Why Do We Say the “Dog Days of Summer”?

Dogs are man's best friend and adorable, but what do they have to do with summer?

Dog days of summerPhoto: Shutterstock

This year, while your summer plans may change due to COVID-19, the weather will still be hot. But what are the dog days of summer, and what do those days have to do with dogs?

“Sirius”-ly surprising origins

The phrase “dog days of summer” actually has nothing to do with man’s best friend. It turns out that the ancient Greeks and Romans weren’t referring to real dogs, but to the great astronomical one, Sirius. The “Dog Star” is located in the constellation Canis Major, known as the Greater Dog, and is the brightest star that shines at night.

Sirius is also known as “Nile Star” or “Star of Isis” because ancient Egyptians noticed 4,000 years ago that around the summer solstice, Sirius would appear to rise before dawn, which would be around the time when the Nile River would flood. Since ancient Egyptian life depended on agriculture, the flowing river would either bring great prosperity or great destruction. (Check out these fascinating facts about the summer solstice.)

Why “dog days of summer”?

What does the star Sirius have to do with the “dog days of summer”? According to the Farmer’s Alamanac, during summer months and specifically on July 23rd, Sirius is extraordinarily bright and, in certain parts of the world, rises and sets with the sun at that time. Ancient Romans believed this shining star contributed to the sun’s heat, thus referring to this extremely hot period as diēs caniculārēs, or “dog days.” The term evolved to mean the 20 days before and after July 23rd, or July 3 to August 11, to coincide with Sirius aligning with the sun. However, depending on where you are in the world, the astronomical dog days and the rising of Sirius vary.

You could also find references to the “dog days of summer” in ancient texts. “If you go back even as far as Homer, The Iliad, it’s referring to Sirius as Orion’s dog rising, and it describes the star as being associated with war and disaster,” Jay B. Holberg, author of Sirius: Brightest Diamond in the Night Sky and senior research scientist at the University of Arizona Lunar & Planetary Laboratory tells National Geographic. “All throughout Greek and Roman literature, you found these things.” The phrase “dog days” has since taken on various meanings after its translation from Latin to English. (These are the Latin words you use every day without knowing it!)

What does science have to do with it?

Even though Sirius is extremely bright, the extra heat is not because of any additional radiation from this star. The earth rotates around the sun and the Earth’s tilted axis is why we experience seasons. The sun is hotter in certain hemispheres because the sun’s rays hit the Earth in a more direct way due to Earth’s tilt. For example, if the North Pole is tilted toward the sun, that translates to hotter days in the Northern Hemisphere. Similarly, if the South Pole is tilted toward the sun, that means it’s winter in the Northern Hemisphere. However, stars shift, the earth wobbles, and the “dog days” of ancient Greece aren’t the same “dog days” of now, and the “dog days” of now won’t be the same as thousands of years from now.

How do you see Sirius?

To find Sirius in the night sky on winter evenings, locate the Orion constellation and then find Orion’s belt. Once you’ve located the three stars that make up Orion’s belt, move in a line down to the left until you find Sirius.

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Originally Published on Reader's Digest