When Double-Star Research Began

Abstract

The first occurrence of the name ‘double star’ happened to have been — by modern definitions — a mistake: in Ptolemy’s star catalogue (2nd century AD), the two stars v1 and v2 Sagittarii — separated by 14 min arc, and independent — were called $\delta i\pi \lambda ous$ Several of the Arab star names refer jointly to two or more neighboring stars easily separated by the naked eye. On the other hand, the well-known naked-eye pair Mizar and Alcor in the Big Dipper received separate names, probably because their difference of brightness did not make them appear as a ‘pair’ of stars.