Yes, the phrase is linked to the brightest star in the night sky during summertime in the Northern Hemisphere: Sirius, the Dog Star. It's called the Dog Star because it's the brightest star in the constellation Canis Major, which is Latin for 'Large Dog'.
It also happens to be the brightest star visible from Earth (if you discount our own sun, of course).
|Sirius the Dog Star, to the left of the constellation Orion|
photo courtesy nevets183, flickr Creative Commons 3.0
The Romans are usually credited with being the originators of the phrase “Dog Days” but the phrase was actually used by the Greeks prior to the time of Rome, and can be found in one of Aristotle's many works – Aristotle's Physics.
It was believed that Sirius added to the heat coming from our own sun and was responsible for the brutally hot days in July and August.
But if you want to find the first time the star and the dog are linked together, you have to travel an even greater distance back in time: try Egypt, circa 3,000 B.C.E.!
Egyptian hieroglyphs are the first known association of the star as a dog, though its reason has been lost in antiquity.
The one thing we do know is that, back in 3,000 BCE, the rising of Sirius in the morning sky occurred at the same time as sunrise. It also coincided with Egypt's summer solstice, which heralded the beginning of Egypt's new year.
Sirius slowly lost its "synch" with sunrise over the centuries due to a stellar phenomenon known as the precession of the equinoxes.
|Fun in the sun! Photo by peyri, Creative Commons|
(Does anyone think that might have something to do with "evil" tempers brought on by sweltering temperatures? Especially considering there wasn't a nice, friendly A/C unit in sight back then?)
A book written in 1813 entitled Clavis Calendaria references the Dog Days as a time when:
“the Sea boiled, the Wine turned sour, Dogs grew mad, and all other creatures became languid; causing to man, among other diseases, burning fevers, hysterics, and phrensies.”
(oooh, we like the word 'phrensies'! That's Allie's new word to describe her state of mind every time Faraday pounces on her.)
|A great way to cool off: a run by the sea|
photo by arbyreed, Creative Commons
The Roman calendar had the Dog Days listed as running from July 24 through August 24.
Many modern European cultures still have the month-long period listed on their calendars in the same way.
Dog Days even made it into the Old Farmer's Almanac.
It lists the Dog Days from July 3rd to August 11th - a period of 40 days which coincides both with the ancient heliacal (or sunrise) rising of the star Sirius, and the days with the least rainfall in the Northern Hemisphere.
The Weather Channel