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Astronomical Unit)
The astronomical unit (AU) is a unit of distance, approximately equal to the mean distance between Earth and Sun. The currently accepted value of the AU is 149,597,870,691 metres (about 150 million kilometres or 93 million miles).
Earth's orbit is not a circle but an ellipse; originally, the AU was defined as the length of the semimajor axis of said orbit. For greater precision, the International Astronomical Union in 1976 defined the AU as the distance from the Sun at which a particle of negligible mass, in an unperturbed circular orbit, would have an orbital period of 365.2568983 days (a Gaussian year). The AU is thus defined as 149,597,870,691 metres.
At the time the AU was introduced, its actual value was very poorly known, but planetary distances in terms of AU could be determined from heliocentric geometry and Kepler's laws of planetary motion. The value of the AU was first estimated by Jean Richer and Giovanni Domenico Cassini in 1672. By measuring the parallax of Mars from two locations on the Earth, they arrived at a figure of about 140 million kilometers.
The first good measurement on the distance between earth and the sun was made by Eratosthenes in around 200 BC, by studying lunar eclipses, his result was 804,000,000 stadia. If we use the common attic stadion this translates to roughly 148,740,000,000 m.
A somewhat more accurate estimate can be obtained by observing the transit of Venus.
This method was devised by Edmond Halley, and applied to the transits of Venus observed in 1761 and 1769, and then again in 1874 and 1882.
Another method involved determining the constant of aberration, and Simon Newcomb gave great weight to this method when deriving his widely accepted value of 8.80" for the solar parallax (close to the modern value of 8.794148").
The discovery of the near-Earth asteroid 433 Eros and its passage near the Earth in 1900–1901 allowed a considerable improvement in parallax measurement. More recently very precise measurements have been carried out by radar and by telemetry from space probes.
While the value of the astronomical unit is now known to great precision, the value of the mass of the Sun is not, because of uncertainty in the value of the gravitational constant. Because the gravitational constant is known to only five or six significant digits while the positions of the planets are known to 11 or 12 digits, calculations in celestial mechanics are typically performed in solar masses and astronomical units rather than in kilograms and kilometers. This approach makes all results dependent on the gravitational constant. A conversion to SI units would separate the results from the gravitational constant, at the cost of introducing additional uncertainty by assigning a specific value to that unknown constant.
It is known that the mass of the Sun is very slowly decreasing, and therefore the period of a body at a given distance is increasing. This implies that the AU is getting smaller (very slowly) over time.
Examples
- Pluto is 39.5 AU from the Sun.
- Jupiter is 5.2 AU from the Sun.
- Mean diameter of Betelgeuse is 2.57 AU.
- The Moon is 0.0026 AU from the Earth.
Some approximate conversion factors:
See also:
parsec and light year,
conversion of units,
orders of magnitude
References
- E. Myles Standish. "Report of the IAU WGAS Sub-group on Numerical Standards". In Highlights of Astronomy, I. Appenzeller, ed. Dordrecht: Kluwer Academic Publishers, 1995. (Complete report available online: PostScript (http://ssd.jpl.nasa.gov/iau-comm4/iausgnsrpt.ps). Tables from the report also available: Astrodynamic Constants and Parameters (http://ssd.jpl.nasa.gov/astro_constants.html))
External links
cs:Astronomická jednotka
cy:Uned seryddol
da:Astronomisk enhed
de:Astronomische Einheit
et:Astronoomiline ühik
es:Unidad astronómica
fr:Unité astronomique
it:Unità Astronomica
lb:Astronomesch Eenheet
hu:Csillagászati egység
nl:Astronomische eenheid
ja:天文単位
pl:Jednostka astronomiczna
sl:Astronomska enota
fi:Astronominen yksikkö
zh-cn:天文单位距离
zh-tw:天文單位
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