Dog Days of Summer

By Dennis Mammana

August 31, 2017 4 min read

Week of Sept. 3-9, 2017

As much of the Earth's Northern Hemisphere swelters under the oppressive heat of late summer, pay close attention and I'll bet that before the season is out, you'll hear someone refer to this time of year as the "dog days of summer."

Now, one might guess that this term comes from the seemingly lethargic behavior of our canine companions in the late-summer heat, but one would be barking up the wrong tree. The term's origin — like that of many everyday phrases — lies among the stars.

The ancients, particularly those of the Mediterranean, knew late summer not only for its scorching heat but also for the disease and discomfort that accompanied it. It was a time when meat spoiled rapidly and food poisoning could take a serious toll on the population. Even rabies seemed to be more common, at least in folklore.

Early sky watchers kept watch on the heavens in their attempts to correlate celestial and terrestrial activity. They noticed that during this brutally hot season, the star Sirius rose around the same time as the sun (its "heliacal rising," as we call it today), and the two moved across the daytime sky together.

Sirius, the most luminous star in the night sky, is well-known as the Dog Star because it marks the constellation Canis Major, the Great Dog. Many in olden times believed that it was the heat of brilliant Sirius coupled with that of the sun that produced the scorching summertime temperatures — the "dies caniculares," or "days of the dog star," as the Romans called them. Over time, the link between the late summer heat, Sirius and the phrase became ingrained in culture.

Granted, Sirius is much hotter than our sun, but at a distance of 53 trillion miles, the heat it provides us is negligible. However, ideas such as this die hard; in fact, I wouldn't be surprised if some people still believe its connection today, despite it being debunked more than 20 centuries ago by the Greek astronomer Geminus.

"It is generally believed that Sirius produces the heat of the Dog Days," he wrote, "but this is an error, for the star merely marks a season of the year when the Sun's heat is the greatest."

Nowadays, if you wish to watch the heliacal rising of Sirius, you've got to wait until August. This is because the Earth wobbles on its axis — an effect called precession — and over several millennia, the positions of celestial objects shift slightly. But if stars were to shine in broad daylight, we would see those of winter — including brilliant Sirius — shining overhead at noon.

Of course, if you'd like to see Sirius in a dark sky, you'll have to wait a few months until the sun no longer appears along roughly the same line of sight; that occurs during the winter months, and when it does, Sirius appears as a sparkling diamond rising in the east at sunset.

With or without Sirius in the sky, summertime is hot. Enjoy the warmth while it lasts because, believe me, it won't be long before we're all whining about the cold!

 The heliacal rise of Sirius.
The heliacal rise of Sirius.

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