The “Dog Days” of summer are from July 3 to August 11 each year. They’re usually the hottest and most unbearable days of the season. But wait, what does it have to do with canines? Is it because the heat makes the day “not fit for a dog.” Is it because the heat drives dogs mad. But where does the term come from? And what does it have to do with dogs?
The phrase is a reference to Sirius, the Dog Star. During the “Dog Days” period, the Sun occupies the same region of the sky as Sirius, the brightest star visible from any part of Earth. Sirius is a part of the constellation Canis Major, the Greater Dog.
In the summer, Sirius rises and sets with the Sun. On July 23rd, specifically, it is in conjunction with the Sun, and because the star is so bright, the ancient Romans believed it actually gave off heat and added to the Sun’s warmth, accounting for the long stretch of sultry weather. They referred to this time as diēs caniculārēs, or “dog days.”
That means, meteorologically speaking, summer starts June 1 and ends August 31. So, the hottest days of summer – the “dog days” – run from July 3 until August 11 and the phrase is a reference to Sirius, the “dog star,” the brightest star in the night sky. Sirius is the fifth-nearest star, 8.7 light-years from Earth. It seems many civilizations noticed this blazing orb in the sky and attributed powers to it.
Ancient Egyptians called Sirius ‘Sothis’ and noticed that around the time of the summer solstice it would rise at dawn, just as the sun was rising. The presence of these two bright stars took on cultural significance, as the Nile River typically flooded at this time and Egyptians greatly depended on agriculture around the Nile.
Later, the Greeks called the star Seirios (Sirius) or Σείριος, meaning “glowing” or “scorching” in ancient Greek. The ancient Greeks noticed there was an approximate 40-day period when Sirius and the sun were both in the sky and believed that to be the cause of extreme heat.
It was the Romans who came up with the phrase “days of the dog star,” to describe those really hot days. The Romans also added Sirius to the constellation Canis Major (the “Greater Dog” in Latin). By the 1500s, “dies caniculares” was shortened and translated to “dog days,” and used throughout the English-speaking world.
You could actually find references to the “dog days of summer” in ancient texts. “If you go back even as far as Homer, The Iliad, it’s referring to Sirius as Orion’s dog rising, and it describes the star as being associated with war and disaster,” Jay B. Holberg, author of Sirius: Brightest Diamond in the Night Sky and senior research scientist at the University of Arizona Lunar & Planetary Laboratory, tells National Geographic. “All throughout Greek and Roman literature, you found these things.” The phrase “dog days” has since taken on various meanings after its translation from Latin to English.
Thus, the term Dog Days of Summer came to mean the 20 days before and 20 days after this alignment of Sirius with the Sun—July 3 to August 11 each year. While this period usually is the hottest stretch of summer, the heat is not due to any added radiation from Sirius, regardless of its brightness. The heat of summer is simply a direct result of the Earth’s tilt.
(Source: Science Magazine; Farmers Almanac: Readers Digest)