Editing Arcturus (section)

==Observation==
[[File:BootesCC.jpg|thumb|left|upright|Arcturus is the brightest star in the constellation of [[Boötes]].]]

With an [[apparent visual magnitude]] of&nbsp;−0.05, Arcturus is the brightest star in the [[northern celestial hemisphere]] and the [[list of brightest stars|fourth-brightest star]] in the night sky,<ref name=100greatest/> after [[Sirius]] (−1.46&nbsp;apparent magnitude), [[Canopus (star)|Canopus]] (−0.72) and [[α Centauri]] (combined magnitude of −0.27). However, α Centauri AB is a [[binary star]], whose components are each fainter than Arcturus. This makes Arcturus the third-brightest individual star, just ahead of α Centauri&nbsp;A (officially named ''Rigil Kentaurus''), whose apparent magnitude {{nobr|is −0.01}}.<ref name=schaaf/> The French mathematician and astronomer [[Jean-Baptiste Morin (mathematician)|Jean-Baptiste Morin]] observed Arcturus in the daytime with a telescope in 1635. This was the first recorded full daylight viewing for any star other than the [[Sun]] and [[supernova]]e. Arcturus has been seen at or just before sunset with the naked eye.<ref name=schaaf/>

Arcturus is visible from both of [[Earth]]'s hemispheres as it is located 19° north of the [[celestial equator]]. The star [[culmination|culminates]] at midnight on April 27, and at 9 p.m. on June 10 being visible during the late northern spring or the southern autumn.<ref>Schaaf, p. 257.</ref> From the [[northern hemisphere]], an easy way to find Arcturus is to follow the arc of the handle of the [[Big Dipper]] (or Plough in the [[United Kingdom|UK]]). By continuing in this path, one can find [[Spica]], "Arc to Arcturus, then spike (or speed on) to Spica".<ref name=rao2007/><ref name=earthsky/> Together with the bright stars [[Spica]] and [[Regulus]] (or [[Denebola]], depending on the source), Arcturus is part of the [[Spring Triangle]] [[Asterism (astronomy)|asterism]]. With [[Cor Caroli]], these four stars form the [[Great Diamond]] asterism.

Ptolemy described Arcturus as ''subrufa'' ("slightly red"): it has a B-V color index of +1.23, roughly midway between [[Pollux (star)|Pollux]] (B-V +1.00) and [[Aldebaran]] (B-V +1.54).<ref name=schaaf/>

[[η Boötis]], or Muphrid, is only 3.3 [[light-year]]s distant from Arcturus, and would have a visual magnitude −2.5, about as bright as [[Jupiter]] at its brightest from Earth, whereas an observer on the former system would find Arcturus with a magnitude -5.0, slightly brighter than [[Venus]] as seen from Earth, but with an orangish color.<ref name=schaaf/>