Dog Days of Summer: How Constellations and Ancient Rome Gave Us One of the English Language’s Most Enduring Idioms

I want to start with something that genuinely surprised me when I looked it up: the phrase “dog days of summer” has absolutely nothing to do with dogs lying around panting in the heat. That is what most people assume, including me, and it is wrong. The phrase comes from a star. Specifically, from the brightest star in the night sky and the ancient civilizations that connected its seasonal appearance to a period of intense heat, bad luck, drought, disease, and general misfortune.

The star is Sirius. The star’s name, in ancient Greek, means “scorching.” The constellation it anchors is Canis Major, Latin for “big dog.” The Romans called the hottest weeks of summer dies caniculares, “days of the dog star.” By the 1500s, English speakers had shortened this to “dog days.” The phrase is still in daily use today, 2,000 years after the Romans coined it and 5,000 years after the Egyptians first tracked the star.

Sirius and Why Ancient People Cared So Much About It

Sirius is the brightest star in the night sky, almost twice as bright as the second-brightest star. It is part of the constellation Canis Major and sits about 8.6 light years from Earth. For most of the year in the Northern Hemisphere, it is visible at night. But around midsummer, it rises and sets with the sun, which means it disappears from the night sky because the sun’s light drowns it out.

Then, in late July, something changes. Sirius begins to appear again on the eastern horizon just before dawn. This is called the heliacal rising, the moment when a star becomes visible again after a period of invisibility during which the sun’s light obscures it. For ancient observers, the heliacal rising of Sirius was a significant calendrical event, a reliable marker that showed up at the same time each year.

The ancient Egyptians tracked it most carefully. For them, the heliacal rising of Sirius, which they called Sothis, coincided with the annual flooding of the Nile. The flood was not a disaster in ancient Egypt. It was the agricultural event that made the whole civilization possible, depositing rich silt across the flood plain and making the land fertile. Sirius rising before dawn meant the flood was coming. The appearance of Sirius was, for Egyptian farmers, the single most important astronomical event of the year.

The Greeks noticed a different correlation. For them, the heliacal rising of Sirius coincided not with the Nile flood but with the hottest, most oppressive period of summer. Homer wrote about it in the Iliad around the 8th century BCE, describing Sirius as “brightest of all but an evil portent, bringing heat and fevers to suffering humanity.” Hesiod, writing around the same period, mentioned the star rising in midsummer as a marker of brutal heat.

Dies Caniculares and What the Romans Made of It

The Romans took the Greek astronomical tradition and added Roman thoroughness. They identified the period of intense heat that followed the heliacal rising of Sirius as dies caniculares, the dog days. They were specific about what this period was like. Brady’s Clavis Calendaria, citing classical sources, summarized Roman belief about the dog days this way: “the Sea boiled, the Wine turned sour, Dogs grew mad, and all other creatures became languid; causing to man, among other diseases, burning fevers, hysterics, and phrensies.”

The dog angle is interesting. The Romans associated Sirius’s appearance with dogs behaving strangely, partly because of a genuine observation: dogs pant excessively in heat, foam at the mouth, and in extreme cases develop rabies, which is more likely to present dangerously in warm weather when people spend more time outdoors and encounter animals more frequently. The connection between summer heat and mad dogs was observationally real, even if the causal mechanism the Romans attributed to it, the influence of a star, was not.

The Romans also connected Sirius to Orion’s hunting dogs in their mythology. Orion, the hunter, had two dogs: Canis Major, the big dog, and Canis Minor, the small dog. Sirius, the Dog Star, anchored the big dog. This mythological frame meant the star was already associated with hunting, tracking, and predatory intensity before it accumulated its hotter and more sinister associations.