Where the phrase 'dog days of summer' comes from: a CofC professor explains
CHARLESTON, S.C. (WCIV) - It's hot outside. The heat and humidity that become the norm for South Carolina lead to heat index readings of over 100 almost every afternoon. These are the days often referred to as "the dog days of summer."
The common phrase that conjures up images of dogs lazily dozing in the afternoon shade doesn't have much to do with dogs at all. In fact, its origins go way back to ancient Greece and Rome.
"It has to deal with the star Sirius and the constellation Canis Major which Sirius is a part of," said Dr. Jon Hakkila, a physics and astronomy professor at the College of Charleston. "From July 3 to Aug. 11, the star Sirius, the brightest star in the sky, the dog star, the brightest star in the constellation Canis Major, the greater dog, moved into the direction of the sun."
Normally the constellation is only visible in the winter months, but Greeks and Romans knew the constellation and star moved toward the sun in the late summer and called those days the "dog days."
"So, both the sun and Sirius were up during the same time, during that time period [in late summer]," said Hakkila. "And the Greeks and Romans believed that made things hotter than at any other time of the year."
Since that time period, science has progressed to where we better understand what makes summer hot (the sun's angle is a big part of it). What's also progressed is Canis Major's march across the sky.
Due to the moon's gravitational pull on the Earth, the Earth's tilt on its axis wobbles, said Hakkila. That means that what we see in the summer now will gradually shift, over the time period of about 13,000 years, and will become what we see in the winter.
This slow wobble means that Canis Major isn't centered on the sun in the late summer like it used to be. What hasn't changed is the use of the term "the dog days of summer" to describe hot, lazy summer afternoons.