खगोलीय इकाई

**खगोलीय इकाई** (**AU** या **au** या **a.u.** या यद कदा **ua**) लम्बाई की इकाई है, जो लगभग 150 मिलियन किलोमीटर है और पृथ्वी से सूर्य की दूरी पर आधारित है। इसकी सही सही मान है 149,597,870,691 ± 30 मीटर (लगभग 150 मिलियन किलोमीटर या 93 मिलियन मील).

इसका चिन्ह *ua* अन्तर्राष्ट्रीय भार एवं माप ब्यूरो द्वारा अनुमोदित  लेकिन अंग्रेजी बोलते देशों में इसका उल्टा रूप - *au* - प्रचलित है।

## इतिहास
## उदाहरण
दूरियां लगभग औसत दूरुयां हैं। खगोलीय पिण्डों की दूरियां अपनी कक्षा में विचरण के दौरान बदल सकती हैं, जिसको ध्यान रखना अनिवार्य है।

* The Earth is 1.00 ± 0.02 AU from the Sun.
* The Moon is 0.0026 ± 0.0001 AU from the Earth.
* Mars is 1.52 ± 0.14 AU from the Sun.
* Jupiter is 5.20 ± 0.05 AU from the Sun.
* Pluto is 39.5 ± 9.8 AU from the Sun.
* The Kuiper Belt begins at roughly 35 AU
* Beginning of Scattered disk at 45 AU (10 AU overlap with Kuiper Belt)
* Ending of Kuiper Belt at 50-55 AU
* 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.
* 94 AU: Termination shock between Solar winds/Interstellar winds/Interstellar medium.
* 100 AU: Heliosheath
* 105 AU: As of February 2008, Voyager 1 is the furthest of any human-made objects from the Sun.
* 100-150 AU: Ending of Scattered Disk
* 500-3000 AU: Beginning of Hills cloud/"Inner Oort Cloud"
* 20,000 AU: Ending of Hills Cloud/"Inner Oort Cloud", beginning of "Outer Oort Cloud"
* 50,000 AU: possible closest estimate of the "Outer Oort Cloud" limits (1.0 ly)
* 100,000 AU: possible farthest estimate of the "Outer Oort Cloud" limits (1.6 ly).
* 125,000 AU: maximum extent of influence of the Sun's gravitational field (Hill/Roche sphere). beyond this is true interstellar space. This distance is roughly 1.8-2.0 light-years.
* Proxima Centauri (the nearest star to Earth, excluding our own Sun) is ~268 000 AU away from the Sun.
* The mean diameter of Betelgeuse is 2.57 AU.
* The distance from the Sun to the centre of the Milky Way is approximately 1.7 AU.

Some conversion factors:
* 1 AU = 149 597 870.691 ± 0.030&nbsp;km ≈ 92 955 807&nbsp;mi ≈ 8.317 light minutes ≈ 499 light-seconds
* 1 light-second ≈ 0.002 AU
* 1 gigametre ≈ 0.007 AU
* 1 light-minute ≈ 0.120 AU
* 1 microparsec ≈ 0.206 AU
* 1 terametre ≈ 6.685 AU
* 1 light-hour ≈ 7.214 AU
* 1 light-day ≈ 173.263 AU
* 1 milliparsec ≈ 206.265 AU
* 1 light-week ≈ 1212.84 AU
* 1 light-month ≈ 5197.9 AU
* 1 light-year ≈ 63,241 AU
* 1 parsec ≈ 206,265 AU

## सन्दर्भ
* E. Myles Standish. "Report of the IAU WGAS Sub-group on Numerical Standards". In *Highlights of Astronomy*, I. Appenzeller, ed. Dordrecht: Kluwer Academic Publishers, 1995. *(Complete report available online: [https://web.archive.org/web/20060208115515/http://ssd.jpl.nasa.gov/iau-comm4/iausgnsrpt.ps PostScript]. Tables from the report also available: [https://web.archive.org/web/20080906180801/http://ssd.jpl.nasa.gov/astro_constants.html Astrodynamic Constants and Parameters])*
* D. D. McCarthy ed., IERS Conventions (1996), IERS Technical Note 21, Observatoire de Paris, July 1996

## बाहरी कडि़यां
* *[https://web.archive.org/web/20080525161241/http://www.sil.si.edu/exhibitions/chasing-venus/intro.htm Chasing Venus, Observing the Transits of Venus]* Smithsonian Institution Libraries
* Units outside the SI *(at the [[NIST]] web site)*
* Recommendations concerning Units *(at the [[International Astronomical Union|IAU]] web site)*
* Solar Mass Loss, the Astronomical Unit, and the Scale of the Solar System *(a discussion of the relation between the AU and other quantities)*
* AU may need to be redefined *(measuring distances in AU will become increasingly imprecise as the Sun radiates away its energy)* New Scientist, *6 फ़रवरी 2008*

श्रेणी:खगोल शास्त्र में लम्बाई की इकाइयाँ
श्रेणी:खगोलिकी में मापन इकाईयाँ