![]() The distance from the Sun to the center of the Milky Way is approximately 1.7×10 9 AU.The mean diameter of Betelgeuse is 2.57 AU.Proxima Centauri (the nearest star) is ~268 000 AU away from the Sun.As of August 2006, Voyager 1 is 100 AU from the Sun, the furthest of any man-made object. ![]() 90377 Sedna's orbit ranges between 76 and 942 AU from the Sun Sedna is currently (as of 2006) about 90 AU from the Sun.Jupiter is 5.20 ± 0.05 AU from the Sun.The Moon is 0.0026 ± 0.0001 AU from the Earth.The Earth is 1.00 ± 0.02 AU from the Sun.It has to be taken into consideration that the distances between celestial bodies change in time due to their orbits and other factors. The distances are approximate mean distances. 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. This approach makes all results dependent on 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. 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. More recently very precise measurements have been carried out by radar and by telemetry from space probes. 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. It was strongly advocated by Edmond Halley and was applied to the transits of Venus observed in 17, and then again in 18.Īnother 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"). This method was devised by James Gregory and published in his Optica Promata. By measuring the parallax of Mars from two locations on the Earth, they arrived at a figure of about 140 million kilometers.Ī somewhat more accurate estimate can be obtained by observing the transit of Venus. The value of the AU was first estimated by Jean Richer and Giovanni Domenico Cassini in 1672. Using the Greek stadium of 185 to 190 meters, the former translation comes to a far-too-low 755,000 km, whereas the second translation comes to 148.7 to 152.8 million km (accurate within two percent).Īt 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. This has been translated either as 4,080,000 stadia (1903 translation by Edwin Hamilton Gifford), or as 804,000,000 stadia (edition of Édouard des Places, dated 1974-1991). His estimate was based on the angle between the half moon and the sun, which he calculated to be 87°.Īccording to Eusebius of Caesarea in the Praeparatio Evangelica, Eratosthenes found the distance to the sun to be "σταδιων μυριαδας τετρακοσιας και οκτωκισμυριας" (literally "of stadia myriads 40"). The International Astronomical Union recommends "au", and the international standard ISO 31-1 uses "AU."Īristarchus 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. The abbreviation "ua" is recommended by the Bureau International des Poids et Mesures, but in the United States and other anglophone countries the reverse lettering (AU or au) is more common. More accurately, it is the distance at which the heliocentric gravitational constant (the product GM ☉) is equal to (0.017 202 093 95)² AU³/d². In 1976, the International Astronomical Union revised the definition of the AU for greater precision, defining it as the distance from the center of the Sun at which a particle of negligible mass, in an unperturbed circular orbit, would have an orbital period of 365.2568983 days (one Gaussian year). The astronomical unit was originally defined as the length of the semimajor axis of the Earth's elliptical orbit around the Sun.
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