Theta Eridani
Encyclopedia
Theta 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 (latitude
s > 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. Consequently, θ Eridani itself is known as
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 is approximately 120 light-year
s.
Ptolemy
described Acamar as a first-magnitude star, a fact which is all the more enigmatic since the brighter star Achernar
can be found just above the horizon in Alexandria
where Ptolemy lived and would have been easily visible to merchants trading farther up the Nile. Al Sufi in the 10th century listed it as one of the thirteen brightest stars in the sky, and Ulugh Beg
from Persia in the 15th century also listed it as a first-magnitude star.
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
Eridanus
Eridanus (constellation)
Eridanus is a constellation. It is represented as a river; its name is the Ancient Greek name for the Po River. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations. It is the sixth largest of the modern...
. It has the traditional name Acamar from the Arabic
Arabic language
Arabic is a name applied to the descendants of the Classical Arabic language of the 6th century AD, used most prominently in the Quran, the Islamic Holy Book...
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
Achernar
Achernar , sometimes spelled Achenar, is the brightest star in the constellation Eridanus and the ninth-brightest star in the night sky. Of the top ten apparent brightest stars —Sirius, Canopus, Alpha Centauri, Arcturus, Vega, Capella, Rigel, Procyon, Achernar and Betelgeuse—Achernar is the hottest...
, a star which shares the same Arabic etymology
Etymology
Etymology is the study of the history of words, their origins, and how their form and meaning have changed over time.For languages with a long written history, etymologists make use of texts in these languages and texts about the languages to gather knowledge about how words were used during...
. Achernar is not visible from the Greek isles (latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
s > 33° North), hence the choice of Acamar as the river's end during the time of Hipparchus
Hipparchus
Hipparchus, the common Latinization of the Greek Hipparkhos, can mean:* Hipparchus, the ancient Greek astronomer** Hipparchic cycle, an astronomical cycle he created** Hipparchus , a lunar crater named in his honour...
and later Ptolemy
Ptolemy
Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...
.
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
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
as Postrema Fluminis.
In Chinese
Chinese language
The Chinese language is a language or language family consisting of varieties which are mutually intelligible to varying degrees. Originally the indigenous languages spoken by the Han Chinese in China, it forms one of the branches of Sino-Tibetan family of languages...
, , meaning Celestial Orchard
Net (Chinese constellation)
The Net mansion is one of the Twenty-eight mansions of the Chinese constellations. It is one of the western mansions of the White Tiger.-Asterisms:...
, 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
Upsilon2 Eridani
Upsilon2 Eridani is a G-type giant star in the constellation of Eridanus. It has an apparent visual magnitude of approximately 3.817 and is approximately 209 light-years from the Earth...
and υ1 Eridani. Consequently, θ Eridani itself is known as
Properties
Acamar is a double starDouble star
In observational astronomy, a double star is a pair of stars that appear close to each other in the sky as seen from Earth when viewed through an optical telescope. This can happen either because the pair forms a binary star, i.e...
with some evidence suggesting it is part of a multiple star system
Star system
A star system or stellar system is a small number of stars which orbit each other, bound by gravitational attraction. A large number of stars bound by gravitation is generally called a star cluster or galaxy, although, broadly speaking, they are also star systems.-Binary star systems:A stellar...
. The main star, θ1 Eri, is of the spectral class
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
A4 and has a +3.2 apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
. Its companion star, θ2 Eri, is of the spectral class A1 and has an apparent magnitude of +4.3. The angular separation
Angular distance
In mathematics and all natural sciences , the angular distance between two point objects, as observed from a location different from either of these objects, is the size of the angle between the two directions originating from...
of the two stars is equal to 8.3 arcseconds. Acamar's distance from Earth is approximately 120 light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s.
Ptolemy
Ptolemy
Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...
described Acamar as a first-magnitude star, a fact which is all the more enigmatic since the brighter star Achernar
Achernar
Achernar , sometimes spelled Achenar, is the brightest star in the constellation Eridanus and the ninth-brightest star in the night sky. Of the top ten apparent brightest stars —Sirius, Canopus, Alpha Centauri, Arcturus, Vega, Capella, Rigel, Procyon, Achernar and Betelgeuse—Achernar is the hottest...
can be found just above the horizon in Alexandria
Alexandria
Alexandria is the second-largest city of Egypt, with a population of 4.1 million, extending about along the coast of the Mediterranean Sea in the north central part of the country; it is also the largest city lying directly on the Mediterranean coast. It is Egypt's largest seaport, serving...
where Ptolemy lived and would have been easily visible to merchants trading farther up the Nile. Al Sufi in the 10th century listed it as one of the thirteen brightest stars in the sky, and Ulugh Beg
Ulugh Beg
Ulugh Bek was a Timurid ruler as well as an astronomer, mathematician and sultan. His commonly-known name is not truly a personal name, but rather a moniker, which can be loosely translated as "Great Ruler" or "Patriarch Ruler" and was the Turkic equivalent of Timur's Perso-Arabic title Amīr-e...
from Persia in the 15th century also listed it as a first-magnitude star.
See also
- List of stars in Eridanus
- Ptolemy's PtolemyClaudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...
AlmagestAlmagestThe Almagest is a 2nd-century mathematical and astronomical treatise on the apparent motions of the stars and planetary paths. Written in Greek by Claudius Ptolemy, a Roman era scholar of Egypt,...
and his arguments in favor of a spherical Earth - Class A Stars
- Acamar in fiction