Astronomical Unit
The astronomical unit (AU or au) is a unit used to measure distances, especially between the objects of the solar system. It is historically based on the distance between Earth and Sun.
An astronomical unit is approximately 149 597 870.7 km (or , or )
A light year is approximately 63 241 AU.
In English and some other languages, the symbol "AU" or "to" is used, but the symbol a is recommended by the International Bureau of Weights and Measures , the international standard ISO / IEC 80000 and the International Astronomical Union.
Summary |
Accuracy of the definition
As a first approximation (assuming negligible mass planets to the Sun), the Earth has an orbit elliptically around the Sun , the temporal law is contained in the laws of Kepler , for greater clarity, we take into account interactions between planets and the force exerted by the planets on the Sun. It therefore appears that the Earth is not at a constant distance from the Sun.
To obtain a fixed value, it has originally been defined as the semi-major axis of the orbit Earth, which corresponds to the time average of the Earth-Sun distance (average distance over a certain period ).
In 1976 , she was defined as the distance from the Sun of a particle of mass negligible .
Numeric value current
These definitions, combined with radar observations and tracking of space probes, have evaluated the astronomical unit (149 597 870.700 0.003) km.
By extension, we consider an astronomical unit is approximately 150 million kilometers.
This represents a journey lasting just over 8 minutes at the speed of light.
Increased Secular
Astronomers Georgy A. Krasinsky and Victor A. Brumberg revealed by radio-metric measures the distance between the Earth and planets of the solar system, increasing the value of the astronomical unit, about 15 meters per century .
Usage
The astronomical unit is used for expressing distances within the solar system or planetary systems. Moreover, because of the methods of triangulation used to measure the distance of stars, which are based on sighting the diameter of Earth's orbit, the astronomical unit is the source of the definition of the parsec , the latter being defined as the distance at which one astronomical unit subtends an angle of an arc second. It is a unit outside the international system (SI), but in common use in astronomy.
Distances within the solar system
Average distance of the planets of the solar system the Sun
- Mercury : 0.38 AU
- Venus : 0.72 AU
- Earth : 1 AU
- March : 1.52 AU
- Jupiter : 5.21 au
- Saturn : 9.52 AU
- Uranus : 19.16 AU
- Neptune : 30.11 AU
(These values are rounded to the nearest hundredth)
Other distances from the Sun
- Asteroid Belt : 2 to 3.5 ua
- Pluto : from 29 to 49 AU
- Kuiper Belt : 30 to 55 AU
- Probe Voyager 1 : 114.2329 au (30 July 2010)
- Probe Voyager 2 : 139.9886 au (30 July 2010)
- Oort cloud : about 50 000 AU
References
- Notes
- The term "negligible mass" is a borderline case of two body problem in which one of the two masses exerts force on the other.
- References
- Conventions SIRT (Service International Earth Rotation and Reference Systems), DD McCarthy ed. Technical Note 21, Observatoire de Paris , July 1996.
- * Krasinsky GA and Brumberg VA , Secular Increase of Astronomical Unit from Analysis of the Major Planet Motions, and Its Interpretation Celestial Mechanics and Dynamical Astronomy 90: 267-288, (2004)
- Therese Encrenaz et al, Solar System, ed. EDP / CNRS, 2003, p. 274, extract online
See also
Related articles
- Light year
- Parsec
- Table of astrophysical constants
- Transit of Venus , a classical method of measuring the astronomical unit
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