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After completing his studies, Nicolaus Copernicus returned to Prussia and took on the position of secretary to his uncle Lucas Watzenrode, who was at the time the bishop of Warmia. During this time he lived at the bishop's castle at Lidzbark Warminski (Heilsberg) and started his research on the heliocentric model of the universe. Copernicus obtained a position as a burgher of Warmia in the Collegiate Church of the Holy Cross in Wroclaw (Breslau) in Bohemia, and he kept this position for most of his life while carrying out his studies as an amateur astronomer. Copernicus was a polymath, serving as economic administrator of Warmia from 1516 to 1521, and as head of the Royal Polish forces for Olsztyn (Allenstein) castle when it was besieged by the Teutonic Knights during the Polish-Teutonic War of 1519-21. He also worked as a diplomat on behalf of the bishop of Warmia and adviser to Duke Albert of Prussia, especially in the fields of monetary reforms, where he was charged with determining and negotiating who had the right to mint coins. Copernicus was also called on for his medical skills, even diagnosing and saving the life of one of Duke Albert's counselors.

Copernicus is most famous for proposing the heliocentric model of the universe. He first formulated it in a six-page, handwritten text, the "Com-mentariolus" (Little Commentary), preceding his famous six-volume work, De revolutionibus orbium coelestium, published in 1543 over three decades or more, although the exact date of the "Commentari-olus" is not known. "Commentariolus" contained seven main assumptions:

• There is no one center for all the celestial circles or spheres.

• The center of the Earth is not the center of the universe, but only of gravity and of the lunar sphere.

• All the spheres revolve about the Sun as their midpoint, and therefore the Sun is the center of the universe.

• The ratio of the Earth's distance from the Sun to the height of the firmament (the heavens) is so much smaller than the ratio of the Earth's radius to its distance from the sun that the distance from the Earth to the sun is imperceptible in comparison with the height of the firmament.

• Whatever motion appears in the firmament arises not from any motion of the firmament, but from the Earth's motion. The Earth together with its circumjacent elements performs a complete rotation on its fixed poles in a daily motion, while the firmament and highest heaven abide unchanged.

• What appear to observers on Earth as motions of the sun arise not from its motion but from the motion of the Earth and its sphere, which revolves about the Sun like any other planet. The Earth has, then, more than one motion.

• The apparent retrograde and direct motion of the planets arises not from their motion but from the Earth's. The motion of the Earth alone, therefore, suffices to explain so many apparent inequalities in the heavens.

In the period between the publication of his two major works, Copernicus's ideas were discussed among many scholars and clergy of the time, including the archbishop of Capua, Nicholas Shonberg, who encouraged Nicolaus to communicate his work and discoveries to scholars and to share his writings at the earliest possible moment.

While Copernicus was still finishing De revo-lutionibus orbium coelestium, a student of mathematics named Georg Joachim Rheticus came to work with him, as arranged by Phillip Melanchton, a professor from Prussia (later part of Germany). Over a period of two years Rheticus studied with Copernicus and wrote a book about Copernicus's work, Narratio prima (First account), which many scholars read who as a result began to appreciate Copernicus's works. Rheticus followed this in 1542 with a well-received book on trigonometry outlining Copernicus's ideas in this field. Having seen his ideas generally well accepted and not criticized by the clergy at the time, Copernicus agreed to publish his major works through the printer Johannes Petreius in Nuremberg. He published De revolutionibus orbim coelestium in 1543 in six volumes categorized by the following:

• general vision of the heliocentric theory, and a summarized exposition of his idea of the world

• mainly theoretical, presents the principles of spherical astronomy and a list of stars (as a basis for the arguments developed in the subsequent books)

• mainly dedicated to the apparent motions of the Sun and to related phenomena

• description of the Moon and its orbital motions

• concrete exposition of the new system

Copernicus died after a long illness on May 24, 1543, the same year that his treatise was published. Legend has it Copernicus awoke from a stroke-induced coma and looked at his book as it was placed in his hands, then died peacefully. He was buried at Frombork Cathedral in northern Poland.

Copernicus's ideas were not only revolutionary, but were quite different from those advocated by the Catholic Church. Despite this, the books caused only minor controversy at first, until three years later in 1546, when Giovanni Maria Tolosani, a Dominican priest, denounced the work, stating that it would have been condemned earlier if the chief censor of the Catholic Church at the time (Bartholo-meo Spina) had not died while reviewing it. In 1616 the Roman Catholic Church issued a decree that suspended De revolutionibus orbim coelestium until it could be corrected, on the basis that it opposed Holy Scripture. The Italian astronomer Galileo Galilei (1564-1642), who supported Copernicus's ideas, was investigated by Cardinal Bellamine on the orders of Pope Paul V and in 1633 was convicted of grave suspicion of heresy for "following the position of Copernicus, which is contrary to the true sense and authority of Holy Scripture" (Papal Condemnation [Sentence] of Galileo, June 22, 1633 [translated from the Latin], in Giorgio de Santillana, The Crime of Galileo, University of Chicago Press, 1955). Galileo was placed under house arrest for the rest of his life. The church's influence was strong in this era—another follower of Copernicus's ideas, Giordano Bruno, was condemned and burned at the stake on February 17, 1600, for being a heretic. The censorship of Copernicus's views continued for centuries, with the original De revolutionibus orbim coelestium remaining on the Index of Prohibited Books published by the Catholic Church in 1758. The revolutionary book was finally dropped in the 1835 edition of prohibited works, nearly 300 years after its initial publication.

See also astronomy; Galilei, Galileo; solar system.

FURTHER READING

Armitage, Angus. The World of Copernicus. New York: Mentor Books, 1951.

Bienkowska, Barbara. The Scientific World of Copernicus: On the Occasion of the 500th Anniversary of His Birth, 1473-1973. New York: springer, 1973. Koyre, Alexandre. The Astronomical Revolution: Coper-nicus-Kepler-Borelli. Ithaca, N.Y.: Cornell university Press, 1973.

Rosen, Edward. Copernicus and His Successors. London: Hambledon Press, 1995.

coral Corals are invertebrate marine fossils of the phylum Cnidaria characterized by radial symmetry and a lack of cells organized into organs. They are related to jellyfish, hydroids, and sea anemones, all of which possess stinging cells. Corals are the best preserved of this phylum because they secrete a hard, calcareous skeleton. The animal is basically a simple sac with a central mouth, surrounded by tentacles, that leads to a closed stomach. Cnidarians are passive predators, catching food that wanders by in their tentacles. Corals and other cnidarians produce alternating generations of two body forms. Medusae are forms that reproduce sexually to form polyps, the asexual forms from which the medusae may bud. Corals belong to a subclass of the anthozoan cnidarians known as the Zooantharia. The jellyfish belong to the scyphozoa class, and the hydrozoa class includes both fresh- and saltwater cnidarians dominated by the polyp stage.

Corals can live in a range of conditions from shallow tidal pools to 19,700 feet (6,000 m) depth. They have a cylindrical or conical skeleton secreted by the polyp-stage organism, which lives in the upper exposed part of the structure. The skeleton is characterized by radial ridges known as septa that join the skeleton's outer wall (the theca), and may have flat floors that were periodically secreted by the polyp.

Corals range from the Early ordovician Tabulata forms, joined in the Middle ordovician by the rugose corals. They both experienced a major extinction in the Late Devonian, from which the rugose forms recovered stronger. Both forms became extinct in the Early Triassic and were replaced by modern coral forms known as scleractinia, which apparently arose independently from different soft-bodied organisms.

Most corals grow in colonial communities and form reefs that provide numerous advantages, including shelter for larvae and young stages. Reefs are framework-supported carbonate mounds built by carbonate-secreting organisms, or in some instances

Coral reef in the Red Sea, Egypt (Specta, Shutterstock, Inc.)

any shallow ridge of rock lying near the surface of the water. Reefs contain a plethora of organisms that together build a wave-resistant structure that reaches up to just below the low-tide level in the ocean waters and provide shelter for fish and other organisms. The spaces between the framework are typically filled by skeletal debris, which together with the framework become cemented together to form a wave-resistant feature that shelters the shelf from high-energy waves. Modern corals can survive only in shallow waters that range in temperature from 77 to 84°F (25-29°C), at depths of fewer than 300 feet (90 m). Reef organisms (presently consisting mainly of zooxanthellae) can survive only in the photic zone, so reef growth is restricted to the upper 328 feet (100 m) of the seawater. Reefs have various forms including fringing, barrier, and atoll reefs.

Reefs are built by a wide variety of organisms, including red algae, mollusks, sponges, and cnidar-ians (including corals). The colonial scleractinia corals are presently the principal reef builders, producing a calcareous external skeleton characterized by radial partitions known as septa. Inside the skeleton are soft-bodied animals called polyps, containing symbiotic algae that are essential for the life cycle of the coral and for building the reef structure. The polyps contain calcium bicarbonate that is broken down into calcium carbonate, carbon dioxide, and water. The calcium carbonate is secreted to the reefs building its structure, whereas the algae pho-tosynthesize the carbon dioxide to produce food for the polyps.

There are several different types of reefs, classified by their morphology and relationship to nearby landmasses. Fringing reefs grow along and fringe the coast of a landmass and are often discontinuous. They typically have a steep outer slope, an algal ridge crest, and a flat, sand-filled channel between the reef and the main shoreline. Barrier reefs form at greater distances from the shore than fringing reefs and are generally broader and more continuous than fringing reefs. They are among the largest biological structures on the planet—for instance, the Great Barrier Reef of Australia is 1,430 miles (2,300 km) long. A wide, deep lagoon typically separates barrier reefs from the mainland. All these reefs show a zonation from a high-energy side on the outside or windward side of the reef, and typically grow fast, and have a smooth outer boundary. In contrast the opposite side of the reef receives little wave energy and may be irregular, poorly developed, or grade into a lagoon. Many reefs also show a vertical zonation in the types of organisms present, from deep water to shallow levels near the sea surface.

Atolls or atoll reefs form circular-, elliptical-, or semicircular-shaped islands made of coral reefs that rise from deep water; atolls surround central lagoons, typically with no internal landmass. Some atolls do have small central islands, and these, as well as parts of the outer circular reef, are in some cases covered by forests. Most atolls range in diameter from half a mile to more than 80 miles (1-130 km) and are most common in the western and central Pacific Ocean basin and in the Indian Ocean. The outer margin of the semicircular reef on atolls is the most active site of coral growth, since it receives the most nutrients from upwelling waters on the margin of the atoll. On many atolls coral growth on the outer margin is so intense that the corals form an overhanging ledge from which many blocks of coral break off during storms, forming a huge talus slope at the base of the atoll. Volcanic rocks, some of which lie more than half a mile (1 km) below current sea level, underlie atolls. since corals can grow only in shallow water fewer than 65 feet (20 m) deep, the volcanic islands must have formed near sea level, grown coral, and subsided with time, with the corals growing at the rate that the volcanic islands were sinking.

Charles Darwin proposed such an origin for atolls in 1842 based on his expeditions on the Beagle from 1831 to 1836. He suggested that volcanic islands were first formed with their peaks exposed above sea level. At this stage coral reefs were established as fringing reef complexes around the volcanic island. He suggested that with time the volcanic islands subsided and were eroded, but that the growth of the coral reefs kept up with the subsidence. In this way as the volcanic islands sank below sea level, the coral reefs continued to grow and eventually formed a ring circling the location of the former volcanic island. When Darwin proposed this theory in 1842, he did not know that ancient eroded volcanic mountains underlay the atolls he studied. More than 100 years later, drilling confirmed his prediction that volcanic rocks would be found beneath the coralline rocks on several atolls.

With the advent of plate tectonics in the 1970s, the cause of the subsidence of the volcanoes became apparent. When oceanic crust is created at midocean ridges, it is typically about 1.7 miles (2.7 km) below sea level. With time, as the oceanic crust moves away from the midocean ridges, it cools and contracts, sinking to about 2.5 miles (4 km) below sea level. In many places on the seafloor small volcanoes form on the oceanic crust a short time after the main part of the crust formed at the midocean ridge. These volcanoes may stick above sea level a few hundred meters. As the oceanic crust moves away from the midocean ridges, these volcanoes subside below sea level. If the volcanoes happen to be in the tropics where corals can grow, and if the rate of subsidence is slow enough for the growth of corals to keep up with subsidence, then atolls may form where the volcanic island used to be. If corals do not grow or cannot keep up with subsidence, then the island subsides below sea level and the top of the island gets scoured by wave erosion, forming a flat-topped mountain that continues to subside below sea level. These flat-topped mountains are known as guyots, many of which were mapped by military surveys during exploration of the seafloor associated with military operations of World War II.

Reefs are extremely sensitive and diverse environments, and cannot tolerate large changes in temperature, pollution, turbidity, or water depth. Reefs have also been subject to mining and destruction for navigation, and have even been sites of testing nuclear bombs in the Pacific. Thus, human-induced and natural changes in the shoreline environment pose a significant threat to the reef environment.

Reefs are rich in organic material and have high primary porosity, so they are a promising target for many hydrocarbon exploration programs. Reefs are well represented in the geological record, with examples including the Permian reefs of west Texas; the Triassic of the European Alps, the Devonian of western Canada, Europe, and Australia; and the Pre-cambrian of Canada and South Africa. Organisms that produced the reefs have changed dramatically with time, but, surprisingly, the gross structure of the reefs has remained broadly similar.

See also divergent plate margin processes.

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