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|
Theta Eridani (θ Eri, θ Eridani) is a star in the constellation Eridanus. It has the traditional name Acamar from the Arabic آخِر النَّهْر Ākhir an-nahr which means "the end of the river". Historically, Acamar represented the end of the constellation Eridanus. 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), 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 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. Consequently, θ Eridani itself is known as 天園六 (Tiān Yuán liù, English: the Sixth Star of Celestial Orchard.)
Acamar is a double star with some evidence suggesting it is part of a multiple star system. 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. Acamar's distance from Earth as measured by the Hipparcos astrometry satellite is approximately 120 light-years.
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.
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Jim Kaler's Stars: Acamar