The Origin of "Dog Days"

There is a constellation (pattern of stars) in the sky which the ancients fancied looked like a dog. Actually, there are three such constellations: Canis Major (the Big Dog), Canis Minor (the Little Dog), and Canes Venatici (the Hunting Dogs). But here, we will be focusing on Canis Major, the Big Dog. Woof!

To see how the ancient peoples may have thought this group of stars looked like a dog, one can reference the image at right, which is a screenshot from Stellarium planetarium software and enhanced by me with my PaintShopPro™ software. One can see the head, body, legs, and tail of the dog, with the star Sirius shining brightly as the jewel on the collar of the creature. That gives Sirius its nickname, "The Dog Star."

The star Sirius (pronounced just like the English word "serious") is the brightest star in the night sky as seen from Earth. Technically, of course, our own Sun is also a star, so it is the brightest, but hey... of all the starts that are not our own Sun, Sirius is the brightest. 😁 Only the Sun, Moon, and four of the planets (Venus, Mars, Jupiter, & Mercury) are brighter. This gives Sirius some measure of importance.

The ancient Greek word for the Sun was Hλιος (Hēlios), and from that we get the word heliacal, which means: with, or near, the sun. When a celestial object can be briefly glimpsed right before the sun rises, it is said to be rising "with the sun" or in a heliacal manner. The ancient peoples noticed that the heliacal rising of Sirius happened about the same time as the hottest period of the year was beginning in the Northern Hemisphere. Thence, the name "Dog Days" after the "Dog Star," Sirius.

^{• SCREENSHOT FROM STELLARIUM, ENHANCED BY ME •}

There is some discrepancy as to exactly when Dog Days begins. Although the heliacal rising of Sirius is a discrete physical event that happens with mathematical accuracy, it occurs at different times from different latitudes on Earth. Obstructed horizons can prevent the star's heliacal rising from being directly observed. Heliacal risings of Sirius occur every 365¼ days, which harmonizes nicely with the Julian calendar but not the modern-day Gregorian calendar. Lastly, some people who don't understand the origin of the term "Dog Days" seem to want to fix the "official date" to certain calendar dates, regardless of when the heliacal rising of Sirius happens, and different people have a different opinion on when that "should" be.

But, now at least everyone who has read this knows the story behind the name, and can help spread the word. If nothing else, it is something interesting to discuss on a front porch with a glass of cold lemonade on a sultry summer evening.

Stellarium is a free open-source planetarium software for computers. Versions are available for Linux, Mac, and Windows computers. I have used it for years and love it. For more information, visit: stellarium.org