en>K6ka: Reverted 1 edit by 114.207.85.64 (talk) to last revision by Rchandra. (TW)

2014-04-21T16:25:40Z

Reverted 1 edit by 114.207.85.64 (talk) to last revision by Rchandra. (TW)

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			[[Image:ParallaxeV2.png\|thumb\|thumb\|right\|Stellar parallax motion from annual parallax]]
			[[Image:Stellarparallax parsec1.svg\|thumb\|right\|Stellar parallax is the basis for the [[parsec]], which is the distance from the [[Sun]] to an [[astronomical object]] which has a [[parallax]] angle of one [[arcsecond]]. (1 [[astronomical unit\|AU]] and 1 [[Parsec\|pc]] are not to scale, 1 pc = ~206265 AU)]]

			'''Stellar parallax''' is the effect of [[parallax]] on distant stars in [[astronomy]]. It is parallax on an interstellar scale, and it can be used to determine the distance of Earth to another star directly with accurate [[astrometry]]. It was the subject of much debate in astronomy for hundreds of years, but was so difficult it was only achieved for a few of the nearest stars in the early 19th century. Even in the 21st century, stars with parallax measurements are relatively close on a galactic scale, and most distance measurements are calculated by [[red-shift]] or other methods.

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			Because other stars are far away, the angle for measurement is small and the [[skinny triangle]] approximation can be applied, the distance to an object (measured in [[parsec]]s) is the [[Reciprocal (mathematics)\|reciprocal]] of the parallax (measured in [[arcsecond]]s): <math>d (\mathrm{pc}) = 1 / p (\mathrm{arcsec}).</math> For example, the distance to [[Proxima Centauri]] is 1/0.7687={{convert\|1.3009\|pc\|ly}}.<ref name="apj118"/> The first successful measurement of stellar parallax was made by [[Friedrich Bessel]] in 1838 for the star [[61 Cygni\|61 Cygni]] using a Fraunhofer [[heliometer]] at [[Königsberg Observatory]].<ref name=ZG44>{{harvnb\|Zeilik\|Gregory\|1998 \| loc=p. 44}}.</ref><ref name=hirschgoogle>[http://books.google.com/books?id=CW6tqdhVMJoC&pg=PA259&dq=heliometer+61+cygni&hl=en&ei=rDGaTJfTBoH6lweFoKht&sa=X&oi=book_result&ct=result&resnum=5&ved=0CDcQ6AEwBA#v=onepage&q=heliometer%2061%20cygni&f=false Alan W. Hirshfeld - '''Parallax: The Race to Measure the Cosmos''' (2002) - Page 259, Google Books 2010]</ref>

			==Early theory and attempts==
			Stellar parallax is so small (as to be unobservable until the 19th century) that it was used as a scientific argument against [[heliocentrism]] during the [[early modern age]]. It is clear from [[Euclid\|Euclid's]] [[geometry]] that the effect would be undetectable if the stars were far enough away, but for various reasons such gigantic distances involved seemed entirely implausible: it was one of [[Tycho Brahe]]'s principal objections to [[Copernican heliocentrism]] that in order for it to be compatible with the lack of observable stellar parallax, there would have to be an enormous and unlikely void between the orbit of Saturn and the eighth sphere (the fixed stars).<ref>See p.51 in ''The reception of Copernicus' heliocentric theory: proceedings of a symposium organized by the Nicolas Copernicus Committee of the International Union of the History and Philosophy of Science'', Torun, Poland, 1973, ed. Jerzy Dobrzycki, International Union of the History and Philosophy of Science. Nicolas Copernicus Committee; ISBN 90-277-0311-6, ISBN 978-90-277-0311-8</ref>

			[[James Bradley]] first tried to measure stellar parallaxes in 1729. The stellar movement proved too insignificant for his [[telescope]], but he instead discovered the [[aberration of light]],<ref>[http://books.google.com/books?id=JARaN7Jmm9cC&pg=PA184&dq=Starlight+astronomy+astrometry&hl=en&ei=6K6kTPGCNoOglAf5wZDXDA&sa=X&oi=book_result&ct=result&resnum=1&ved=0CDEQ6AEwAA#v=onepage&q=Starlight%20astronomy%20astrometry&f=false Robert K. Buchheim - '''The sky is your laboratory: advanced astronomy projects for amateurs'''(2007) - Page 184, Google Books 2010]</ref> the [[nutation]] of the Earth’s axis, and did a cataloging of 3222 stars.

			==19th and 20th centuries==
			[[Image:Koenigsberg helio.jpg\|thumb\|right\|Bessel's heliometer]]

			Stellar parallax is most often measured using [[annual parallax]], defined as the difference in position of a star as seen from the Earth and Sun, i. e. the angle subtended at a star by the mean radius of the Earth's orbit around the Sun. The [[parsec]] (3.26 [[light-year]]s) is defined as the distance for which the annual parallax is 1 [[arcsecond]]. Annual parallax is normally measured by observing the position of a star at different times of the [[year]] as the Earth moves through its orbit. Measurement of annual parallax was the first reliable way to determine the distances to the closest stars. The first successful measurements of stellar parallax were made by [[Friedrich Bessel]] in 1838 for the star [[61 Cygni\|61 Cygni]] using a [[heliometer]].<ref name="ZG44" /><ref>Bessel, FW, "[http://www.ari.uni-heidelberg.de/gaia/documents/bessel-1838/index.html Bestimmung der Entfernung des 61sten Sterns des Schwans]" (1838) ''[[Astronomische Nachrichten]]'', vol. 16, pp. 65-96.</ref>

			Being very difficult to measure, only about 60 stellar parallaxes had been obtained by the end of the 19th century, mostly by use of the [[filar micrometer]]. [[Astrograph]]s using astronomical [[photographic plate]]s sped the process in the early 20th century. Automated plate-measuring machines<ref>[http://cdsweb.cern.ch/record/1107461 CERN paper on plate measuring machine] USNO StarScan</ref> and more sophisticated computer technology of the 1960s allowed more efficient compilation of [[star catalogue]]s. In the 1980s, [[charge-coupled device]]s (CCDs) replaced photographic plates and reduced optical uncertainties to one milliarcsecond.

			Stellar parallax remains the standard for calibrating other measurement methods (see [[Cosmic distance ladder]]). Accurate calculations of distance based on stellar parallax require a measurement of the distance from the Earth to the Sun, now based on [[radar]] reflection off the surfaces of planets.<ref>{{harvnb\|Zeilik\|Gregory\|1998\|loc=§ 22-3}}.</ref>

			The angles involved in these calculations are very small and thus difficult to measure. The nearest star to the Sun (and thus the star with the largest parallax), [[Proxima Centauri]], has a parallax of 0.7687 ± 0.0003 arcsec.<ref name="apj118">{{cite journal
			\| author=Benedict, G. Fritz ''et al.''
			\| title=Interferometric Astrometry of Proxima Centauri and Barnard's Star Using HUBBLE SPACE TELESCOPE Fine Guidance Sensor 3: Detection Limits for Substellar Companions
			\| journal=The Astronomical Journal
			\| year=1999 \| volume=118 \| issue=2 \| pages=1086–1100
			\| bibcode=1999astro.ph..5318B
			\| doi=10.1086/300975
			\| ref=harv \|arxiv = astro-ph/9905318 }}</ref> This angle is approximately that [[subtended]] by an object 2 centimeters in diameter located 5.3 kilometers away.

			==Space astrometry for parallax==
			In 1989 the satellite [[Hipparcos]] was launched primarily for obtaining parallaxes and [[proper motion]]s of nearby stars, increasing the reach of the method tenfold. Even so, Hipparcos is only able to measure parallax angles for stars up to about 1,600 [[light-year]]s away, a little more than one percent of the diameter of the [[Milky Way Galaxy]]. The [[European Space Agency]]'s [[Gaia mission]], launched 19 December 2013, will be able to measure parallax angles to an accuracy of 10 micro[[arcsecond]]s, thus mapping nearby stars (and potentially planets) up to a distance of tens of thousands of light-years from earth.<ref>{{cite web \| last = Henney \| first = Paul J. \| title = ESA's Gaia Mission to study stars \| publisher = Astronomy Today \| date = \| url = http://www.astronomytoday.com/exploration/gaia.html \| accessdate = 2008-03-08}}</ref>

			==Other baselines==
			The motion of the Sun through space provides a longer baseline that will increase the accuracy of parallax measurements, known as [[secular parallax]]. For stars in the Milky Way disk, this corresponds to a mean baseline of 4 A.U. per year, while for halo stars the baseline is 40 A.U. per year. After several decades, the baseline can be orders of magnitude greater than the Earth-Sun baseline used for traditional parallax. However, secular parallax introduces a higher level of uncertainty because the relative velocity of other stars is an additional unknown. When applied to samples of multiple stars, the uncertainty can be reduced; the precision is inversely proportional to the [[square root]] of the sample size.<ref>{{cite arxiv
			\| last=Popowski \| first=Piotr \| coauthors=Gould, Andrew
			\| date=1998-01-29
			\| title=Mathematics of Statistical Parallax and the Local Distance Scale
			\| eprint=astro-ph/9703140
			\| class=astro-ph }}</ref>

			==Other parallax in astronomy==
			Other uses of the term parallax in astronomy, with different meanings are the [[Photometric parallax method]], [[Spectroscopic parallax]], and [[Dynamical parallax]].

			==See also==
			*[[Cosmic distance ladder]]
			*[[Moving cluster method]]
			*[[Proper motion]]
			*[[Apparent place]]

			==References==
			{{Reflist}}
			{{refbegin}}
			*{{Cite book \| last=Hirshfeld \| first=Alan w. \|title=Parallax: The Race to Measure the Cosmos \|location=New York\|publisher = W. H. Freeman \|year=2001 \|isbn = 0-7167-3711-6 \| ref=harv \| postscript=<!--None-->}}
			*{{Cite book \| last=Whipple \| first=Fred L. \| year=2007 \| title=Earth Moon and Planets \| isbn=1-4067-6413-2 \| publisher=Read Books \| ref=harv \| postscript=<!--None--> }}.
			*{{Cite book \| last=Zeilik \| first=Michael A. \| last2=Gregory \| first2=Stephan A. \| title=Introductory Astronomy & Astrophysics \| edition=4th \| year=1998 \| publisher=Saunders College Publishing \| isbn=0-03-006228-4 \| ref=harv \| postscript=<!--None--> }}.
			{{refend}}

			==Further reading==
			*{{cite journal\|author1=Dyson, F. W.\|title=Measurement of the distances of the stars\|journal=The Observatory\|volume=38\|pages=292\|year=1915\|bibcode=1915Obs....38..292D}}

			[[Category:Astrometry]]
			[[Category:Parallax]]

en>Mgowiki at 19:22, 17 April 2012

2012-04-17T19:22:11Z

New page

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en>K6ka: Reverted 1 edit by 114.207.85.64 (talk) to last revision by Rchandra. (TW)

en>Rchandra: Spelling/grammar correction

en>Mgowiki at 19:22, 17 April 2012