The ability to resolve Mizar and Alcor with the naked eye is often quoted as a test of eyesight, although even people with quite poor eyesight can see the two stars. Arabic literature says that only those with the sharpest eyesight can see the companion of Mizar. The 14thcentury Arabian lexicographer Firuzabadi called it "Our Riddle", while the 13thcentury Persian astronomical writer Zakariya al-Qazwini said that "people tested their eyesight by this star." Humboldt wrote of it as being seen with difficulty, and Arago similarly alluded to it. Astronomer Sir Patrick Moore suggested that this in fact refers to another star that lies visually between Mizar and Alcor. This star is occasionally known as "Ludwig's Star", it was observed on 2 December 1722 by the German astronomer Johann Georg Liebknecht (23 April 1679 – 17 September 1749) and named in honour of his patron the Landgrave Ludwig of Hessen-Darmstadt. Liebknecht thought it was a planet, but it had already been observed in exactly the same position by Benedetto Castelli (1577–1643) approximately a century earlier in 1616, which indicated it was a background star.