any structure containing telescopes and auxiliary instruments with which to observe celestial objects. Observatories can be classified on the basis of the part of the electromagnetic spectrum in which they are designed to observe. The largest number of observatories are optical; i.e., they are equipped to observe in and near the region of the spectrum visible to the human eye. Some other observatories are instrumented to detect cosmic emitters of radio waves, while still others, known as orbiting astronomical observatories, consist of Earth satellites (satellite observatory) that carry special telescopes and detectors to study celestial sources of such forms of high-energy radiation as gamma rays and X rays from high above the atmosphere.

Optical observatories have a long history. The predecessors of astronomical observatories were monolithic structures that tracked the positions of the Sun, Moon, and other celestial bodies for timekeeping or calendrical purposes. The most famous of these ancient structures is the Stonehenge, constructed in England over the period from 2500 BC to 1700 BC. At about the same time, astrologer-priests in Babylonia observed the motions of the Sun, Moon, and planets from atop their terraced towers known as ziggurats (ziggurat). No astronomical instruments appear to have been used. The Indians of the Yucatan Peninsula in Mexico carried out the same practice at El Caracol, a dome-shaped structure somewhat resembling a modern optical observatory. There is again no evidence of any scientific instrumentation, even of a rudimentary nature.

Perhaps the first observatory that utilized instruments for accurately measuring the positions of celestial objects was built about 150 BC on the island of Rhodes by the greatest of the pre-Christian astronomers, Hipparchus. There he discovered precession and developed the magnitude system used to indicate the brightness of celestial objects. The true predecessors of the modern observatory were those established in the Islāmic (Islāmic world) world. Observatories were built at Damascus and Baghdad as early as the 9th–10th century AD. A splendid one was built at Marāgheh (now in Iran) about AD 1260, and substantial modifications in Ptolemaic astronomy were introduced there. The most productive Islāmic observatory was that erected by the Timurid prince Ulūgh Beg at Samarkand in about 1420; he and his assistants made a catalog of stars from observations with a large quadrant. The first notable premodern European observatory was that at Uraniborg on the island of Hven, built by King Frederick of Denmark for Tycho Brahe in AD 1576.

The first optical telescope used to study the heavens was constructed in 1609 by Galileo Galilei, using information from Flemish pioneers in lens-making. The first major centres for astronomical study utilized a telescope movable only in one plane, with motion solely along the local meridian (the “transit,” or “meridian circle”). Such centres were founded in the 18th and 19th centuries at Greenwich (London), Paris, Cape Town, and Washington, D.C. By timing the passage of stars as the local meridian was swept past them by the Earth's rotation, astronomers were able to improve the accuracy of position measurements of celestial objects from a few minutes of arc (before the advent of the telescope) to less than a tenth of a second of arc.

One notable observatory built and operated by an individual was that of William Herschel (Herschel, Sir William), assisted by his sister Caroline, in Slough, Eng. Known as Observatory House, its largest instrument had a mirror made of speculum metal, with a diameter of 122 cm (48 inches) and a focal length of 17 m (40 feet). Completed in 1789, it became one of the technical wonders of the 18th century.

Today, the site of the world's largest grouping of optical telescopes is atop Kitt Peak, near Tucson, in southern Arizona, U.S. Most of the telescopes are a part of the Kitt Peak National Observatory. Most notable among this array of instruments are the 4-metre (157-inch) Mayall telescope and the McMath solar telescope, the largest of its type in the world.

The largest modern-day optical telescopes are the 6-metre (236-inch) reflector on Mount Pastukhov, in the Caucasus Mountains in the Soviet Union, and the 5-metre (200-inch) Hale Telescope on Palomar Mountain, near San Diego, Calif., U.S. Also noteworthy is the revolutionary Multiple Mirror Telescope (MMT Observatory) atop Mount Hopkins, near Tucson; it has an aperture of 4.5 m (177 inches).

The ability to observe the universe in the radio (radio and radar astronomy) region of the spectrum was developed during the 1930s. The American engineer Karl Jansky detected radio signals from the centre of the Milky Way Galaxy in 1931 by means of a linear directional antenna. Soon thereafter, the American engineer and astronomer Grote Reber (Reber, Grote) constructed a prototype of the radio telescope, a bowl-shaped antenna 9.4 m (31 feet) in diameter.

Today's radio telescopes are capable of observing at most wavelength regions from a few millimetres to about 20 metres. They vary in construction, though they are typically huge movable dishes. The world's largest steerable dish is the 96-metre (315-foot) telescope at Jodrell Bank (Jodrell Bank Observatory), Cheshire, Eng. The largest single-unit radio telescope is located at Arecibo (Arecibo Observatory), in Puerto Rico. Lying level in a rounded-out hollow in the mountains, the main antenna of this instrument has a diameter of 304 m (about 1,000 feet). Limited aiming capability is allowed by the Earth's motion and by some movement of the overhanging antenna. One other significant radio telescope is the Very Large Array (VLA), operated by the National Radio Astronomy Observatory. Located near Socorro, N.M., U.S., the VLA is composed of 27 individual radio telescopes, each of which is 25 m (81 feet) in diameter. These instruments are not only steerable but also movable over railroad tracks in the shape of a large Y. Each arm of the Y is 21 km (13 miles) long. The purpose of the VLA is to obtain extremely high-resolution imaging of cosmic radio sources. The resolving ability of a telescope, whether radio or optical, improves with increasing diameter. The individual dishes of the VLA work in precise unison to fabricate a large radio telescope having an effective diameter of 27 km (16.7 miles).

With the advent of the space age, the capability of astronomical instruments to orbit above the Earth's absorbing and distorting atmosphere has enabled astronomers to build telescopes sensitive to regions of the electromagnetic spectrum besides those of visible light and radio waves. Since the 1960s, orbiting observatories have been launched to observe gamma rays (Orbiting Solar Observatory and the Small Astronomy Satellite-2), X rays (Uhuru and the High Energy Astronomical Observatory), ultraviolet radiation (International Ultraviolet Explorer), and infrared radiation (Infrared Astronomical Satellite). Several of these functions were combined in the Hubble Space Telescope, launched in 1990.