Theta Eridani, Latinized from θ Eridani, is a binary system in the constellation of Eridanus with a combined apparent magnitude of 2.88.^[6] Its two components are designated θ¹ Eridani, formally named Acamar /ˈækəmɑːr/ (the traditional name of the system),^[13][14] and θ² Eridani. The system's distance from the Sun based on parallax measurements is approximately 165 light-years.

Quick Facts Observation data Epoch J2000 Equinox J2000, Apparent magnitude (V) ...

Theta Eridani

Location of θ Eridani (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Eridanus
θ1 Eri
Right ascension	02h 58m 15.6764s[1]
Declination	−40° 18′ 16.839″[1]
Apparent magnitude (V)	3.18[2]
θ2 Eri
Right ascension	02h 58m 16.4037s[3]
Declination	−40° 18′ 16.906″[3]
Apparent magnitude (V)	4.11[2]
Characteristics
θ1 Eri
Spectral type	A3IV-V[4]
U−B color index	+0.14[5]
B−V color index	+0.128±0.012[6]
θ2 Eri
Spectral type	A1V[4]
B−V color index	+0.08[5]
Variable type	suspected[7]
Astrometry
θ1 Eri
Radial velocity (Rv)	+11.9±2.6[6] km/s
Proper motion (μ)	RA: −57.132[1] mas/yr Dec.: +23.058[1] mas/yr
Parallax (π)	19.9047 ± 0.2062 mas[1]
Distance	164 ± 2 ly (50.2 ± 0.5 pc)
θ2 Eri
Proper motion (μ)	RA: −50.502[3] mas/yr Dec.: +16.613[3] mas/yr
Parallax (π)	19.5270 ± 0.1372 mas[3]
Distance	167 ± 1 ly (51.2 ± 0.4 pc)
Absolute magnitude (MV)	−0.59 (combined)[6]
Details
θ1 Eri
Mass	2.6[8] M☉
Radius	4.85[lower-alpha 1] R☉
Luminosity	145[6] L☉
Temperature	8,200[8] K
Rotational velocity (v sin i)	65.1[9] km/s
θ2 Eri
Mass	1.95[10] M☉
Radius	2.7[11] R☉
Luminosity	37[11] L☉
Surface gravity (log g)	3.83[10] cgs
Temperature	8,293[10] K
Metallicity [Fe/H]	−0.197[10] dex
Rotational velocity (v sin i)	102.5[12] km/s
Other designations
Theta Eri, θ Eri, CD−40° 771, HIP 13847, SAO 216113, CCDM J02583-4018, WDS 02583-4018
θ1 Eri: Acamar, HD 18622, HR 897
θ2 Eri: HD 18623, HR 898
Database references
SIMBAD	θ1 Eri
	θ2 Eri

Close

Theta Eridani is the system's Bayer designation; θ¹ and θ² Eridani those of its two components.

The system bore the traditional name Acamar, derived from the Arabic آخِر النَّهْر ^{[citation needed]} Ākhir an-nahr, which means "the end of the river", via a Roman-alphabet handwriting misread "rn" to "m". In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)^[15] to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.^[16] It approved the name "Acamar" for θ¹ Eridani on 20 July 2016 and it is now so entered in the IAU Catalog of Star Names.^[14]

The term "Ākhir an-nahr", or "Achr al Nahr", appeared in the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, which was translated into Latin as Postrema Fluminis.^[17]

Historically, Acamar represented the end of the constellation Eridanus.^[8][18] Now that distinction is held by the star Achernar, which shares the same Arabic etymology. Achernar is not visible from the Greek isles (latitudes > 33° North),^[19] hence the choice of Acamar as the river's end during the time of Hipparchus and, later, Ptolemy.

In Chinese, 天園 (Tiān Yuán), meaning Celestial Orchard, refers to an asterism consisting of Theta Eridani, Chi Eridani, Phi Eridani, Kappa Eridani, HD 16754, HD 23319, HD 24072, HD 24160, Upsilon⁴ Eridani, Upsilon³ Eridani, Upsilon² Eridani and Upsilon¹ Eridani.^[20] Consequently, the Chinese name for Theta Eridani itself is 天園六 (Tiān Yuán liù, English: the Sixth Star of Celestial Orchard).^[21]

Theta Eridani is a binary system with some evidence suggesting it is part of a multiple star system.^[7][22] The main star, θ¹ Eridani, is of spectral class A4 with a +3.2 apparent magnitude. Its companion star, θ² Eridani, is of spectral class A1 with an apparent magnitude of +4.1. The angular separation of the two stars is equal to 8.3 arcseconds.^[8]