I AM A SCORPIO

Astronomy
I INTRODUCTION
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, clusters of galaxies, and quasars. Modern astronomy is divided into several branches: 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.
II ANCIENT ORIGINS
The 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, the need to establish the proper times for planting and harvesting crops and for religious celebrations and to find bearings and positions on long trading journeys or voyages. see Archaeoastronomy.
To ancient peoples the sky showed many regularities of behaviour. The bright Sun, which divided day from night, 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 the stars could be seen to follow a similar course, seeming to rotate in permanent groupings, called constellations, around a fixed point in the sky, which is known as the celestial pole.
People noticed that daytime and night-time were unequal in length. On long days, seen from the North Temperate Zone, 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. 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. see Ecliptic.
Further study showed that the Sun, Moon, and five bright planets 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. Because neither an entire month nor an entire year contains an exact number of days the calendar-makers assigned to different months or years different numbers of days, having a long-range average that is close to the true value. 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, sometimes move westward, or retrograde, for varying durations. The planets seem to follow an eastward course erratically, with periodic loops in their paths. Since ancient times, people have 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, but has no scientific basis.
III 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 went further. 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 found how to represent this cycle arithmetically and 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 the 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 Erech (Uruk) on the Euphrates River, bear the name of Naburiannu, who flourished around 491 BC, or Kidinnu, who flourished around 379 BC—astrologers who may have invented the systems of calculation.
IV 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 ploughing, 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. About 450 BC the Greeks began a fruitful study of planetary motions. Philolaus, who flourished during the 5th century BC and was a follower of Pythagoras, believed that the Earth, Sun, Moon, and planets all moved around a central fire hidden from view by an interposed counter-Earth. 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 revolved around the Earth, rotating daily. 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.
Another Greek, Aristarchus of Samos, believed that motions in the sky could be explained by the hypothesis that the Earth turns on its axis once every 24 hours and, along with the other planets, moves around the Sun. This explanation was rejected by most Greek philosophers, who regarded the large, heavy Earth as a motionless globe around which the light, incorporeal celestial bodies revolve. This theory, known as the geocentric system, remained largely unchallenged for about 2,000 years.
The Greeks combined their celestial theories with carefully planned observations. The astronomers Hipparchus, in the 2nd century BC, and Ptolemy, in the 2nd century AD, determined the positions of about 1,000 bright stars and used this star chart as a background for measuring the planetary motions. They postulated that the planets, Sun, and Moon moved around a series of eccentric circles, with the Earth near a common centre. To explain the periodic variations in the speed of the Sun and Moon and the retrograde motions of the planets, they proposed that each of these bodies revolved uniformly around a second circle, called an epicycle, the centre of which moved around the first circle. By proper choice of the diameters and speeds for the two circular motions ascribed to each body, its observed motion could usually be represented. In some cases a third circle was required. This technique was described by Ptolemy in his great work the Almagest (see Ptolemaic System). Another thinker, who, like Ptolemy, kept the tradition of Greek astronomy alive in Alexandria in the early centuries of the Christian era, was Hypatia, a follower of Plato. She wrote commentaries on mathematical and astronomical topics and is regarded today as the first noteworthy female scientist and philosopher of the West.
Greek astronomy was later transmitted eastward to the Syrians, the Hindus, and the Arabs. The Arabian astronomers compiled new star catalogues in the 9th and 10th centuries and subsequently developed tables of planetary motion. The Arabs were good observers, but they made few useful contributions to astronomical theory. Medieval European astronomy began to flourish in the 13th century, when Arabic translations of Ptolemy’s Almagest filtered into Western Europe. 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.
V 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 investigating astronomy, however, and he made a new star catalogue from personal observations. He is most famous for his great work On the Revolutions of the Heavenly Bodies (1543), in which he analysed critically the Ptolemaic theory of an Earth-centred 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 the Italian physicist Galileo found evidence to support it. Long an admirer of Copernicus’s work, Galileo found evidence to support the idea after the invention, or reinvention, of the telescope in the Netherlands early in the 17th century. 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 orbiting around Jupiter, forming a miniature version of the Copernican solar system. Galileo also made the sensational discovery that the Milky Way is made up of a myriad of individual stars. He began to speak and write in favour of the Copernican system, but ran into trouble with the ecclesiastical authorities, and was tried in Rome for heresy in 1632. Although he was found guilty and forced to repudiate his beliefs and writings, large parts of Europe were by then no longer under the influence of the Catholic Church, and the powerful theory could not be suppressed.
VI 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. Using the data compiled by Brahe, his German assistant, Johannes Kepler, formulated the laws of planetary motion, after discovering 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 are related to their periods of revolution.
The English physicist Isaac Newton advanced a simple principle to explain Kepler’s laws of planetary motion. 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 inversely on the squares of the distances between them, provides the basis for the physical interpretation of Kepler’s laws. Although others had speculated that an inverse square law might explain the elliptical orbits of the planets, it was Newton who proved mathematically that only such a law could do so, and who introduced the idea that this law of gravitation applies to every object in the universe.
VII ASTRONOMY BECOMES A SCIENCE
In the 18th century Edmond Halley used a combination of Newton’s and Kepler’s laws to explain the orbits of comets, and successfully predicted the return of the comet that now bears his name. Telescopes also improved rapidly, and made new observations possible—as early as 1659, the Dutch polymath Christiaan Huygens had discovered the true nature of the rings of Saturn, explaining them as rings around the planet, not pieces sticking out of the planet. In 1675 Giovanni Cassini, an Italian-born astronomer, discovered the gap in the rings of Saturn that is still known as the Cassini division.
Newton had left one great puzzle in his theory of planetary orbits. Although one planet on its own would orbit the Sun forever in an elliptical orbit, Newton himself had not been able to prove that the orbit would still be stable when it was influenced by the gravitational pull of other planets, as well as the Sun. The French mathematician and astronomer Pierre Laplace solved this puzzle in the mid-1780s. He showed that although the orbits of the planets are affected by the gravity of the other planets, the changes are cyclic. For example, at that time the orbit of Jupiter was slowly getting smaller, while the orbit of Saturn was slowly getting bigger. Laplace proved that this is part of a cycle 929 years long in which the orbits grow larger and smaller.
In 1773, from studies of the way stars seem to be moving across the sky, William Herschel inferred that the Sun and solar system are all moving through space in the direction of the constellation Hercules, and in 1781 Herschel discovered the seventh planet of the solar system, Uranus. The first known asteroid, or minor planet, Ceres, was discovered in 1801; Neptune, the eighth major planet, was discovered in 1846. The first stellar distance, determined by parallax, was measured in 1838, to the star 61 Cygni. However, the greatest change in astronomy in the 19th century (apart from continuing improvements in telescopes) was the development of spectroscopy by the German physicists Gustav Kirchhoff and Robert Bunsen, building on the work of Joseph von Fraunhofer, which made it possible to study the composition of the stars and determine their temperatures from the lines that can be seen in their light when it is spread out to form a spectrum using a prism. Thanks to the Doppler effect, the changing positions of the lines in the spectrum even make it possible to measure the speed with which a star is moving.
Knowledge of the solar system continued to grow. The ninth planet, Pluto, was discovered in 1930 by the American astronomer Clyde William Tombaugh. The number of known natural satellites increased dramatically when unmanned probes flew past the outer planets. As of 2003, the tally of known natural moons is: Earth, 1; Mars, 2; Jupiter, 47; Saturn, 30; Uranus, 20; Neptune, 11; and Pluto, 1. These numbers may continue to increase as astronomers get better views of the planets. Thousands of asteroids have been followed as they move around the Sun, mostly between the orbits of Mars and Jupiter. Several hundred comets have been catalogued and studied in detail by probes, such as the Giotto probe sent to Comet Halley in 1986 and the Stardust probe launched towards Comet Wild-2 in 1999. Countless smaller bodies exist as stony and metallic meteoroids. Yet at the beginning of the 21st century, the greatest development of astronomy so far has been in our understanding of objects beyond the solar system.
VIII MODERN ASTRONOMY
Throughout the 20th century, larger and larger reflecting telescopes were built. Early studies with these instruments showed, in the 1920s, that the entire Milky Way system, containing hundreds of billions of stars, is just one galaxy among many millions of similar “islands in space”. In the second half of the 20th century, developments in physics led to new kinds of astronomical instruments, some of which have been placed on unmanned Earth-orbiting satellite observatories. These instruments are sensitive to a wide range of wavelengths of electromagnetic radiation, including the gamma-ray, X-ray, ultraviolet, infrared, and radio regions of the spectrum. Astronomers now study not only planets, stars, and galaxies but also interstellar clouds that are the birthplaces of new stars, cold dust grains that are invisible in the optical regions, dim failed stars called brown dwarfs, quasars, which are energetic nuclei of galaxies that almost certainly contain black holes, and background radiation originating from the big bang that is beginning to yield information about the origin of the universe. see Gamma-Ray Astronomy; Infrared Astronomy; Radar Astronomy; Radio Astronomy; Space Exploration; Ultraviolet Astronomy; X-Ray Astronomy.
IX STARS
The nearest star, Alpha Centauri, is about 260,000 times farther from the Earth than is the Sun. It is 4.3 light years away; a light year is the distance light would travel in one year, at a speed of 300,000 km/s. The entire Milky Way Galaxy is about 100,000 light years across, but only some 1,000 light years thick.
All stars are hot, gaseous bodies like the Sun, but differ from it and from one another in minor ways. The most important physical data about a star are its intrinsic brightness, size, mass, and chemical composition. Although all stars seem much fainter than the Sun because of their great distances from the Earth, some of them are intrinsically brighter than the Sun (see Magnitude). Star masses can be determined directly for the Sun and for pairs of stars, in binary systems, that are seen to orbit around each other. Astronomers apply the law of gravitation and Kepler’s laws to determine the stellar masses mathematically. Of the 50 nearest stars for which information is fairly complete, 10 per cent are brighter, larger, and more massive than the Sun. Spectroscopic studies show that the stars are composed largely of hydrogen.
The source of the vast quantities of energy radiated by the Sun was long a mystery. The Sun emits energy at the rate of 3.8 × 1026 watts. Geological evidence shows that life has existed on Earth for billions of years, indicating that solar energy must have been expended at about its present rate for that long. Even before the processes by which the Sun generates energy were understood, however, the pioneering astrophysicist Arthur Eddington was able, in the 1920s, to work out the internal structure of stars from the laws of physics. No matter how the Sun generates its energy, for example, it is straightforward to calculate how hot it must be inside from its known mass and the rate at which it is radiating. This enabled Eddington to work out the temperature at the heart of the Sun, about 15 million degrees C (27 million degrees F).
In the 1930s it was clear that the energy generation process must involve the conversion of hydrogen into helium inside the Sun, and in 1938 the American physicist Hans Bethe showed how this could be achieved in a series of nuclear interactions called the proton-proton chain. This process involves the conversion of some of the mass of the original hydrogen nuclei into energy. Overall, over 4 million tonnes of mass are converted into pure energy at the heart of the Sun each second, in line with the equation, devised by Albert Einstein, E = mc2. This says that mass and energy are interchangeable, and that the energy E locked up in a mass m is equal to the mass multiplied by the square of c, the speed of light.
Observation and theory combine to reveal the principal steps in the life cycle of a star. The protostar begins to condense from inside a relatively dense and cool cloud of interstellar gas. This contraction converts gravitational energy into heat, and this internal heating increases the temperature at the heart of the star to the point where nuclear reactions can begin (a temperature of about 15 million degrees C/27 million degrees F). The heat generated by these reactions then halts the contraction, and the star settles down to a period as a hydrogen-burning main-sequence star. In the case of a star with about the same mass as the Sun, the main-sequence phase lasts about 10 billion years. Near the end of its lifetime, such a star expands to a red giant state, expels gas from its atmosphere, contracts again, and then shrinks and cools as a dense white dwarf star, containing about as much mass as the Sun in a volume roughly the same as the Earth. Stars with more mass run through their life cycles more quickly; those that end their time on the main sequence with more than about ten times as much mass as the Sun explode as supernovae. see Star: Evolution of Stars.
In 1967 a British radio astronomer, Jocelyn Bell, discovered rapidly varying radio signals coming from well-defined points on the sky. The objects producing this noise were named pulsars, a contraction of pulsating radio sources, but were soon shown to be rotating compact objects, spinning several times a second and sweeping beams of radio noise around the sky like a radio lighthouse. In order to spin so rapidly without breaking apart, pulsars must consist of matter even more condensed than white dwarfs, with about as much mass as our Sun packed into a sphere less than 10 km (6 mi) across, about as big as a large mountain on Earth. A pulsar is a fast-spinning neutron star, a tightly packed mass of neutrons, like a single huge atomic nucleus. It is the densest object in the universe apart from a black hole. Pulsars are formed as the remnants of supernova explosions.
Even a mass of neutrons cannot hold itself up against the inward tug of gravity if the mass left over from the supernova explosion is more than about three times the mass of Sun. Any neutron star that tries to form with more mass than this will collapse to form a black hole, which has such a strong gravitational field 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 hot gas swirling in a ring as it fell into the black hole. The X-ray source was a member of a binary system, so its mass could be estimated, and turned out to be above the limit of stability for a neutron star. Since then several black hole candidates have been identified. There is also strong evidence that much larger black holes, containing hundreds of millions of times as much mass as our Sun, lie at the hearts of many galaxies, and may provide the power source for quasars.
X 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. He discovered not only the planet Uranus but also several satellites and many double stars, in addition to myriad star clusters and nebulae. His counts of stars in different regions of the heavens convinced Herschel that the Sun is one of a vast cloud of stars, a cloud shaped like a grindstone. According to his analogy, a person living on a small planet near the Sun deep inside the “grindstone” looks towards its rim and is able to see a belt of faint, distant stars, which is called the Milky Way, stretching completely around the sky; looking above or below, the person is able to see relatively few nearby stars.
Modern investigations confirm this picture, except that the solar system is now known to be about two thirds of the way out from the centre. The name “Milky Way” or “galaxy” is often applied to the whole system. The stars in the system are all gravitationally bound and orbiting about a distant centre. A knowledge of star distances is of primary importance in studying the structure of the Milky Way. The parallax method of determining these distances can be applied only to the nearest stars. A particular class of stars exists, the Cepheid variables, which vary in brightness with periods that depend on their intrinsic intensities. Comparison of the observed brightness of such a star with the intrinsic brightness calculated from its period provides a means of determining its distance. Following the discovery of the relation between period and luminosity by Henrietta Swan Leavitt, 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/s (186,300 mi/s), would require 400,000 years to traverse the Milky Way from edge to edge of its extended halo (described below). The visible spiral is approximately 100,000 light years across. Altogether, the Milky Way consists of about 100 billion stars orbiting about a common centre. The Sun, located about 30,000 light years from the centre of the Milky Way, travels at a speed of about 210 km/s (130 mi/s) and completes an entire revolution approximately every 250 million years.
The Milky Way includes great quantities of dust and gas 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 electronics engineer Karl G. Jansky discovered in 1931-1932 that radio waves are emitted from the Milky Way. Later study traced this radiation partly to interstellar matter and partly to discrete sources, originally called radio stars. Radio waves emitted by distant parts of the Milky Way can penetrate interstellar matter that 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 barred spiral galaxy with a flattened central bulge of old stars, an outer disc of both older stars and hot young stars that make up the spiral arms, and a large, extended halo of faint stars.
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 it was indicated that a supermassive black hole lies at the centre, as indeed there appears to be in the centre of most galaxies.
Advances in observation techniques led to the opening of a new branch of astronomy in the mid-1990s. In 1995 a Swiss team of astronomers announced the discovery of the first extrasolar planet orbiting a Sun-like star. They had deduced the planet's mass, orbital period, and distance from the star by measuring small wobbles in the star's axis of rotation caused by the pull of the planet's mass. By the beginning of the 21st century, over 100 such planets had been discovered, including, in 1999, the first multiple-planet system. While the technique was initially only sensitive enough to detect planets that were the size of Jupiter and orbiting closer to their star than the Earth is to the Sun, refinements are expected eventually to enable it to detect planets the size of Earth. Indeed, two smaller planets, about the size of Saturn, were discovered early in 2000. A different approach aims to image extrasolar planets directly, by designing space-based telescopes equipped with a central baffle to block the glare of the parent star's light. By the end of the 1990s a number of such designs had been proposed. The first visual proof of the existence of an extrasolar planet was announced at the end of 1999, when a tiny dip in the light of a star was observed just at the time when a planet was due to pass in front of it. Such observations allow much more information to be obtained about the planetary body, in particular its diameter.
XI THE COSMOS
The Milky Way is only one of many galaxies that populate the known universe. By 1924 studies conducted by the American astronomer Edwin Hubble had shown that the spiral nebulae are individual galaxies like the Milky Way, located at very great distances. Although some galaxies have a spiral form, like the Milky Way, other galaxies are ellipsoidal, without spiral arms; others still are of irregular shape, sometimes showing traces of spiral arms.
Spectral analysis of the light from the galaxies shows that the stars making up these systems are composed of the chemical elements known on Earth, dominated, like the Sun, by hydrogen. Such analysis also demonstrates a red shift, indicating that the galaxies (or rather, clusters of galaxies) are all moving away from each other: the more distant a galaxy, the faster its recession. Although this red shift is analogous to the Doppler effect, it is explained by the general theory of relativity as evidence that the universe is expanding, with space itself stretching, not that the galaxies are flying apart through space. This means that the universe originated from an extremely hot, dense state in an outburst called the big bang. Space and time, as well as energy and matter, were created in the big bang, which was not an explosion at a point in space.
Radiation filling the universe has been cooling ever since the big bang; its present temperature is about 3 K (3° C above absolute zero, or about -454° F). Radiation of this temperature, coming from all directions in the sky, was discovered in 1965 by the American physicists Arno Penzias and Robert Wilson, and is the best indicator of the early history of the universe. Einstein’s theory of gravitation, the general theory of relativity, predicted that the universe would be found to be expanding, although Einstein himself resisted this conclusion; the big bang description of the universe is essentially based on general relativity. A combination of theory and observation indicates that the big bang occurred at least 13 billion years ago.
Quasars, discovered in the 1960s, are the energetic nuclei of very distant galaxies. They are so bright that a quasar masks the light from the surrounding galaxy, just as the light from a pocket torch could not be seen in the glare from a searchlight. Often quasars occur in extremely distant clusters of galaxies. The spectral lines of quasars display very large red shifts, which indicates that some of these objects are receding from us at speeds of 80 per cent of the speed of light or more. Because the red shift is proportional to distance, this means that they are among the most distant of cosmological objects, and that we see them by light that left them when the universe was only about 7 per cent of its present age, some 1 billion years after the big bang. In 1998 astronomers announced that observations of supernovae in distant galaxies had a higher red shift than predicted, suggesting that the expansion of the universe is actually accelerating.
However, everything described above may be no more than the tip of the universal iceberg. From the way that galaxies move within clusters, astronomers calculate that they are being held in the gravitational grip of at least 10 times, and perhaps 100 times, as much matter as we can see in the form of bright stars and galaxies. Observations in 1998 hinted at an inverse relationship between dark and normal matter, with the smallest, faintest galaxies having motions that indicated the presence of the greatest amount of dark matter. The nature of this dark matter is still a mystery, and determining its composition will be one of the most important problems facing astronomers in the 21st century.
Microsoft ® Encarta ® Encyclopedia 2004. © 1993-2003 Microsoft Corporation. All rights reserved.
Zodiac
Zodiac, imaginary belt in the celestial sphere, extending about 8° on either side of the ecliptic, the apparent path of the Sun among the stars. The width of the zodiac was determined originally so as to include the orbits of the Sun and Moon and of the five planets (Mercury, Venus, Mars, Jupiter, and Saturn) that were known to the people of ancient times. The zodiac is divided into 12 sections of 30° each, which are called the signs of the zodiac. Starting with the vernal equinox and then proceeding eastwards along the ecliptic, each of the divisions is named after the constellation situated within its limits in the 2nd century BC. The names of the zodiacal signs are: Aries, the Ram; Taurus, the Bull; Gemini, the Twins; Cancer, the Crab; Leo, the Lion; Virgo, the Virgin; Libra, the Balance; Scorpio, the Scorpion; Sagittarius, the Archer; Capricornus, the Goat; Aquarius, the Water Bearer; and Pisces, the Fishes. Because of the precession of the equinoxes about the ecliptic, a 26,000-year cycle, the first point of Aries retrogrades about 1° in 70 years, so that the sign Aries today lies in the constellation Pisces. In about 24,000 years, when the retrogression will have completed the entire circuit of 360°, the zodiacal signs and constellations will again coincide. See Astronomy.
It is believed that the zodiacal signs originated in Mesopotamia as early as 2000 BC. The Greeks adopted the symbols from the Babylonians and passed them on to the other ancient civilizations. The Egyptians assigned other names and symbols to the zodiacal divisions. The Chinese also adopted the 12-fold division, but called the signs rat, ox, tiger, hare, dragon, serpent, horse, sheep, monkey, hen, dog, and pig. Independently, the Aztec people devised a similar system.
See also Astrology and separate articles on the signs of the zodiac.
Microsoft ® Encarta ® Encyclopedia 2004. © 1993-2003 Microsoft Corporation. All rights reserved.
Scorpio (astrology)
Scorpio (astrology), the eighth sign of the zodiac, symbolized by a scorpion. According to astrologers, people whose birthdays fall between October 23 and November 21 are born under the sun sign of Scorpio. The planet Pluto rules Scorpio, which is a water sign.
Astrologers consider Scorpios to be energetic, passionate, deep, intuitive, and secretive, with a great deal of self-control. They also believe that Scorpios can be wilful, stubborn, and easily made jealous. Scorpios are thought to be keen observers of people, potentially calculating and manipulative. Seeing more of people's deepest motivations than others do, they have a tendency to be cynical. They are sensitive and never forget a hurt or a slight—for the typical Scorpio, forgiveness can be difficult.
Astrologers consider Scorpio perhaps the most extreme of all signs. The intensity and focus of Scorpios gives them great ability to see a project through despite all obstacles. Their strong leadership qualities, incisive analytic abilities, energy, and desire for financial security can make them motivated career people. Many Scorpios also like to flirt with danger and push themselves and those close to them to their limits. Professions traditionally associated with Scorpio include forensic science, law enforcement or detective work, the military, medicine, psychology, and business. Famous Scorpio subjects include Prince Charles, Marie Antoinette, Richard Burton, Charles de Gaulle, Pablo Picasso, and Billy Graham. See also Astrology; Horoscope.
Microsoft ® Encarta ® Encyclopedia 2004. © 1993-2003 Microsoft Corporation. All rights reserved.
Horoscope
Horoscope, chart of the position of the Sun, Moon, and planets from a given latitude and longitude on Earth at a given moment, usually that of birth. The construction of the horoscope is based on the Ptolemaic system, in which the Earth is stationary and the heavenly bodies move around it. Astrologers have divided the heavens into 12 sections, or houses, each thought to be ruled by a different sign of the zodiac, the band of the sky through which the Sun, Moon, and planets move. Once the houses occupied by the heavenly bodies have been established, their traditional astrological characteristics, modified by the geometrical relationship between them, are used to divine the character, and foretell events in the life, of the individual for whom the horoscope has been drawn.
Microsoft ® Encarta ® Encyclopedia 2004. © 1993-2003 Microsoft Corporation. All rights reserved.
Astrology
I INTRODUCTION
Astrology, a system based on the belief that events on Earth are represented by the positions and movements of astronomical bodies, particularly the Sun, Moon, planets, and stars. The word "astrology" derives from the Greek astron (star) and logos (word, study). Astrologers maintain that the position of astronomical bodies at the exact moment of a person’s birth and the subsequent movements of the bodies reflect that person’s character and, therefore, destiny. The celestial patterns are interpreted so as to understand, plan, or predict events on Earth. They are deemed to be associated with the characteristics of individuals.
Astrologers use charts known as horoscopes, which map the position of astronomical bodies at certain times, and are based on the signs of the Zodiac; these are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces.
Astrology is one of the oldest known forms of study and its continuous history can be traced back over 4,000 years to the ancient Middle East. The astrology used in India and the West share common origins, but most cultures have developed their own forms of mystical cosmology, and at least two, China (by 2000 BC)and the Mayan and Aztec cultures of central America, also developed complex systems of astrology. These people may have observed that the movements of certain astronomical bodies, particularly the Sun, affected the change of seasons and the success of crop harvests. It was astrology, and the hope of predicting the future, that led to systematic study of the heavens, and to the development of the science of astronomy—as alchemy led to chemistry. SeeArchaeoastronomy.
Astrology’s current popularity dates from the late 19th century, and the invention of the distinctive twelve-sign newspaper horoscope column in 1930 made it an entrenched part of modern mass media and popular culture.
II HISTORY
A Middle-Eastern Origins
Astrology can be traced back to the earliest literate urban civilization in Mesopotamia (modern Iraq) before 2000 BC, although its origins may date from earlier attempts to create calendars in order to regulate civil and religious life or to divine the future. Deer antler markings dating from 15000 BC may indicate lunar phases, while the earliest post holes at Stonehenge in southern England are dated to 8000 BC. Little is known about what kind of astrology may have been practised by Neolithic people, or by later peoples who left no written records, such as the Celts.
The first complete surviving astrological text is the Venus Tablet, a collection of omens on the planet Venus compiled in the reign of the Babylonian king Amisaduqa, around 1650 BC. A thousand years later the Assyrian emperor Ashurbanipal collected all the known astrological tablets together in a single collection, the Enuma Anu Enlil. Originally all astrology seems to have been devoted to the king and the state, and the astrologer’s task was to offer political advice. The first known individual birth chart dates from the Persian period, 410 BC.
Developments in Egypt followed a different course and it has been argued that, as the inhabitants of the Nile delta lived in a relatively secure environment, they felt little need to forecast the future and therefore did not require a complicated predictive astrology. Instead, they evolved an elaborate astral theology in which the Sun was the symbol of the supreme creator god Amon or, in earlier times creator goddesses such as Sekhmet or Hathor. The constellation Orion represented the god Osiris, of whom the Pharaoh was regarded as an incarnation, while the star Sirius represented his consort, Isis. The great monuments of Egyptian history tend to be aligned with the Sun or stars. The Great Pyramid (c. 2600 BC), for example, was aligned with Sirius and Orion.
B Ancient Greece
The classical Greeks devised the philosophical and technical basis of the astrology still in use today. The Athenian philosopher Plato (c. 428-347 BC) argued that the entire universe was divine and could be seen as an image of God, who placed the planets in the sky in order that his nature, ideas, and intentions might be understood. Aristotle (c. 384-322 BC) originated the concept of celestial causes and influences, although he believed that the planets were “secondary causes” responsible for transmitting God’s will to humanity. By the 1st century BC the Greeks had developed the horoscope of 12 zodiacal signs, 12 houses, 7 planets, and 5 major aspects that is the basis of modern horoscope interpretation. The principal figure was Claudius Ptolemy (c. 120 AD), whose Tetrabiblos summarized much of the astrological teaching of the time and was to become a standard text in medieval and Renaissance Europe.
C India
Astrology is known in India as Jyotish and was originally used to fix the most auspicious dates for the performance of religious rituals and the regulation of the sacred calendar prior to 2000 BC. However, the bulk of technical Indian astrology appears to have been imported from the Hellenized Greek world which, under the conquests of Alexander the Great (356-323 BC), extended eastwards to the Indus valley.
D Roman Empire
Astrology became a standard part of life in the Roman Empire from the 1st to 4th centuries BC and was used extensively by the emperors, who came to regard themselves as incarnations of the Sun God: the main festival of the cult of Sol Invictus, the reborn and unconquered Sun, was December 25. Astrology was also central to the Mithraicreligion, which was popular among the Roman legions, and in which the planets, as spiritual entities, were seen as a path to God and spiritual enlightenment.
E Christianity and Islam
The ambivalence towards astrology evident in the Old Testament, combined with its associations with paganism in the Roman Empire, led to hostility from many Christians, notably St Augustine (354-430), who attacked it in his Confessions. Christians were particularly concerned that astrological determinism conflicted with the individual’s freedom to choose between right and wrong or to seek salvation.
This, combined with the collapse of the Roman Empire in western Europe in 476, moved the focus of attention east to India and Persia. From here astrology was rediscovered by the Arabs after their conquest of the Middle East.
European scholars began to relearn astrology from the Islamic world in the late 10th century, and by the 12th century it was a central part of the new world view, studied at university as a single subject with astronomy. Thomas Aquinas (c. 1225-1274) reconciled astrology with Christianity by arguing that while the soul, and hence the individual’s freedom to make moral choices, was responsible to God, their physical needs and passions, and hence matters of war and peace, disease, or economics were also influenced by the stars.
As long as astrologers respected the primacy of Christian theology and ecclesiastical power, the subject was an accepted part of natural philosophy. In the absence of developed science, technology, and medicine, it was seen as an essential part of medical diagnosis and treatment, an aid to political power used by kings and popes, a topic for philosophical speculation, a means to arrange marriages or make money, and a tool for forecasting the future.
F The Scientific Revolution
By the late 17th century, European astrology had lost its intellectual respectability. It has been argued that this was due to the scientific and astronomical discoveries of Galileo, Isaac Newton, and Nicolaus Copernicus, which showed that the Sun was the centre of the solar system, and that other aspects of the solar system were not arranged as previously thought. These discoveries, along with the growth of materialism and the scientific method, led to the rejection of astrology by many Western scientists and philosophers (seeScience, Philosophy of). Another argument is that astrology suffered from its association with radical and popular political movements that sprang up in the early 17th century.
G Nineteenth-Century Revival
The revival of interest in astrology began at the end of the 18th century in England, and benefited from parallel developments such as the spread of spiritualism and discovery of eastern religions. The most notable figure was Alan Leo (1860-1917), a British astrologer who wrote extensively. Leo began the shift of psychological astrology away from the delineation of fixed personality characteristics defined at birth towards a dynamic system in which the individual can grow and develop. He popularized the aphorism “character is destiny”, arguing that astrological prediction was dependent on the individual’s personality and behaviour.
The psychologist Carl Jung studied astrology and discussed it throughout his published writings, thereby contributing to the evolution of a humanistic astrology based heavily on psychoanalysis, and to the emergence of professional astrologers who work primarily as psychological counsellors.
H Eastern Astrology
Astrology remains an integral part of religious and social life in India, particularly in arranged marriages. However, it is used to time and forecast events with greater precision than in the West, and uses a sidereal, or star-based, zodiac based on slightly different coordinates.
Chinese astrology is as complex as its Western and Indian counterparts and is closely connected to traditional medicine and feng shui, the alignment of buildings with the natural environment. It has a system of 12 “year rulers”: these are the Rat, Ox, Tiger, Hare or Rabbit, Dragon, Snake, Sheep, Monkey, Rooster, Horse, Dog, and Pig. There are 10 Heavenly Stems and 12 Earthly Branches. Together these make a 60-year cycle. The lunar month, day, and hour of birth are also taken into consideration in a Chinese horoscope. There are character types associated with elements (such as wood and air). There are also “mansions” (Chinese, gong) into which people are born. Charts of the 60-year cycle are produced in Hong Kong and other parts of the Chinese world.
III ASTROLOGICAL TECHNIQUES
Traditionally, astrology was divided into two branches: natural, which dealt with planetary influences, and judicial, in which the astrologer’s interpretation was crucial. Judicial astrology has four sub-branches: natal (the casting of charts for individuals); horary (the answering of specific questions); electional (the timing of events); and mundane (the study of history and politics).
A The Zodiac
Most Western astrology relies on the tropical zodiac, the division of the ecliptic (the Sun’s annual path through the sky as seen from the Earth) into twelve equal divisions, beginning with the “Aries point”, the Sun’s location on the spring equinox (the autumn equinox in the southern hemisphere), usually March 21.
B Horoscopes
A horoscope is set for the time of the event to be studied. The sign rising over the eastern horizon is known as the Rising Sign or Ascendant and is the basis of the twelve houses that represent different areas of life. The ten planets—the Sun, the Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto—are then placed in the houses and signs. Their location and aspects (the distances between them, seen from the Earth) then provide the basis for interpretation.
The signs, houses, planets, and aspects are generally seen as symbols that can be used to describe personality and anticipate future trends. Few astrologers believe that the planets exert a causal influence. More common is Jung’s concept of synchronicity (meaningful coincidence), in which celestial patterns coincide with events on earth at significant moments. A similar classical concept is that the heavens and earth are inextricably linked, and therefore any movement in the former corresponds to change in the latter.
C Science and Astrology
Traditionally, astrology and astronomy were viewed as complementary sciences. However, astrology is no longer regarded as a science by many, because its claims are almost impossible to test empirically in controlled laboratory conditions, and it can not meet the scientific need for reproducibility. It is argued that it can therefore be neither proved nor disproved.
However, there have been a number of attempts to prove statistically astrological claims about personality. The most notable was the research by the French statistician Michel Gauquelin (1928-1991) in collaboration with his wife Françoise. Using vast banks of data on the birth charts of individuals successful in various fields, he claimed his studies showed that certain planets correlate statistically to outstanding professional success when they are either rising over the eastern horizon at birth or culminating overhead. The most significant was the “Mars effect”, linking the position of Mars at birth to sporting success. The Mars Effect remains the subject of academic argument and is not supported by most scientists.
Scientific opinion in general sees astrology as a means of satisfying psychological needs and, as a practice, as lacking any objective basis. They point to the “Barnum effect”, which claims most people agree with most statements made about them as long as they are sufficiently general. Such criticism has, however, had only a marginal effect on astrology’s spread, and most western countries have astrological societies and schools that train professionals. Astrology has also become increasingly popular in former communist countries, particularly Russia.

Contributed By:
Nick Campion
Microsoft ® Encarta ® Encyclopedia 2004. © 1993-2003 Microsoft Corporation. All rights reserved.
Pluto (mythology)
Pluto (mythology), in Roman mythology, god of the dead, the husband of Proserpine, and the Latin counterpart of the Greek god Hades. Pluto assisted his two brothers, Jupiter and Neptune, in overthrowing their father, Saturn. In dividing the world among them, Jupiter chose the Earth and the heavens as his realm, Neptune became the ruler of the sea, and Pluto received as his kingdom the lower world, in which he ruled over the shades of the dead. He was originally considered a fierce and unyielding god, deaf to prayers and unappeased by sacrifices. In later cults and popular belief the milder and more beneficent aspects of the god were stressed. Believed to be the bestower of the blessings hidden in the earth, such as mineral wealth and crops, Pluto was also known as Dis or Orcus, the giver of wealth.
Microsoft ® Encarta ® Encyclopedia 2004. © 1993-2003 Microsoft Corporation. All rights reserved.