Canis Major and Sirius: the star of the dog days and the Nile
Sirius, brightest star of the night sky: the Greek dog days, the Nile flood heralded by Sothis and Laelaps, the hound that never failed to catch its prey.
No star in the night sky shines as brightly as Sirius. White and sparkling, low on the horizon on winter evenings, it seems to change colour every moment because of the atmosphere. It lies in the constellation of Canis Major, the faithful companion of Orion, and for many ancient peoples it was a fundamental reference point in the calendar. Its Greek name, Seirios, probably means “scorching” or “glowing”, and its fame is tied as much to heat as to light.
Orion's dog
In the Iliad, Homer compares Achilles running towards Troy to the brightest star, the one “that men call Orion's dog”. The poet adds that it is an evil sign, because it brings fever to mortals. Hesiod too, in Works and Days, recalls Sirius as the star that parches the head and knees in the hottest days of summer.
This association arises from a precise observation: in Greece the heliacal rising of Sirius, that is, its first appearance at dawn after a period of invisibility, took place in high summer. It was believed that the star added its own heat to that of the Sun. The Romans called that period dies caniculares, “the days of the little dog”, from which comes the Italian word “canicola” and the English “dog days”, still used for the most intense summer heat.
Sothis and the flooding of the Nile
In Egypt Sirius was worshipped as the goddess Sopdet, whom the Greeks called Sothis. In antiquity its heliacal rising coincided with the start of the annual flooding of the Nile, on which the fertility of the fields depended. For this reason its reappearance at dawn had a deep religious and calendrical meaning, and some texts treat it as the beginning of the new year.
Since the Egyptian civil year lasted exactly 365 days, with no leap years, the rising of Sirius slipped slowly by one day every four years, returning to the same date after about 1,460 years. This period is called the Sothic cycle. The Latin writer Censorinus, in De die natali, records that one of these cycles began in AD 139, and historians have used this information to reconstruct Egyptian chronology.
Laelaps and the Teumessian fox
Among the Greek myths about Canis Major, one of the most singular concerns Laelaps, a dog destined to catch any prey. The same destiny, however, belonged to the fox of Teumessus, in Boeotia, which could never be caught. When Laelaps was set upon the fox, an endless paradox arose. Zeus solved the problem by turning both animals to stone and, according to some versions, placing the dog among the stars.
In modern times Sirius has revealed another secret. In 1844 the astronomer Friedrich Bessel deduced from the wobbles in its motion the presence of an invisible companion, which was first observed in 1862 by Alvan Graham Clark. Sirius B is a white dwarf, a very small and extremely dense star, among the first of its kind to be identified.
Canis Major and its chief star thus unite the image of the faithful dog following the hunter and that of the scorching star marking the rhythm of the seasons. From the flooding of the Nile to the summer dog days, Sirius has measured for millennia the time of fields and rites, earning a unique place in the history of sky-watching.