Astronomical unit
| Astronomical unit | |
|---|---|
The red line indicates the Earth–Sun distance, which on average is about 1 astronomical unit
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| Unit system |
Astronomical system of units (Accepted for use with the SI) |
| Unit of | length |
| Symbol | au or ua |
| 1 au or ua in ... | ... is equal to ... |
| metric (SI) units | 149597870700 m |
| imperial & US units | 9.2956×107 mi |
| astronomical units |
4.8481×10−6 pc 1.5813×10−5 ly |
The astronomical unit (symbol au, or ua) is a unit of length, roughly the distance from Earth to the Sun. However, that distance varies as Earth orbits the Sun, from a maximum (aphelion) to a minimum (perihelion) and back again once a year. Originally conceived as the average of Earth's aphelion and perihelion, it is now defined as exactly 149597870700 metres (about 150 million kilometres, or 93 million miles). The astronomical unit is used primarily as a convenient yardstick for measuring distances within the Solar System or around other stars. However, it is also a fundamental component in the definition of another unit of astronomical length, the parsec.
A variety of unit symbols and abbreviations have been in use for the astronomical unit. In a 1976 resolution, the International Astronomical Union (IAU) used the symbol A for the astronomical unit (AU). In 2006, the International Bureau of Weights and Measures (BIPM) recommended ua as the symbol for the unit. In 2012, the IAU, noting "that various symbols are presently in use for the astronomical unit", recommended the use of the symbol "au". In the 2014 revision of the SI Brochure, the BIPM used the unit symbol "au". In ISO 80000-3, the symbol of the astronomical unit is "ua".
Earth's orbit around the Sun is an ellipse. The semi-major axis of this ellipse is defined to be half of the straight line segment that joins the aphelion and perihelion. The centre of the sun lies on this straight line segment, but not at its midpoint. Because ellipses are well-understood shapes, measuring the points of its extremes defined the exact shape mathematically, and made possible calculations for the entire orbit as well as predictions based on observation. In addition, it mapped out exactly the largest straight-line distance that Earth traverses over the course of a year, defining times and places for observing the largest parallax (apparent shifts of position) in nearby stars. Knowing Earth's shift and a star's shift enabled the star's distance to be calculated. But all measurements are subject to some degree of error or uncertainty, and the uncertainties in the length of the astronomical unit only increased uncertainties in the stellar distances. Improvements in precision have always been a key to improving astronomical understanding. Throughout the twentieth century, measurements became increasingly precise and sophisticated, and ever more dependent on accurate observation of the effects described by Einstein's theory of relativity and upon the mathematical tools it used.
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