Everyone knows that the “dog days of summer” occur during the hottest and muggiest part of the season. Webster defines “dog days” as the period between early July and early September when the hot, sultry weather of summer usually occurs in the northern hemisphere. But have you ever wondered where the term originated? Why do we call the hot and muggy days of summer “dog days”?
Well, according to Jerry Wilson, in ancient times, when the night sky wasn’t obscured by smog and artificial lights, people drew images in the sky by “connecting the dots” of stars. The images were dependent upon the culture: The Chinese saw different images than the Native Americans, who saw different images than Europeans. We know these star pictures, mapped by our European ancestors, as constellations.
Most of us are familiar with the constellations Ursa Major and Ursa Minor, the bears; Gemini, the twins; Taurus, the bull: and, among others, Canis Major and Canis Minor, the dogs. The brightest of the stars in Canis Major (the big dog) is Sirius, which is also the brightest star in the night sky. In fact, it is so bright that the ancient Romans thought that the earth received heat from it. Look for it in the southern sky (viewed from northern latitudes) during January.
In the summer, however, Sirius, the “dog star,” rises and sets with the sun. During late July Sirius is in conjunction with the sun, and the ancients believed that its heat added to the sun’s heat, creating a stretch of hot and sultry weather. They named this period, from 20 days before the conjunction to 20 days after, “dog days” after the dog star.
The conjunction of Sirius with the sun varies somewhat with latitude. And the “precession of the equinoxes” (a gradual drifting of the constellations over time) means that the constellations today are not in exactly the same place in the sky as they were in ancient Rome. Today, the “dog days” occur between July 3 and August 11. And although it’s certainly the warmest period of the summer, the heat isn’t caused by added radiation from a far-away star as the ancients believed, but by the earth’s tilt.
So, for those of us in the northern hemisphere, today marks the end of the “dog days” of summer. Thanks, Michael, for reminding me. I feel cooler already!