Roman writers and calendar-keepers associated the days following early July with the heliacal rising of Sirius, the 'Dog Star,' a period the Romans called dies caniculares. They believed the star's proximity to the sun compounded summer's heat and brought disease, lethargy, and even canine rabies, a superstition inherited from Egyptian and Greek astronomy and passed down into the Julian calendar's civic and agricultural rhythms.
Long before modern meteorology, the Mediterranean world tracked the seasons by the stars, and none loomed larger in summer lore than Sirius, the brightest star in the night sky, part of the constellation Canis Major. Egyptian astronomers had first linked its heliacal rising — its reappearance at dawn after weeks of invisibility — to the annual flooding of the Nile, treating it as a sacred calendrical anchor for the entire civilization.
Roman writers, inheriting this astronomical tradition through Greek intermediaries, gave the star's early-July reappearance a more ominous cast. They called the ensuing weeks the dies caniculares, the 'dog days,' and associated them with oppressive heat, crop stress, fevers, and a supposed rise in madness among both men and dogs. Farmers' almanacs and poets alike, including Virgil and later Pliny the Elder, referenced the phenomenon as a real seasonal hazard requiring precaution.
The belief proved remarkably durable, surviving the fall of Rome and persisting in European folk almanacs well into the early modern period, long after astronomers understood that Sirius's rising had no actual causal link to summer heat. The phrase 'dog days of summer' remains in everyday English, an ancient superstition fossilized into idiom.