Epoch J2000 Equinox J2000
|Right ascension||02h 58m 15.67525s|
|Declination||−40° 18′ 16.8524″|
|Apparent magnitude (V)||3.2|
|U−B color index||0.14|
|B−V color index||0.14|
|Radial velocity (Rv)||11.9  km/s|
|Proper motion (μ)||RA: −52.89 mas/yr
Dec.: +21.98 mas/yr
|Parallax (π)||20.23 ± 0.55 mas|
|Distance||161 ± 4 ly
(49 ± 1 pc)
|Absolute magnitude (MV)||0.44|
|Rotational velocity (v sin i)||70 km/s|
|Rotational velocity (v sin i)||90 km/s|
|θ1 Eri: HD 18622, HR 897, Acamar|
|θ2 Eri: HD 18623, HR 898|
Theta Eridani (θ Eridani, abbreviated Theta Eri, θ Eri) is a binary system in the constellation of Eridanus. Its two components are designated θ¹ Eridani, also named Acamar, and θ² Eridani. The system's distance from the Sun as measured by the Hipparcos astrometry satellite is approximately 120 light-years.
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 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) to catalog and standardize proper names for stars. The WGSN approved the name Acamar for θ¹ Eridani on 20 July 2016 and it is now so entered in the IAU Catalog of Star Names.
Historically, Acamar represented the end of the constellation Eridanus. 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), 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, 43 Eridani, Upsilon2 Eridani and Upsilon1 Eridani. Consequently, Theta Eridani itself is known as 天園六 (Tiān Yuán liù, English: the Sixth Star of Celestial Orchard.)
Theta Eridani is a binary system with some evidence suggesting it is part of a multiple star system. 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 +4.3. The angular separation of the two stars is equal to 8.3 arcseconds.
Ptolemy described Theta Eridani as a first-magnitude star, which some have taken to be in fact Achernar, which today appears just above the horizon in Alexandria where Ptolemy lived. Achernar, however, was not visible to Ptolemy (it is from Alexandria today due to precession); its declination in 100CE was -67, making it invisible even at Aswan. Ptolemy's Eridanus thus without doubt ended at Theta Eridani, which rose about 10 degrees above the horizon from Alexandria. This fact besides positively identifying Theta Eridani as the original "end of the river", also strengthens the case for it having been first magnitude in ancient times.
- Perryman, M. A. C.; Lindegren, L.; Kovalevsky, J.; et al. (July 1997), "The Hipparcos Catalogue", Astronomy and Astrophysics, 323: L49–L52, Bibcode:1997A&A...323L..49P
- van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, Bibcode:2007A&A...474..653V, arXiv: , doi:10.1051/0004-6361:20078357
- "ACAMAR -- Star in double system". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2010-02-24.
- Hoffleit; et al. (1991). "Bright Star Catalogue". VizieR (5th Revised ed.). Centre de Données astronomiques de Strasbourg. Retrieved 2010-02-24.
- Kaler, James B. "ACAMAR (Theta Eridani)". Stars. University of Illinois. Retrieved 2010-02-24.
- Setiawan, J.; et al. (July 2004), "Precise radial velocity measurements of G and K giants. Multiple systems and variability trend along the Red Giant Branch", Astronomy and Astrophysics, 421: 241–254, Bibcode:2004A&A...421..241S, doi:10.1051/0004-6361:20041042-1
- "IAU Catalog of Star Names". Retrieved 28 July 2016.
- Perryman, Michael (2010), The Making of History's Greatest Star Map, Heidelberg: Springer-Verlag, doi:10.1007/978-3-642-11602-5
- "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
- Knobel, E. B. (June 1895). "Al Achsasi Al Mouakket, on a catalogue of stars in the Calendarium of Mohammad Al Achsasi Al Mouakket". Monthly Notices of the Royal Astronomical Society. 55: 429. Bibcode:1895MNRAS..55..429K. doi:10.1093/mnras/55.8.429.
- Rogers, J. H. (1998). "Origins of the ancient constellations: II. The Mediterranean traditions". Journal of the British Astronomical Association. 108 (2): 79–89. Bibcode:1998JBAA..108...79R.
- Larry Sessions. "Achernar: End of the River". EarthSky Tonight Post 06-29-2009. Retrieved 2010-02-26.
- (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
- (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表, Hong Kong Space Museum. Accessed on line November 23, 2010.
- NSV 01002, database entry, New Catalogue of Suspected Variable Stars, the improved version, Sternberg Astronomical Institute, Moscow, Russia. Accessed on line February 26, 2010.
- "Acamar". Alcyone Bright Star Catalogue. Retrieved 2010-02-26.
Jim Kaler's Stars: Acamar