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scientific facility especially equipped to detect and record naturally occurring scientific phenomena. Although geological and meteorological observatories exist, the term is generally applied to astronomical observatories.

The Astronomical Observatory

The function of the astronomical observatory is centered around the telescopetelescope,
traditionally, a system of lenses, mirrors, or both, used to gather light from a distant object and form an image of it. Traditional optical telescopes, which are the subject of this article, also are used to magnify objects on earth and in astronomy; other types of
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. In addition to visual and photographic observations of astronomical bodies and phenomena, perhaps the most valuable use of the telescope is in connection with the spectroscopic study of starlight. The total light from a star is separated into its various wavelengths (see spectrumspectrum,
arrangement or display of light or other form of radiation separated according to wavelength, frequency, energy, or some other property. Beams of charged particles can be separated into a spectrum according to mass in a mass spectrometer (see mass spectrograph).
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), and the intensity of each is measured. The temperature and chemical composition of stars can be obtained by this method, as well as information about stellar motion and magnetic fields. Using computers, astronomers can measure the spectra digitally recorded by spectrographs and photometers. Observatories specializing in solar astronomy usually have coronographs and spectroheliographs. Atmospheric limitations on telescopic observations include weather conditions, air turbulence, air glow, pollution, and any source of extraneous illumination. To minimize such conditions optical observatories are generally located at high altitudes in sparsely populated areas.

See articles on specific observatories.

Development of the Astronomical Observatory

Early civilizations, such as those of Babylon, China, and Egypt, recognized the regular and periodic nature of heavenly motions and established primitive observatories to maintain astronomical records. The main purposes of these early observatories were to regulate the calendar and predict the changes of season. Because it was believed that unusual occurrences, such as comets and eclipses, foretold future events on earth, the early observatories also served a religious function, and most of the ancient astronomers were priests. Later observatories were established to compile accurate star charts and an annual ephemerisephemeris
(pl., ephemerides), table listing the position of one or more celestial bodies for each day of the year. The French publication Connaissance de Temps is the oldest of the national astronomical ephemerides, founded in 1679.
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 that would be of use to navigators in determining longitude at sea. For some 600 years, beginning in the 13th cent., Roman Catholic churches included solar observatories to measure the movements of the sun and so determine the correct date for Easter.

The instruments in use before the invention of the telescope include the sextantsextant,
instrument for measuring the altitude of the sun or another celestial body; such measurements can then be used to determine the observer's geographical position or for other navigational, surveying, or astronomical applications.
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, quadrant, astrolabeastrolabe
, instrument probably used originally for measuring the altitudes of heavenly bodies and for determining their positions and movements. Although its origin is ancient and obscure, its invention is frequently ascribed either to Hipparchus or to Apollonius of Perga.
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, and armillary sphere. These are all calibrated sighting devices for determining the angular positions of stars and planets. The armillary sphere was the most sophisticated of these instruments. It was composed of a number of rings corresponding to great circles on the celestial sphere and was used to determine both the right ascension and the declination of a star. The last great observatory of the pretelescopic era was built by Tycho BraheBrahe, Tycho
, 1546–1601, Danish astronomer. The most prominent astronomer of the late 16th cent., he paved the way for future discoveries by improving instruments and by his precision in fixing the positions of planets and stars.
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 at Uranienborg, on the island of Ven, Denmark.

The invention of the telescope in the early 17th cent. revolutionized observational astronomy in two ways. First, the positions and motions of celestial bodies could be measured much more accurately with telescopes than with the earlier instruments. Such data provided a source of precise time signals. Second, the telescope could be used to analyze the physical nature of celestial bodies themselves. Until the 19th cent., telescopic images were inspected visually by highly trained observers who made drawings of what they saw. The development of dry-plate photography, which permitted long exposure times, however, offered a much more sensitive method of recording images. In the late 20th cent., electronic digital detectors utilizing charge-coupled devices (CCDs) superseded the use of film; a CCD can detect the arrival of a single photon of light. A recent development is the extension of astronomical observations to wavelengths outside the visible spectrum. Most important has been the development of radio astronomyradio astronomy,
study of celestial bodies by means of the electromagnetic radio frequency waves they emit and absorb naturally. Radio Telescopes

Radio waves emanating from celestial bodies are received by specially constructed antennas, called radio telescopes,
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, the study of radio waves emitted by celestial bodies.

Because the atmosphere interferes with astronomical observations from the ground, the ideal location for an observatory is beyond the earth's atmosphere. Since the late 20th cent., there has been an increasing emphasis on space-based observatories (see observatory, orbitingobservatory, orbiting,
research satellite designed to study solar radiation, electromagnetic radiation from distant stars, the earth's atmosphere, or the like. Because the atmosphere and other aspects of the earth's environment interfere with astronomical observations from the
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). Several artificial satellitessatellite, artificial,
object constructed by humans and placed in orbit around the earth or other celestial body (see also space probe). The satellite is lifted from the earth's surface by a rocket and, once placed in orbit, maintains its motion without further rocket propulsion.
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 have been equipped with telescopes for infrared, visible, ultraviolet, and X-ray observations. The International Ultraviolet Explorer (IUE) satellite, launched in 1978, is an 18-in. (0.45-meter) space telescope for ultraviolet studies. Launched in 1983, the Infrared Astronomy Satellite (IRAS) discovered some 246,000 infrared sources, as well as several stars around which planetary systems appear to be forming. Skylab was a manned orbiting space observatory (see space explorationspace exploration,
the investigation of physical conditions in space and on stars, planets, and other celestial bodies through the use of artificial satellites (spacecraft that orbit the earth), space probes (spacecraft that pass through the solar system and that may or may not
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). The largest space-based observatory is the Hubble Space TelescopeHubble Space Telescope
(HST), the first large optical orbiting observatory. Built from 1978 to 1990 at a cost of $1.5 billion, the HST (named for astronomer E. P. Hubble) was expected to provide the clearest view yet obtained of the universe from a position some 350 mi (560 km)
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, launched in 1990. Other observatories include the Compton Gamma-Ray Observatory, launched in 1991, and the Chandra X-ray Observatory, launched in 1999. ROSAT [ROentgen SATellite], a joint German-U.S.-British project launched in 1990, studies both X-ray and ultraviolet wavelengths never before imaged from space. It has detected a new class of bright stars that shine only in the ultraviolet part of the spectrum. The Cosmic Background Explorer (1989–93) studied microwave background radiation that no star or other known object could emit—it is believed to have come from the creation of the universe (see cosmologycosmology,
area of science that aims at a comprehensive theory of the structure and evolution of the entire physical universe. Modern Cosmological Theories
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The computer age has also impacted observatories in several ways. Instead of being cast in one piece, reflecting telescope mirrors can be constructed of numerous small segments that move under computer control to focus the light and create an image equivalent to that of a much larger telescope. This has made it economically possible for observatories to reach further and further into the cosmos. Computers have also made it easier to construct and retrieve information from archival data bases. Computer networks are under construction that will make it possible for these archival databases to be made available to other observatories so as to create a virtual observatoryvirtual observatory,
a collection of integrated astronomical data archives and software tools that utilize computer networks to create an environment in which research can be conducted.
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, with gains in productivity and cost-effectiveness for the observatories that participate in it.


(ŏb-zer -vă-tor-ee, -toh-ree) A structure built primarily for astronomical observation, equipped nowadays with optical, radio, and/or infrared telescopes and, in the larger observatories, the associated equipment with which spectrographic, photometric, and other such measurements are made. The sites of modern optical and infrared observatories are selected very carefully so that there is maximum transmission of signals by the atmosphere (i.e. the sky is free from clouds and dust), minimal atmospheric turbulence (i.e. the seeing is optimal), and minimum light pollution (i.e. the night sky is dark). Most are sited in mountainous areas or on volcanic islands where the atmosphere is very thin and the absorption effects of water vapor are reduced. Radio observatories, less hampered by seeing conditions, clouds, and light pollution, must still be isolated from terrestrial radio and electrical interference.

Instruments can now be carried into space, away from the disturbing and absorbing effects of the Earth's atmosphere. The greatest hopes for future astronomical studies lie in space observatories and possible lunar observatories, permitting observations over the entire electromagnetic spectrum with a resolution that is limited only by the size of the telescope.


A structure in which astronomical observations are carried out; a place such as an upper room that affords a wide view, a lookout.



an institution that conducts astronomical and geophysical (magnetic, hydrometeorological, seismic) observations and research. The first observatories were merely sites where observations were conducted. Later, observatories evolved into research institutes that not only observe various phenomena but also study the processes giving rise to the phenomena. (SeeAEROLOGICAL OBSERVATORY, ASTRONOMICAL OBSERVATORIES AND INSTITUTES, GEOPHYSICAL OBSERVATORY, NAVAL OBSERVATORY, and METEOROLOGICAL OBSERVATORY.)


1. A structure, generally with a rotatable dome, in which astronomical observations are carried out.
2. A place, such as an upper room, which affords a wide view; a lookout.


an institution or building specially designed and equipped for observing meteorological and astronomical phenomena
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