Zeta Aquarii (ζ¹ & ζ² Aquarii)
Coordinates, Hipparcos distance (92 ly), 487-year twin F-star binary orbit (2.4″ in 2026), and observing guide for Zeta Aquarii.
Provenance-Backed Scientific Measurements
Every measurement below preserves its unit, uncertainty interval, observational method, reference epoch, quality status, and primary catalog source. See Uncertainty & Provenance for how competing measurements are handled.
| Parameter | Value | Uncertainty | Method & Epoch | Quality Status | Source Archive |
|---|---|---|---|---|---|
| Combined apparent visual magnitude (V) | 3.65 mag | ± 0.01 mag | Johnson UBV photometry (ζ² Aqr = 4.36 mag, ζ¹ Aqr = 4.53 mag) | definitive | Hipparcos New Reduction Catalog |
| Trigonometric parallax | 35.48 mas | ± 1.07 mas | Hipparcos New Reduction + interferometric orbit (28.18 ± 0.85 pc / 91.9 ly) (J2000.0) | definitive | Hipparcos New Reduction Catalog |
| Visual binary orbital period (ζ¹–ζ²) | 177875 d | ± 3650 d | Speckle & adaptive-optics orbit (487 ± 10 yr, a = 3.50″, e = 0.43) | high-confidence | SIMBAD Astronomical Database (CDS) |
| Visual angular separation (2026 epoch) | 0.04 arcmin | ± 0.002 arcmin | Orbital ephemeris (~2.4 arcseconds in 2026, widening after 1978 periastron) | definitive | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Zeta Aquarii (Water Jar Twin F-Star Binary) at RA 22h 28m 49.69s, Dec -00° 01′ 11.9″ within Aquarius, alongside peak meridian transit altitudes for 40°N and 30°S observers. Use the RA/Dec Coordinate Converter to compute exact culmination altitude for your latitude.
Astrophysical Context & Structure
Zeta Aquarii (55 Aquarii) sits at the exact center of the three-pointed 'Water Jar' Y-asterism of northern Aquarius and just 1.2 arcminutes south of the celestial equator (declination −00° 01′ 12″; due to axial precession, it crossed from the Southern into the Northern Celestial Hemisphere in 2004). First recorded as a double star by Christian Mayer in 1777 and independently by William Herschel in 1779, Zeta Aquarii shines at combined visual magnitude 3.65 at a distance of 28.18 ± 0.85 parsecs (91.9 light-years).
In the eyepiece, Zeta Aquarii resembles an autumn version of Porrima (Gamma Virginis): two nearly equal, creamy yellow-white stars—Zeta² Aquarii (magnitude 4.36, spectral type F3 V, ~1.4 solar masses) and Zeta¹ Aquarii (magnitude 4.53, spectral type F6 IV subgiant, ~1.4 solar masses)—orbiting their common center of mass every 487 years along an inclined ellipse.
Having passed through its closest visual separation (1.67 arcseconds) around 1978, the pair has now opened to ~2.4 arcseconds in 2026 along position angle ~156° and will continue widening over the next two centuries toward 5.0 arcseconds. High-contrast infrared adaptive optics also resolved a third component, Zeta¹ Aquarii B, an M0 V red dwarf orbiting Zeta¹ Aquarii every 25.95 years.
Practical Visual Observing Guide
- Locate the compact Y-shaped Water Jar of Aquarius south of the head of Pegasus; Zeta Aquarii is the central vertex where the three branches of the Y meet.
- At 2.4 arcseconds separation of two equal 4.4- and 4.5-magnitude stars, Zeta Aquarii is a superb benchmark test for a 60–80 mm refractor at 120×–160×, producing two touching or cleanly split figure-8 Airy disks.
Catalog Cross-Identifications & Aliases
Common Aliases: ζ Aquarii · ζ¹ & ζ² Aquarii · HD 213051 / HD 213052 · HIP 110960 · 55 Aquarii
| Catalog Authority | Identifier |
|---|---|
| HIP | HIP 110960 |
| HD | HD 213051 |
| SIMBAD | * zet Aqr |
| WIKIDATA | Q13068 |
Related Objects, Comparisons & Guides
Porrima (Gamma Virginis Twin F0V Binary)
Coordinates, Hipparcos distance (38.1 ly), 169-year eccentric F0V+F0V visual binary orbit (3.4″ in 2026), and observing guide for Porrima.
Epsilon Lyrae (The Double Double Star)
Coordinates, Gaia DR3 distance (160 ly), 208″ wide pair and 2.3″/2.4″ tight sub-pairs, and telescope resolution test guide for Epsilon Lyrae.
Fomalhaut (Alpha Piscis Austrini Debris Disk)
Coordinates, Hipparcos distance (25.13 ly), magnitude 1.16, JWST triple debris-belt system, and observing guide for Fomalhaut.
Angular Size, Separation & Field of View
How angular diameter (degrees, arcminutes, arcseconds), spherical angular separation, and small-angle physical diameter relate to telescope field of view.
Celestial Coordinates (RA & Dec) Explained
Practical reference to J2000 equatorial coordinates: Right Ascension (RA in hours, minutes, seconds), Declination (Dec in degrees), and sexagesimal conversion.
Scientific Sources & Dataset Provenance
- Hipparcos New Reduction Catalog (2007 New Reduction): van Leeuwen, F. 2007, A&A, 474, 653 (Validation of the new Hipparcos reduction) — ESA / van Leeuwen (2007)
- SIMBAD Astronomical Database (CDS) (2026-10 release): Wenger, M., et al. 2000, A&AS, 143, 9 (The SIMBAD astronomical database) — Centre de Données astronomiques de Strasbourg