The brightest star in the night sky, Sirius is recorded in the earliest astronomical records. Its displacement from the ecliptic causes this heliacal rising to be remarkably regular compared to other stars, with a period of almost exactly 365.25 days holding it constant relative to the solar year. This occurs at Cairo on 19 July (Julian), placing it just prior to the summer solstice and the onset of the annual flooding of the Nile during antiquity.
Owing to the flood's own irregularity, the extreme precision of the star's return made it important to the ancient Egyptians, who worshipped it as the goddess Sopdet (Ancient Egyptian: Spdt, "Triangle";[a] Greek: Σῶθις, Sō̂this), guarantor of the fertility of their land.[b] The Egyptian civil calendar was apparently initiated to have its New Year “Mesori” coincide with the appearance of Sirius, although its lack of leap years meant that this congruence only held for four years until its date began to wander backwards through the months. The Egyptians continued to note the times of Sirius's annual return, which may have led them to the discovery of the 1460-year Sothic cycle and influenced the development of the Julian and Alexandrian calendars.
The ancient Greeks observed that the appearance of Sirius heralded the hot and dry summer and feared that it caused plants to wilt, men to weaken, and women to become aroused. Due to its brightness, Sirius would have been noted to twinkle more in the unsettled weather conditions of early summer. To Greek observers, this signified certain emanations which caused its malignant influence. Anyone suffering its effects was said to be "star-struck" (ἀστροβόλητος, astrobólētos). It was described as "burning" or "flaming" in literature. The season following the star's reappearance came to be known as the dog days. 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 century BC feature dogs or stars with emanating rays, highlighting Sirius' importance. The Romans celebrated the heliacal setting of Sirius around April 25, sacrificing a dog, along with incense, wine, and a sheep, to the goddess Robigo so that the star's emanations would not cause wheat rust on wheat crops that year.
Ptolemy of Alexandria mapped the stars in Books VII and VIII of his Almagest, in which he used Sirius as the location for the globe's central meridian. He depicted it as one of six red-coloured stars (see the Colour controversy section below). The other five are class M and K stars, such as Arcturus and Betelgeuse.
Bright stars were important to the ancient Polynesians for navigation between the many islands and atolls of the Pacific Ocean. Low on the horizon, they acted as stellar compasses. They also served as latitude markers; the declination of Sirius matches the latitude of the archipelago of Fiji at 17°S and thus passes directly over the islands each night. 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. Just as the appearance of Sirius in the morning sky marked summer in Greece, it marked the onset of winter for the 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. The Hawaiian people had many names for Sirius, including Aa ("glowing"), Hoku-kauopae, Kau-ano-meha (also Kaulanomeha), "Standing-alone-and-sacred", Hiki-kauelia or Hiki-kauilia (the navigational name), Hiki-kau-lono-meha ("star of solitary Lono", the astrological name), Kaulua (also Kaulua-ihai-mohai, "flower of the heavens"), Hiki-kauelia, Hoku-hoo-kele-waa ("star which causes the canoe to sail", a marine navigation name), and Kaulua-lena ("yellow star"). The people of the Society Islands called Sirius variously Taurua-fau-papa, Taurua-nui-te-amo-aha, and Taurua-e-hiti-i-tara-te-feiai. Other names for Sirius included Palolo-mua (Futuna), Mere (Mangaia), Apura (Manihiki), Taku-ua (Marquesas Islands), and Tokiva (Pukapuka). In the cosmology of the Tuamotus, Sirius had various names, including Takurua-te-upuupu, Te Kaha ("coconut fibre"), Te Upuupu, Taranga, and Vero-ma-torutoru ("flaming and diminishing").
The indigenous Boorong people of northwestern Victoria named Sirius as Warepil.
In 1718, Edmond Halley discovered the proper motion of the hitherto presumed "fixed" stars after comparing contemporary astrometric measurements with those given in Ptolemy's Almagest. The bright stars Aldebaran, Arcturus and Sirius were noted to have moved significantly; Sirius had progressed 30 arc minutes (about the diameter of the Moon) southwards in 1,800 years.
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 spectrum of the star and observed a red shift. He concluded that Sirius was receding from the Solar System at about 40 km/s. Compared to the modern value of −5.5 km/s, this was an overestimate and had the wrong sign; the minus means it is approaching the Sun. It's possible that Huggins did not account for the Earth's orbital velocity, which would cause an error of up to 30 km/s.
In his 1698 book, Cosmotheoros, Christiaan Huygens estimated the distance to Sirius at 27664 times the distance of the Earth to the Sun (about 0.437 light years, translating to a parallax of roughly 7.5 arcseconds). There were several unsuccessful attempts to measure the parallax of Sirius: by Jacques Cassini (6 seconds); by some astronomers (including Nevil Maskelyne) using Lacaille's observations made at the Cape of Good Hope (4 seconds); by 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 Bessel (no sensible parallax).
Scottish astronomer Thomas Henderson was the first to gain some meaningful value using his observations made in 1832–1833 and South African astronomer Thomas Maclear's observations made in 1836–1837, and was published in 1839. The value of the parallax was 0.23 arcseconds, and error of the parallax was estimated not to exceed a quarter of a second, or in Henderson's words, "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." Astronomers adopted a value of 0.25 arcseconds for much of the 19th century. It is now known to have a parallax of 0.3792 ± 0.0016 arcseconds and therefore a distance of 1/0.3792 ≅ 2.637 parsecs, showing Henderson's measurement to be accurate.
Discovery of a companion
In 1844, the German astronomer Friedrich Bessel deduced from changes in the proper motion of Sirius that it had an unseen companion. On January 31, 1862, American telescope-maker and astronomer Alvan Graham Clark first observed the faint companion, which is now called Sirius B, or affectionately "the Pup". This happened during testing of an 18.5-inch (470 mm) aperture great refractor telescope for Dearborn Observatory, which was the largest refracting telescope lens in existence at the time, and the largest telescope in the United States. Sirius B's sighting was confirmed on March 8 with smaller telescopes.
The visible star is now sometimes known as Sirius 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 A and a mass of 0.06 M☉. This star would be five to ten magnitudes fainter than the white dwarf Sirius B, which would make it difficult to observe. 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.
In 1915, Walter Sydney Adams, using a 60-inch (1.5 m) reflector at Mount Wilson Observatory, observed the spectrum of Sirius B and determined that it was a faint whitish star. This led astronomers to conclude that it was a white dwarf, the second to be discovered. The diameter of Sirius A was first measured by Robert Hanbury Brown and Richard Q. Twiss in 1959 at Jodrell Bank using their stellar intensity interferometer. In 2005, using the Hubble Space Telescope, astronomers determined that Sirius B has nearly the diameter of the Earth, 12,000 kilometres (7,500 mi), with a mass 102% of the Sun's.
Around AD 150, the Greek astronomer of the Roman period Claudius Ptolemy described Sirius as reddish, along with five other stars, Betelgeuse, Antares, Aldebaran, Arcturus and Pollux, all of which are of orange or red hue. The discrepancy was first noted by amateur astronomer Thomas Barker, squire of Lyndon Hall in Rutland, who prepared a paper and spoke at a meeting of the Royal Society in London in 1760. 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. Thomas Jefferson Jackson See resurrected discussion on red Sirius with the publication of several papers in 1892, and a final summary in 1926. He cited not only Ptolemy but also the poet Aratus, the orator Cicero, and general Germanicus as calling the star red, though acknowledging that none of the latter three authors were astronomers, the last two merely translating Aratus' poem Phaenomena. Seneca had described Sirius as being of a deeper red than Mars. Not all ancient observers saw Sirius as red. The 1st-century poet Marcus Manilius described it as "sea-blue", as did the 4th century Avienus. It was the standard white star in ancient China, and multiple records from the 2nd century BC up to the 7th century AD all describe Sirius as white.
In 1985, German astronomers Wolfhard Schlosser and Werner Bergmann published an account of an 8th-century Lombardic manuscript, which contains De cursu stellarum ratio by St. Gregory of Tours. The Latin text taught readers how to determine the times of nighttime prayers from positions of the stars, and Sirius is described within as rubeola — "reddish". The authors proposed this was further evidence Sirius B had been a red giant at the time. Other scholars replied that it was likely St. Gregory had been referring to Arcturus.
The possibility that stellar evolution of either Sirius A or Sirius B could be responsible for this discrepancy has been rejected by astronomers on the grounds that the timescale of thousands of years is too short and that there is no sign of the nebulosity in the system that would be expected had such a change taken place. An interaction with a third star, to date undiscovered, has also been proposed as a possibility for a red appearance. Alternative explanations are either that the description as red is a poetic metaphor for ill fortune, or that the dramatic scintillations of the star when rising left the viewer with the impression that it was red. To the naked eye, it often appears to be flashing with red, white and blue hues when near the horizon.