Dictionary: astronomy   (ə-strŏn'ə-mē)

1. The scientific study of matter in outer space, especially the positions, dimensions, distribution, motion, composition, energy, and evolution of celestial bodies and phenomena.
2. A system of knowledge or beliefs about celestial phenomena: the various astronomies of ancient civilizations.

[Middle English astronomie, from Old French, from Latin astronomia, from Greek astronomiā : astro-, astro- + -nomiā, -nomy.]

The study of the universe and the objects in it through scientific investigation. Since much of contemporary astronomy uses the laws and methods of physics, the terms “astronomy” and “astrophysics” are usually used interchangeably. However, modern astronomy also uses techniques from many other scientific disciplines, including chemistry, geology, and biology, for which the terms astrochemistry, planetary science, and astrobiology are increasingly used.

The use of geological knowledge and methods in analyzing close-up observations from spacecraft of planets and their satellites and of comets and asteroids closely links the disciplines of astronomy and planetary science. Indeed, the discovery of planets around distant stars holds for even closer relations in the future. Methods of studying molecules in interstellar clouds involve chemical knowledge. Planetary science and astrochemistry come together with astronomy in the search for life outside the solar system, part of the search for extraterrestrial intelligence (SETI). The National Aeronautics and Space Administration (NASA), the United States space agency, has placed a priority on astrobiology, including the investigation of Mars and the bringing of samples back to Earth from Mars. See also Asteroid; Comet; Cosmochemistry; Extraterrestrial intelligence; Interstellar matter; Planet; Planetary physics; Solar system.

Astronomers often lead in employing new technologies, pushing them to the limit in exploring extremely faint signals in various parts of the electromagnetic spectrum. Nearly all astronomical research is now heavily dependent on computers. Astronomical imagery is now dominated by light-sensitive silicon chips known as charge-coupled devices (CCDs), which are approximately 100 times more sensitive than film. Fiber optics are used for a variety of astronomical purposes, including the taking of hundreds of galaxy images simultaneously from the field of view of a telescope and bringing the light to a spectrograph that can produce simultaneous spectra of all the objects. The technology of active optics, in which the shape of a mirror is changed slightly at a high rate (often faster than 1 Hz) to compensate for the blurring of astronomical images caused by the Earth's atmosphere, is being increasingly pursued to eliminate the twinkling of stars. See also Adaptive optics; Fiber-optics imaging.

The opening of the 5-m (200-in.) Hale telescope at the Palomar Observatory on Palomar Mountain, California, in 1948 marked the beginning of a great period of development in optical astronomy. The light-gathering power of this telescope allowed cosmological study that extended most of the way to the beginning of time in the universe. It was joined in the task by several 4-m-class (160-in.) telescopes and by one less successful larger telescope. In the 1990s, new techniques of telescope making allowed the completion of several telescopes in the 10-m (400-in.) class, twice the diameter and thus four times the collecting area of the Hale telescope. The large telescopes have proven useful in taking spectra of the optical counterparts of gamma-ray bursts, proving that they are very far away; and in analyzing the distances to faraway galaxies and in measuring the redshifts of their spectra, leading to the current cosmological models of the expansion of the universe and the tentative conclusion that the rate of expansion is accelerating. See also Cosmology; Hubble constant.

The 1990s saw the thorough use of the vantage points of space for astronomical observation, exemplified by NASA's series of Great Observatories. In 1991 the Compton Gamma-Ray Observatory was launched, and in the following years mapped about one gamma-ray burst per day in addition to many other objects and events. The Hubble Space Telescope was launched in 1990 to study the ultraviolet and visible parts of the spectrum. Its repair in 1993, with secondary mirrors compensating for a focusing problem with the main mirror, brought it to full working order, and a 1996 upgrade included an improved two-dimensional spectrograph and infrared capability. The Chandra X-Ray Observatory, launched in 1999, provides high-resolution x-ray images, and is the same size and scope as Hubble. It studies various types of celestial objects and processes, such as black holes of stellar and galactic sizes. The Space Infrared Telescope Facility, the fourth of this series of Great Observatories, was launched in 2004 and renamed the Spitzer Space Telescope. Smaller spacecraft have also made valuable contributions. See also Black hole; Infrared astronomy; X-ray astronomy; X-ray telescope.

The atmosphere blocks most of the electromagnetic spectrum from reaching the Earth's surface, leaving windows of transparency mostly in the optical and radio parts of the spectrum. Radio astronomers have made the most of their window of transparency with such telescopes as the 100-m (328-ft) fully steerable telescope outside Bonn, Germany; the 330-m (1083-ft) Arecibo dish in Puerto Rico, which has some limited tracking ability, the Very Large Array of radio telescopes in New Mexico, and the Very Long Baseline Array. The ozone layer and other constituents of the atmosphere block the shortest wavelengths from penetrating to the Earth's surface, so observations of gamma rays, x-rays, and most of the ultraviolet region require telescopes in space. See also Ozone; Radio astronomy; Radio telescope.

Much of astronomy involves breaking down the incoming celestial radiation into its component wavelengths, a process known as spectroscopy. Spectroscopic studies can reveal the temperature of an object, the identity and proportions of its chemical elements, and the velocities of its constituents toward and away from the Earth. Light from the Sun and other objects is sometimes polarized, and studies of such polarization can tell about the magnetic fields present or about scattering processes. See also Astronomical spectroscopy; Polarimetry.

The expansive definition of a telescope includes anything used in astronomy to observe the sky. Several neutrino telescopes have been used to detect neutrinos from the Sun and, in one instance, from a supernova. The pace of observation of secondary cosmic rays as well as the few primary cosmic rays that reach the Earth is increasing. A pair of interferometers are being built on Earth to attempt direct detection of such gravitational waves, which should result from such distant events as the merger of two neutron stars. See also Cosmic rays; Neutrino; Solar neutrinos.

Theoretical calculations of the nature of astronomical objects or processes are known as theoretical astrophysics. The availability of supercomputers, powerful and fast computers capable of handling large amounts of data, has led to three-dimensional simulations of, for example, the formation of large-scale structure in the early universe. Models of the oscillations detectable on the Sun's surface through long-time-series observations are used to improve understanding of the solar interior, a process known as helioseismology. See also Helioseismology; Simulation; Supercomputer; Universe.

Laboratory astrophysics involves the measurement of basic parameters that are used in calculations of physical or chemical processes relevant to astronomy, such as cross sections of atomic and molecular collisional excitation and ionization. See also Molecular structure and spectra.

For more information on astronomy, visit Britannica.com.

The science of astronomy (to which Rome contributed nothing) developed comparatively late in Greece and, being for the Greeks essentially a system for predicting the positions of the heavenly bodies, it covered only a part of what we understand by the term. In the classical period interest was confined to observations of the solstice and equinox and of the rising and setting of the most obvious stars and constellations, for the practical ends of navigation and establishing a calendar. The fifth-century Athenian astronomer Meton tried to correlate the lunar year with the solar year in a 19-year cycle (c.435 BC), but it seems that his system was not incorporated into the Athenian calendar. By the fifth century BC it was known to some individuals that the earth is a sphere and that the moon receives its light from the sun, and Anaxagoras understood the principles of eclipses. Not till the fourth century can we be sure that the Greeks identified the five planets known to the ancient world, Venus, Mercury, Mars, Jupiter, and Saturn (see EUDOXUS). In that century became crystallized the picture of the cosmos which was accepted by Aristotle (see 4 (iii)) and given final shape in the second century AD by Ptolemy, who was able to calculate the position of all the known heavenly bodies at a given moment, predict eclipses, and foretell the appearance and disappearances of the planets and fixed stars. At the centre of the cosmos was the earth, a globe suspended in space, whose weight had taken it to the centre. The earth was surrounded by a system of concentric spherical shells, rotating at various speeds and about various axes; on these were carried the moon, sun, planets, and fixed stars. The terrestrial atmosphere reached as far as the moon. The outermost sphere, that of the fixed stars, was composed of the rarest of all elements, the aether, and revolved daily about the earth. Ptolemy in his Almagest gave canonical form to this view of the cosmos, and his work remained uncorrected except in small details throughout all antiquity until Copernicus (1473–1543) and Galileo (1564–1642). For a heliocentric view of the universe see ARISTARCHUS (1). See further the names cited under MATHEMATICS.

Colonial Americans lacked instruments and libraries. They had difficulty communicating with each other and often relied on English correspondents for news of other colonialists. During the seventeenth century, European astronomy was focused on extending Isaac Newton's mathematical description of the solar system, and a few Americans contributed their observations to the Royal Society of London. American observations served European theories.

When Venus passed in front of the sun in 1761 and 1769, transits revealed the distance of the earth from the sun. John Winthrop, professor of mathematics and natural philosophy at Harvard, organized an expedition to Newfoundland to observe the first transit. The Massachusetts Assembly assigned a ship to transport Winthrop's group and Harvard permitted him to take college instruments, provided they were insured against loss or damage. The observations were sent to Europe for analysis.

Winthrop lectured his students that determination of the distance of the earth from the sun would result in a deeper insight into God's wonderful works. Enlightenment faith in the discernable regularity of the universe also encouraged the study of astronomy in early American colleges. In its appeal to the Pennsylvania Assembly for funds to observe the 1769 transit, the American Philosophical Society, founded in Philadelphia in 1743, cited a more utilitarian goal, "the Promotion of Astronomy and Navigation, and consequently of Trade and Commerce." In a period of increasing cultural nationalism, the society also wanted to win recognition for American achievements.

Many of America's astronomers were surveyors. The self-taught American astronomer David Rittenhouse made his living as a clockmaker, but he was also a surveyor. In 1767, he constructed in Philadelphia his famous orrery, or mechanical planetarium, which represented with great precision the motions of the planets around the sun. The Pennsylvania assembly paid for it. The onset of the Revolutionary War suspended hope to build an observatory.

With political independence came a desire for cultural independence. However, little public patronage was forthcoming for astronomy in the early national period. In 1825, President John Quincy Adams pointed out that Europe had 130 "lighthouses of the skies" but the United States none. Yet his request for funds for a national observatory was denied.

The American Academy of Arts and Sciences, founded in Boston in 1780 by John Adams, published astronomical observations by Nathaniel Bowditch, a self-educated Salem seaman. His The New American Practical Navigator (1800) became the most widely used nautical guide, and in 1811, he observed a solar eclipse to improve the determination of the longitude of Cambridge. The European scientific community applauded Bowditch's translation and commentary on Pierre Laplace's Mécanique céleste. The American Academy offered to pay for publication, but Bowditch waited until he could afford to publish it himself.

Through much of the nineteenth century, the United States was a nation in development. While some of their European brethren made observations and contributed to the advance of knowledge, American astronomers often struggled with more mundane problems, including writing textbooks, acquiring books and journals for libraries, and building, equipping, and financing observatories. Elias Loomis, a professor at Western Reserve College in Ohio, at the University of the City of New York, and at Yale University, published An Introduction to Practical Astronomy in 1855 and a A Treatise on Astronomy in 1876, both of which went through numerous editions. At both New York and Yale, Loomis arranged to receive publications from European observatories on an exchange basis. His will left funds to pay observers and publish their results.

College observatories consisted of a small building and telescope intended for the education of undergraduate students, but they could not pay researchers or provide funds for publication. The University of North Carolina built an observatory in 1831, which lasted several years. Observatories were constructed at Yale (1830s), at Williams College (1838), at Western Reserve College (1838), at the Philadelphia High School (1838), at West Point (1839), and at Georgetown (1843). In 1839, Harvard lured William Cranch Bond from his private observatory to supply his own instruments and work for no salary. The great comet of 1843 aroused public interest, which manifested itself in public support to construct and endow the Harvard College Observatory. Harvard ordered from the German firm Merz and Mahler a twin to the Russian Pulkova Observatory's fifteen-inch (lens diameter) telescope, then the largest in the world.

There were also public observatories: the Cincinnati Observatory, whose cornerstone John Quincy Adams laid in 1843, and the Dudley Observatory in Albany, built between 1852–1856. The tribulations of the Cincinnati Observatory illustrate the obstacles to practicing astronomy in mid-nineteenth-century America. Ormsby MacKnight Mitchel, a West Point graduate, moved to Cincinnati and became professor of mathematics, civil engineering, mechanics, and machinery at Cincinnati College. His public lectures on astronomy led to the founding of the Cincinnati Astronomical Society and the municipal Cincinnati Observatory, funded by public subscription. After five hours of teaching, Mitchel would supervise construction of the observatory in the afternoons. Cincinnati purchased an 11.25-inch telescope from Merz and Mahler, but Mitchel spent much of his time displaying the heavens to subscribers, from 4:00 to 10:00 P.M. daily. He tried publishing a journal to raise money for auxiliary instruments and for his salary, the observatory having no endowment for operating expenses, and did make money from a book on popular astronomy and from surveying a railroad route. His observations of singular phenomena, a kind of natural history of the heavens, fell short of a new professional emphasis on measurement and theory, requiring considerable mathematical competence. Economic forces discouraged sustained, structured research.

A few would-be professional astronomers received training and employment with the Coast Survey, established in 1807 in response to commercial interests of seaboard states. In legislation for the Coast Survey in 1832, Congress explicitly declared that it did not authorize construction or maintenance of a permanent astronomical observatory. A decade later, the Naval Observatory was created surreptitiously, as part of the Depot of Charts and Instruments. Not until 1866, however, would the observatory begin a program of fundamental research in astronomy. Meanwhile, the Nautical Almanac, located in Cambridge, Massachusetts, was established under the Naval Observatory budget in 1849. It reported directly to the secretary of the navy, provided training and employment for a few astronomers, and improved navigation and raised America's scientific standing with an annual astronomical almanac more accurate and theoretically advanced than the British Nautical Almanac. Simon New-comb, one of America's best-known scientists at the end of the century, got his start at the Nautical Almanac, and also worked at the Naval Observatory. He analyzed the motions of the moon and planets.

There were also a few private observatories in America. Lewis Rutherfurd, a wealthy New Yorker and trustee of Columbia College, had a nine-inch diameter telescope, and also a small transit instrument belonging to Columbia College at his observatory at Second Avenue and Eleventh Street. The Coast Survey used this observatory in 1848 to determine the longitude of New York. Rutherfurd was a pioneer in astronomical photography. Not until late in the century, though, would individual American fortunes fund the establishment and sustenance of large observatories with systematic programs of scientific investigation carried on by full-time, paid employees.

The second half of the nineteenth century saw advances in telescope production, especially by the Boston firm of Alvan Clark & Sons. Their metal tubes were stiffer yet lighter than wooden telescopes. Larger pieces of optical glass were now available, and the Clarks figured the lens for the world's largest refracting telescope on five occasions: an 18.5-inch lens for the University of Mississippi in 1860, a 26-inch lens for the Naval Observatory in 1873 (with which Asaph Hall discovered Mars's moons in 1877), a 30-inch lens for the Pulkova Observatory in 1883, a 36-inch lens for the Lick Observatory of the University of California in 1887, and a 40-inch lens for the Yerkes Observatory of the University of Chicago in 1897. James Lick, a California land speculator during the gold rush, and Charles Yerkes, a Chicago street car magnate, put up the funds for their eponymous observatories, under university auspices, and Boston investor Percival Lowell directed his own observatory. All three observatories were far removed from cities, and Lick's and Lowell's were on mountain peaks. With the largest telescopes in the best locations, American observatories now surpassed all others.

Growing interest in astrophysics and in distant stars and nebulae encouraged the development of new observatories with large steerable reflecting (light focused by a curved mirror) telescopes suitable for photography and auxiliary instruments for the analysis of starlight. George Ellery Hale founded the Astrophysical Journal in 1895, the American Astronomical and Astrophysical Society in 1899, the Mount Wilson Observatory in 1904, and the International Astronomical Union in 1918. Hale was an early prototype of the high-pressure, heavy-hardware, big-spending, team-organized scientific entrepreneur. In 1902, Andrew Carnegie, rich from innovations in the American steel industry, created the Carnegie Institution of Washington to encourage investigation, research, and discovery in biology, astronomy, and the earth sciences. Its ten million dollars were more than the total of endowed funds for research in all American universities combined. Hale left the Yerkes Observatory to build, with Carnegie money, the Mount Wilson Observatory on a mountain above Los Angeles. There George Willis Ritchey, who accompanied Hale from Yerkes, made the photographic reflecting telescope the basic instrument of astronomical research, constructing a 60-inch telescope in 1908 and a 100-inch telescope in 1919. They were the largest telescopes in the world and revolutionized the study of astronomy. Harlow Shapley found that the system of stars is a hundred times larger than previous estimates and that the sun is far from the center. Edwin Hubble showed that spiral nebulae are independent island universes beyond our galaxy and that the universe is expanding. Cosmology, previously limited to philosophical speculations, joined mainstream astronomy.

The Mount Wilson Observatory depended on its relationship with physicists at the nearby California Institute of Technology for its dominance of astrophysics during the first half of the twentieth century. A scientific education was fast becoming necessary for professional astronomers, as astrophysics came to predominate, and the concerns of professionals and amateurs diverged. As late as the 1870s and 1880s, the self-educated American astronomer Edward Emerson Barnard, an observaholic with indefatigable energy and ocular acuteness, could earn positions at the Lick and Yerkes observatories with visual observations of planetary details and discoveries of comets and moons. Already, however, he was an exception and an anachronism. Soon an advanced academic degree and considerable theoretical understanding were required of professional astronomers in America.

Supposedly, only men could withstand the rigors of observing the heavens all night in unheated telescope domes. Women were first employed to examine photo-graphs of stellar spectra and to catalog the spectra. Edward Pickering, director of the Harvard College Observatory in 1881 and an advocate of advanced study for women, was so exasperated with his male assistant's inefficiency that he declared even his cook could do a better job of copying and computing. Pickering hired her and she did do a better job, as did some twenty more females over the next several decades, recruited for their steadiness, adaptability, acuteness of vision, and willingness to work for low wages. In 1925, Cecilia Payne, a graduate student, determined the relative abundances of eighteen chemical elements found in stellar atmospheres. Her Ph.D. thesis has been lauded as the most brilliant written in astronomy. Her degree, however, was from Radcliffe College, before Harvard granted degrees to women, and in subsequent employment at Harvard she was initially budgeted as "equipment."

Radio astronomy began in America in 1933. Karl Jansky, a radio engineer with the Bell Telephone Company, detected electrical emissions from the center of our galaxy while studying sources of radio noise. Optical astronomers were not interested, nor were Jansky's practical-minded supervisors. Grote Reber, an ardent radio amateur obsessed with distance communication, was interested, and built for a few thousand dollars a 31.4-foot-diameter pointable radio antenna in his backyard in Wheaton, Illinois. In 1940, he reported the intensity of radio sources at different positions in the sky. Fundamental knowledge underlying radio astronomy techniques increased during World War II, especially with research on radar.

Advances in nuclear physics during the war made possible quantitative calculations of the formation of elements in a supposed primeval fireball. The Russian-American physicist George Gamow sought to explain the cosmic abundance of elements as the result of thermo-nuclear reactions in an early hot phase of an expanding universe, consisting of high-energy radiation. In 1963, unaware of Gamow's work, Arno Penzias and Robert Wilson at the Bell Telephone Laboratories detected radiation of cosmic origin. Meanwhile, Robert Dicke at Princeton University had independently thought of the cosmic background radiation and set a colleague to work calculating its strength. When Dicke learned in 1965 of Penzias and Wilson's measurement, he correctly interpreted it as Gamow's predicted radiation. A Nobel Prize went to Penzias and Wilson. Their discovery won general acceptance of the big bang theory and refuted the rival steady state theory.

World War II changed the relationship between science and the state. Radar, missiles, and the atomic bomb established state-sponsored and state-directed research and development. Furthermore, groups of scientists brought together in wartime proved effective. After the war, engineers and physicists with their instruments, techniques, training, and ways of operating moved into astronomy. Then came Sputnik in 1957, the world's first satellite. This Soviet triumph challenged American supremacy in military might and world opinion.

After Sputnik, the National Science Foundation supplied many millions of dollars for construction of the Kitt Peak National Observatory on a mountain near Tucson, Arizona. It is the largest collection of big telescopes in the Northern Hemisphere. Seventeen universities came together in AURA, the Association of Universities for Re-search in Astronomy, to manage the observatory.

Another response to Sputnik was the creation of the National Aeronautics and Space Administration (NASA). Among its accomplishments are automated observatories launched into space, including the Hubble Space Telescope. Its primary mirror is eight feet in diameter. Including recording instruments and guidance system, the telescope weighs twelve tons. It has been called the eighth wonder of the world, and critics say it should be, given its cost of 1.5 billion dollars! The telescope is as much a political and managerial achievement as a technological one. Approval for a large space telescope was won in a political struggle lasting from 1974 to 1977, but not until 1990 were a plethora of problems finally overcome and the telescope launched into space, only to discover that an error had occurred in the shaping of the primary mirror. One newspaper reported "Pix Nixed as Hubble Sees Double." The addition of a corrective mirror solved the problem.

NASA also funds X-ray astronomy. Captured German rockets provided the first proof of X-rays from the sun. Astronomers did not expect to find X-ray sources and were skeptical that brief and expensive rocket-borne experiments were worthwhile. NASA, however, had more money than there were imaginative scientists to spend it, and the military, even more. One imaginative and eager scientist was the Italian-born Riccardo Giacconi, who in 1960, funded by the Air Force Cambridge Research Laboratories, discovered a cosmic X-ray source, and in 1963, detected a second. NASA adjudicates questions of scientific priority and supplies money for space observatories; industry helps build them; universities or consortiums of universities design and operate them and analyze the data. NASA then funded a rocket survey program and a small satellite for X-ray astronomy and in 1978 the Einstein X-ray telescope. Unlike the relatively quiescent universe seen by earth-bound astronomers, the universe revealed to engineers and physicists observing from satellites is violently energetic.

Major changes have occurred in both the size and scope of American astronomy over the centuries, but never more rapidly nor more dramatically than at the beginning of the Space Age. There were some five hundred American astronomers in 1962 and three times that many a decade later. Only four worked on X-rays in 1962 compared to over forty times that many in 1972. Over eighty percent of them were migrants from experimental physics, with expertise in designing and building instruments to detect high-energy particles.

Astronomers now realize that important cosmological features can be explained as consequences of new theories of particle physics, and particle physics increasingly drives cosmology. Conversely, particle physicists, having exhausted the limits of particle accelerators and public funding for yet larger instruments, turn to cosmology for information regarding the behavior of matter under extreme conditions, such as those prevailing in the early universe.

The spectacular rise of American astronomy roughly parallels the remarkable evolution of the nation, itself, from British colonies to world super power. Once limited to visual observations and determining positions, astronomy now includes cosmology, the study of the structure and evolution of the universe, and analysis of the physical and chemical composition of the universe and its components. Once peripheral, now American astronomers, men and women, formally educated in a variety of fields, working in large teams, on systematic long-term projects, and enjoying government patronage, lead world advances in instrumentation, observation, and theory.

Bibliography

Christianson, Gale E. Edwin Hubble: Mariner of the Nebulae. New York: Farrar, Stauss, Giroux, 1995.

Edmundson, Frank K. AURA and its US National Observatories. New York: Cambridge University Press, 1997.

Hetherington, Norriss S. Hubble's Cosmology: A Guided Study of Selected Texts. Tucson, Ariz.: Pachart Publishing, 1996.

Hindle, Brook. David Rittenhouse. New Jersey: Princeton University Press, 1964.

Hoyt, William Graves. Lowell and Mars. Tuscon: University of Arizona, 1976.

Jones, Bessie Judith Zaban, and Lyle Gifford Boyd. The Harvard College Observatory: The First Four Directorships, 1839–1919. Cambridge, Mass. Belknap Press, 1971.

Lankford, John. American Astronomy: Community, Careers, and Power, 1859–1940. Chicago: University of Chicago Press, 1997.

Levy, David H. Clyde Tombaugh: Discoverer of Planet Pluto. Tucson: University of Arizona Press, 1991.

Osterbrock, Donald E. Eye on the Sky: Lick Observatory's First Century. Berkeley: University of California Press, 1988.

———. Yerkes Observatory 1892–1950: The Birth, Near Death, and Resurrection of a Scientific Research Institution. Chicago: University of Chicago, 1997.

———. Pauper and Prince: Ritchey, Hale, & Big American Telescopes. Tucson: University of Arizona Press, 1993.

Sheehan, William. The Immortal Fire Within: The Life and Work of Edward Emerson Barnard. New York: Cambridge University Press, 1995.

Smith, Robert W. The Space Telescope: A Study of NASA, Science, Technology, and Politics. New York: Cambridge, 1989.

Tucker, Wallace, and Karen Tucker. The Cosmic Inquirers: Modern Telescopes and Their Makers. Cambridge: Harvard University Press, 1986.

Warner, Deborah Jean. Alvan Clark & Sons: Artists in Optics. Richmond, Va.: Willmann-Bell, 1995.

—Norriss Hetherington

The movement of the stars and planets has fascinated humans for thousands of years. For the vast majority of ancient astronomers, the stars seemed to be equally distant from Earth in what was an Earth-centered (or "geocentric") cosmos. The ancient Greeks observed that the stars revolved westward around the north celestial pole every twenty-three hours and fifty-six minutes, thus constituting a kind of objective clock. Most envisaged these revolving stars to be located on a sphere, composing an incorruptible, celestial orb that could easily be contrasted with the world of change, generation, and corruption on Earth (to which irregular meteorological phenomena such as comets were also understood to belong). Against the backdrop of this outer sphere, the Sun was seen to move along a path termed the ecliptic, while the planets moved within eight degrees of this path. These usually moved eastward, though occasionally they moved in the opposite direction, thus exhibiting retrograde motion. Many Greek astronomers suggested that the Sun could be placed on the equator of an inner sphere that revolved once a year, hence constituting a second sphere in addition to the stellar orb. Following the theories of the fourth-century-B.C.E. scholar Eudoxus, astronomers and natural philosophers became increasingly committed to the idea that the motions of each planet could be accounted for by means of their own specific, homocentric spheres.

Defending an Earth-Centered Universe

The retrograde motion of the planets offended against the apparent simplicity of heavenly motions, as well as the dictum that their orbits were circular. From the third century B.C.E., astronomers began to conceive of planets rotating in small circles (epicycles) around a point that moved on a carrier orbit (or deferent). This accounted for the apparent motions of the planets, including their retrograde motion, although others followed Aristotle in positing the existence of homocentric spheres. After Ptolemy's composition of the Almagest in the second century C.E., the movements of the planets could be accurately represented by means of techniques involving the use of epicycles, deferents, eccentrics (whereby planetary motion is conceived as circular with respect to a point displaced from Earth), and equants (a device that posits a constant angular rate of rotation with respect to a point displaced from Earth). These approaches, and arguments over the reality of the celestial orbs, continued to be refined in the following centuries by first Muslim and then Christian astronomers. The geocentric cosmos was readily appropriated by Christians, many of whom believed that God existed outside the stellar sphere and (as evidenced by Dante's fourteenth-century poem Divine Comedy) that angels turned the epicycles and spheres in the intervening spaces.

Astronomy had a primarily practical function, and by the thirteenth century it played a central role in determining the dates of religious festivals. Ptolemy's mathematically sophisticated Almagest, with various commentaries, formed the basis for advanced astronomy in university curricula from this period onward. In the late fifteenth century, the arrival of the printing press coincided with the transmission to western Europe of a number of important Greek mathematical and astronomical manuscripts. These became a new focal point for doing astronomy, as ancient texts could be seen and assessed in their original form (as opposed to being translated into Latin from Syriac or Arabic). Georg von Peuerbach (1423–1461) and his pupil Regiomontanus produced important works that formed the basis of astronomical training for the next century.

Astronomy was equally significant for providing data from which astrological predictions could be made (judicial astrology). The effects that heavenly actions had on earthly events, and the manner in which they were carried out, constituted a problem for most human societies. Astrology was a major part of both daily life and intellectual culture, and was deeply implicated in politics, medicine, and agriculture. However, its use was increasingly questioned by elites in the late sixteenth century; both Catholics and Protestants argued that the notion that the relations between stars and planets influenced or governed human behavior detracted from the primary Christian belief in free will. Nevertheless, this argument was often expressed as a condemnation of bad astrology, the implication being that astrology was a credible art or science that could be reformed. However, although it was still taken seriously by Johannes Kepler at the beginning of the seventeenth century, it was almost universally despised by social elites a hundred years later.

In Aristotle's De Caelo, heavenly bodies were treated as physical objects and thus as a branch of natural philosophy, or physics. Astronomy thus enjoyed a somewhat bifurcated position within the university system, taught partly in tandem with medicine and astrology, and partly as a higher-level, more technically difficult mathematical enterprise. This mirrored the scholarly distinction between the treatment of the physical reality of the supposedly "crystalline" spheres and such ideas as the epicycles, the actual existence of which was debatable. When Nicolaus Copernicus (1473–1543) published his heliocentric (Sun-centered) De Revolutionibus Orbium Coelestium (On the revolutions of the heavenly orbs) of 1543, a number of astronomers believed that his system could be treated as another convenient set of devices for explaining the celestial phenomena without any commitment to the reality of the general cosmology and physics that was prominent in the first part of the book. Indeed, the Lutheran theologian Andreas Osiander (1498–1552) reassured readers in an anonymous preface to the work that Copernicus had merely devised "hypotheses" that facilitated the calculation of celestial positions.

The Copernican Revolution

However, Copernicus had indeed upset the standard disciplinary division in universities, and he implied that a mathematical astronomer was entitled to speak about the physical nature of things—the traditional preserve of the more prestigious role of the natural philosopher. Not only did he offer the first modern heliocentric system (although he appealed to the authority of ancient heliocentric systems such as that of Pythagoras), but he asserted that the demonstrations in his work could only be understood by mathematically competent astronomers. He condemned the uncertainty of the calendar, and also of the inability of astronomers to determine the precise motions of the heavenly bodies. He lambasted the inconsistent use of homocentric circles and epicycles, and argued that the systems produced by recent astronomers lacked aesthetic credibility. Instead, with all the different techniques and philosophies in play, they had produced a sort of astronomical "monster."

Although he retained epicycles, Copernicus dispensed with the equant and showed that retrograde motion could be explained by means of a heliocentric system. Indeed, contra Osiander, he could hardly have been more assertive about the truth of his Sun-centered cosmos. He proffered tentative physical explanations of why objects on a revolving earth would not be ejected into the heavens, and argued that the stellar sphere had to be an enormous distance further away than that accepted by traditional astronomers. This meant that his inability to detect stellar parallax (the feature by which the apparent position of a star would vary depending on the time of year, since an observer on Earth would view it from a different position) would not be an argument against his system. Ominously, he also suggested that biblical passages that appeared to support geocentricity were the result of the author "accommodating" his discourse to the capacities of ordinary people. Momentous as it now seems, Copernicus's system had only a handful of adherents in the sixteenth century, although Erasmus Reinhold's Prutenic Tables of 1551, which were based on methods pioneered by Copernicus, were used to produce the Gregorian calendar (decreed by Pope Gregory XIII) in 1582.

In the last three decades of the sixteenth century, the Danish astronomer Tycho Brahe (1546–1601) was single-handedly responsible for two major initiatives in astronomy. First, he devised a "geoheliocentric" system that, for the half-century before Copenicus's system was broadly accepted, provided the major alternative to a geocentric cosmos. In this, the Sun again revolved around the Earth, with the five planets (not including the Moon) revolving around the Sun; since the Martian and solar orbits intersected, this system could not accommodate the reality of the spheres. Tycho rejected a heliocentric system, partly because he could not abide the massive distances required by Copernicus's system, partly because he could not find stellar parallax, and partly because he was strongly opposed to heliocentrism on scriptural grounds.

Most important, Tycho made a series of naked-eye observations, conducted from 1574 as part of a monumental observation program at his observatory (Uraniborg) on the island of Hven. He designed and made new instruments that were as much as ten times more accurate than those of his predecessors. He also conceived of a means of using each of his devices to cross-check results obtained from other instruments in his collection. His instruments were less cumbersome than previous examples and he introduced new techniques for more precisely dividing them into minutes and seconds of a degree. He turned himself and his workplace—which had its own printing press—into the hub of an extensive network of correspondents, and he used this and personal contacts to train a number of the best astronomers of the next generation. Two of his measurements, linked to exceptional celestial events in the 1570s, demonstrated the precision of his observations. First, he determined the extreme distance from Earth of the terrifying supernova of 1572, and second, he showed for the first time that the comet of 1577 existed beyond the lunar sphere and hence was technically part of the heavens. Independent of any implications of De Revolutionibus, both seemed to suggest that the celestial sphere could no longer be considered immutable.

As Tycho's work came to a close, Johannes Kepler (1571–1630) burst on to the stage with his heliocentric Mysterium Cosmographicum (The secret of the universe) of 1596. Kepler was trained at Tübingen by the pro-Copernican Michael Mästlin (1550–1631), and throughout his adult life remained committed to a heliocentric system. In the Mysterium he famously attempted to show that the distances between the planets could be represented by nested regular solids, although this ultimately failed to fit the astronomical data produced by Tycho and his colleagues. At the end of 1600 Kepler traveled to Prague to work with Tycho, who had only recently been appointed imperial mathematician to Rudolf II. When Tycho died within the year, Kepler gained access to his data concerning the orbit of Mars. Tycho had observed the Martian orbit with astonishing accuracy, finding a discrepancy between the observed orbit and an ideal circular trajectory of eight feet that could not be explained by instrument error, and Kepler sought strenuously over the next few years to provide a harmonious and geometrically satisfying orbit to fit these observations. After many different shapes had been tried, he decided that Mars and the other planets traveled in ellipses, one of whose foci was the Sun, and that each planet swept out equal areas in equal times (considering the area to be drawn out by a line linking the planet to the Sun).

Kepler published his first two laws of planetary motion in his Astronomia Nova (New astronomy) of 1609, although they are merely a handful of many mathematical relations that he posed for planetary orbits. In this work he also appealed to the magnetic philosophy devised by the English physicist William Gilbert in his De Magnete (On the magnet) of 1600, in order to explain the physical causes of heavenly motion. He suggested that the planets, which all traveled in the same direction and (virtually) in the same plane, were controlled by a motive force emanating from the Sun. His third and final law, first enunciated in his Harmonice Mundi (Harmony of the world) of 1619, is more complicated and states that the square of the mean orbital period of a planet is proportional to the cube of its mean distance from the Sun. Kepler combined a Platonic-Pythagorean concern for the reality of harmonies and ratios with a magnetic physics, but from a disciplinary point of view he was explicitly asserting the right of an astronomer such as himself to produce a "celestial physics" that gave the true (non-Aristotelian) causes of heavenly motion. In a work on astrology of 1601, he also attempted to uncover the physical causes underlying the influence that planetary conjunctions had on earthly activities.

The Scientific Revolution

In 1609, the same year that Kepler published his first two laws, Galileo Galilei (1564–1642) used a combination of lenses to look at the heavens, having heard of a similar invention that had been introduced the year before. Within weeks, he had deduced that the Moon had mountains and valleys (and was thus not perfectly smooth), had noted that the Milky Way was actually composed of numerous stars, and proposed that Jupiter possessed four satellites that revolved around it as the Moon revolved around Earth. For his extraordinary discoveries, which appeared in Sidereus Nuncius (Starry messenger) of 1610, he was rewarded with the position of court philosopher to his onetime pupil, Cosimo de' Medici (Cosimo II), grand duke of Tuscany. Galileo was now a court intellectual to rival Johannes Kepler, who had succeeded Tycho Brahe as imperial mathematician to Rudolf II. Given the courtly locations of both Kepler and Galileo, twentieth- and twenty-first-century historians have pointed to the prevalence of nonuniversity locations as the most important settings for innovative astronomy. Kepler's friendly stance toward Galileo (he wrote a book praising Galileo's discoveries within months of the work appearing) ensured that there was minimal animosity between them, although this may have been helped by Galileo's apparent complete ignorance of Kepler's own discoveries.

In the next two and a half decades, adherence to the Copernican system met with determined opposition and, notoriously, René Descartes felt obliged to suppress his not sufficiently anti-Copernican Le monde (The world) in 1633 soon after Galileo's pro-Copernican Dialogue had been condemned by the Roman Inquisition in July. However, the vast majority of astronomers and natural philosophers were avowed Copernicans by the middle of the seventeenth century, and even those, such as the Jesuits, who were slow to accept the Copernican worldview, made substantial contributions to astronomy. Indeed, Galileo's successes can be overstated as contributions to the demise of the notion of a perfect celestial sphere, for previous events and observations had begun to shake confidence in heavenly incorrigibility. Of all his discoveries, only his sightings of the phases of Venus provided overt support for Copernicus's system, although Tycho's system could also account for them. Nevertheless, Galileo's visually striking discoveries—and their implications—were easily understood by a new audience of scholars and gentlemen, and they had a dramatic impact on writers and poets such as John Donne (1572–1631).

It is important to note, as Allan Chapman has properly argued, that the great theoretical advances in cosmology achieved by Kepler, Isaac Newton, and others were facilitated and indeed made possible by improvements in angular astronomical measurement and not by superior visual acuity. Tycho's advances in instrument design and observational accuracy were made possible by the cadre of excellent craftsmen he had at his disposal, and similarly, John Flamsteed, the first British astronomer royal, whose observations were to prove crucial for Newton's enunciation of universal gravitation, had innovative and highly skilled instrument makers working with him. Only with patient astronomy of this sort could precise measurements be made of celestial magnitudes such as distance and size.

In the 1660s telescopic sights were added to quadrants and a zenith sector in the attempt by the French to measure the length of a degree of meridian in France. In Restoration England a number of episodes occurred that acted as a spur to creating an alliance between accurate measurement and theoretical innovation. The Royal Society of London was founded in 1660, followed by the Royal Greenwich Observatory—founded to aid navigation and the determination of longitude by improving astronomy—in 1675. The Observatory was badly stocked with instruments at first, but gradually, with some private support and with the help of occasionally brilliant suggestions from Robert Hooke (1635–1703) for automating observations, Flamsteed was able to build up a stock of the best instruments then available. By the 1690s he had a degree clock that allowed star positions to be measured with extraordinary accuracy, and the ten-to-twelveseconds error of his mural arc (that is, a large quadrant set on a wall) was a sixfold improvement on the accuracy of Tycho's instruments. Flamsteed used telescopic sights on his instruments but also a filar micrometer, that is, a system of thin wires, minutely movable by means of a carefully graduated screw, that could be placed inside a telescope to finesse its accuracy.

Isaac Newton (1642–1727) developed an early interest in astronomy and became famous initially because of his development of a reflecting telescope in the late 1660s. Flamsteed's data was crucial for Newton in the winter of 1684–1685, when the latter was trying to determine what mutual influence Jupiter and Saturn might have on each other, and again in late 1685, when Newton wanted three items of data (accurate to a minute) on the path of the Great Comet of 1680–1681. This data would constitute crucial evidence for the cosmological system that Newton published in his momentous Principia Mathematica (The mathematical principles of natural philosophy) of 1687, for he could now analyze observed deviations from perfect elliptical orbits by means of his concept of universal gravitation. Perhaps of equal significance were the observations Flamsteed put his way in 1694–1695 when Newton had another go at the Moon. This ultimately unprofitable endeavor was part of an effort to solve the (insoluble) three-body problem of the mutual interactions of Sun, Moon, and Earth, all of which Newton later described as the most difficult science he ever did. The pair fell out irreconcilably soon after this, and Newton behaved abominably toward the astronomer royal, practically stealing Flamsteed's laboriously crafted star catalog by claiming it as the property of the state. Not the least of Newton's actions was to downgrade and even efface (in his Principia) the contributions made by Flamsteed, who had generously provided the observations that allowed Newton to corroborate and then rework his supreme theory. Whatever his personal dealings with others, Newton's theory provided the basic theory of the heavens that we now take to be true, and his achievements included the recognition that some comets travel in periodic elliptical orbits.

By the early eighteenth century, the London instrument-making trade was widely held to produce the highest quality instruments; Pierre-Louis Moreau de Maupertuis (1698–1759), for example, took a zenith sector and clock constructed by the outstanding London instrument maker George Graham (1673–1751) to Lapland in 1736–1737. It was this expedition that went furthest in determining the shape of Earth, confirming Newton's calculation that it was an oblate spheroid (flattened at the poles). In England, Graham and others made instruments for the astronomers royal who followed Flamsteed, namely Edmond Halley (in 1720) and James Bradley (in 1742). Bradley, who discovered stellar aberration in 1727 and who confirmed Newton's analysis of the extent of the nutation of Earth's axis in the 1740s, combined access to the best instruments of the day with an obsession for accuracy. By the middle of the eighteenth century, measurements were confined to the meridian, and, among other activities, experiments were being undertaken to better ascertain longitude and latitude—an activity seen by the British, the French, and many other naval powers as essential for improving navigation. With massively expensive instrumentation that only large institutions could afford, astronomy had changed beyond all recognition from the medieval period. Religious and other value systems no longer placed barriers on believing in and publishing particular accounts of the cosmos, and all serious intellectuals were heliocentrists.

Bibliography

Primary Sources

Copernicus, Nicolaus. On the Revolutions of the Heavenly Spheres. Translated by A. M. Duncan. Newton Abbott, U.K., and New York, 1976.

Descartes, René. The World and Other Writings. Translated and edited by Stephen Gaukroger. Cambridge, U.K., and New York, 1998.

Galilei, Galileo. Sidereus Nuncius; or, The Sidereal Messenger. Translated by Albert Van Helden. Chicago and London, 1989.

Kepler, Johannes. Mysterium Cosmographicum: The Secret of the Universe. Translated by A. M. Duncan. New York, 1981.

——. New Astronomy. Translated by William H. Donahue. Cambridge, U.K., and New York, 1992.

Secondary Sources

Chapman, Allan. Dividing the Circle: The Development of Critical Angular Measurement in Astronomy, 1500–1850. 2nd ed. Chichester, U.K., and New York, 1995.

Donahue, William H. The Dissolution of the Celestial Spheres. New York, 1981.

Dreyer, J. L. E. A History of Astronomy from Thales to Kepler. Rev. ed., with foreword by W. H. Stahl. New York, 1953.

Jardine, Nicholas. The Birth of History and Philosophy of Science: Kepler's A Defence of Tycho against Ursus, with Essays on its Provenance and Significance. Cambridge, U.K., and New York, 1984.

King, Henry C. The History of the Telescope. New York, 1979.

Kuhn, Thomas S. The Copernican Revolution: Planetary Astronomy in the Development of Western Thought. Cambridge, Mass., and London, 1957.

Newman, William R., and Anthony Grafton, eds. Secrets of Nature: Astrology and Alchemy in Early Modern Europe. Cambridge, Mass., and London, 2001.

Schechner Genuth, Sara. Comets, Popular Culture, and the Birth of Modern Cosmology. Princeton, 1997.

Stephenson, Bruce. Kepler's Physical Astronomy. Princeton, 1994.

Thoren, Victor E., with contributions by John R. Christianson. The Lord of Uraniborg: A Biography of Tycho Brahe. Cambridge, U.K., and New York, 1990.

—ROB ILIFFE

The science that deals with the universe beyond the Earth. It describes the nature, position, and motion of the stars, planets, and other objects in the skies, and their relation to the Earth.

Quotes:

"Astronomy is perhaps the science whose discoveries owe least to chance, in which human understanding appears in its whole magnitude, and through which man can best learn how small he is." - Georg C. Lichtenberg

"Adam inquires concerning celestial motions, is doubtfully answered, and exho