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Astronomy, science dealing with all the celestial bodies in the universe, including the planets and their satellites, comets and meteors, the stars and interstellar matter, the star systems known as galaxies, and clusters of galaxies. Modern astronomy is divided into several branches, namely, astrometry, the observational study of the positions and motions of these bodies; celestial mechanics , the mathematical study of their motions as explained by the theory of gravitation; astrophysics, the study of their chemical composition and physical condition from spectrum analysis and the laws of physics; and cosmology, the study of the universe as a whole.

Ancient Origins Curiosity of ancient peoples concerning day and night and the sun, moon, and stars led eventually to the observation that the heavenly bodies appear to move in a regular manner that is useful in defining time and direction on the earth. Astronomy grew out of problems originating with the first civilizations, that is, the need to establish with precision the proper times for planting and harvesting crops and for religious celebrations and to find bearings and latitudes on long trading journeys or voyages.
To ancient peoples the sky exhibited many regularities of behavior. The bright sun, which divided daytime from nighttime, rose every morning from one direction, the east, moved steadily across the sky during the day, and set in a nearly opposite direction, the west. At night more than 1000 visible stars followed a similar course, appearing to rotate in permanent groupings, called constellations, around a fixed point in the sky, which is known as the north celestial pole.
In the North Temperate Zone, people noticed that daytime and nighttime were unequal in length. On long days the sun rose north of east and climbed high in the sky at noon; on days with long nights the sun rose south of east and did not climb so high at noon. Observation of the stars that appear in the west after sunset or in the east before sunrise showed that the relative position of the sun among the stars changes gradually. The Egyptians may have been the first to discover that the sun moves completely around the sphere of the fixed stars in approximately 365 days and nights.
Further study showed that the sky also holds the moon and five bright planets. These bodies, together with the sun, move around the star sphere within a narrow belt called the zodiac. The moon traverses the zodiac quickly, overtaking the sun about once every 29.5 days, the period known as the synodic month. Star watchers in ancient times attempted to arrange the days and either the months or the years into a consistent time system, or calendar. Inasmuch as neither an entire month nor an entire year contains an exactly integral number of days, the calendar makers assigned to successive months or years different numbers of days, having a long-range average that would approximate the true value. Thus, the modern calendar provides for 97 leap years in every 400-year period, so that the average number of days per year is 365.2425, very close to the astronomically determined number, which is 365.24220.
The sun and moon always traverse the zodiac from west to east. However, the five bright planets—Mercury, Mars, Venus, Jupiter and Saturn, which also have a generally eastward motion against the background of the stars—move westward, or retrograde, for varying durations during each synodic period. Thus, the planets appear to pursue an eastward course erratically, with periodic loops in their paths. In ancient times, people imagined that celestial events, especially the planetary motions, were connected with their own fortunes. This belief, called astrology, encouraged the development of mathematical schemes for predicting the planetary motions and thus furthered the progress of astronomy during ancient times.

Babylonian Astronomy
Interesting constellation maps and useful calendars were developed by several ancient peoples, notably the Egyptians, the Mayans, and the Chinese, but the Babylonians accomplished even greater achievements. To perfect their calendar, they studied the motions of the sun and moon. It was their custom to designate as the beginning of each month the day after the new moon, when the lunar crescent first appears after sunset. Originally this day was determined by observations, but later the Babylonians wanted to calculate it in advance. About 400 BC they realized that the apparent motions of the sun and moon from west to east around the zodiac do not have a constant speed. These bodies appear to move with increasing speed for half of each revolution to a definite maximum and then to decrease in speed to the former minimum. The Babylonians attempted to represent this cycle arithmetically by giving the moon, for example, a fixed speed for its motion during half its cycle and a different fixed speed for the other half. Later they refined the mathematical method by representing the speed of the moon as a factor that increases linearly from the minimum to maximum during half of its revolution and then decreases to the minimum at the end of the cycle. With these calculations of the lunar and solar motions, Babylonian stargazers could predict the time of the new moon and the day on which the new month would begin. As a by-product, they knew the daily positions of moon and sun for every day during the month. In a similar manner the planetary positions were calculated, with both their eastward and retrograde motions represented. Archaeologists have unearthed hundreds of cuneiform tablets that show these calculations. A few of these tablets, which originated in the cities of Babylon and Uruk, on the Euphrates River, bear the name of Naburiannu (flourished about 491 BC) or Kidinnu (flourished about 379 BC), astrologers who may have invented the systems of calculation.

Greek Astronomy
The ancient Greeks made important theoretical contributions to astronomy. The Odyssey of Homer refers to such constellations as the Great Bear, Orion, and the Pleiades and describes how the stars may serve as a guide in navigation. The poem Works and Days by Hesiod informs the farmer which constellations rise before dawn at different seasons of the year to indicate the proper times for plowing, sowing, and harvesting. Scientific contributions are associated with the names of the Greek philosophers Thales of Miletus and Pythagoras of Samos, but none of their own writings survive. The legend that Thales correctly predicted a total solar eclipse on May 28, 585 BC, is probably apocryphal. About 450 BC the Greeks began a fruitful study of planetary motions. Philolaus (flourished 5th century BC), a follower of Pythagoras, believed that the earth, sun, moon, and planets all moved around a central fire hidden from view by an interposed counterearth. According to his theory, the revolution of the earth around the fire every 24 hours accounted for the daily motions of the sun and stars. About 370 BC the astronomer Eudoxus of Cnidus explained observed motions by the supposition that a huge sphere bearing the stars on its inner surface moved around the earth at its center in a daily rotation. In addition, to account for solar, lunar, and planetary motions, he assumed that inside the star sphere were many interconnected transparent spheres that revolved in various ways.
Probably the most original ancient observer of the heavens was Aristarchus of Samos, a Greek. He believed that motions in the sky could be explained by the hypothesis that the earth turns around on its axis once every 24 hours and, along with the other planets, revolves around the sun. This explanation was rejected by most Greek philosophers, who regarded the big, heavy earth as a motionless globe around which the light, incorporeal bodies revolve. This theory, known as the geocentric system, remained virtually unchallenged for about 2000 years.
In the 2nd century AD, the Greeks combined their celestial theories with carefully planned observations. The astronomers Hipparchus and Ptolemy determined the positions of about 1000 bright stars and used this star chart as a background for measuring the planetary motions. Abandoning the spheres of Eudoxus for a more flexible system of circles, they postulated a series of eccentric circles with the earth near a common center to represent the general eastward motions at varying speeds of the sun, moon, and planets around the zodiac. To explain the periodic variations in the speed of the sun and moon and the retrogressions of the planets, they postulated that each of these bodies revolved uniformly around a second circle, called an epicycle, the center of which was situated on the first. By proper choice of the diameters and speeds for the two circular motions ascribed to each body, its observed motion usually could be represented. In some cases a third circle was required. This technique was described by Ptolemy in his great work the Almagest Hypatia, a follower of Plato, wrote commentaries on mathematical and astronomical topics and is regarded today as the first female astronomer.

Medieval Astronomy
Greek astronomy was transmitted eastward to the Syrians, the Hindus, and the Arabs. The Arabian astronomers compiled new star catalogs in the 9th and 10th centuries and subsequently developed tables of planetary motion. Although the Arabs were good observers, they made few useful contributions to astronomical theories. In the 13th century, Arabic translations of Ptolemy's Almagest filtered into western Europe, stimulating interest in astronomy. Initially, Europeans were content to make tables of planetary motions, based on Ptolemy's system, or to write short popular digests of his theory. Later the German philosopher and mathematician Nicholas of Cusa and the Italian artist and scientist Leonardo da Vinci questioned the basic assumptions of the centrality and immobility of the earth.

The Copernican Theory
The history of astronomy took a dramatic turn in the 16th century as a result of the contributions of the Polish astronomer Nicolaus Copernicus. He was educated in Italy and was a canon of the Roman Catholic church. He spent most of his life pursuing astronomy, however, and he made a new star catalog from personal observations. He is most famous for his great work On the Revolution of Heavenly Bodies (1543), in which he analyzed critically the Ptolemaic theory of an earth-centered universe and showed that the planetary motions can be explained by assuming a central position for the sun rather than for the earth. Little attention was paid to the Copernican, or heliocentric, system until Galileo discovered evidence to support it. Long a secret admirer of Copernicus's work, Galileo saw his chance to test the Copernican theory of a moving earth when the telescope was invented in the Netherlands. He made (1609) a small refracting telescope, turned it skyward, and discovered the phases of Venus, indicating that this planet revolves around the sun; he also discovered four moons revolving around Jupiter, as well as the rings of Saturn. Convinced that some bodies, at least, do not circle the earth, he began to speak and write in favor of the Copernican system. His attempts to publicize the Copernican system caused him to be tried by the ecclesiastical authorities. Although he was forced to repudiate his beliefs and writings, the powerful theory could not be suppressed.

The Newtonian Theory
From the scientific viewpoint, the Copernican theory was only a rearrangement of the planetary orbits, as conceived by Ptolemy. The ancient Greek theory of planets moving around circles at fixed speeds was retained in the Copernican system. From 1580 to 1597 the Danish astronomer Tycho Brahe observed the sun, moon, and planets at his island observatory near Copenhagen and later in Germany. Based on the data compiled by Brahe, his German assistant, Johannes Kepler, formulated the laws of planetary motion, stating that the planets revolve around the sun, not in circular orbits with uniform motion but in elliptical orbits at varying speeds, and that their relative distances from the sun can be determined from the observed periods of revolution. The British physicist Sir Isaac Newton advanced a simple principle to explain Kepler's laws of planetary motion. By mathematical reasoning, he argued that an attractive force exists between the sun and each of the planets. This force, which depends on the masses of the sun and planets and on the distances between them, provides the basis for the physical interpretation of Kepler's laws. Newton's mathematical discovery is called the law of universal gravitation.

Modern Astronomy
After Newton's time, astronomy branched out in several directions. With his law of gravitation, the old problem of planetary motion was studied anew as celestial mechanics. Improved telescopes permitted the scanning of planetary surfaces, the discovery of many faint stars, and the measurement of stellar distances . In the 19th century a new instrument, the spectroscope, yielded information about the chemical composition and motions of heavenly bodies.
During the 20th century, increasingly larger reflecting telescopes have been built, including one with a mirror 236 in. (6 m) in diameter. Studies with these instruments have revealed the structure of huge distant assemblages of stars, called galaxies, and of clusters of galaxies. In the second half of the 20th century, developments in physics led to new classes of astronomical instruments, some of which have been placed on earth-orbiting satellite observatories. These instruments are sensitive to a wide variety of radiation wavelengths, including the gamma-ray, X-ray, ultraviolet, infrared, and radio regions of the electromagnetic spectrum. Astronomers now study not only planets, stars, and galaxies but also plasmas (hot, ionized gases) surrounding double stars, interstellar regions that are the birthplaces of new stars, cold dust grains that are invisible in the optical regions, energetic nuclei of galaxies that may contain black holes, and photons originating from the big bang that may yield information about the early history of the universe.

The Solar System
Newton's law of gravitation postulated an attractive force between the sun and each of the planets in order to explain Kepler's laws of elliptical motion. It also implied, however, that much smaller forces must exist between the planets themselves and between the sun and other bodies such as comets. The interplanetary gravitational forces cause the orbits of the planets to deviate from simple elliptical motion. Most of these irregularities, predicted on the basis of Newton's theory, could be observed only with the telescope.
Observation of planet positions was improved as a result of the development of more accurate astronomical instruments and photographic techniques. Correspondingly, mathematical calculations enable present-day astronomers to predict planetary positions years in advance, with an accuracy approximating that of the observed positions. Electronic computing machines are now extensively utilized for such calculations. With the use of the telescope many new members of the solar system were discovered, including the planet Uranus in 1781 by the British astronomer Sir William Herschel; the planet Neptune in 1846 independently by the British astronomer John Couch Adams and the French astronomer Urbain Jean Joseph Leverrier; and Pluto in 1930 by the American astronomer Clyde William Tombaugh. The number of known natural satellites is increasing as unmanned probes fly by the outer planets. The earth has 1 natural moon; Mars, 2; Jupiter, 16; Saturn, more than 20; Uranus, 15; Neptune, 8; and Pluto, 1. These numbers may continue to increase as astronomers get better views of the planets. More than 1600 asteroids have been followed as they move around the sun, mostly between the orbits of Mars and Jupiter. Several hundred separate comets are cataloged. Countless smaller bodies exist as stony and metallic meteoroids.
The chemical analysis and physical study of inaccessible heavenly bodies were made possible by the invention of the spectroscope in 1814 by the German physicist Joseph von Fraunhofer and the subsequent discovery that every chemical element exhibits a unique set or sets of spectral lines. Analyses of planetary and stellar spectra have demonstrated that heavenly bodies are composed of the same chemical elements known on earth. Spectroscopic studies also provide clues about such conditions as the surface temperatures, surface gravities, and motions of the heavenly bodies. Instrument-bearing satellites, which have approached Mercury, Venus, Mars, Jupiter, Saturn, and Uranus in the 1970s and '80s to gather chemical and physical data from these bodies, have discovered rings about Jupiter and new moons of that planet, Saturn, and Uranus, and have supplied information that casts doubt on the possible presence of life on other planets in the solar system. These planets appear to be too hot, too cold, or too dry or to have atmospheres too inhospitable to life as conceived by humans.

Nearby Stars
Before the invention of the telescope the stars were regarded as merely a convenient backdrop for scanning the wanderings of the sun, moon, and planets. For the modern astronomer equipped with telescope and spectroscope, the study of the stars is a challenging aspect of astronomy.
Basic to the study of a star is the knowledge of its distance from the earth, which is found by measuring the position of the star in the sky at intervals six months apart, when the earth is on opposite sides of its orbit. As the earth swings around the sun, the star appears to shift back and forth in the sky. This annual shift, called parallax, can be used to determine the distance from the earth of a star near the sun. The greater the distance, the smaller is the parallax of the star. The nearest star, Alpha Centauri, is about 260,000 times farther from the earth than is the sun. The first star distances were measured independently by three astronomers in 1838.
All stars are hot, gaseous bodies like the sun, but differ from it and from one another in various ways. The most important physical data about a star are its intrinsic brightness, size, mass, and chemical composition. Although all fixed stars appear much fainter than the sun because of their great distances from the earth, some of them are intrinsically brighter . Star masses can be determined directly for the sun and for pairs of stars, such as eclipsing binaries, that exhibit a mutual revolution, similar to the motions of the planets around the sun. Assuming that these revolutions are due to gravitation, astronomers apply the law of gravitation to determine the stellar masses mathematically. Of the 50 nearest stars for which information is fairly complete, 10 percent are brighter, larger, and more massive than the sun. Spectroscopic studies show that the majority of the stars are composed largely of hydrogen.
The source of the vast energy radiated by the sun was long a mystery. The sun produces 3.86 × 1026 watts (5.18 × 1023 hp). Geological evidence shows that life has existed on earth for some billion years, indicating that solar energy must have been expended at about its present rate for hundreds of millions of years. In 1938 the American physicist Hans Bethe advanced the theory that solar energy is produced by the nuclear fusion of hydrogen atoms into helium. His discovery helped pave the way for the development of a nuclear- fusion hydrogen bomb approximately 15 years later.
Stars at least 1.4 times more massive than the sun pass through their entire life cycle much faster than the sun. Optical telescopes have revealed the principal steps in this cycle. First the star begins to condense from inside but generally near one edge of a dense molecular cloud, or “cocoon.” This condensation initiates a period of contracting and internal heating followed by a long period as a main-sequence star. Near the end of its lifetime, the star expands to a red giant state, contracts back to the main sequence, and degenerates to a white dwarf.
In the 1960s the British radio astronomer Jocelyn Bell discovered rapidly varying signals coming from starlike objects. Studies by the British radio astronomer Antony Hewish showed these to be pulsating sources, named pulsars, that consist of matter even more condensed than white dwarfs. A pulsar is apparently the last stage in the life cycle before final extinction as a black hole, which is matter so dense that nothing, not even radiation, can escape from it. In 1974 the existence of a black hole in the constellation Cygnus was suggested by detection of X radiation from gas accelerated to nearly the speed of light as it fell into the black hole. Since that time other possibilities have been proposed, including tremendous black holes located at the center of intensely radiating galaxies. No black hole, however, has yet been confirmed to exist.

The Galaxy
In the late 18th century, Sir William Herschel constructed the largest reflecting telescopes of his day and used them to explore the heavens. The first serious student of the universe, he discovered not only the planet Uranus but also a number of satellites and many double stars, in addition to myriad star clusters and nebulas (see NEBULA). His counts of stars in different regions of the heavens convinced Herschel that the sun is one of a vast cloud of stars arranged like the grains of abrasive in a grindstone. According to his analogy, a person living on a small planet near the sun deep inside the grindstone looks toward its edges and is able to see a belt of faint, distant stars, which is called the Milky Way, or the earth's galaxy, stretching completely around the sky; looking above or below, the person is able to see relatively few nearby stars.
Modern investigations confirm that the Milky Way is a galaxy of stars that are all gravitationally bound and rotating about a distant center. Of primary importance in studying the structure of the Milky Way is a knowledge of star distances. The parallax method of determining these distances can be applied only to a few thousand of the nearest stars. A special class of stars exists, the Cepheid variables, which vary in brightness in periods that depend on their intrinsic intensities. Comparison of the observed brightnesses with the known intrinsic brightness of these stars provides a means of determining their distances. Following the discovery of the relation between period and luminosity by the U.S. astronomer Henrietta Swan Leavitt, U.S. astronomer Harlow Shapley used the Cepheid variables, scattered throughout the Milky Way, to measure its size. A ray of light, moving at a speed of about 300,000 km/sec (about 186,000 mi/sec), would require 400,000 years to traverse the Milky Way from edge to edge of its extended halo (described below). The visible spiral is somewhat less than half as wide. Altogether, the Milky Way consists of about a million million stars rotating about a common center. The sun, located about 30,000 light-years from the center of the Milky Way, travels at a speed of about 210 km/sec (about 130 mi/sec) and completes an entire revolution approximately every 200 million years.
The Milky Way includes great quantities of dust and gas particles scattered between the stars. This interstellar matter intercepts the visible light emitted by distant stars so that observers on earth cannot view in detail distant parts of the Milky Way. A new branch of astronomy was initiated when the American electronic engineer Karl G. Jansky discovered in 1932 that radio waves are emitted in the Milky Way. Later study traced this radiation partly to interstellar matter and partly to discrete sources, formerly called radio stars. Radio waves emitted by distant parts of the Milky Way can penetrate interstellar matter, which is opaque to visible light, and thus enable astronomers to observe regions hidden to optical instruments. Such observations have revealed the Milky Way to be a spiral galaxy with a flattened bulge of old stars, an outer disk of hot young stars that make up the spiral arms, and a large, extended halo of faint stars. From observations of the outer disk by radio telescope in 1986, astronomers saw, for the first time in history, the birth of a star, in the constellation Ophiuchus, or the Serpent Bearer, 500 light-years away. The nucleus of the Milky Way was until recently a mysterious region, obscured from view by dark clouds of interstellar dust. Astronomers began getting their first detailed picture of the region in 1983, when the Infrared Astronomy Satellite (IRAS) was launched. Freed from the obscuring effects of the earth's atmosphere, sensors aboard IRAS recorded in unprecedented detail the positions and shapes of the myriad sources of infrared energy that occupy the heart of the Milky Way. Among these was discovered one massive object, not a star and too compact to be a star cluster, that may yet prove to be a black hole.

The Cosmos
Despite its vast size, the Milky Way is only one of many great star systems, called galaxies, that populate the known universe. Studies conducted by the American astronomer Edwin Hubble resolved in 1924 the question as to the nature of the spiral nebulae, showing them to be individual galaxies like the Milky Way but located at very great distances. Some galaxies have a spiral form, like the Milky Way; other galaxies are spheroidal, without the spiral arms, or of irregular shape. The largest optical telescope in the world, the 387-in (9.82-m) Keck Telescope at Mauna Kea Observatory in Hawaii, has revealed galaxies as far away as several billion light-years.
Spectrum analysis of the light from exterior galaxies shows that the stars making up these systems are composed of the same chemical elements known on earth. Somewhat unexpectedly, it also demonstrates that the galaxies are all moving away from the Milky Way: the more distant a galaxy, the faster its recession . This is currently taken as evidence that the universe is expanding, and that it originated from an extremely hot, dense state of matter by an explosion called the big bang . The possible conditions that could have initiated the explosion are treated in a cosmological theory of the early 1980s known as the inflationary theory. Big bang radiation has been cooling ever since; its present temperature is about 3 K above absolute zero (about -454° F). Radiation of this temperature, coming from all directions, was discovered in 1965 by the American physicists Arno Penzias and Robert W. Wilson, and is currently the best indicator of the early history of the universe. Albert Einstein's relativistic theory of gravitation also supports the big bang theory.
Quasars, discovered in the 1950s with the use of radio telescopes, are believed by most astronomers to be the energetic nuclei of very distant galaxies. For reasons not yet known, they have brightened so much that they mask the light from their underlying galaxies. Often they occur in extremely distant clusters of galaxies. The spectral lines of quasars display very large red shifts, which would indicate that these objects are traveling away from earth's galaxy at speeds in the range of 80 percent of the speed of light. Their apparent great speed also means that they are among the most distant of cosmological objects. A quasar 12 billion light-years distant was discovered in 1991 by astronomers using the 200-in. (5.08-m) reflector at Palomar Observatory.


  • Todd Gross's Page of Astronomy
  • Astronomy and Astrophysics Guide
  • Astronomy Live - STARLAB: Portable Planetarium
  • Ireland Astronomy
  • INFRARED ASTRONOMY (IPAC Educational Outreach)
  • TASS Motherpage
  • The Astronomy Cafe A must see!
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