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2.Development of Modern Astronomy


4.observatory, orbiting probe

*** Introduction
branch of science that studies the motions and natures of celestial bodies, such as planets , stars , and galaxies ; more generally, the study of matter and energy in the universe at large.
Ancient Astronomy
Astronomy is the oldest of the physical sciences. In many early civilizations the regularity of celestial motions was recognized, and attempts were made to keep records and predict future events. The first practical function of astronomy was to provide a basis for the calendar , the units of month and year being determined by astronomical observations. Later, astronomy served in navigation and timekeeping. The Chinese had a working calendar as early as the 13th cent. BC About 350 BC, Shih Shen prepared the earliest known star catalog, containing 800 entries. Ancient Chinese astronomy is best known today for its observations of comets and supernovas . The Babylonians, Assyrians, and Egyptians were also active in astronomy. The earliest astronomers were priests, and no attempt was made to separate astronomy from astrology . In fact, an early motivation for the detailed study of planetary positions was the preparation of horoscopes.
Greek Innovations
The highest development of astronomy in the ancient world came with the Greeks in the period from 600 BC to AD 400. The methods employed by the Greek astronomers were quite distinct from those of earlier civilizations, such as the Babylonian. The Babylonian approach was numerological and best suited for studying the complex lunar motions that were of overwhelming interest to the Mesopotamian peoples. The Greek approach, on the contrary, was geometric and schematic, best suited for complete cosmological models. Thales, an Ionian philosopher of the 6th cent. BC, is credited with introducing geometrical ideas into astronomy. Pythagoras, about a hundred years later, imagined the universe as a series of concentric spheres in which each of the seven ˇ°wanderersˇ± (the sun, the moon, and the five known planets) were embedded. Euxodus developed the idea of rotating spheres by introducing extra spheres for each of the planets to account for the observed complexities of their motions. This was the beginning of the Greek aim of providing a theory that would account for all observed phenomena. Aristotle (384-322 BC) summarized much of the Greek work before him and remained an absolute authority until late in the Middle Ages. Although his belief that the earth does not move retarded astronomical progress, he gave the correct explanation of lunar eclipses and a sound argument for the spherical shape of the earth.
The Alexandrian School and the Ptolemaic System
The apex of Greek astronomy was reached in the Hellenistic period by the Alexandrian school. Aristarchus (c.310-c.230 BC) determined the sizes and distances of the moon and sun relative to the earth and advocated a heliocentric (sun-centered) cosmology. Although there were errors in his assumptions, his approach was truly scientific; his work was the first serious attempt to make a scale model of the universe. The first accurate measurement of the actual (as opposed to relative) size of the earth was made by Eratosthenes (284-192 BC). His method was based on the angular difference in the sun's position at the high noon of the summer solstice in two cities whose distance apart was known.
The greatest astronomer of antiquity was Hipparchus (190-120 BC). He developed trigonometry and used it to determine astronomical distances from the observed angular positions of celestial bodies. He recognized that astronomy requires accurate and systematic observations extended over long time periods. He therefore made great use of old observations, comparing them to his own. Many of his observations, particularly of the planets, were intended for future astronomers. He devised a geocentric system of cycles and epicycles (a compounding of circular motions) to account for the movements of the sun and moon. Ptolemy (AD 85-165) applied the scheme of epicycles to the planets as well. The resulting Ptolemaic system was a geometrical representation of the solar system that predicted the motions of the planets with considerable accuracy. Among his other achievements was an accurate measurement of the distance to the moon by a parallax technique. His 13-volume treatise, the Almagest, summarized much of ancient astronomical knowledge and, in many translations, was the definitive authority for the next 14 centuries.

Development of Modern Astronomy
The Copernican Revolution

After the fall of Rome, European astronomy was largely dormant, but significant work was carried out by the Muslims and the Hindus. It was by way of Arabic translations that Greek astronomy reached medieval Europe. One of the great landmarks of the revival of learning in Europe was the publication (1543) by Nicolaus Copernicus (1473-1543) of his De revolutionibus orbium coelestium ( On the Revolutions of the Celestial Spheres ). According to the Copernican system , the earth rotates on its axis and, with all the other planets, revolves around the sun. The assertion that the earth is not the center of the universe was to have profound philosophical and religious consequences. Copernicus's principal claim for his new system was that it made calculations easier. He retained the uniform circular motion of the Ptolemaic system, but by placing the sun at the center, he was able to reduce the number of epicycles. Copernicus also determined the sidereal periods (time for one revolution around the sun) of the planets and their distance from the sun relative to the sun-earth distance.
Brahe and Kepler
The great astronomer Tycho Brahe (1546-1601) was principally an observer; a conservative in matters of theory, he rejected the notion that the earth moves. Under the patronage of King Frederick II, Tycho established Uraniborg, a superb observatory on the Danish island of Hveen. Over a period of 20 years (1576-97), he and his assistants compiled the most accurate and complete astronomical observations to that time. At his death his records passed to Johannes Kepler (1571-1630), who had been his last assistant. Kepler spent nearly a decade trying to fit Tycho's observations, particularly of Mars, into an improved system of heliocentric circular motion. At last, he conceived the idea that the orbit of Mars was an ellipse with the sun at one focus. This led him to the three laws of planetary motion that bear his name.
Galileo's Telescope
Galileo Galilei (1564-1642) made fundamental discoveries in both astronomy and physics; he is perhaps best described as the founder of modern science. Galileo was the first to make astronomical use of the telescope . His discoveries of the four largest moons of Jupiter and the phases of Venus were persuasive evidence for the Copernican cosmology. His discoveries of craters on the moon and blemishes on the sun ( sunspots ) discredited the ancient belief in the perfection of the heavens. These findings were announced in The Sidereal Messenger, a small book published in 1610. Galileo's Dialogue on the Two Chief Systems of the World (1632) was an eloquent argument for the Copernican system over the Ptolemaic. However, Galileo was called before the Inquisition and forced to renounce publicly all doctrines considered contrary to Scripture.
Astrophysical Discoveries
Isaac Newton (1642-1727), possibly the greatest scientific genius of all time, succeeded in uniting the sciences of astronomy and physics . His laws of motion and theory of universal gravitation provided a physical, dynamic basis for the merely descriptive laws of Kepler. Until well into the 19th cent., all progress in astronomy was essentially an extension of Newton's work. Edmond Halley 's prediction that the comet of 1682 would return in 1758 was refined by A. C. Clairault, who included the perturbing effects of Jupiter and Saturn on the orbit to calculate the nearly exact date of the return of the comet. In 1781, William Herschel accidentally discovered a new planet, eventually named Uranus. Discrepancies between the observed and theoretical orbits of Uranus indicated the existence of a still more distant planet that was affecting Uranus's motion. J. C. Adams and U. J. J. Leverrier independently calculated the position where the new planet, Neptune, was actually discovered (1846). Similar calculations for a large ˇ°Planet Xˇ± led in 1930 to the discovery of a small planet with the most distant orbit, Pluto.
By the early 19th cent., the science of celestial mechanics had reached a highly developed state at the hands of Leonhard Euler, J. L. Lagrange, P. S. Laplace, and others. Powerful new mathematical techniques allowed solution of most of the remaining problems in classical gravitational theory as applied to the solar system. In 1801, Giuseppe Piazzi discovered Ceres, the first of many asteroids . When Ceres was lost to view, C. F. Gauss applied the advanced gravitational techniques to compute the position where the asteroid was subsequently rediscovered. In 1838, F. W. Bessel made the first measurement of the distance to a star; using the method of parallax with the earth's orbit as a baseline, he determined the distance of the star 61 Cygni to be 60 trillion mi (about 10 light-years ), a figure later shown to be 40% too large.

Modern Techniques, Discoveries, and Theories
Astronomy was revolutionized in the second half of the 19th cent. by the introduction of techniques based on photography and spectroscopy. Interest shifted from determining the positions and distances of stars to studying their physical composition. The dark lines in the solar spectrum that had been observed by W. H. Wollaston and Joseph von Fraunhofer were interpreted in an elementary fashion by G. R. Kirchhoff on the basis of classical physics, although a complete explanation came only with the quantum theory. Between 1911 and 1913, Ejnar Hertzsprung and H. N. Russell studied the relation between the colors and luminosities of typical stars. With the construction of ever more powerful telescopes, the boundaries of the known universe constantly increased. E. P. Hubble's study of the distant galaxies led him to conclude that the universe is expanding. Using Cepheid variables as distance indicators, Harlow Shapley determined the size and shape of our galaxy, the Milky Way . During World War II Walter Baade defined two ˇ°populationsˇ± of stars, and suggested that an examination of these different types might trace the spiral shape of our own galaxy. In 1951 a Yerkes Observatory group led by William W. Morgan detected evidence of two spiral arms in the Milky Way galaxy.
Various rival theories of the origin and overall structure of the universe, e.g., the big bang and steady state theories, have been formulated. Albert Einstein's theory of relativity plays a central role in all modern cosmological theories. In 1963, the moon passed in front of the radio source 3C-273, allowing Cyril Hazard to calculate the exact position of the source. With this information, Maarten Schmidt photographed the object's spectrum using the 200-in. (5-m) reflector on Palomar Mt., then the world's largest telescope. He interpreted the result as coming from an object, now known as a quasar , at an extreme distance and receding from us at a substantial fraction of the speed of light. In 1967 Antony Hewish and Jocelyn Bell Burnell discovered a radio source a few hundred light years away featuring regular pulses at intervals of about 1 second with an accuracy of repetition of one-millionth of a second. This was the first discovered pulsar , a rapidly spinning neutron star emitting lighthouse-type beams of energy, the end result of the death of a star in a supernova explosion.
The discovery by Karl Jansky in 1931 that radio signals were emitted by celestial bodies initiated the science of radio astronomy . Most recently, the frontiers of astronomy have been expanded by space exploration . Perturbations and interference from the earth's atmosphere make space-based observations necessary for infrared , ultraviolet , gamma-ray , and X-ray astronomy . The Surveyor and Apollo spacecraft of the late 1960s and early 1970s helped launch the new field of astrogeology. A series of interplanetary probes, such as Mariner 2 (1962) and 5 (1967) to Venus, Mariner 4 (1965) and 6 (1969) to Mars, and Voyager 1 (1979) and 2 (1979), provided a wealth of data about Jupiter, Saturn, Uranus, and Neptune; more recently, the Magellan probe to Venus (1990) and the Galileo probe to Jupiter (1995) have continued this line of research. The Hubble Space Telescope , launched in 1990, has made possible visual observations of a quality far exceeding those of earthbound instruments.

scientific facility especially equipped to detect and record naturally occurring scientific phenomena. Although geological and meteorological observatories exist, the term is generally applied to astronomical observatories.
The Astronomical Observatory
The function of the astronomical observatory is centered around the telescope . In addition to visual and photographic observations of astronomical bodies and phenomena, perhaps the most valuable use of the telescope is in connection with the spectroscopic study of starlight. The total light from a star is separated into its various wavelengths, and the intensity of each is measured. The temperature and chemical composition of stars can be obtained by this method, as well as information about stellar motion and magnetic fields. Using computers, astronomers can measure the spectra digitally recorded by spectrographs and photometers. Observatories specializing in solar astronomy usually have coronographs and spectroheliographs. Atmospheric limitations on telescopic observations include weather conditions, air turbulence, air glow, pollution, and any source of extraneous illumination. To minimize such conditions optical observatories are generally located at high altitudes in sparsely populated areas.
Development of the Astronomical Observatory
Early civilizations, such as those of Babylon, China, and Egypt, recognized the regular and periodic nature of heavenly motions and established primitive observatories to maintain astronomical records. The main purposes of these early observatories were to regulate the calendar and predict the changes of season. Because it was believed that unusual occurrences, such as comets and eclipses, foretold future events on earth, the early observatories also served a religious function, and most of the ancient astronomers were priests. Later observatories were established to compile accurate star charts and an annual ephemeris that would be of use to navigators in determining longitude at sea. For some 600 years, beginning in the 13th cent., Roman Catholic churches included solar observatories to measure the movements of the sun and so determine the correct date for Easter.
The instruments in use before the invention of the telescope include the sextant , quadrant, astrolabe , and armillary sphere. These are all calibrated sighting devices for determining the angular positions of stars and planets. The armillary sphere was the most sophisticated of these instruments. It was composed of a number of rings corresponding to great circles on the celestial sphere and was used to determine both the right ascension and the declination of a star. The last great observatory of the pretelescopic era was built by Tycho Brahe at Uranienborg, on the island of Ven, Denmark.
The invention of the telescope in the early 17th cent. revolutionized observational astronomy in two ways. First, the positions and motions of celestial bodies could be measured much more accurately with telescopes than with the earlier instruments. Such data provided a source of precise time signals. Second, the telescope could be used to analyze the physical nature of celestial bodies themselves. Until the 19th cent., telescopic images were inspected visually by highly trained observers who made drawings of what they saw. The development of dry-plate photography, which permitted long exposure times, however, offered a much more sensitive method of recording images. In the late 20th cent., electronic digital detectors utilizing charge-coupled devices (CCDs) superseded the use of film; a CCD can detect the arrival of a single photon of light. A recent development is the extension of astronomical observations to wavelengths outside the visible spectrum. Most important has been the development of radio astronomy , the study of radio waves emitted by celestial bodies.
Because the atmosphere interferes with astronomical observations from the ground, the ideal location for an observatory is beyond the earth's atmosphere. The past two decades have seen increasing emphasis on space-based observatories. Several artificial satellites have been equipped with telescopes for infrared, visible, ultraviolet, and X-ray observations. The International Ultraviolet Explorer (IUE) satellite, launched in 1978, is an 18-in. (0.45-meter) space telescope for ultraviolet studies. Launched in 1983, the Infrared Astronomy Satellite (IRAS) discovered some 246,000 infrared sources, as well as several stars around which planetary systems appear to be forming. Skylab was a manned orbiting space observatory. The largest space-based observatory is the Hubble Space Telescope , launched in 1990. Other observatories include the Compton Gamma-Ray Observatory, launched in 1991, and the Chandra X-ray Observatory, launched in 1999. ROSAT [ RO entgen SAT ellite], a joint German-U.S.-British project launched in 1990, studies both X-ray and ultraviolet wavelengths never before imaged from space. It has detected a new class of bright stars that shine only in the ultraviolet part of the spectrum. The Cosmic Background Explorer (1989-93) studied microwave background radiation that no star or other known object could emitit is believed to have come from the creation of the universe.

observatory, orbiting
research satellite designed to study solar radiation, electromagnetic radiation from distant stars, the earth's atmosphere , or the like. Because the atmosphere and other aspects of the earth's environment interfere with astronomical observations from the ground, especially in the ultraviolet and infrared portions of the spectrum, the decades since the 1960s have seen increasing emphasis on space-based observatories. The U.S. Orbiting Solar Observatory (OSO) program, comprising eight satellites launched between 1962 and 1971, was one of the earliest series of orbiting observatories; it studied the sun's atmosphere and the sunspot cycle. Also beginning in 1962 and extending through 1979 were the launches of the six satellites in Great Britain's Ariel program, which concentrated on solar ultraviolet and X radiation. The Orbiting Geophysical Observatory (OGO) program consisted of six satellites, launched between 1964 and 1969, that provided data on the earth's atmosphere, ionosphere , and magnetosphere and on the solar wind . The Orbiting Astronomical Observatory (OAO) program comprised four satellites, launched between 1966 and 1972, that studied astronomical phenomena at ultraviolet and X-ray wavelengths inaccessible to earthbound equipment.
In the following years, a large number of satellites were launched to study solar and galactic radio waves, X rays, gamma rays, and ultraviolet rays. In addition to the United States a number of countries participated, among them the Netherlands with ANS-1 (1974-76), which studied soft and hard X radiation; India with Aryabhata (1975), which returned atmospheric data for only four days before being silenced by a power failure; Japan with Hakucho (1979-85) and Tenma (1981-84), both of which studied X radiation; and the European Space Agency (ESA) with Exosat (1983-86), an X-ray observatory. This period also saw the first cooperative efforts, such as the International Ultraviolet Explorer (IUE), a joint effort of the United States, ESA, and Great Britain (1978-96), which returned data on ultraviolet radiation for 18 years.
ROSAT [ Ro entgen Sat ellite] (1990-99), a joint German-U.S.-British project, studied both X-ray and ultraviolet wavelengths never before imaged from space. It detected a new class of bright stars that shine only in the ultraviolet part of the spectrum and X-ray emissions from comets. The Cosmic Background Explorer (1989-93) studied microwave background radiation that no star or other known object could emitit is believed to have come from the creation of the universe. The Infrared Space Observatory (ISO; 1995-98), launched by ESA, found water in the Orion nebula and in the atmospheres of the giant planets and Titan, found fluoride molecules in interstellar space, and studied the ˇ°coolˇ± galaxies first seen by the Infrared Astronomy Satellite (IRAS) in 1983. Another European-built orbiting observatory, the Solar and Heliospheric Observatory (SOHO), was launched by NASA in 1995. After reaching a position about 950,000 mi (1.5 million km) from the earth, where the gravitational attraction of the earth and the sun are in balance (called a Lagrangian point), SOHO initiated a program of solar physics studies, such as the solar wind and solar plumes.
To fully explore the cosmos it is necessary to collect and analyze radiation emitted by phenomena throughout the entire electromagnetic spectrum. Toward that end, NASA proposed the concept of great observatories, a series of four orbiting observatories designed to conduct astronomical studies over many different wavelengths. An important aspect of the program was to overlap the operations phases of the missions to enable astronomers to make concurrent observations of an object at different spectral wavelengths. The first member of the program and the largest orbiting observatory is the Hubble Space Telescope (HST), which was deployed by a space shuttle in 1990 and repaired in orbit in 1993. Subsequent servicing missions in 1997 and 1999 (with another planned for 2002) added capabilities to the HST, which observes the universe at ultraviolet, visual, and near-infrared wavelengths. The second great observatory, the Compton Gamma-Ray Observatory, was launched and deployed by a shuttle in 1991; it continues to collect data on gamma-ray bursts, which are some of the most violent physical processes in the universe. The third great observatory, the Chandra X-ray Observatory, formerly called the Advanced X-ray Astrophysics Facility, was deployed from a shuttle and boosted into a high earth orbit in 1999; it focuses on such objects as black holes, quasars, and high-temperature gases throughout the X-ray portion of the electromagnetic spectrum. The Space Infrared Telescope Facility (SIRTF) represents the fourth and final element in the great observatory program; to be launched 2002, SIRTF will fill an important gap in wavelength coverage not available from earthbound telescopes.

space probe
space vehicle carrying sophisticated instrumentation but no crew, designed to explore various aspects of the solar system. Unlike an artificial satellite , which is placed in more or less permanent orbit around the earth, a space probe is launched with enough energy to escape the gravitational field of the earth and navigate among the planets. Radio-transmitted commands and on-board computers provide the means for midcourse corrections in the space probe's trajectory; some advanced craft have executed complex maneuvers on command from earth when many millions of miles away in space. Radio contact between the control station on earth and the space probe also provides a channel for transmitting data recorded by on-board instruments back to earth. Instruments carried by space probes include radiometers, magnetometers, and television cameras sensitive to infrared, visible, and ultraviolet light; there also may be special detectors for micrometeors, cosmic rays, gamma rays, and solar wind. A probe may be directed to orbit a planet, to soft-land instrument packages on a planetary surface, or to fly by as close as a few thousand miles from one or more planets. The particulars of trajectory and instrumentation of each space probe are tailored around the mission's scientific and technological objectives; the data provided by a single space probe may require months or even years of analysis. Much has been learned from probes about the origins, composition, and structure of various bodies in the solar system. Scientists trying to understand the earth's weather by constructing theoretical models of global weather systems make use of the knowledge that is gained concerning the atmospheres and meteorology of the planets. Because conditions on other planets are simpler than on earth, scientists can check each of their hypotheses separately in isolation from complicating factors. The earliest space probes in the U.S. space program were the Mariner series, which investigated Mars, Venus, and Mercury, and the Pioneer series, which explored the outer planets. Pioneer 10 was the first human-made object to entirely escape the solar system. Several Viking space probes voyaged to Mars in the late 1970s, mapping the planet and searching for life. The Voyager probes, launched in 1977, returned spectacular photos and data from brushes by Jupiter, Saturn, Uranus, Neptune, and their moons. The Magellan spacecraft succeeded in orbiting Venus in 1990, returning a radar map of the planet's hidden surface. The Japanese probes Sakigake and Suisei and the European Space Agency's (ESA) probe Giotto both rendezvoused with Halley's comet in 1986, and Giotto also came within 125 mi (200 km) of the nucleus of the comet Grigg-Skjellerup in 1992. The U.S. probe Ulysses returned data about the poles of the sun in 1994, and the ESA Solar and Heliospheric Observatory ( SOHO ) was orbited in 1995. Launched in 1989, the Galileo spacecraft followed a circuitous route that returned data about Venus (1990), the moon (1992), and the asteroids 951 Gaspra (1991) and 243 Ida (1993) before it reached Jupiter in 1995 and sent a small probe into the Jovian atmosphere to study its composition. Over the next two years it orbited Jupiter 11 times, returning data about the planet's atmosphere and also about Jupiter's largest moons, Io, Ganymede, Europa, and Callisto; its mission was extended to include further studies of Europa, the volcanic moon Io, and the thunderstorms of Jupiter. The joint U.S.-ESA probe Cassini, launched in 1997, will explore Saturn and some of its moons. Upon its arrival in 2004, Cassini will send a probe (called Huygens ) into the atmosphere of Saturn's largest moon, Titan. Over three years, Cassini will conduct detailed studies of Saturn's atmosphere, rings, and magnetosphere; conduct close-up studies of Saturn's satellites Iapetus, Dione, and Enceladus; and characterize Titan's atmosphere and surface. The Mars Pathfinder and Mars Global Surveyor, both of which arrived at the red planet in 1997, were highly successful, the former in analyzing the Martian surface and the latter in mapping it. Both the Mars Climate Orbiter and Mars Polar Lander, however, were lost upon their arrival at Mars in 1999, setting NASA's Mars exploration program back by at least two years. The NEAR (for Near Earth Asteroid Rendezvous) -Shoemaker probe returned data about the asteroid Mathilde as it flew by in 1997 and the asteroid Eros as it orbited it in 1999 and 2000 and then landed on its surface in 2001, returning unparalleled data about a minor planet.

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