Behind the Forecast: Dog days of Summer

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Published: Jun. 14, 2019 at 7:51 AM EDT
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LOUISVILLE, Ky. (WAVE) - The phrase “the dog days of summer” is less about meteorology and cynology, the study of dogs, and more about astronomy.

Most associate the "dog days of summer" with the hottest part of the year.

Ancient Greeks realized that the hottest days occurred when the star Sirius rose and set with the sun, according to This 40-day stretch typically occurred in the early summer. The Ancient Greeks thought that Sirius warmth combined with the sun's to bring about the scorching conditions. Sirius was Orion the hunter's dog according to Greek mythology.

The Ancient Romans also noticed that the summer’s hottest days were during this same time period and called it the “dies caniculares” which translates to “days of the dog star.” Sirius was placed by the Ancient Romans in the constellation Canis Major which is Latin for “Greater Dog”.

Louisville’s hottest months are typically July and August when high temperatures average near 88°. The city’s all-time high 107° occurred on July 24, 1901, July 28, 1930 and on July 14, 1936.


These dates fall squarely in inside the July 3 to August 11 stretch defined as the official dog days of summer.

While many thought the heat ushered in a descent into madness, droughts and plagues, one ancient astronomer had a different idea. The Greek astronomer Geminus wrote: “It is generally believed that Sirius produces the heat of the ‘dog days,’ but this is an error, for the star merely marks a season of the year when the sun’s heat is the greatest.”

The stars in our sky shift independently of the calendar so that means the hottest days don’t always coincide with the rise and set of Sirius. Also the date of Sirius’s appearance changes with latitude so that means the astronomical dog days change depending on your location.

Adler Planetarium astronomer Larry Ciupik told National Geographic that the “calendar is fixed according to certain events, but the stars have shifted according to the way that the Earth wobbles so in about 50-some years, the sky shifts about one degree.” Because of this the dog days that the Ancient Greeks and Romans experienced aren’t the same as today!

Bradley Schaefer, Louisiana State University professor of physics and astronomy explained it to National Geographic this way: “In 26,000 years, the dog days would completely move all around the sky. Roughly 13,000 years from now, Sirius will be rising with the sun in mid-winter."

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