The Bayer designation Tau Hydrae (τ Hya, τ Hydrae) is shared by two star systems in the equatorial constellation of Hydra. The two stars are separated by 1.74° in the sky.
The stars τ1 Hya and τ2 Hya, along with ι Hya and the 5th‑magnitude 33 Hya (A Hydrae), were Ptolemy's Καμπή (Kampē); but Kazwini knew them as ʽUḳdah, the Knot.[1] According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, Uḳdah were the title for four stars: τ1 Hya as Uḳdah I, τ2 Hya as Uḳdah II, 33 Hya as Uḳdah III and ι Hya as Uḳdah IV [2]
In Chinese, Chinese: 柳宿 (Chinese: Xīng Sù), meaning Star (asterism), refers to an asterism consisting of τ1 and τ2 Hydrae, Alphard, ι Hydrae, 26 Hydrae, 27 Hydrae, HD 82477 and HD 82428.[3] Consequently, τ1 and τ2 Hydrae are known as Chinese: 星宿二 (Chinese: Xīng Sù èr, English: the Second Star of Star) and Chinese: 星宿三 (Chinese: Xīng Sù sān, English: the Third Star of Star) respectively.[4]
. 1963 . Richard Hinckley Allen . Star Names: Their Lore and Meaning . 2010-12-12 . Reprint . Dover Publications Inc . New York, NY . 0-486-21079-0 . 250 . registration .