of the experience.

Cascading waterfalls plunge into waiting pools of water on the Havasupai Indian reservation, near the lower end of the canyon. The canyon was home to at least seven Native American tribes, and the Hopi Indians still regard the canyon as their place of emergence onto the Earth and the place where their dead return. Their predecessors left behind more than 3,000 archaeological sites and artifacts as old as 10,000 years.

Home to 88 species of mammals and 315 species of birds, the Grand Canyon was declared a national monument in 1908, when President Theodore Roosevelt decided to protect it in perpetuity; in 1919 the U.S. Congress authorized the expansion and upgrading of the monument to a national park.

Grand Canal

THE GRAND CANAL is one of the wonders of the world. Built in the 7th century by CHINA’s Sui Dynasty, it stretches about 1,100 mi (1,800 km) across eastern China—about the same distance as MIAMI, FLORIDA, to NEW YORK CITY, or LOS ANGELES, CALIFORNIA, to Vancouver, CANADA. The canal was built to transport rice and silk from the rich and fertile south to the dry and barren north, particularly to soldiers protecting the frontier and the northern capital, BEIJING. Major cities grew up along its route, which remain major commercial centers to this day, although much of the canal is now too small or too full of silt to carry major boat traffic.

The Grand Canal, Da Yunhe in Mandarin Chinese, has been called the “Bridle on the Dragons” since it crossed China’s five major east-west rivers and helped control perennial flooding in the region. Throughout China’s history, dynasties could rise and fall depending on their ability to control these floods; natural catastrophe was a sign of displeasure from the heavens and frequently led to change in regime. Sections of the canal were built as early as the 4th century B.C.E., though most of it was undertaken in the 7th century C.E. by the notoriously cruel Emperor Yangdi: The forced labor of millions of peasants accomplished a nearly complete canal in only six years. It linked the southern city of Hangzhou to Xi’an (then the largest city in the world), and from there trade passed overland via the SILK ROAD to western Asia and Europe. The canal still begins at Hangzhou, capital of the 13th-century Song Dynasty, which was visited by Marco Polo, who described it as the most magnificent city in the world. When the Mongols (Yuan Dynasty) set up a new capital at Beijing in the next century, the canal was extended further north to feed the capital and its armies with rice from the south.

Heading north from its terminus at Hangzhou, the Grand Canal crosses the provinces of Zhejiang and Jiangsu, where it passes through the city of Suzhou, sister to Hangzhou. Both cities are famous for their gardens and natural beauty (Suzhou is sometimes referred to as the “Venice of the East” because of its extensive canals). The canal forms the main street for commercial cities like Wuxi and Xuzhou, where hundreds of small junks compete for space with larger barges, loading and unloading products—silks, rice, green tea—onto train lines that reach far into the Chinese interior.

The canal then intersects the Changjiang (Yangzi River) about the midpoint between two of China’s most important cities, Shanghai and Nanjing. Following the contours of the North China Plain, the canal is at its deepest and widest, enough for 600-ton riverboats. Small rice plots surround the canal for miles. The landscape begins to change as the waterway enters Shandong Province: plaster and tile houses of the south give way to mud and thatch in the north, wheat replaces rice as the dominant crop, and mules replace water buffalo as the major domesticated animal. This region is heavily affected by drought and floods, and much of the region’s trade has shifted to railroads, leaving many sections of the canal completely dry, with much of its water used for irrigation. The canal no longer enters the Huang (Yellow) River, which has risen about 20 ft (6 m) on silt deposits, kept in by dikes. From the Huang River, the canal works its way north through Hebei Province to Tianjin, the port city for Beijing. Formerly connecting Beijing to its port, the canal is now unnavigable between the two cities.

Golan Heights

THE GOLAN HEIGHTS (in Arabic, Al-Jaw-lan) is a hilly region in southwest SYRIA, bordering nearby regions of LEBANON and JORDAN, with a highest point of 7,297 ft (2,224 m). The area is one of the most heavily disputed areas in the history of the 20th century. When the state of ISRAEL emerged after the vote of the United Nations and the war of independence in 1948, the Golan Heights, under Syrian rule, were used as a military tactical point against Israel. Up to the war of 1967, Syria used the Golan Heights to attack Israeli villages from the Sea of Galilee to the Lebanese border with sporadic artillery fire. The Golan Heights were heavily fortified by the Syrians, but the Israelis were informed about these activities by their master spy, Elie Cohen. The Israelis were able to take the Golan Heights in little more than 24 hours during the 1967 war, and the Syrians lost their decisive strategic asset for shelling large parts of northern Israel.

In the war of 1973, the Israelis were taken by surprise, including on the Syrian front. After heavy losses, the Israelis, in an unprecedented turn of the tide, drove out the conquering Syrians and were only stopped by political pressures from the UNITED STATES to go all the way to DAMASCUS, the Syrian capital. After political negotiations brokered by U.S. Secretary of State Henry Kissinger, Israel and Syria signed a disengagement agreement in 1974. The Israelis gave back the Syrian land conquered in 1973 and even part of the Golan Heights area (especially the city of Kuneitra).

The economic importance of the Golan Heights is negligible (apart from some tourism, including winter sport facilities in the Mount Hermon area), but the strategic importance of the Golan Heights is gigantic. By its prominent position, the Golan Heights can control many parts of northern Israel. Therefore, the Israeli parliament made the Golan Heights part of Israel in 1981. U.S.-brokered negotiations in the 1990s for a peace settlement between Syria and Israel failed, as the Syrian side demanded the return of the Golan Heights before any peace settlement, and the Israeli side ruled out discussions on a change of sovereignty of this strategic asset without any durable peace settlement with Syria. The Golan Heights remain a hotspot of international tension and could well become the setting for another war, although the area has limited immediate military presence as a consequence of the agreements of the 1970s.

Gobi Desert

THE 650,000-square-mi (1,600,000-square-km) Gobi is the largest desert in CHINA and MONGOLIA; both the Taklamakan Desert (in XINJIANG) and the Changtang Desert (in TIBET and Qinghai) are only half the size. Only the SAHARA is bigger than the Gobi among the world’s deserts. The Gobi separates the Republic of Mongolia from the province of Inner Mongolia in the People’s Republic of China (PRC). In Mongolia, the Gobi occupies all the southern districts (aimak) of the country: eastern Gobi, central Gobi, southern Gobi, Baian Khongor, and Gobi Altai. In China, the Gobi Desert extends from Manchuria to Gansu and Xinjiang.

It consists of several plateaus: Hulun Buir, Xilin Gol, and Ulanqab in the east and the Alashan plateau in the west of Inner Mongolia. The Yinshan Mountains and the Hexi corridor form the southern edge of the Gobi Desert.

Athough vast, the Gobi peneplain is neither the driest nor the warmest desert in western China. The Siberian high-pressure system keeps the Gobi dry and very cold in winter; in summer, the monsoon winds bring a few inches of precipitation to the region. The meager vegetation and the brief growing season account for the barren appearance of the Gobi. Topographical profiles of Inner Mongolia display a succession of denuded hills covered with gobi pebbles and basins covered with sandy gobi gravel. Gobi or govi usually means “desert” in Mongol, although an absolute desert would be called tjall or “waste.” Gobi designates also the winderoded small basins that are scattered in the desert. Chinese geographers establish a difference between sandy desert (shamo) and gravel desert (gobi). They call gobi the gray surface of hard pebbles that covers dilluvial plains. Sand dunes are indeed rare in the Gobi Desert.

The Gobi enjoyed a semitropical climate during the Cretaceous period (145 to 160 million years ago). The paleontologist Roy Chapman Andrews organized, in the 1920s, the Central Asiatic expeditions of the American Museum of Natural History. He discovered rich repositories of dinosaur fossils in what is now a forbidding desert, with badlands and salt flats. The two characters in Chinese to transcribe Gobi are ge, “halberd,” and bi, “wall.” Combined, they suggest that this type of desert constitutes either a dangerous obstacle or a formidable protection. No major road crosses the Gobi

Desert except for the imperial highway from Kalgan (Zhangjiakou) to Ulaanbator and the much more recent Transmongolian railway that links Beijing and Ulaanbator via Baotou. The Gobi plays today an unexpected role in space-based international communications as the launch site for Chinese Long March rockets; the Jiuquan launch site lies in the western Gobi Desert (Gansu province).