Science of the Dog Days of Summer


Where do we get the term “the dog days of summer?”

Does it refer to the hottest days of Summer, when even dogs go on strike due to the heat, refusing to do their job of lying on the floor?


Look to the Skies

Alas, no. The answer is “in our stars” or, rather, in a constellation. Astronomically, the time beginning July 3rd and running through August 11th is associated with the rising – above the eastern horizon around dawn – of the star system Sirius, commonly called the “Dog Star.” I’m serious when I say the Greeks connected this occurrence with heat, lethargy, thunderstorms, mad dogs, and generally bad luck. Its association with fever could only be remedied with more cowbell.


What’s In A Name?

Sirius, Σείριος in Greek, transliterated to Latin Seirios, means “glowing” or “scorching.” We borrow “the puppy days” from the Latin dies caniculares into English.

In astronomy, the star is Alpha Canis Majoris, part of the constellation Canis Major, the “Greater Dog.”


canis major constellation map.svg

Sirius in Canis Major


Ancient astronomers noted it as the brightest star in the sky due to its luminosity and proximity to Earth: it’s a mere 2.64 parsecs away. That’s only 8.6 light-years away; we’re practically neighbors.


angular map of fusors around sol within 9ly (large)

Our neighbor, Sirius


It’s twice the size of our local star, the Sun, and 25 times more luminous.

  • The ancient Egyptians recognized this star rise as the annual flooding of the Nile.
  • The Greeks saw it as the morning star, heralding the wilting of plants and men to weaken. This is where we get the term “star-struck.”
  • The Romans would sacrifice a dog along with a sheep, wine, and incense, believing it would prevent wheat rust.


Due to the change in the constellations’ appearance in the sky over time, caused by the Earth’s wobbly rotation, Sirius used to rise closer to the Summer Solstice. Now, it occurs in the July/August timeframe.


sirius orion


Sirius in Homer‘s Iliad

Homer mentions Sirius as a metaphor for the bright light of Achilles‘ approach to Troy during the Trojan War:

Sirius rises late in the dark, liquid sky
On summer nights, star of stars,
Orion’s Dog they call it, brightest
Of all, but an evil portent, bringing heat
And fevers to suffering humanity.


The Patron Saint of Dogs

st rochus

St. Roch, Bílá Hora, Prague (1751)

Saint Roch, who lived in early 14th century Majorca (modern Spain), is the patron saint of dogs. Legend has it that when he fell ill with the plague, he was saved by a dog, who supplied him with bread and licked his wounds. He’s often depicted accompanied by a dog.

He is commemorated on August 16, possibly as a connection to the “dog days of summer.”



Bill Petro, your friendly neighborhood historian


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About billpetro

Bill Petro writes articles on history, technology, pop culture, and travel. He has been a technology sales enablement executive with extensive experience in Cloud Computing, Automation, Data Center, Information Storage, Big Data/Analytics, Mobile, and Social technologies.

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