Summer’s Gone to the Dogs
But brilliant Sirius isn’t to blame
For kids heading back to school, summer has truly gone to the dogs. But neither that nor your canine companion panting on the cold basement floor is why the hottest days of the year are referred to as the Dog Days of summer. The answer shines in the heavens in the form of a star more than 81⁄2 light years away.
Sirius, the Dog Star, is the brightest in the heavens and is the main star in the constellation Canis Major — the big dog. For us in the Northern Hemisphere, the star Sirius is lost behind the sun from July 3 to August 11. It is only toward the end of July that the Dog Star emerges to the east of the sun before dawn. Our ancient ancestors noticed that the hottest days of summer coincided with the alignment of our sun and this brilliant light and concluded that Sirius was causing the extra heat.
Both the ancient Mesopotamians and the Egyptians called Sirius the Dog Star. For the Egyptians, its return marked the new year and foretold the annual flooding of the Nile. In Greek mythology, Sirius’ Canis Major is one of Orion’s two hunting dogs, the other being Canis Minor and its own brilliant star Procyon. Roman farmers sacrificed a dog each year at this time to appease the Sirius and ensure a good harvest.
Now, Sirius has returned to our pre-dawn skies. When it rises in the east-southeast around 5am this week, the only brighter objects will be the moon and Jupiter. Even Mars, not far from Jupiter in the east, pales against the Dog Star, shining at magnitude +1.6 compared to Sirius at magnitude –1.4. Twinkling a brilliant blue-white, Sirius is easy to spot on its own, but the three aligned belt stars of Orion point straight down to it.
Sirius is the fifth-closest star to us. It is twice as large as our sun and burns almost twice as hot. Yet it emits 25 times as much energy, much of it outside the spectrum of light visible to our unaided eyes.