Theta Eridani

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Acamar, θ Eri
Eridanus constellation map.png
Acamar is the θ star in the lower right of the map
Observation data
Epoch J2000      Equinox J2000
Constellation Eridanus
Right ascension 02h 58m 15.67525s[1][2]
Declination −40° 18′ 16.8524″[1][2]
Apparent magnitude (V) 3.2[3]
Spectral type A3IV-V[3]
U−B color index 0.14[4]
B−V color index 0.14[4]
Variable type suspected
Radial velocity (Rv) 11.9 [3] km/s
Proper motion (μ) RA: −52.89[1][2] mas/yr
Dec.: +21.98[1][2] mas/yr
Parallax (π) 20.23 ± 0.55[1][2] mas
Distance 161 ± 4 ly
(49 ± 1 pc)
Absolute magnitude (MV) 0.44
θ1 Eri
Mass 2.6[5] M
Radius 16.0[6] R
Luminosity 96[5] L
Temperature 8,200[5] K
Rotation 569 days[6]
Rotational velocity (v sin i) 70[3] km/s
θ2 Eri
Mass 2.4[5] M
Luminosity 36[5] L
Temperature 9,200[5] K
Rotational velocity (v sin i) 90[3] km/s
Other designations
θ1 Eri, HD 18622, HIP 13847, HR 897, CCDM J02583-4018A, FK5 106, NSV 01002, SAO 216113, WDS 02583-4018A
θ2 Eri, HD 18623, HIP 13847, HR 898, CCDM J02583-4018B, NSV 01002, SAO 216114, WDS 02583-4018B
Database references

Theta Eridani (θ Eri, θ Eridani) is a star in the constellation Eridanus. It has the traditional name Acamar from the Arabic آخِر النَّهْر[citation needed] Ākhir an-nahr which means "the end of the river". Historically, Acamar represented the end of the constellation Eridanus.[5][7] Now that distinction is held by the star Achernar, a star which shares the same Arabic etymology. Achernar is not visible from the Greek isles (latitudes > 33° North),[8] hence the choice of Acamar as the river's end during the time of Hipparchus and later Ptolemy.

The term Ākhir an-nahr or Achr al Nahr was appeared in the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, which was translated into Latin as Postrema Fluminis.[9]

In Chinese, 天園 (Tiān Yuán), meaning Celestial Orchard, refers to an asterism consisting of θ Eridani, χ Eridani, φ Eridani, κ Eridani, HD 16754, HD 23319, HD 24072, HD 24160, υ4 Eridani, 43 Eridani, υ2 Eridani and υ1 Eridani.[10] Consequently, θ Eridani itself is known as 天園六 (Tiān Yuán liù, English: the Sixth Star of Celestial Orchard.)[11]

Acamar is a double star with some evidence suggesting it is part of a multiple star system.[12][13] The main star, θ1 Eri, is of the spectral class A4 and has a +3.2 apparent magnitude. Its companion star, θ2 Eri, is of the spectral class A1 and has an apparent magnitude of +4.3. The angular separation of the two stars is equal to 8.3 arcseconds.[5] Acamar's distance from Earth as measured by the Hipparcos astrometry satellite is approximately 120 light-years.[1][2][14]

Ptolemy described Acamar 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 Acamar, which rose about 10 degrees above the horizon from Alexandria. This fact besides positively identifying Acamar as the original "end of the river", also strengthens the case for Acamar having been first magnitude in ancient times.


  1. ^ a b c d e f 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 
  2. ^ a b c d e f van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357 
  3. ^ a b c d e "ACAMAR -- Star in double system". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2010-02-24. 
  4. ^ a b Hoffleit; et al. (1991). "Bright Star Catalogue". VizieR (5th Revised ed.). Centre de Données astronomiques de Strasbourg. Retrieved 2010-02-24. 
  5. ^ a b c d e f g h Kaler, James B. "ACAMAR (Theta Eridani)". Stars. University of Illinois. Retrieved 2010-02-24. 
  6. ^ a b 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 
  7. ^ 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. 
  8. ^ Larry Sessions. "Achernar: End of the River". EarthSky Tonight Post 06-29-2009. Retrieved 2010-02-26. 
  9. ^ 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. 
  10. ^ (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.
  11. ^ (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.
  12. ^ NSV 01002, database entry, New Catalogue of Suspected Variable Stars, the improved version, Sternberg Astronomical Institute, Moscow, Russia. Accessed on line February 26, 2010.
  13. ^ "Acamar". Alcyone Bright Star Catalogue. Retrieved 2010-02-26. 
  14. ^ Perryman, Michael (2010), The Making of History's Greatest Star Map, Heidelberg: Springer-Verlag, doi:10.1007/978-3-642-11602-5 

External links[edit]

Jim Kaler's Stars: Acamar

Coordinates: Sky map 02h 58m 15.70s, −40° 18′ 17.0″