History of Astronomy

Astronomy, History of, history of the science that deals with all the celestial bodies in the universe. Astronomy includes the study of planets and their satellites, comets and meteors, stars and interstellar matter, star systems known as galaxies, and clusters of galaxies. The field of astronomy has developed from simple observations about the movement of the Sun and Moon into sophisticated theories about the nature of the universe. See also Astronomy.

II. Ancient Origins

The curiosity of ancient peoples about day and night and the Sun, Moon, and stars led eventually to observations that the heavenly bodies appear to move in a regular manner. This movement proved to be useful in defining time, location, and directions on Earth. Astronomy grew out of problems originating with the first civilizations. Ancient peoples needed to establish the proper times for planting and harvesting crops and for religious celebrations. The movement of celestial bodies helped them keep track of time and helped them find bearings on long trading journeys or voyages.

The sky exhibited many regularities of behavior, which ancient astronomers noticed and recorded. 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 1,000 visible stars followed a similar course. The stars appeared to rotate in permanent groups, called constellations, around a fixed point in the sky.

In the middle latitudes of the northern hemisphere, people noticed that daytime and nighttime were unequal in length for most of the year. On long days in the summer, the Sun rose north of east and climbed high in the sky at noon; the winter had long nights and the Sun rose south of east and did not climb so high at noon. People also observed that the stars that appear in the west after sunset or in the east before sunrise changed gradually from night to night. The pattern of stars repeated itself every 365 days.

Ancient astronomers saw that the sky also held 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 moves around 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. Neither an entire month nor an entire year contains an exact whole number of days; the calendar makers assigned different numbers of days to successive months or years. Even though individual months or years were not the same length, they averaged out to approximate the true value.

The Sun and Moon always move along the zodiac from west to east. The five bright planets—Mercury, Mars, Venus, Jupiter and Saturn—also have a generally eastward motion against the background of the stars. However, the planets move westward, or retrograde, for varying durations during each synodic period. Thus, the planets appear to have an erratic eastward course, 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.

III. Babylonian Astronomy

Interesting constellation maps and useful calendars were developed by several ancient peoples, notably the Egyptians, the Maya, and the Chinese, but the Babylonians accomplished even greater achievements. The Babylonian civilization thrived from the 18th to the 6th century BC in what is now Iraq (see Babylon). To perfect their calendar, they studied the motions of the Sun and Moon. They designated the day after the new moon as the beginning of each month. The new moon occurs when the side of the Moon away from Earth is lit by the Sun, so the side of the Moon facing Earth is dark. Originally calendar makers determined this day by observations, but later the Babylonians wanted to calculate it in advance.

About 400 BC, by which time Babylonia was a part of Persia, Babylonian astronomers 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 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 Uruk (Erech) on the Euphrates River, bear the name of Naburiannu or Kidinnu, astrologers who may have invented the systems of calculation.

IV. Greek Astronomy

The civilization of ancient Greece extended from about 1400 to about 300 BC. The ancient Greeks made important theoretical contributions to astronomy. The Odyssey, an epic poem traditionally attributed to Greek author Homer and probably written in the 8th century BC, refers to star groups such s the Great Bear, Orion, and the Pleiades and describes how the stars serve as a guide in navigation. The poem Works and Days by Greek poet Hesiod informs farmers about which constellations rise before dawn at different seasons of the year, indicating the proper times for plowing, sowing, and harvesting.

Scientists associate many important scientific contributions with Thales of Miletus and Pythagoras of Sámos, but none of the writings of these Greek philosophers survive. The legend that Thales correctly predicted a total solar eclipse on May 28, 585 BC, is of dubious origin. About 450 BC the Greeks began a fruitful study of planetary motions. Philolaus, a follower of Pythagoras, believed that Earth, the Sun, the Moon, and the planets all moved around a central fire. People on Earth could not see the fire because a body called counterearth moved around the fire between the fire and Earth. According to his theory, the revolution of 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 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 Sámos, a Greek. He believed that motions in the sky could be explained by the hypothesis that 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 2,000 years.

In the 2nd century AD, at the beginning of the Hellenistic period of Greek civilization, the Greeks combined their celestial theories with carefully planned observations. The astronomers Hipparchus and Ptolemy determined the positions of about 1,000 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 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 backward movement 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.

V. 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 Earth.

VI. 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 Earth.

Little attention was paid to the Copernican, or heliocentric, system until Italian astronomer 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. In 1609 Galileo made 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 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.

VII. Kepler's Laws and 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 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.

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.

VIII. Toward 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 were built, with mirrors as large as 390 in (1,000 cm) in diameter. Studies with these instruments 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 were sensitive to a wide variety of radiation wavelengths, including the gamma-ray, X ray, ultraviolet, infrared, and radio regions of the electromagnetic spectrum. Astronomers began to 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.

IX. The Solar System

(see also 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 enabled later astronomers to predict planetary positions years in advance, with an accuracy approximating that of the observed positions. Computers helped perform more and more complicated calculations, enabling more and more accurate results.

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. Earth has 1 natural moon; Mars, 2; Jupiter, 16; Saturn, at least 18; Uranus, 18; 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 approached Mercury, Venus, Mars, Jupiter, Saturn, and Uranus in the 1970s and 1980s to gather chemical and physical data. Such spacecraft discovered rings about Jupiter and new moons of that planet, Saturn, and Uranus. These satellites also supplied information that cast 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.

X. 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. After the invention of the telescope, stars became a topic of astronomy in their own right.

A. Measuring Brightness

Early astronomers divided the visible stars into six classes, depending on their relative brightness. The measurement of brightness was called magnitude. The brightest stars were given a magnitude of zero and the dimmest stars were said to have a magnitude of six. As telescopes came into use, astronomers were able to see stars that are much dimmer than those visible with the unaided eye. Astronomers extended the magnitude scale farther and farther, with dimmer stars having higher magnitudes.

Magnitude was the only measurement of a star's brightness until the 19th century, when scientists developed instruments to measure the actual amount of light that reaches Earth from a star. By the 1850s scientists knew more about the response of human eyes to light and about the actual brightness of stars. When human eyes compare two objects, one of which releases twice as much light as the other, they do not see the brighter object as twice as bright. A large difference in brightness results in a relatively small difference in magnitude. Astronomers redefined magnitude as a multiple of the logarithm of the brightness of an object. A logarithm is a mathematical way of representing large changes with small values. For example, the logarithm of 100 is 2, and the logarithm of 1,000 is 3. The number 1,000 is ten times bigger than the number 100, but the logarithm, or log, of 1,000 is only one more than the log of 100.

B. Measuring Distance

Basic to the study of a star is the knowledge of its distance from Earth, which is found by measuring the position of the star in the sky at intervals six months apart, when Earth is on opposite sides of its orbit. As 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 Earth to a star relatively near the Sun. The greater the distance, the smaller is the parallax of the star. The nearest star system, Alpha Centauri, is about 270,000 times farther from Earth than is the Sun. The first star distances were measured independently by three astronomers in 1838.

C. Composition and Energy of Stars

The source of the vast energy that the Sun and other stars radiate was long a mystery. The Sun produces 3.86 × 1026 watts of power (5.18 × 1023 horsepower). 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 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 cycles much faster than the Sun. Optical telescopes have revealed the principal steps in the life cycles of such stars. 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 a calm state similar to that of a main-sequence star, and eventually 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 a rotating neutron star, which is the final stage for some stars that are more massive than the Sun. Very massive neutron stars or neutron stars that pull mass from a neighboring star may collapse further into a black hole, which contains 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. In 1994 the Hubble Space Telescope provided the first convincing evidence that such a black hole exists, at the center of the galaxy M 87. By measuring the acceleration of gases in the vicinity of the black hole, scientists estimate that it has a mass 2.5 billion to 3.5 billion times that of the Sun. Astronomers found huge black holes at the centers of several more galaxies, and many scientists believe that almost every galaxy may have a supermassive black hole at its center.

In 1983 astronomers discovered that the nearby star Vega has a solar system. Vega is surrounded by a disk of dust and rock that may be in the process of forming planets. In 1995 a pair of American astronomers found the first conclusive evidence for a fully formed planet around another Sunlike star, 51 Pegasi. By 1998 astronomers knew of about 20 stars with orbiting planets and about ten more stars surrounded by disks of dust. Before these discoveries, most astronomers believed that the probability of finding planets around Sunlike stars was very slim. Many astronomers now think that solar systems are relatively common.

XI. The Galaxy

In the late 18th century British astronomer 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.

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 the amount of light that they actually emit (as opposed to the amount of their light that reaches Earth). Comparison of the amount of light that reaches Earth with the actual amount of light emitted by these stars provides a means of determining their distances. Following the discovery of the relation between period and luminosity by the American astronomer Henrietta Swan Leavitt, American astronomer Harlow Shapley used the Cepheid variables, which are scattered throughout the Milky Way, to measure the Milky Way's 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 trillion 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.

Until the 1980s the nucleus of the Milky Way was 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 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.

XII. 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; and still others are of irregular shape. One of the largest optical telescopes in the world, the 387-in (982-cm) Keck Telescope at Mauna Kea Observatory in Hawaii, has revealed galaxies more than 10 billion light-years from Earth.

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 -273.16° C/-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 (Background Radiation). 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 (508-cm) reflector at Palomar Observatory.

Contributed By:

Fred Lawrence Whipple, M.A., Ph.D. Professor Emeritus of Astronomy, Harvard University. Senior Scientist, Smithsonian Institution Astrophysical Observatory. Author of Earth, Moon, and Planets.

Vera C. Rubin, M.A., Ph.D. Staff Member, Department of Terrestrial Magnetism, Carnegie Institution of Washington. Associate Editor, Astrophysical Journal Letters. Member, Editorial Board, Science.

"Astronomy, History of," Microsoft® Encarta® Online Encyclopedia 2001 http://encarta.msn.com © 1997-2000 Microsoft Corporation. All Rights Reserved.