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# astronomical unit

a unit of length, equal to the mean distance of the earth from the sun: approximately 93 million miles (150 million km). Abbr.: AU
[1900-05]

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▪ unit of measurement
Comparative data for the Sun, planets, and other solar system objectsa unit of length effectively equal to the average, or mean, distance between Earth and the Sun. Alternately, it can be considered the length of the semimajor axis—i.e., the length of half of the maximum diameter—of Earth's elliptical orbit around the Sun. An astronomical constant defined in terms of a form of Kepler's third law of planetary motion (Kepler's laws of planetary motion), the unit has a value of 149,597,870 km (92,955,808 miles). The astronomical unit provides a convenient way to express and relate distances of objects in the solar system and to carry out various astronomical calculations. For example, stating that the planet Jupiter is 5.2 AU (5.2 Earth distances) from the Sun and that Pluto is nearly 40 AU gives ready comparisons of the distances of all three bodies. For a listing of the average orbital distances of solar system bodies in astronomical units and kilometres, see columns 1–3 of the table (Comparative data for the Sun, planets, and other solar system objects).

In principle, the easiest way to determine the value of the astronomical unit would be to measure the Earth-Sun distance directly by means of the parallax method. In this approach, two observers stationed at the ends of a long, accurately known baseline—ideally, a baseline as long as Earth's diameter—would simultaneously record the position of the Sun against the essentially motionless background of the distant stars. Comparison of the observations would reveal an apparent shift, or angular (parallax) displacement, of the Sun against the remote stars. A simple trigonometric relationship incorporating this angular value and the baseline length then could be used to find the Earth-Sun distance. In practice, however, the method cannot be applied, because the Sun's intense glare blots out the background stars needed for the parallax measurement.

By the 17th century, astronomers understood the geometry of the solar system and the motion of the planets well enough to develop a proportional model of objects in orbit around the Sun, a model that was independent of a particular scale. To establish the scale for all orbits and to determine the astronomical unit, all that was needed was an accurate measurement of the distance between any two objects at a given instant. In 1672 the Italian-born French astronomer Gian Domenico Cassini (Cassini, Gian Domenico) made a reasonably close estimate of the astronomical unit based on a determination of the parallax displacement of the planet Mars—and thus its distance to Earth. Later efforts made use of widely separated observations of the transit of Venus (eclipse) across the Sun's disk to measure the distance between Venus and Earth. In 1932, determination of the parallax displacement of the asteroid Eros as it made a close approach to Earth yielded what was at the time a very precise value for the astronomical unit. Since the mid-20th century, astronomers have further refined their knowledge of the dimensions of the solar system and the value of the astronomical unit through a combination of radar ranging of Mercury, Venus, and Mars; laser ranging of the Moon (making use of light reflectors left on the lunar surface by Apollo (Apollo program) astronauts); and timing of signals returned from spacecraft as they orbit or make close passes of objects in the solar system.

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Universalium. 2010.

### Look at other dictionaries:

• astronomical unit — n. Astron. a unit of length based on the mean distance of the earth from the sun, c. 149.6 million km ( c. 93 million mi): light travels this distance in c. 8.3 minutes: abbrev. AU …   English World dictionary

• Astronomical unit — This article is about unit of length. For the full system of units, see Astronomical system of units. 1 astronomical unit = SI units 149.60×10^6 km 149.60×10^9 m Astronomical units 4.8481 …   Wikipedia

• astronomical unit — (ua or au or AU)    a unit of distance used by astronomers to measure distances in the Solar System. One astronomical unit equals the average distance from the center of the Earth to the center of the Sun (mathematically, it is the length of the… …   Dictionary of units of measurement

• astronomical unit — UK / US noun [countable] Word forms astronomical unit : singular astronomical unit plural astronomical units astronomy a unit for measuring distance in space …   English dictionary

• Astronomical Unit — noun a unit of length used for distances within the solar system; equal to the mean distance between the Earth and the Sun (approximately 93 million miles or 150 million kilometers) (Freq. 1) • Syn: ↑AU • Hypernyms: ↑astronomy unit * * *… …   Useful english dictionary

• Astronomical Unit —    A unit of distance used to measure orbits and trajectories within the solar system. An Astronomical Unit is the average distance between the Sun and Earth. One AU is roughly equal to 149,600,000 km (92,956,000 mi.). To give you an idea of the… …   The writer's dictionary of science fiction, fantasy, horror and mythology

• astronomical unit — astronominis vienetas statusas T sritis Standartizacija ir metrologija apibrėžtis Apibrėžtį žr. priede. priedas( ai) Grafinis formatas atitikmenys: angl. astronomical unit vok. astronomische Einheit, f rus. астрономическая единица, f pranc. unité …   Penkiakalbis aiškinamasis metrologijos terminų žodynas

• astronomical unit — astronominis vienetas statusas T sritis fizika atitikmenys: angl. astronomical unit vok. Astronomische Einheit, f rus. астрономическая единица, f pranc. unité astronomique, f …   Fizikos terminų žodynas

• astronomical unit — unit of distance equal to the distance from the earth to the sun Units of Measurement …   Phrontistery dictionary

• astronomical unit — /ˌæstrənɒmɪkəl ˈjunət/ (say .astruhnomikuhl yoohnuht) noun a unit, defined as 149 597 870 km, approximately equal to the mean distance between the centre of the earth and the centre of the sun, used as a unit of distance in astronomy. Symbol: AU… …   Australian English dictionary