Editing Double star (section)

==Observation of double stars==
[[File:Masquerading as a double star.jpg|thumb|Astronomers have mistakenly reported observations of a double star in place of [[J 900]] and a faint star in the constellation of Gemini.<ref>{{cite news|title=Masquerading as a double star|url=http://www.spacetelescope.org/images/potw1312a/|access-date=25 March 2013|newspaper=ESA/Hubble Picture of the Week}}</ref> ]]

Observation of visual double stars by visual measurement will yield the ''separation'', or angular distance, between the two component stars in the sky and the ''[[position angle]]''. The position angle specifies the direction in which the stars are separated and is defined as the bearing from the brighter component to the fainter, where north is 0°.<ref name=argyle2>p. 2, ''Observing and Measuring Double Stars'', Bob Argyle, ed., London: Springer-Verlag, 2004, {{ISBN|1-85233-558-0}}.</ref> These measurements are called ''measures''. In the measures of a visual binary, the position angle will change progressively and the separation between the two stars will oscillate between maximum and minimum values. Plotting the measures in the plane will produce an ellipse. This is the ''apparent orbit'', the projection of the orbit of the two stars onto the celestial sphere; the true [[orbit]] can be computed from it.<ref name=argyle5355>p. 53–67, ''Observing and Measuring Double Stars'', Bob Argyle, ed., London: Springer-Verlag, 2004, {{ISBN|1-85233-558-0}}.</ref> Although it is expected that the majority of catalogued visual doubles are visual binaries,<ref name=Heintz12>{{cite book | last=Heintz | first=W. D. | year=1978 | page=[https://archive.org/details/DoubleStars/page/12 12] | title=Double Stars | publisher=D. Reidel Publishing Company | location=Dordrecht | isbn=90-277-0885-1 | url=https://archive.org/details/DoubleStars/page/12 }}</ref> orbits have been computed for only a few thousand of the over 100,000 known visual double stars.<ref name=wdsintro>"Introduction and Growth of the WDS", [http://ad.usno.navy.mil/wds/wdstext.html#intro The Washington Double Star Catalog] {{Webarchive|url=https://web.archive.org/web/20080917194922/http://ad.usno.navy.mil/wds/wdstext.html#intro |date=2008-09-17 }}, Brian D. Mason, Gary L. Wycoff, and William I. Hartkopf, Astrometry Department, [[United States Naval Observatory]], accessed on line August 20, 2008.</ref><ref>[http://ad.usno.navy.mil/wds/orb6.html Sixth Catalog of Orbits of Visual Binary Stars] {{Webarchive|url=https://web.archive.org/web/20090412084731/http://ad.usno.navy.mil/wds/orb6.html |date=2009-04-12 }}, William I. Hartkopf and Brian D. Mason, [[United States Naval Observatory]], accessed on line August 20, 2008.</ref>