# Astronomical unit

AUastronomical unitsAUsdistance from the Earth to the Sunastronomical units (AU)astronomical unit (AU)distance to the Sununit distance 215 astronomical unit (AU)
The astronomical unit (symbol: au, ua, or AU) is a unit of length, roughly the distance from Earth to the Sun.wikipedia
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### Parsec

Mpcpckpc
It is also a fundamental component in the definition of another unit of astronomical length, the parsec.
A parsec is defined as the distance at which one astronomical unit subtends an angle of one arcsecond, which corresponds to 1⁄648000 astronomical units.

### Sun

solarSolThe Sun
The astronomical unit (symbol: au, ua, or AU) is a unit of length, roughly the distance from Earth to the Sun.
The solar constant is equal to approximately 1368 W/m2 (watts per square meter) at a distance of one astronomical unit (AU) from the Sun (that is, on or near Earth).

### Earth's orbit

its orbitorbitEarth orbits
Earth's orbit around the Sun is an ellipse.
The average distance between Earth and the Sun is 149.60 million km (92.96 million mi), and one complete orbit takes days (1 sidereal year), during which time Earth has traveled 940 million km (584 million mi).

### Solar System

outer Solar Systeminner Solar SystemSol system
The astronomical unit is used primarily for measuring distances within the Solar System or around other stars.
As the contracting nebula rotated faster, it began to flatten into a protoplanetary disc with a diameter of roughly 200 AU and a hot, dense protostar at the centre.

### Gaussian gravitational constant

It stated that "the astronomical unit of length is that length (A) for which the Gaussian gravitational constant (k) takes the value 0.01720209895 when the units of measurement are the astronomical units of length, mass and time".
and its value in radians per day follows by setting Earth's semi-major axis (the astronomical unit, a.u.) to unity, :(rad/day) = 0.5 ·(a.u.) -1.5

### Light-year

light yearlight yearsly
|≈ 15.812507 millionths (15.812507E-6) of a light-year
The unit most commonly used in professional astrometry is the parsec (symbol: pc, about 3.26 light-years; the distance at which one astronomical unit subtends an angle of one second of arc).

### Unit of length

lengthunits of lengthL
The astronomical unit (symbol: au, ua, or AU) is a unit of length, roughly the distance from Earth to the Sun.
astronomical unit au. Defined as 149597870700 m. Approximately the distance between the Earth and Sun.

### Cosmic distance ladder

standard candlestandard candlesdistance
The astronomical unit is typically used for stellar system scale distances, such as the size of a protostellar disk or the heliocentric distance of an asteroid, whereas other units are used for other distances in astronomy.
Direct distance measurements are based upon the astronomical unit (AU), which is the distance between the Earth and the Sun.

### Parallax

trigonometric parallaxsolar parallaxmotion parallax
In addition, it mapped out exactly the largest straight-line distance that Earth traverses over the course of a year, defining times and places for observing the largest parallax (apparent shifts of position) in nearby stars. The parsec (parallax arcsecond) is defined in terms of the astronomical unit, being the distance of an object with a parallax of 1 arcsecond.
To ascertain the scale, it is necessary only to measure one distance within the Solar System, e.g., the mean distance from the Earth to the Sun (now called an astronomical unit, or AU).

### Solar mass

mass of the SunSun's masssolar masses
With the definitions used before 2012, the astronomical unit was dependent on the heliocentric gravitational constant, that is the product of the gravitational constant G and the solar mass.
Because Earth follows an elliptical orbit around the Sun, the solar mass can be computed from the equation for the orbital period of a small body orbiting a central mass. Based upon the length of the year, the distance from Earth to the Sun (an astronomical unit or AU), and the gravitational constant (

### Eratosthenes

Eratosthenes of Cyrenea seminal experimentEratosthenis
According to Eusebius of Caesarea in the Praeparatio Evangelica (Book XV, Chapter 53), Eratosthenes found the distance to the Sun to be "σταδιων μυριαδας τετρακοσιας και οκτωκισμυριας" (literally "of stadia myriads 400 and 80000") but with the additional note that in the Greek text the grammatical agreement is between myriads (not stadia) on the one hand and both 400 and 80000 on the other, as in Greek, unlike English, all three (or all four if one were to include stadia) words are inflected.
Additionally, he may have accurately calculated the distance from the Earth to the Sun and invented the leap day.

### Apsis

perihelionperigeeapogee
However, that distance varies as Earth orbits the Sun, from a maximum (aphelion) to a minimum (perihelion) and back again once a year.
At perihelion, the Earth's center is about 0.98329 astronomical units (AU) or 147098070 km from the Sun's center.

### Lunar distance (astronomy)

lunar distancesLDlunar distance
The book On the Sizes and Distances of the Sun and Moon, which has long been ascribed to Aristarchus, says that he calculated the distance to the Sun to be between 18 and 20 times the distance to the Moon, whereas the true ratio is about 389.174.
The measurement is also useful in characterizing the lunar radius, the mass of the Sun and the distance to the Sun.

### Minute and second of arc

masarcsecondarc second
The parsec (parallax arcsecond) is defined in terms of the astronomical unit, being the distance of an object with a parallax of 1 arcsecond.
an object of diameter 725.27 km at a distance of one astronomical unit,

### International System of Units

SISI unitsSI unit
In 1983, the International Committee for Weights and Measures (CIPM) modified the International System of Units (SI, or "modern" metric system) to make the metre defined as the distance travelled in a vacuum by light in 1/299792458 second.
minute, hour, day, degree of arc, minute of arc, second of arc, hectare, litre, tonne, astronomical unit

### Speed of light

clight speedvelocity of light
Because all photons move at the speed of light in vacuum, a fundamental constant of the universe, the distance of an object from the probe is calculated as the product of the speed of light and the measured time.
It is customary to express the results in astronomical units (AU) per day.

### IAU (1976) System of Astronomical Constants

adopted a new definition
In 1976, in order to establish a yet more precise measure for the astronomical unit, the IAU formally adopted a new definition.
The astronomical unit of length is known as the astronomical unit (A or au), which in the IAU(1976) system is defined as the length for which the gravitational constant, more specifically the Gaussian gravitational constant k expressed in the astronomical units (i.e. k 2 has units A 3 S −1 D −2 ), takes the value of 0.017 202 098 95 . This astronomical unit is approximately the mean distance between the Earth and the Sun. The value of k is the angular velocity in radians per day (i.e. the daily mean motion) of an infinitesimally small mass that moves around the Sun in a circular orbit at a distance of 1 AU.

### ISO 80000-3

defined by standardISO 80000-3:2006ISO standard
In the non-normative Annex C to ISO 80000-3 (2006), the symbol of the astronomical unit is "ua".
Units of distance: light year (the distance travelled in one year by light in vacuum; abbreviation l.y.; 1 l.y. ≈ 9.460 730 × 10 15 m), astronomical unit (the mean distance of the Earth from the Sun, 1 ua ≈ 1.495 978 706 91(30) × 10 11 m), parsec (the distance at which 1 ua subtends an angle of 1″ (second of arc); 1 pc ≈ 206 264.8 ua ≈ 30.856 78 × 10 15 m)

### Star system

multiple star systemmultiple systemstriple star
The astronomical unit is typically used for stellar system scale distances, such as the size of a protostellar disk or the heliocentric distance of an asteroid, whereas other units are used for other distances in astronomy.
Alpha Centauri is a triple star composed of a main binary yellow dwarf pair (Alpha Centauri A and Alpha Centauri B), and an outlying red dwarf, Proxima Centauri. Both A and B form a physical binary star, designated as Alpha Centauri AB, α Cen AB, or RHD 1 AB, where the AB denotes this is a binary system. The moderately eccentric orbit of the binary can make the components be as close as 11 AU or as far away as 36 AU. Proxima is much further away (~15,000 AU) from α Cen AB than they are to each other. Although this distance is still comparatively small to interstellar distances, it is still debatable whether Proxima, whose orbital period would be more than 500,000 years, is gravitationally bound to α Cen AB.

### Numerical model of the Solar System

numerical models
When simulating a numerical model of the Solar System, the astronomical unit provides an appropriate scale that minimizes (overflow, underflow and truncation) errors in floating point calculations.
For example for distances in the Solar System the astronomical unit is most straightforward.

### Near-Earth object

near-Earth asteroidNEOnear-Earth
The discovery of the near-Earth asteroid 433 Eros and its passage near Earth in 1900–1901 allowed a considerable improvement in parallax measurement.
By convention, a Solar System body is a NEO if its closest approach to the Sun (perihelion) is less than 1.3 astronomical units (AU).

### Metre

metermmetres
= 149597870700 metres (exactly)
Long distances are usually expressed in km, astronomical units (149.6 Gm), light-years (10 Pm), or parsecs (31 Pm), rather than in Mm, Gm, Tm, Pm, Em, Zm or Ym; "30 cm", "30 m", and "300 m" are more common than "3 dm", "3 dam", and "3 hm", respectively.

### Arthur Robert Hinks

Arthur HinksArthur R. HinksMr. Arthur Hinks
As an astronomer, he is best known for his work in determining the distance from the Sun to the Earth (the astronomical unit) from 1900–1909: for this achievement, he was awarded the Gold Medal of the Royal Astronomical Society and was elected a fellow of the Royal Society.

### Transit of Venus

transits of VenusVenus1761 transit of Venus
A somewhat more accurate estimate can be obtained by observing the transit of Venus.
Horrocks's observations allowed him to make a well-informed guess as to the size of Venus, as well as to make an estimate of the mean distance between the Earth and the Sun — the astronomical unit.

### Kuiper belt

KBOKuiper belt objectsKuiper-belt
; epoch as of January 2015.)Distances but not sizes are to scaleSource: Minor Planet Center, http://www.cfeps.net/ and others