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## Astronomical unit is a distance between the Sun and Earth. |

The The symbol "ua" is recommended by Bureau International des Poids et Mesure, but in the United States and other anglophone countries the reverse usage is more common. The International Astronomical Union recommends "au" and international standard ISO 31-1 uses "Astronomical unit".
Earth's orbit is not a circle but an ellipse; originally, the Astronomical unit was defined as the Length of the Semimajor axis of said orbit. For greater precision, the International Astronomical Union in 1976 defined the Astronomical unit as the distance from the centre of 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). More accurately, it is the distance such that the heliocentric gravitational constant (the product
Aristarchus of Samos estimated the distance to the Sun to be about 20 times the distance to the moon, whereas the true ratio is about 390. His estimate was based on the angle between the half moon and the sun, which he estimated as 87º. According to Eusebius of Caesarea in the At the time the Astronomical unit was introduced, its actual value was very poorly known, but planetary distances in terms of Astronomical unit could be determined from heliocentric geometry and Kepler's laws of planetary motion. The value of the Astronomical unit 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. 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 kilometres. 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.
The distances are approximate mean distances. It has to be taken into consideration that the distances between celestial bodies change in time due to their orbits and other factors. - The Earth is 1.00 ± 0.02 Astronomical unit from the Sun.
- The Moon is 0.0026 ± 0.0001 Astronomical unit from the Earth.
- Mars is 1.52 ± 0.14 Astronomical unit from the Sun.
- Jupiter is 5.20 ± 0.05 Astronomical unit from the Sun.
- Pluto is 39.5 ± 9.8 Astronomical unit from the Sun.
- 90377 Sedna's orbit ranges between 76 and 942 Astronomical unit from the Sun; Sedna is currently (as of 2006) about 90 Astronomical unit from the Sun.
- As of August 2006, Voyager 1 is 100 Astronomical unit from the Sun, the furthest of any man-made object.
- The mean diameter of the Solar System, including the Oort cloud, is approximately 10
^{5}Astronomical unit. - Proxima Centauri (the nearest star) is ~268 000 Astronomical unit away from the Sun.
- The mean diameter of Betelgeuse is 2.57 Astronomical unit.
- The distance from the Sun to the centre of the Milky Way is approximately 1.7×10
^{9}Astronomical unit. - The Earth is actually 147,104,753 km away from the Sun on 29 December and 152,091,803 km away from the Sun on 30 June.
Some conversion factors: - 1 Astronomical unit = 149 597 870.691 ± 0.030 km ˜ 92 955 807 miles ˜ 8.317 light minutes ˜ 499 light-seconds.
- 1 light-second ˜ 0.002 Astronomical unit.
- 1 gigameter ˜ 0.00(6) Astronomical unit.
- 1 light-minute ˜ 0.120 Astronomical unit.
- 1 microparsec ˜ 0.206 Astronomical unit.
- 1 terameter ˜ 6.685 Astronomical unit.
- 1 light-hour ˜ 7.214 Astronomical unit.
- 1 light-day ˜ 173.263 Astronomical unit.
- 1 milliparsec ˜ 206.265 Astronomical unit.
- 1 light-week ˜ 1212.84 Astronomical unit.
- 1 light-month ˜ 5197.9 Astronomical unit.
- 1 Light-year ˜ 63 241 Astronomical unit.
- 1 pc ˜ 206 265 Astronomical unit.
- Units outside the SI
*(at the NIST web site)*. - Recommendations concerning Units
*(at the IAstronomical unit web site)*. - Solar Mass Loss, the Astronomical Unit, and the Scale of the Solar System
*(a discussion of the relation between the Astronomical unit and other quantities)*.
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