Mizar and Alcor

Mizar and Alcor are two stars forming a naked eye double in the handle of the Big Dipper (or Plough) asterism in the constellation of Ursa Major.

Mizar and its fainter companion Alcor are actually a four-star binary system consisting of two pairs of double stars that are gravitationally bound to each other.

In fact, Mizar was the first known binary star system, claimed to be discovered by Italian astronomer Giovanni Battista Riccioli in 1650.

[9] Although the statement has been made that Alcor was not known to the ancient Greeks, there is an old story that it was the lost Pleiad Electra, which had wandered here from her companions and became Alopex, the Fox.

[1] The Persian astronomer Al Biruni (973–1048 AD) mentioned its importance in the family life of the Arabs on the 18th day of the Syrian month Adar, the March equinox; and a modern story of that same people makes it the infant of the walidan (mother?)

[11] The ability to resolve Mizar and Alcor with the naked eye is often quoted as a test of eyesight and that has been confirmed by modern experimental research.

Astronomer Patrick Moore suggested that this in fact refers to another star that lies visually between Mizar and Alcor.

[16] Liebknecht thought it was a planet, but it had already been observed in exactly the same position by Benedetto Castelli approximately a century earlier, which indicated it was a background star.

Agnes M. Clerke wrote: "The Arabs in the desert regarded it as a test of penetrating vision; and they were accustomed to oppose "Sohail" to "Soha" (Canopus to Alcor) as occupying respectively the highest and lowest posts in the celestial hierarchy.