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Kinematics

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All components of Alpha Centauri display significant proper motion against the background sky. Over centuries, this causes their apparent positions to slowly change. Proper motion was unknown to ancient astronomers. Most assumed that the stars are permanently fixed on the celestial sphere, as stated in the works of the philosopher Aristotle. In 1718, Edmond Halley found that some stars had significantly moved from their ancient astrometric positions. In the 1830s, Thomas Henderson discovered the true distance to Alpha Centauri by analysing his many astrometric mural circle observations. He then realised this system also likely had a high proper motion. In this case, the apparent stellar motion was found using Nicolas Louis de Lacaille's astrometric observations of 1751–1752, by the observed differences between the two measured positions in different epochs. Calculated proper motion of the centre of mass for Alpha Centauri AB is about 3620 mas (milli-arcseconds) per year toward the ...
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Stellar system

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Alpha Centauri is a triple star system, with its two main stars, Alpha Centauri A and Alpha Centauri B, being a binary component. The AB designation, or older A×B , denotes the mass centre of a main binary system relative to companion star(s) in a multiple star system. AB-C refers to the component of Proxima Centauri in relation to the central binary, being the distance between the centre of mass and the outlying companion. Because the distance between Proxima (C) and either of Alpha Centauri A or B is similar, the AB binary system is sometimes treated as a single gravitational object. Orbital properties edit The A and B components of Alpha Centauri have an orbital period of 79.91 years. Their orbit is moderately eccentric, e  = 0.5179; their closest approach or periastron is 11.2 AU (1.68 billion km), or about the distance between the Sun and Saturn; and their furthest separation or apastron is 35.6 AU (5.33 billion km), about the distance between the Sun and Pluto. The most recent ...
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Observational history

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Alpha Centauri is listed in the 2nd-century star catalog of Ptolemy. He gave its ecliptic coordinates, but texts differ as to whether the ecliptic latitude reads 44° 10′ South or 41° 10′ South . (Presently the ecliptic latitude is 43.5° South , but it has decreased by a fraction of a degree since Ptolemy's time due to proper motion.) In Ptolemy's time, Alpha Centauri was visible from Alexandria, Egypt, at 31° N, but, due to precession, its declination is now –60° 51′ South , and it can no longer be seen at that latitude. English explorer Robert Hues brought Alpha Centauri to the attention of European observers in his 1592 work Tractatus de Globis , along with Canopus and Achernar, noting: Now, therefore, there are but three Stars of the first magnitude that I could perceive in all those parts which are never seene here in England. The first of these is that bright Star in the sterne of Argo which they call Canobus. The second Achernar is in the end of Eridanus. The third Alph...
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