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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. 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. In Chinese, (), 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.〔 ''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.〕 Consequently, θ Eridani itself is known as (, (英語:the Sixth Star of Celestial Orchard).)〔 (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.〕 Acamar is a double star with some evidence suggesting it is part of a multiple star system.〔NSV 01002, database entry, (New Catalogue of Suspected Variable Stars, the improved version ), Sternberg Astronomical Institute, Moscow, Russia. Accessed on line February 26, 2010.〕〔(【引用サイトリンク】 title=Acamar )〕 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.〔〔〔 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. ==References== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Theta Eridani」の詳細全文を読む スポンサード リンク
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