astronomical unit (AU)

Length of the semimajor axis of Earth's orbit around the Sun, 92,955,808 mi (149,597,870 km), often defined simply as the average distance from Earth to the Sun. Direct measurement through the parallax method cannot be used for accurate determinations, because the Sun's glare blots out the light of the background stars necessary to make the measurement. The most precise values have been obtained by measuring the distance from Earth to other objects orbiting the Sun. This indirect method requires an accurate proportional mathematical model of the solar system; once the distance to one planet or other object is determined, then the distance to the Sun can be calculated.

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Structure containing telescopes and other instruments for observing celestial objects and phenomena. Observatories can be classified by the part of the electromagnetic spectrum they can receive. Most are optical, observing in and near the region of the visible spectrum. Some are equipped to detect radio waves; others (space observatories) are Earth satellites and other spacecraft that carry special telescopes and detectors to study celestial sources of high-energy radiation (e.g., gamma rays, ultraviolet radiation, X-rays) from above the atmosphere. Stonehenge may have been an early predecessor of the optical observatory. Perhaps the first observatory that used instruments to accurately measure the positions of celestial objects was built circa 150 BC by Hipparchus. The first notable premodern European observatory was that at Uraniborg, built for Tycho Brahe in 1576. Observatory House, in Slough, Eng., built and operated by William Herschel (see Herschel family), was one of the technical wonders of the 18th century. Today the world's largest groupings of optical telescopes are atop Mauna Kea, in Hawaii, and Cerro Tololo, in Chile. Other major observatories include Arecibo Observatory; Mount Wilson Observatory; Palomar Observatory; and Royal Greenwich Observatory.

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Astronomical observatory, oldest scientific institution in Britain, founded for navigational purposes in 1675 by Charles II at Greenwich, England. Its main contributions have been in navigation, timekeeping, determination of star positions, and almanac publication. In 1767 it began publishing The Nautical Almanac, based on the time at the longitude of Greenwich; its popularity among navigators led in part to the Greenwich meridian's being made Earth's prime meridian and the starting point for international time zones in 1884 (see Greenwich Mean Time).

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Astronomical observatory located atop Mount Wilson, near Pasadena, California, U.S. Founded in 1904 by George Ellery Hale (1868–1938), it was operated jointly with Palomar Observatory as Hale Observatories (1948–80). Its largest optical telescope, with a diameter of 100 in. (2.5 m), enabled Edwin Hubble and his associates to discover evidence of an expanding universe and to estimate its size.

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Astronomical observatory near Arecibo, P.R., site of the world's largest single-unit radio telescope (as opposed to multiple telescope interferometers such as the Very Large Array). The telescope dish, 1,000 ft (300 m) across, is built into a valley; celestial sources are tracked across the sky by moving secondary structures suspended about 500 ft (150 m) above the dish. The observatory has produced detailed radar maps of the surface of Venus and near-Earth asteroids (see Earth-crossing asteroid), made detailed studies of Earth's ionosphere, and made major contributions to studies of pulsars and hydrogen gas in galaxies.

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This article is about the Sextant as used for astrometry. For the navigator's sextant, see Sextant.

Sextants for astronomical observations were used primarily for measuring the positions of stars. They are little used today, having been replaced over time by transit telescopes, astrometry techniques, and satellites such as Hipparcos.

There are two types of astronomical sextants, mural instruments and frame-based instruments.

Mural Sextants

Mural sextants are a special case of a mural instrument. Many were made that were quadrants rather than sextants. They were a kind of speciality of medieval Muslim astronomers to whom the credit of building the first mural sextants is attributed.

The first known mural sextant was constructed in Ray, Iran, by Abu-Mahmud al-Khujandi in 994.. To measure the obliquity of the ecliptic , al-Khujandī invented a device that he called al-Fakhri sextant (al-suds al Fakhrī), a reference to his patron; Buwayhid ruler, Fakhr al Dawla (976-997). The main improvement incorporated in al-Fakhri sextant over earlier instruments was bringing the precision of reading to seconds while older instruments could only be read in degrees and minutes. This fact was confirmed by al-Birūni, al-Marrākushī and al-Kāshī. This instrument was a sixty-degree arc on a wall aligned along a meridian (north-south line). Al Khujandi’s instrument was larger than previous such instruments; it had a radius of about twenty meters.

Ulugh Beg constructed a Fakhri Sextant that had a radius of 40.4 meters, the largest instrument of its type in 16th century. Seen in the image on the right, the arc was finely constructed with a staircase on either side to provide access for the assistants who performed the measurements.

Framed sextants

A sextant based on a large metal frame had an advantage over a mural instrument in that it could be used at any orientation. This allows the measure of angular distances between astronomical bodies.

These instruments differ substantially from a navigator's sextant in that the latter is a reflecting instrument. The navigator's sextant uses mirrors to bring the image of the sun, moon or a star to the horizon and measure the altitude of the object. Due to the use of the mirrors, the angle measured is twice the length of the instrument's arc. Hence, the navigator's sextant measures 120° on an arc with an included angle of 60°. By comparison, the astronomical sextants are large and measure angles directly - a 60° arc will measure at most 60°.


These large sextants are made primarily of wood, brass or a combination of both materials. The frame is heavy enough to be stiff and provide reliable measures without flexural changes in the instrument compromising the quality of the observation. The frame is mounted on a support structure that holds it in position while in use. In some cases, the position of the sextant can be adjusted to allow measurements to be made with any instrument orientation. Owing to the size and weight of the instrument, attention was paid to balancing it so that it could be moved with ease.

Observations were typically made with an alidade, though newer versions could use a telescope. In some cases, a system of counter-weights and pulleys were used to allow the observer to manipulate the instrument in spite of its size.


These instruments were used in much the same way as smaller instruments, with effort possibly scaled due to the size. Some of the instruments might have needed more than one person to operate.

If the sextant is permanently fixed in position, only the position of the alidade or similar index need be determined. In that case, the observer moved the alidade until the object of interest is centred in the sights and then reads the graduations marked on the arc.

For instruments that could be moved, the process was more complex. It was necessary to sight the object with two lines. The edge of the instrument would typically be supplied with sights and the instrument was aligned with one of the two objects of interest. The alidade was then aligned with the second object as well. Once each object was centred in one set of sights, the reading could be taken. This could be a challenge for a moving star observed with a very large instrument as a single person might not be able to confirm both sights with ease; an assistant was a great benefit. The illustration of the Hevelius instrument to the right shows how two persons would use such a sextant. In the image, Elisabetha is aligning the instrument while Johannes sets the alidade.

Well-known framed sextants


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