Editing Epsilon Eridani (section)

== Observational history ==
[[File:Epsilon Eridani location.png|right|thumb|Above, the northern section of the Eridanus constellation is delineated in green, while [[Orion (constellation)|Orion]] is shown in blue. Below, an enlarged view of the region in the white box shows the location of Epsilon Eridani at the intersection of the two lines.|alt=The upper photograph shows a region of many point-like stars with coloured lines marking the constellations. The lower image shows several stars and two white lines.]]

=== Cataloguing ===
Epsilon Eridani has been known to astronomers since at least the 2nd century AD, when [[Claudius Ptolemy]] (a [[Greek astronomy|Greek astronomer]] from [[History of Alexandria|Alexandria]], [[Egypt (Roman province)|Egypt]]) included it in his catalogue of more than a thousand stars. The catalogue was published as part of his astronomical treatise the ''[[Almagest]]''. The constellation [[Eridanus (constellation)|Eridanus]] was named by Ptolemy – {{langnf|grc|Ποταμού|River}}, and Epsilon Eridani was listed as its thirteenth star. Ptolemy called Epsilon Eridani {{langnf|grc|ό τών δ προηγούμενος|a foregoing of the four}} (here {{lang|grc|δ}} is the number four). This refers to a group of four stars in Eridanus: [[Gamma Eridani|γ]], [[Pi Eridani|π]], [[Delta Eridani|δ]] and ε (10th–13th in Ptolemy's list). ε is the most western of these, and thus the first of the four in the apparent daily motion of the sky from east to west. Modern scholars of Ptolemy's catalogue designate its entry as ''"P 784"'' (in order of appearance) and ''"Eri 13"''. Ptolemy described the star's [[apparent magnitude|magnitude]] as 3.{{r|Baily1843|Verbunt2012}}

Epsilon Eridani was included in several star catalogues of [[Astronomy in medieval Islam|medieval Islamic]] astronomical treatises, which were based on Ptolemy's catalogue: in [[Abd al-Rahman al-Sufi|Al-Sufi]]'s ''[[Book of Fixed Stars]]'', published in 964, [[Abū Rayḥān al-Bīrūnī|Al-Biruni]]'s ''Mas'ud Canon'', published in 1030, and [[Ulugh Beg]]'s ''[[Zij-i Sultani]]'', published in 1437. Al-Sufi's estimate of Epsilon Eridani's magnitude was 3. Al-Biruni quotes magnitudes from Ptolemy and Al-Sufi (for Epsilon Eridani he quotes the value 4 for both Ptolemy's and Al-Sufi's magnitudes; original values of both these magnitudes are 3). Its number in order of appearance is 786.{{r|Al-Biruni1030}} Ulugh Beg carried out new measurements of Epsilon Eridani's coordinates in [[Ulugh Beg Observatory|his observatory]] at [[Samarkand]], and quotes magnitudes from Al-Sufi (3 for Epsilon Eridani). The modern designations of its entry in Ulugh Beg's catalogue are ''"U 781"'' and ''"Eri 13"'' (the latter is the same as Ptolemy's catalogue designation).{{r|Baily1843|Verbunt2012}}

In 1598 Epsilon Eridani was included in [[Tycho Brahe]]'s star catalogue, republished in 1627 by [[Johannes Kepler]] as part of his ''[[Rudolphine Tables]]''. This catalogue was based on Tycho Brahe's observations of 1577–1597, including those on the island of [[Hven]] at his observatories of [[Uraniborg]] and [[Stjerneborg]]. The sequence number of Epsilon Eridani in the constellation Eridanus was 10, and it was designated {{Langnf|la|Quae omnes quatuor antecedit|which precedes all four}}; the meaning is the same as Ptolemy's description. Brahe assigned it magnitude 3.{{r|Baily1843|Verbunt2010a}}

Epsilon Eridani's [[Bayer designation]] was established in 1603 as part of the ''[[Uranometria]]'', a star catalogue produced by German celestial cartographer [[Johann Bayer]]. His catalogue assigned letters from the [[Greek alphabet]] to groups of stars belonging to the same visual magnitude class in each constellation, beginning with alpha (α) for a star in the brightest class. Bayer made no attempt to arrange stars by relative brightness within each class. Thus, although Epsilon is the fifth letter in the Greek alphabet,<ref name=jha17_50_189 /> the star is the [[List of stars in Eridanus|tenth-brightest in Eridanus]].<ref name=ybsc1991 /> In addition to the letter ε, Bayer had given it the number 13 (the same as Ptolemy's catalogue number, as were many of Bayer's numbers) and described it as {{lang-for|la|Decima septima|the seventeenth}}.{{#tag:ref|This is because Bayer designated 21 stars in the northern part of Eridanus by preceding along the 'river' from east to west, starting from β ({{lang|la|Supra pedem Orionis in flumine, prima}}, meaning ''above [[Rigel|the foot]] of [[Orion (constellation)|Orion]] in the river, the first'') to the twenty-first, σ ({{lang|la|Vigesima prima}}, that is ''the twenty-first''). Epsilon Eridani was the seventeenth in this sequence. These 21 stars are: β, λ, ψ, b, ω, μ, c, ν, ξ, ο (two stars), d, A, γ, π, δ, ε, ζ, ρ, η, σ.<ref name="Bayer1603" />|group=note|name=bayer}} Bayer assigned Epsilon Eridani magnitude 3.{{r|Bayer1603}}

In 1690 Epsilon Eridani was included in the star catalogue of [[Johannes Hevelius]]. Its sequence number in constellation Eridanus was 14, its designation was {{langnf|la|Tertia|the third}}, and it was assigned magnitude 3 or 4 (sources differ).{{r|Baily1843|Verbunt2010b}} The star catalogue of English astronomer [[John Flamsteed]], published in 1712, gave Epsilon Eridani the [[Flamsteed designation]] of 18&nbsp;Eridani, because it was the eighteenth catalogued star in the constellation of Eridanus by order of increasing [[right ascension]].<ref name=SIMBAD /> In 1818 Epsilon Eridani was included in [[Friedrich Bessel]]'s catalogue, based on [[James Bradley]]'s observations from 1750–1762, and at magnitude 4.{{r|Bessel1818}} It also appeared in [[Nicolas Louis de Lacaille]]'s catalogue of 398 principal stars, whose 307-star version was published in 1755 in the {{lang|fr|Ephémérides des Mouvemens Célestes, pour dix années, 1755–1765}},{{r|Lacaille1755}} and whose full version was published in 1757 in {{lang|la|Astronomiæ Fundamenta}}, Paris.{{r|Lacaille1757}} In its 1831 edition by [[Francis Baily]], Epsilon Eridani has the number 50.{{r|Baily1831}} Lacaille assigned it magnitude 3.{{r|Lacaille1755|Lacaille1757|Baily1831}}

In 1801 Epsilon Eridani was included in {{lang|fr|[[Histoire céleste française]]}}, [[Joseph Jérôme Lefrançois de Lalande]]'s catalogue of about 50,000 stars, based on his observations of 1791–1800, in which observations are arranged in time order. It contains three observations of Epsilon Eridani.<ref name=lalande group=note>1796 September 17 (page 246), 1796 December 3 (page 248) and 1797 November 13 (page 307)</ref>{{r|Lalande1801}} In 1847, a new edition of Lalande's catalogue was published by Francis Baily, containing the majority of its observations, in which the stars were numbered in order of [[right ascension]]. Because every observation of each star was numbered and Epsilon Eridani was observed three times, it got three numbers: 6581, 6582 and 6583.{{r|Baily1847}} (Today numbers from this catalogue are used with the prefix "Lalande", or "Lal".{{r|Lal}}) Lalande assigned Epsilon Eridani magnitude 3.{{r|Lalande1801|Baily1847}} Also in 1801 it was included in the catalogue of [[Johann Bode]], in which about 17,000 stars were grouped into 102 constellations and numbered (Epsilon Eridani got the number 159 in the constellation Eridanus). Bode's catalogue was based on observations of various astronomers, including Bode himself, but mostly on Lalande's and Lacaille's (for the southern sky). Bode assigned Epsilon Eridani magnitude 3.{{r|Bode1801}} In 1814 [[Giuseppe Piazzi]] published the second edition of his star catalogue (its first edition was published in 1803), based on observations during 1792–1813, in which more than 7000 stars were grouped into 24 hours (0–23). Epsilon Eridani is number 89 in hour 3. Piazzi assigned it magnitude 4.{{r|Piazzi1814}} In 1918 Epsilon Eridani appeared in the [[Henry Draper Catalogue]] with the designation HD&nbsp;22049 and a preliminary spectral classification of K0.<ref name=ahco91_1 />

=== Detection of proximity ===
Based on observations between 1800 and 1880, Epsilon Eridani was found to have a large [[proper motion]] across the [[celestial sphere]], which was estimated at three [[arcsecond]]s per year ([[angular velocity]]).<ref name=mras48_77 /> This movement implied it was relatively close to the Sun,<ref name=belkora2002 /> making it a star of interest for the purpose of [[stellar parallax]] measurements. This process involves recording the position of Epsilon Eridani as Earth moves around the Sun, which allows a star's distance to be estimated.<ref name=mras48_77 /> From 1881 to 1883, American astronomer [[William Lewis Elkin|William L. Elkin]] used a [[heliometer]] at the [[Royal Observatory at the Cape of Good Hope]], South Africa, to compare the position of Epsilon Eridani with two nearby stars. From these observations, a parallax of {{nowrap|0.14 ± 0.02 arcseconds}} was calculated.<ref name=gill1893 /><ref name=gill1884 /> By 1917, observers had refined their parallax estimate to 0.317&nbsp;arcseconds.<ref name=apj46_313 /> The modern value of 0.3109&nbsp;arcseconds is equivalent to a distance of about {{convert|10.50|ly|pc}}.<ref name=aaa474_2_653 />

=== Circumstellar discoveries ===

[[File:Epsilon eridani dustring.gif|right|thumb|Submillimeter wavelength image of a ring of dust particles around Epsilon Eridani (above centre). The brightest areas indicate the regions with the highest concentrations of dust.|alt=An uneven, multi-coloured ring arranged around a five-sided star at the middle, with the strongest concentration below centre. A smaller oval showing the scale of Pluto's orbit is in the lower right.]]

Based on apparent changes in the position of Epsilon Eridani between 1938 and 1972, [[Peter van de Kamp]] proposed that an unseen companion with an orbital period of 25&nbsp;years was causing gravitational [[Perturbation (astronomy)|perturbations]] in its position.<ref name=aj79_491 /> This claim was refuted in 1993 by [[Wulff-Dieter Heintz]] and the false detection was blamed on a systematic error in the [[photographic plate]]s.<ref name=aj105_3_1188 />

Launched in 1983, the [[space telescope]] [[IRAS]] detected [[Infrared astronomy|infrared]] emissions from stars near to the Sun,<ref name=apj278_L1 /> including an [[infrared excess|excess infrared emission]] from Epsilon Eridani.<ref name=pasp97_885 /> The observations indicated a disk of fine-grained [[cosmic dust]] was orbiting the star;<ref name=pasp97_885 /> this [[debris disk]] has since been extensively studied. Evidence for a planetary system was discovered in 1998 by the observation of asymmetries in this dust ring. The clumping in the dust distribution could be explained by gravitational interactions with a planet orbiting just inside the dust ring.<ref name=apj506_2_L133 />

In 1987, the detection of an orbiting planetary object was announced by Bruce Campbell, Gordon Walker and Stephenson Yang.<ref name=hesser1987 /><ref name=Campbell1988 /> From 1980 to 2000, a team of astronomers led by [[Artie P. Hatzes]] made [[radial velocity]] observations of Epsilon Eridani, measuring the [[Doppler spectroscopy|Doppler shift of the star along the line of sight]]. They found evidence of a planet orbiting the star with a period of about seven years.<ref name=apj544_2_L145 /> Although there is a high level of noise in the radial velocity data due to magnetic activity in its [[photosphere]],<ref name=iau202 /> any periodicity caused by this magnetic activity is expected to show a strong correlation with variations in [[emission line]]s of ionized calcium (the [[Calcium#H and K lines|Ca II H and K lines]]). Because no such correlation was found, a planetary companion was deemed the most likely cause.<ref name=apj133_6_2442 /> This discovery was supported by [[astrometric]] measurements of Epsilon Eridani made between 2001 and 2003 with the [[Hubble Space Telescope]], which showed evidence for [[gravitational perturbation]] of Epsilon Eridani by a planet.<ref name=aj132_2206 />

=== SETI and proposed exploration ===
In 1960, physicists [[Philip Morrison]] and [[Giuseppe Cocconi]] proposed that [[extraterrestrial civilisations]] might be using radio signals for communication.<ref name=gugliucci20100524 /> [[Project Ozma]], led by astronomer [[Frank Drake]], used the [[Tatel Telescope]] to search for such signals from the nearby [[Solar-type star|Sun-like stars]] Epsilon Eridani and [[Tau Ceti]]. The systems were observed at the [[Hydrogen line|emission frequency of neutral hydrogen]], 1,420&nbsp;MHz (21&nbsp;cm). No signals of intelligent extraterrestrial origin were detected.<ref name=heidmann_dunlop1995 /> Drake repeated the experiment in 2010, with the same negative result.<ref name=gugliucci20100524 /> {{Anchor|In fiction}}Despite this lack of success, Epsilon Eridani made its way into science fiction literature and television shows for many years following news of Drake's initial experiment.<ref name=marschall_maran2009 />

In ''Habitable Planets for Man'', a 1964 [[RAND Corporation]] study by space scientist Stephen H. Dole, the probability of a [[habitable planet]] being in orbit around Epsilon Eridani were estimated at 3.3%. Among the known nearby stars, it was listed with the 14 stars that were thought most likely to have a habitable planet.<ref name=dole1964 />

[[William I. McLaughlin]] proposed a new strategy in the search for extraterrestrial intelligence ([[SETI]]) in 1977. He suggested that widely observable events such as [[nova]] explosions might be used by intelligent extraterrestrials to synchronise the transmission and reception of their signals. This idea was tested by the [[National Radio Astronomy Observatory]] in 1988, which used outbursts of [[Nova Cygni 1975]] as the timer. Fifteen days of observation showed no anomalous radio signals coming from Epsilon Eridani.<ref name=baas20_1043 />

Because of the proximity and Sun-like properties of Epsilon Eridani, in 1985 physicist and author [[Robert L. Forward]] considered the system as a plausible target for [[interstellar travel]].<ref name=jsr22_345 /> The following year, the [[British Interplanetary Society]] suggested Epsilon Eridani as one of the targets in its [[Project Daedalus]] study.<ref name=jbis29_94 /> The system has continued to be among the targets of such proposals, such as [[Project Icarus (Interstellar Probe Design Study)|Project Icarus]] in 2011.<ref name=aa20110125 />

Based on its nearby location, Epsilon Eridani was among the target stars for [[Project Phoenix (SETI)|Project Phoenix]], a 1995 [[microwave]] survey for signals from extraterrestrial intelligence.<ref name=henry_asp19930916 /> The project had checked about 800 stars by 2004 but had not yet detected any signals.<ref name=whitehouse_bbc20040325 />