Editing Sirius (section)

== Observational history ==
{{See also|Winter Triangle}}
[[File:Sopdet.svg|thumb|right|upright|A [[Ancient Egyptian deities|Neteru]] image of [[Sopdet]], Egyptian goddess of Sirius and the fertility of the [[Nile]], pictured with a star upon her head]]
{{hiero|Sirius<br/>''{{lang|egy|[[wikt:spdt|Spdt]]}}''|<hiero>X1:N14-M44</hiero>|align=right}}
As the brightest star in the night sky, Sirius appears in some of the earliest astronomical records. Its displacement from the [[ecliptic]] causes its heliacal rising to be remarkably regular compared to other stars, with a period of almost exactly 365.25&nbsp;days holding it constant relative to the [[solar year]]. This rising occurs at [[Cairo]] on 19&nbsp;July ([[Julian calendar|Julian]]), placing it just before the onset of the [[flooding of the Nile|annual flooding]] of the [[Nile]] during antiquity.<ref name="Wendorf2001"/> Owing to the flood's own irregularity, the extreme precision of the star's return made it important to the [[ancient Egypt]]ians,<ref name="Wendorf2001"/> who worshipped it as the goddess [[Sopdet]] ({{langx|egy|[[wikt:spdt|Spdt]]}}, "Triangle";{{efn|
Compare the meaning of the Egyptian name with Sirius's completion of the [[Winter Triangle]] [[asterism (astronomy)|asterism]], joining the other two brightest stars of the northern winter sky, [[Betelgeuse]] and [[Procyon]].}} {{langx|grc|{{math|{{linktext|Σῶθις}}}}}}}, ''Sō̂this''), guarantor of the fertility of their land (see [[Sothic cycle]]). As Sirius is visible together with the constellation of [[Orion (constellation)|Orion]], the Egyptians worshiped Orion as the god [[Sah (god)|Sah]], the husband of Sopdet, with whom she had a son, the sky god [[Sopdu]]. The goddess Sopdet was later [[syncretization|syncretized]] with the goddess [[Isis]], Sah was linked with [[Osiris]] (which is by some suggested as a root for the name of Sirius),<ref name="Brosch2008-21"/> and Sopdu was linked with [[Horus]]. The joining of Sopdet with Isis would allow [[Plutarch]] to state that "The soul of Isis is called Dog by the Greeks", meaning Sirius worshiped as Isis-Sopdet by Egyptians was named the Dog by the Greeks and Romans. The 70&nbsp;day period of the absence of Sirius from the sky was understood as the passing of Sopdet-Isis and Sah-Osiris through the [[duat|Egyptian underworld]].<ref name="Holberg2007-4-5"/>

The [[ancient Greeks]] observed that the appearance of Sirius as the morning star heralded the hot and dry summer and feared that the star caused plants to wilt, men to weaken, and women to become aroused.<ref name="Holberg2007-19"/> Owing to its brightness, Sirius would have been seen to [[Scintillation (astronomy)|twinkle]] more in the unsettled weather conditions of early summer. To Greek observers, this signified emanations that caused its malignant influence. Anyone suffering its effects was said to be "star-struck" ({{lang|grc|{{math|ἀστροβόλητος}}}}, ''astrobólētos''). It was described as "burning" or "flaming" in literature.<ref name="Holberg2007-20"/> The season following the star's reappearance came to be known as the "dog days".<ref name="Holberg2007-16-17"/> The inhabitants of the island of [[Ceos]] in the [[Aegean Sea]] would offer sacrifices to Sirius and [[Zeus]] to bring cooling breezes and would await the reappearance of the star in summer. If it rose clear, it would portend good fortune; if it was misty or faint then it foretold (or emanated) pestilence. Coins retrieved from the island from the 3rd&nbsp;century BC feature dogs or stars with emanating rays, highlighting Sirius's importance.<ref name="Holberg2007-20"/>

The Romans celebrated the heliacal setting of Sirius around 25&nbsp;April, [[Animal sacrifice|sacrificing]] a dog, along with incense, wine, and a sheep, to the goddess [[Robigo]] so that the star's emanations would not cause [[Rust (fungus)|wheat rust]] on wheat crops that year.<ref name="Ovid"/>

Bright stars were important to the ancient [[Polynesians]] for navigation of the Pacific Ocean. They also served as latitude markers; the declination of Sirius matches the latitude of the archipelago of [[Fiji]] at [[17th parallel south|17°S]] and thus passes directly over the islands each [[sidereal day]].<ref name="Holberg2007-25"/> Sirius served as the body of a "Great Bird" constellation called ''Manu'', with Canopus as the southern wingtip and [[Procyon]] the northern wingtip, which divided the Polynesian night sky into two hemispheres.<ref name="Holberg2007-25-26"/> Just as the appearance of Sirius in the morning sky marked summer in Greece, it marked the onset of winter for the [[Māori people|Māori]], whose name ''Takurua'' described both the star and the season. Its culmination at the [[winter solstice]] was marked by celebration in [[Hawaii]], where it was known as ''Ka'ulua'', "Queen of Heaven". Many other Polynesian names have been recorded, including ''Tau-ua'' in the [[Marquesas Islands]], ''Rehua'' in New Zealand, and ''Ta'urua-fau-papa'' "Festivity of original high chiefs" and ''Ta'urua-e-hiti-i-te-tara-te-feiai'' "Festivity who rises with prayers and religious ceremonies" in Tahiti.<ref name=henry1907>{{cite journal|last=Henry|first=Teuira |author-link=Teuira Henry|date=1907|title=Tahitian Astronomy: Birth of Heavenly Bodies|journal=The Journal of the Polynesian Society |volume=16|issue=2|pages=101–04|jstor=20700813}}</ref>

=== Kinematics ===
In 1717, [[Edmond Halley]] discovered the [[proper motion]] of the hitherto presumed ''[[fixed stars]]''<ref name="Aitken1942"/> after comparing contemporary [[astrometry|astrometric]] measurements with those from the second century AD given in Ptolemy's ''[[Almagest]]''. The bright stars [[Aldebaran]], [[Arcturus]] and Sirius were noted to have moved significantly; Sirius had progressed about 30 [[arcminutes]] (about the diameter of the Moon) to the southwest.<ref name="Holberg2007-41-42"/>

In 1868, Sirius became the first star to have its velocity measured, the beginning of the study of celestial [[radial velocities]]. Sir [[William Huggins]] examined the [[stellar spectrum|spectrum]] of the star and observed a [[red shift]]. He concluded that Sirius was receding from the Solar System at about 40&nbsp;km/s.<ref name="Daintith1994"/><ref name="Huggins1868"/> Compared to the modern value of −5.5&nbsp;km/s, this was an overestimate and had the wrong sign; the minus sign (−) means that it is approaching the Sun.<ref name="Hearnshaw2014">{{Cite book |last=Hearnshaw |first=John B. |url=https://books.google.com/books?id=HNbSAgAAQBAJ&pg=PA88 |title=The analysis of starlight: two centuries of astronomical spectroscopy |date=2014 |publisher=Cambridge Univ. Pr |isbn=978-1-107-03174-6 |edition=2nd |location=New York, NY |page=88}}</ref>

=== Distance ===
In his 1698 book, ''Cosmotheoros'', [[Christiaan Huygens]] estimated the distance to Sirius at 27,664&nbsp;times the [[Astronomical unit|distance from the Earth to the Sun]] (about 0.437&nbsp;light-year, translating to a parallax of roughly 7.5&nbsp;arcseconds).<ref>{{cite book |last=Huygens |first=C. |author-link=Christiaan Huygens |year=1698 |title={{math|ΚΟΣΜΟΘΕΩΡΟΣ}}, sive De terris cœlestibus earumque ornatu conjecturae |publisher=Apud A. Moetjens, bibliopolam |location=The Hague |pages=137 |language=la |url=https://catalog.lindahall.org/permalink/01LINDAHALL_INST/19lda7s/alma991342203405961}}</ref> There were several unsuccessful attempts to measure the [[Stellar parallax|parallax]] of Sirius: by [[Jacques Cassini]] (6&nbsp;seconds); by some astronomers (including [[Nevil Maskelyne]])<ref name="Maskelyne1761">{{cite journal |last=Maskelyne |first=N. |author-link=Nevil Maskelyne |year=1759 |title=LXXVIII. A proposal for discovering the annual parallax of Sirius |journal=[[Philosophical Transactions of the Royal Society]] |volume=51 |pages=889–895 |bibcode=1759RSPT...51..889M |doi=10.1098/rstl.1759.0080|doi-access=free}}</ref> using [[Nicolas-Louis de Lacaille|Lacaille]]'s observations made at the [[Cape of Good Hope]] (4&nbsp;seconds); by [[Giuseppe Piazzi|Piazzi]] (the same amount); using Lacaille's observations made at [[Paris]], more numerous and certain than those made at the Cape (no sensible parallax); by [[Friedrich Bessel|Bessel]] (no sensible parallax).<ref name="Henderson1840">{{cite journal |last=Henderson |first=T. |year=1840 |title=On the parallax of Sirius |journal=[[Memoirs of the Royal Astronomical Society]] |volume=11 | pages=239–248 |bibcode=1840MmRAS..11..239H}}</ref>

Scottish astronomer [[Thomas Henderson (astronomer)|Thomas Henderson]] used his observations made in 1832–1833 and South African astronomer [[Thomas Maclear]]'s observations made in 1836–1837, to determine that the value of the parallax was 0.23&nbsp;[[arcsecond]], and error of the parallax was estimated not to exceed a quarter of a second, or as Henderson wrote in 1839, "On the whole we may conclude that the parallax of Sirius is not greater than half a second in space; and that it is probably much less."<ref name=henderson>{{cite journal |last=Henderson |first=T. |author-link=Thomas Henderson (astronomer) |year=1839 |title=On the parallax of Sirius |journal=Monthly Notices of the Royal Astronomical Society |volume=5 |issue=2 |pages=5–7 |bibcode=1839MNRAS...5....5H |doi=10.1093/mnras/5.2.5 |doi-access=free}}</ref> Astronomers adopted a value of 0.25&nbsp;arcsecond for much of the 19th&nbsp;century.<ref name="Holberg2007-45"/> It is now known to have a parallax of nearly {{val|0.4|u=arcseconds}}.

The Hipparcos parallax for Sirius indicates a distance of {{val|8.60|u=light years}}, statistically accurate to plus or minus 0.04&nbsp;[[light year]]s.<ref name=aaa474_2_653/> Sirius&nbsp;B is generally assumed to be at the same distance. Sirius&nbsp;B has a [[Gaia Data Release 3]] parallax with a much smaller statistical margin of error, giving a distance of {{val|8.709|0.005|u=light years}}, but it is flagged as having a very large value for astrometric excess noise, which indicates that the parallax value may be unreliable.<ref name=Gaia3/>

=== Discovery of Sirius B ===
[[File:Sirius A and B Hubble photo.jpg|thumb|right|[[Hubble Space Telescope]] image of Sirius&nbsp;A and Sirius&nbsp;B. The white dwarf can be seen to the lower left. The [[diffraction spikes]] and concentric rings are [[Point spread function|instrumental effects]]. Sirius B is approximately one thousand times fainter than Sirius A.]]

In a letter dated 10&nbsp;August 1844, the German astronomer [[Friedrich Bessel|Friedrich Wilhelm Bessel]] deduced from changes in the proper motion of Sirius that it had an unseen companion.<ref name="Bessel1844"/> On 31 January 1862, American telescope-maker and astronomer [[Alvan Graham Clark]] first observed the faint companion, which is now called Sirius&nbsp;B.<ref name="Flammarion1877" /> This happened during testing of an {{convert|18.5|in|adj=on}} aperture [[great refractor]] telescope for [[Dearborn Observatory]], which was one of the largest refracting telescope lenses in existence at the time, and the largest telescope in the [[United States]].<ref name=craig/> Sirius&nbsp;B's sighting was confirmed on 8&nbsp;March with smaller telescopes.<ref>{{cite book |title=Appletons' annual cyclopaedia and register of important events of the year: 1862 |date=1863 |publisher=D. Appleton & Company |location=New York |page=176 |url=https://archive.org/stream/1862appletonsan02newyuoft#page/n183/mode/1up}}</ref>

The visible star is now sometimes known as Sirius&nbsp;A. Since 1894, some apparent orbital irregularities in the Sirius system have been observed, suggesting a third very small companion star, but this has never been confirmed. The best fit to the data indicates a six-year orbit around Sirius&nbsp;A and a mass of {{Solar mass|0.06}}. This star would be five to ten magnitudes fainter than the white dwarf Sirius&nbsp;B, which would make it difficult to observe.<ref name="Benest1995"/> Observations published in 2008 were unable to detect either a third star or a planet. An apparent "third star" observed in the 1920s is now believed to be a background object.<ref name="BonnetBidaud2008"/>

In 1915, [[Walter Sydney Adams]], using a {{Convert|60|in|m|adj=on}} reflector at [[Mount Wilson Observatory]], observed the [[stellar spectrum|spectrum]] of Sirius&nbsp;B and determined that it was a faint whitish star.<ref name="Adams1915"/> This led astronomers to conclude that it was a white dwarf—the second to be discovered.<ref name="Holberg2005"/> The diameter of Sirius&nbsp;A was first measured by [[Robert Hanbury Brown]] and [[Richard Q. Twiss]] in 1959 at [[Jodrell Bank]] using their stellar [[intensity interferometer]].<ref name="Hanbury1958"/> In 2005, using the [[Hubble Space Telescope]], astronomers determined that Sirius&nbsp;B has nearly the diameter of the Earth, {{convert|12000|km}}, with a mass 102% of the Sun's.<ref name="Barstow2005"/>

=== Colour controversy ===
[[File:Szintillation.Sirius.480.webm|thumb|[[Twinkling]] of Sirius ([[apparent magnitude]] = −1.5) in the evening shortly before upper [[culmination]] on the southern [[meridian (astronomy)|meridian]] at a height of 20&nbsp;degrees above the horizon. During 29 seconds Sirius moves on an arc of 7.5 minutes from the left to the right.]]
Around the year 150&nbsp;AD,<ref name="Holberg2007-157"/> [[Ptolemy|Claudius Ptolemy]] of Alexandria, an ethnic Greek Egyptian astronomer of the Roman period, mapped the stars in Books&nbsp;VII and VIII of his ''[[Almagest]]'', in which he used Sirius as the location for the globe's central meridian.<ref name="Holberg2007-32"/> He described Sirius as reddish, along with five other stars, [[Betelgeuse]], [[Antares]], [[Aldebaran]], [[Arcturus]], and [[Pollux (star)|Pollux]], all of which are at present observed to be of orange or red hue.<ref name="Holberg2007-157"/> The discrepancy was first noted by amateur astronomer [[Thomas Barker (meteorologist)|Thomas Barker]], squire of [[Lyndon, Rutland|Lyndon Hall]] in [[Rutland]], who prepared a paper and spoke at a meeting of the [[Royal Society]] in London in 1760.<ref name="Ceragioli1995"/> The existence of other stars changing in brightness gave credibility to the idea that some may change in colour too; Sir [[John Herschel]] noted this in 1839, possibly influenced by witnessing [[Eta Carinae]] two years earlier.<ref name="Holberg2007-158"/> [[Thomas Jefferson Jackson See|Thomas J.J. See]] resurrected discussion on red Sirius with the publication of several papers in 1892, and a final summary in 1926.<ref name="Holberg2007-161"/> He cited not only Ptolemy but also the poet [[Aratus]], the orator [[Cicero]], and general [[Germanicus]] all calling the star red, though acknowledging that none of the latter three authors were astronomers, the last two merely translating Aratus's poem ''Phaenomena''.<ref name="Holberg2007-162"/> [[Seneca the Younger|Seneca]] had described Sirius as being of a deeper red than [[Mars]].<ref name="Whittet1999"/> It is therefore possible that the description as red is a poetic metaphor for ill fortune. In 1985, German astronomers Wolfhard Schlosser and Werner Bergmann published an account of an 8th-century [[Lombardy|Lombardic]] manuscript, which contains ''De cursu stellarum ratio'' by St.&nbsp;[[Gregory of Tours]]. The Latin text taught readers how to determine the times of nighttime prayers from positions of the stars, and a bright star described as ''rubeola'' ("reddish") was claimed to be Sirius. The authors proposed this as evidence that Sirius&nbsp;B had been a red giant at the time of observation.<ref name="Schlosser1985"/> Other scholars replied that it was likely St.&nbsp;Gregory had been referring to [[Arcturus]].<ref name="McCluskey1987" /><ref name="VanGent1987"/>

It is notable that not all ancient observers saw Sirius as red. The 1st-century poet [[Marcus Manilius]] described it as "sea-blue", as did the 4th-century [[Avienius]].<ref name="Holberg2007-163"/> Furthermore, Sirius was consistently reported as a white star in ancient [[China]]: a detailed re-evaluation of Chinese texts from the 2nd&nbsp;century BC up to the 7th&nbsp;century AD concluded that all such reliable sources are consistent with Sirius being white.<ref name="Jiang1992"/><ref name="Jiang1993" />

Nevertheless, historical accounts referring to Sirius as red are sufficiently extensive to lead researchers to seek possible physical explanations. Proposed theories fall into two categories: intrinsic and extrinsic. Intrinsic theories postulate a real change in the Sirius system over the past two millennia, of which the most widely discussed is the proposal that the white dwarf Sirius B was a red giant as recently as 2000 years ago. Extrinsic theories are concerned with the possibility of transient reddening in an intervening medium through which the star is observed, such as might be caused by dust in the [[interstellar medium]], or by particles in the [[Atmosphere of Earth|terrestrial atmosphere]].

The possibility that [[stellar evolution]] of either Sirius&nbsp;A or Sirius&nbsp;B could be responsible for the discrepancy has been rejected on the grounds that the timescale of thousands of years is orders of magnitude too short and that there is no sign of the nebulosity in the system that would be expected had such a change taken place.<ref name="Whittet1999"/> Similarly, the presence of a third star sufficiently luminous to affect the visible colour of the system in recent millennia is inconsistent with observational evidence.<ref name="Kuchner2000"/> Intrinsic theories may therefore be disregarded. Extrinsic theories based on reddening by [[interstellar dust]] are similarly implausible. A transient dust cloud passing between the Sirius system and an observer on Earth would, indeed redden the appearance of the star to some degree, but reddening sufficient to cause it to appear similar in colour to intrinsically red bright stars such as Betelgeuse and Arcturus would also dim the star by several magnitudes, inconsistent with historical accounts: indeed, the dimming would be sufficient to render the colour of the star imperceptible to the human eye without the aid of a telescope.<ref name="Whittet1999"/>

Extrinsic theories based on optical effects in the Earth's atmosphere are better supported by available evidence. [[Scintillation (astronomy)|Scintillations]] caused by [[Turbulence|atmospheric turbulence]] result in rapid, transient changes in the apparent colour of the star, especially when observed near the horizon, although with no particular preference for red.<ref>{{Cite web |last=King |first=Bob |date=December 22, 2014 |title=Have A Sirius-ly Scintillating Holiday! |url=https://skyandtelescope.org/observing/sirius-ly-scintillating-holiday12222014/#google_vignette |website=Sky & Telescope |publisher=AAS Sky Publishing LLC}}</ref> However, systematic reddening of the star's light results from [[Absorption (electromagnetic radiation)|absorption]] and [[scattering]] by particles in the atmosphere, exactly analogous to the redness of the Sun at [[sunrise]] and [[sunset]]. Because the particles that cause reddening in the Earth's atmosphere are different (typically much smaller) than those that cause reddening in the interstellar medium, there is far less dimming of the starlight, and in the case of Sirius the change in colour can be seen without the aid of a telescope.<ref name="Whittet1999"/> There may be cultural reasons to explain why some ancient observers might have reported the colour of Sirius preferentially when it was situated low in the sky (and therefore apparently red). In several Mediterranean cultures, the local visibility of Sirius at [[heliacal rising]] and setting (whether it appeared bright and clear or dimmed) was thought to have astrological significance and was thus subject to systematic observation and intense interest. Thus Sirius, more than any other star, was observed and recorded while close to the horizon. Other contemporary cultures, such as Chinese, lacking this tradition, recorded Sirius only as white.<ref name="Whittet1999"/>