US customary /
The astronomical unit (AU or au or a.u. or
sometimes ua) is a unit of
nearly equal to the
semi-major axis of
around the Sun.
The currently accepted value of the AU is 149 597 870 691
± 30 metres (about 150
million kilometres or 93 million miles).
The symbol "ua" is recommended by the
Bureau International des Poids et Mesures
, but in the United States and other
anglophone countries the reverse usage is more common. The
International Astronomical Union recommends "au"
international standard ISO 31-1
Originally, the AU was defined as the
length of the
semimajor axis of the Earth's elliptical orbit. In
International Astronomical Union revised the definition of the AU for greater
precision, defining it 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 (one
Gaussian year). More accurately, it is the distance at which the
gravitational constant (the product GM☉) is equal to
(0.017 202 093 95)² AU³/d².
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 87°.
Eusebius of Caesarea in the
Eratosthenes found the distance to the sun to be "σταδιων μυριαδας
τετρακοσιας και οκτωκισμυριας" (literally "of stadia myriads 400 and 80000").
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). Using the Greek stadium of 185 to 190
metres, 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 2%).
At 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
Kepler's laws of planetary motion. The value of the AU was first estimated
Jean Richer and
Giovanni Domenico Cassini in
1672. By measuring
Mars from two locations on the Earth, they arrived at a figure of about 140
A somewhat more accurate estimate can be obtained by observing the
transit of Venus. This method was devised by
James Gregory and published in his
Optica Promata. It was strongly advocated by
Edmond Halley and was applied to the transits of Venus observed in
1769, and then
again in 1874 and
Another method involved determining the constant of
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
allowed a considerable improvement in parallax measurement. More recently very
precise measurements have been carried out by
radar and by
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
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
- The Earth is
1.00 ± 0.02 AU from the
- The Moon is
0.0026 ± 0.0001 AU from the Earth.
Mars is 1.52 ± 0.14 AU from the Sun.
is 5.20 ± 0.05 AU from the Sun.
- Pluto is 39.5
± 9.8 AU from the Sun.
Sedna's orbit ranges between 76 and 942 AU from the Sun; Sedna is currently
(as of 2006)
about 90 AU from the Sun.
- As of August 2006,
is 100 AU from the Sun, the furthest of any man-made object.
Proxima Centauri (the nearest
star) is ~268 000
AU away from the Sun.
- The mean diameter of
is 2.57 AU.
- The distance from the Sun to the centre of the
is approximately 1.7×109 AU.
- The Earth is
actually 147,104,753 km away from the
December and 152,091,803 km away from the Sun on
Some conversion factors:
- 1 AU = 149 597 870.691 ± 0.030 km ≈ 92 955 807 miles ≈ 8.317
minutes ≈ 499
light-second ≈ 0.002 AU
gigameter ≈ 0.00(6) AU
light-minute ≈ 0.120 AU
microparsec ≈ 0.206 AU
terameter ≈ 6.685 AU
light-hour ≈ 7.214 AU
light-day ≈ 173.263 AU
milliparsec ≈ 206.265 AU
light-week ≈ 1212.84 AU
light-month ≈ 5197.9 AU
light-year ≈ 63 241 AU
- 1 parsec ≈
206 265 AU