Zeta Aquarii
Zeta Aquarii (ζ Aquarii, ζ Aqr) is the Bayer designation for a triple star system, the central star of the "water jar" asterism in the equatorial constellation of Aquarius. The combined apparent visual magnitude of this system is 3.65, which is readily visible to the naked eye. Parallax measurements yield a distance estimate of around from Earth. Etymology. In the catalogue of stars in the "Calendarium of Al Achsasi Al Mouakket", this star was designated "Achr al Achbiya" (آخر ألأخبية - "ākhir al-akhbiya"), which was translated into Latin as "Postrema Tabernaculorum", meaning "the end of luck of the  homes (tents)". This star, along with γ Aqr (Sadachbia), π Aqr (Seat) and η Aqr (Hydria), were "al-Akḣbiya" (الأخبية), the Tents. In Chinese, (), meaning "Tomb", refers to an asterism consisting of ζ Aquarii, γ Aquarii, η Aquarii and π Aquarii. Consequently, the Chinese name for ζ Aquarii itself is (, .) Properties. The binary was measured by William Herschel in 1779 however Christian Mayer, listed an earlier observation in his first double-star catalog in 1784 so its true discoverer is not clear. The two stars have an orbital period of about 587 years. The semimajor axis is 3.8 arcseconds and  they have an orbital eccentricity of 0.40. The orbital plane is inclined by 138.2° to the line of sight. The brighter component, ζ Aquarii A (also called ζ Aquarii), is a yellow-white-hued F-type main sequence star with an apparent magnitude of +4.42. Its companion, ζ Aquarii B (also called ζ Aquarii), is a yellow-white-hued F-type subgiant with an apparent magnitude of +4.51. The fact that their brightness is so similar makes the pair easy to measure and resolve. Zeta Aquarii A is known to be an astrometric binary system, as it undergoes regular  perturbations from its orbit. It has a 26-year orbital period and a semimajor axis of 0.11″. The secondary's mass is . Zeta Aquarii is currently a northern hemisphere object. In 2004 it was directly above the celestial equator, and before that it was located south of it.