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Arcturus (α Boo, α Boötis, Alpha Boötis) is the brightest star in the constellation Boötes. To the naked-eye, orangey-yellow Arcturus has a visual magnitude of −0.04, making it the fourth brightest star in the night sky, after -1.46 magnitude Sirius,-0.86 magnitude Canopus and -0.27 magnitude Alpha Centauri. However, Alpha Centauri is a bright binary star, whose unresolved components to the naked eye are both fainter than Arcturus. This makes Arcturus as the third brightest individual star, just ahead of Alpha Centauri A (α Cen A), whose visual magnitude is −0.01.

Arcturus is visible from both hemispheres in the sky, as it is located near the celestial equator. The star culminates at midnight on about the 30th April, being visible during the northern spring or the southern autumn. From the northern hemisphere, an easy way to find Arcturus is to follow the arc of the handle of the Big Dipper. By continuing in this path, one can find SpicaVirginis) as well—hence the maxim, "Arc to Arcturus, then speed on to Spica." The star is also a member of the Local Interstellar Cloud.


Observational history

As one of the brightest stars in the sky, Arcturus has been significant to observers since antiquity. In Ancient Greece, the star's celestial activity was supposed to portend tempestuous weather. For citations, see Plautus Rudens prol. 71 and Lewis and Short's Latin Dictionary and its entry for Arcturus.

Prehistoric Polynesian navigators knew Arcturus as Hōkūleʻa, the "Star of Joy". Arcturus is the zenith star of the Hawaiian Islands. Using Hōkūleʻa and other stars, the Polynesians launched their double-hulled canoes from Tahiti and the Marquesas Islands. Traveling east and north they eventually crossed the equator and reached the latitude at which Arcturus would appear directly overhead in the summer night sky. Knowing they had arrived at the exact latitude of the island chain, they sailed due west on the trade winds to landfall. If Hōkūleʻa could be kept directly overhead, they landed on the southeastern shores of the Big Island of Hawaiʻi. For a return trip to Tahiti the navigators could use Sirius, the zenith star of that island. Since 1976, the Polynesian Voyaging Society's Hōkūle‘a has crossed the Pacific Ocean many times under navigators who have incorporated this wayfinding technique in their non-instrument navigation.

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