When you think about classic summer weather, what says it better than the “Dog Days of Summer?”

This phrase likely conjures up the ideas, memories, feeling of those long, steamy days, blending into the sticky, sultry nights. July witnesses the culmination of the hot, humid weather, as temperatures reach their annual maximum during the third week of the month.

Admittedly, many of us visualize an actual dog, finding some cool, shady shelter through the heat of the afternoon. But Dog Days find their origin in the skies, specifically the night sky, hosting the stars and planets. Here, among the illuminated orbs shines the brightest of all of them, the star Sirius, affectionately knick-named “the Dog Star.” Don’t make plans to look for it on the next clear night. Sirius takes center stage during the winter nights, in the company of the great Orion. So how does this star figure into our lazy, hazy crazy days of July?

STJ_AnnualMeanT.png
Orion_to_Sirius.png

(Diagram showing Orion in Sirius in the winter sky)

The Greeks, constantly searching for explanations for the workings of the natural world, knew that the hottest days of summer occurred well after the sun had reached its annual peak on the Summer Solstice. This period of time from mid-July through mid-August coincided with the period of time when Sirius joined the Sun in the skies. The Greeks reasoned that since Sirius was the brightest star, its appearance added to the heat of the Sun, creating the hottest conditions of the summer. In fact, the Greeks named this star, meaning “glowing” or “scorching,” which could be a reference to the brilliance of the star, or possibly its connection to summer’s heat and humidity.

There are a number of references from Greek writers, including Homer, who mentions in his work, The Illiad:

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 Greek poet Hesiod also made note of Sirius:

When Sirius parches head and knees,

And the body is dried up by reason of the heat,

Then sit in the shade and drink.

bust-of-homer.jpg

Marble bust of Homer. 

The specific period of time called the Dog Days varied from different sources from both the Greeks and the Romans, often beginning or ending with something astronomers call its heliacal (sun-related) rising. Each summer, the Earth’s orbit allows Sirius to return to the night skies, barely visible as the dawn’s twilight brightened just before sunrise. Today, the common definition starts on July 3, and continues to Aug. 11, starting when Sirius makes its closest approach to the sun and ending when it becomes visible in the brightening twilight of dawn. This heliacal rising originates much earlier, extending back to the ancient Egyptians, who observed Sirius for different reasons.

The Egyptian interest in the heliacal rising of Sirius focused on marking the time when the Nile River flooded, which happened very close to the time that the star Sirius reappeared in the skies. The Egyptians called this star Sopdet, which also represented their goddess of the same name, often associated with their great goddess Isis. The annual flooding of the Nile, the result of rains through its tropical headwaters in Ethiopia, brought water and nutrients to the Nile Valley, creating a strip of fertile land that supported a tremendously productive agricultural region. Without the Nile, and its annual flooding, the Egyptian civilization would have never developed. The connection to Sirius can’t be underestimated since the star signaled this life-giving event. It was so significant that formed the basis for their calendar year, accurate enough that they knew the length of the year to be 365 and one-quarter days.

While the term Dog Days persists, as well as its imagery and associations, we know the star adds nothing to the heat of summer. The difference in time from the longest days of the year near the Summer Solstice, and the warmest days near the end of July comes from the phenomenon called seasonal lag. The heating of our atmosphere actually comes from the Sun heating the surface of the Earth. That surface, both land, and ocean, doesn’t heat up instantly, but instead continues to warm well past the Summer Solstice. The ocean, covering over 70 percent of the Earth’s surface, doesn’t reach its greatest warmth until September in the northern hemisphere, but the decreasing daylight comes into play by the time we reach August. The balance point comes during the last week of July when our local temperatures average a morning temperature near 55 degrees, and the afternoon’s top out at 80 degrees.

With our long winters, especially so this past winter, most of us will welcome these Dog Days, knowing that they bring wonderful days to enjoy the lake, ripen the corn and tomatoes, and melt those frozen memories.