This Is Where the Phrase “Dog Days of Summer” Came From 

When I hear the phrase “the dog days of summer,” I see dogs lazing about in any shade they can find, tongues lolling. The phrase remains a favorite in our lexicon—in part because dogs make for such vivid mental imagery. But its origin has little to do with actual canines; it’s more about celestial ones. 

How did dogs and hot summer days get all mixed up?

Sirius, the brightest star we can see from Earth, is a part of the Canis Major constellation, or the Greater Dog. Historically, from mid-July through August in the Northern Hemisphere, Sirius rose alongside the sun. The Romans and the Greeks referred to this time as “dies caniculares” and kynádes hēmérai, respectively. Those phrases translate to “days of the dog star” and “dog days.” 

The Greek poet Hesiod, writing in the 8th century BCE, was among the first to document the phenomenon. In his Works and Days, he wrote, “When the piercing power and sultry heat of the sun abate, and almighty Zeus sends the autumn rains, and men’s flesh comes to feel far easier—for then the star Sirius passes over the heads of men, who are born to misery, only a little while by day and takes greater share of night.” 

Or more plainly: When Sirius’s visibility shifted from day to night, Hesiod counseled that the worst heat was over and more comfortable weather was on its way. 

The Romans shared in the Greeks’ astronomical observation and used it to guide their agricultural efforts, planning harvests around this period. The Roman calendar officially recognized the dog days as spanning from July 24 to August 24, though these dates varied through regions and eras.

Did this have anything to do with earthbound dogs?

It was Pliny the Elder, writing from Rome, who brought the phrase back to actual dogs. In Natural History, he wrote: “Who is there that does not know that the vapor of the sun is kindled by the Dog-Star?” and warned that dogs are “disposed to become rabid” during this period (this is about as scientific an observation as the kindling of sun vapors, no disrespect to Mr. the Elder).

What role did the Ancient Greeks and Romans believe Sirius played in determining the weather?

As Pliny suggests, the Romans and Greeks believed that during this time of year, Sirius had an active role in amplifying the heat along with the sun, since they both rose together. Eventually, the literal connection between Sirius and summer heat was debunked. Sirius’s proximity in the sky to the sun during summer is purely coincidental and has no effect on the temperature down here. The real cause of summer’s heat lies in the Earth’s axial tilt and the angle of solar radiation. Sirius’s rise has even shifted over millennia; The Old Farmer’s Almanac clocked Sirius’s heliacal rise and fall from July 3 to August 11, over 20 days earlier than it was in ancient Greek and Roman times.

These days, the phrase “Dog Days of Summer” tends to refer to any time in mid- to late summer, when the days are long and the heat drags on. We like to think of it as the time to brush up on the best indoor games to play with dogs, how to protect their paws from hot pavement, and the best ways to avoid heat stroke. That is, all the things you can do to keep your dog safe during those dog days.