I’ve just come off a week’s vacation. The first weekend was almost unbearably hot and humid, and not once but twice, I overheard someone mention the ‘dog days of summer’.

Hmmmm….So why do we call the hot, sometimes dry-sometimes sticky July/August weather ‘dog days’?

The expression dog days of summer refers to the sultry days of the northern hemisphere summer that coincide with heliacal rising of the constellation Sirius. This constellation, also known as Orion’s Dog or the Dog Star, is the brightest star in the night sky. It can be seen almost everywhere on the Earth’s horizon and at this time of year, in the Northern Hemisphere, it rises in conjunction with the sun.

The ancient Greeks observed that the appearance of Sirius heralded the hot and dry summer, and feared that it caused plants to wilt, men to weaken, and women to become aroused (woo hoo!). The Romans knew these days as dies caniculares, and the star Sirius was called Canicula, “little dog.” Homer even referenced Sirius in the Iliad when describing the approach of Achilles toward Troy, associating it with oncoming heat, fevers and evil:

Sirius rises late in the dark, liquid sky
On summer nights, star of stars,
Orion’s Dog they call it, brightest
Of all, but an evil portent, bringing heat
And fevers to suffering humanity.

Because Sirius is so bright and rises and sets with the sun at this time of year, the Ancient Greeks and Romans believed that its heat added to the heat of the sun, creating a stretch of hot and sultry weather. They named this period of time, from 20 days before the conjunction to 20 days after, “dog days” after the dog star.

Today, the dog days of summer occur during the period between July 3 and August 11. Although it is certainly the warmest period of the summer, the heat is not due to the added radiation from a far-away star, regardless of its brightness. The heat of summer is a direct result of the earth’s tilt.