The sultry “dog days” of summer might spark visions of listless canines baking in the oppressive heat, but the phrase has nothing to do with panting pooches. Instead, it’s a throwback to the time when ancient civilizations tracked the seasons by looking to the sky.

The ancient Greeks noticed that summer’s most intense heat occurred during the approximate 40-day period in the early summer when Sirius, the brightest star in the sky, rose and set with the sun. To them it was simple math. The daytime addition of the warmth of Sirius—ancient Greek for “glowing” or “scorcher”—to the blaze of the sun equaled extreme heat.

According to Greek mythology, Sirius was the dog of the hunter Orion, and the ancient Romans placed the star in the constellation Canis Major (Latin for “Greater Dog”). The Romans thus referred to the sweltering period when the rising of the sun and Sirius converged as the “dies caniculares” or “days of the dog star.” By the 1500s, the English world began to call the same summertime point on the astronomical calendar as the “dog days.”

Due to a wobble in the Earth’s rotation that shifts the position of the stars in the night sky, the dates of the “dog days” now fall several weeks later on the calendar than they did thousands of years of ago. The ancient Egyptians 5,000 years ago noticed Sirius’s heliacal rising, when it was visible just before sunrise, just prior to the annual flooding of the Nile River and the summer solstice.

Today, the precise dates vary by latitude, but the Old Farmer’s Almanac reports the traditional timing of the “dog days” in the United States is between July 3 and August 11. In approximately 10,000 years, the date of the heliacal rising of Sirius will fall back so late on the calendar that future civilizations in the northern hemisphere will experience the “dog days” of winter.