Why Are ‘Dog Days of Summer’ the Hottest?

The muggy “dog days of summer” might recall visions of listless canines lying around baking in the searing heat, but the moniker has very little to do with panting pooches. Most people believe the term simply refers to the hottest days of the year when dogs (and their parents) experience severe heat-induced sluggishness, but the origin is much more complex.

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The dog days have less to do with your best friend than with a star named Sirius—also known as the Dog Star. Named after the mythical dog belonging to the hunter Orion, Sirius is the brightest star shining in the night sky. Ancient Romans placed the star in the constellation Canis Major (or “Greater Dog” in Latin). For 40 days in summer, the Dog Star rises and sets at very much the same time as the sun.

At times when ancient civilizations tracked the seasons by looking up into the night sky, astronomy was simple math. The addition of the warmth of Sirius—ancient Greek for “glowing” or “scorcher”—to the daytime blaze of the sun equaled extreme heat. The ancient Greeks noticed that this period seemed to coincide with the summer’s hottest, most sweltering days, they went on to assume that Sirius was the one giving off additional heat in the daytime. The Romans then began to refer to the 40-day heat wave period as “dies caniculares” (literally “days of the dog star”) By the 1500s, the English-speaking world had gone on to shorten the phrase to “dog days.”

In reality, now we know that the earth’s tilt is the cause of extreme summertime heat. Our view of the stars also shifts gradually due to a wobble in the Earth’s rotation over time, and the dog days of summer have changed on the calendar too. The dates also change with latitude—farther north, Sirius rises later in the year than it does in the south.

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The effects of summer heat and rainfall patterns are constant, but they too vary with latitude and location depending upon many factors. London in the UK which is far up north than Calgary in Canada has a milder climate impact thanks to the presence of the sea and the warm Gulf Stream current.

Various computations of the dog days place their start anywhere between 3 July to 15 August and lasting for anywhere from 30 to 61 days. The star Sirius follows a period of almost exactly 365¼ days in-between risings, that is largely consistent with the Julian but not the Gregorian calendar.

Nowadays in the United States, the dog days of summer last from July 3 to August 1. The dates of dog days tend to get somewhat delayed in the year over a span of millennia. In approximately 10,000 years, the dates of the heliacal rising of Sirius will have fallen back so late on the calendar that future civilizations in the northern hemisphere will most likely experience the “dog days” of winter.

Related Post: What your pet might be trying to tell you

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