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An astronomical unit (international designation: au) is a historically established unit of distance measurement in astronomy, approximately equal to the average distance from the Earth to the Sun. Why "average"? Because the Earth does not move around the Sun in a regular circular orbit - at the extreme points, the distance from the Earth to the Sun varies from 147.5 to 152.5 million kilometers. The astronomical unit is often rounded up to 150 million kilometers. This is quite justified when it is used simply for comparative estimation of distances in the solar system.
Light travels this distance in about 500 seconds (8 minutes 20 seconds).
It is mainly used to measure the distances between objects in the solar system,
extra solar systems, and also between the components of binary stars.
In September 2012, the 28th General Assembly of the International Astronomical Union in Beijing decided to tie the astronomical unit to the International System of Units (SI). An astronomical unit is, by definition, exactly 149,597,870,700 meters. In addition, the IAU decided to standardize the international designation of the astronomical unit: "au". Sometimes the designations "a. u."or "AU". There is also the international standard ISO 80000-3, which recommends the use of the designation "ua".
In accordance with the decision of the 10th General Assembly of the IAU in 1976, the astronomical unit was defined as the radius of the circular orbit of a test body in isotropic coordinates, the angular velocity of which, neglecting all bodies of the solar system except the Sun, would be exactly equal to 0.017 202 098 95 radians on ephemeris days. In the IERS 2003 constant system, the astronomical unit was assumed to be 149,597,870.691 km.
Since the advent of the heliocentric system, and especially Keplerian celestial mechanics, the relative distances in the solar system (excluding the too close Moon) have become known with good accuracy. Since the Sun is the central body of the system, and the Earth, which moves in an almost circular orbit, is the location of observers, it was natural to take the radius of this orbit as a unit of measurement. However, there was no way to reliably measure the value of this unit, that is, to compare it with terrestrial scales. The Sun is too far away to reliably measure parallax from Earth. The distance to the Moon was known, but based on the data known in the 17th century, it was not possible to estimate the ratio of the distances to the Sun and the Moon - observation of the Moon does not give the required accuracy, and the ratio of the masses of the Earth and the Sun was also not known.
In 1672, Giovanni Cassini, together with his collaborator Jean Richet, measured the parallax of Mars. Since the parameters of the orbit of the Earth and Mars were measured with high accuracy, it became possible to estimate the value of the astronomical unit - in modern units they got about 140 million km. Subsequently, refined measurements of the astronomical unit were carried out using the passages of Venus across the solar disk. The approach of the asteroid Eros to the Earth in 1901 and the measurement of its parallax made it possible to obtain an even more accurate estimate.
The astronomical unit was also refined using planetary radar. The location of Venus in 1961 established that the astronomical unit is 149,599,300 km. Possible error did not exceed 2000 km. The re-radaring of Venus in 1962 made it possible to reduce this uncertainty and clarify the value of the astronomical unit: it turned out to be 149,598,100 ± 750 km. It turned out that before the 1961 location, the value of a. e. was known with an accuracy of 0.1%.
Long-term measurements of the distance from the Earth to the Sun recorded its slow increase at a rate of about 15 meters per hundred years (which is an order of magnitude higher than the accuracy of modern measurements). One of the reasons may be the loss of mass by the Sun (due to the solar wind), but the observed effect significantly exceeds the calculated values.
- The orbital radius of Neptune, the most distant planet in the solar system, is about 30 AU.
- As of April 23, 2016, the Voyager 1 spacecraft was at a distance of 134.75 AU. from the Sun, moving away from it at a speed of 3.6 a.
u. at year. It is the most distant from the Earth and the fastest moving object created by man.
The distance to our nearest star, Proxima Centauri, is about 270,000 AU.