The “dog days,” I always thought, were those summer days so devastatingly hot that even dogs would lie around on the asphalt, panting.

Many people today use the phrase to mean something like that—but originally, the phrase actually had nothing to do with dogs, or even with the lazy days of summer. Instead, it turns out, the dog days refer to the dog star, Sirius, and its position in the heavens.

To the Greeks and Romans, the “dog days” occurred around the day when Sirius appeared to rise just before the sun, in late July. They referred to these days as the hottest time of the year, a period that could bring fever, or even catastrophe.

“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,” said Jay B. Holberg , author of Sirius: Brightest Diamond in the Night Sky and senior research scientist at the University of Arizona Lunar & Planetary Laboratory . “All throughout Greek and Roman literature, you found these things.”

The phrase “dog days” was translated from Latin to English about 500 years ago. Since then, it has taken on new meanings.

“Now people come up with other explanations for why they’re called the ‘dog days’ of summer, [like] this is when dogs can go crazy,” said Anne Curzan , an English professor at the University of Michigan .

“This is a very human tendency,” she said. When we don’t know the origin of a phrase, we come up with a plausible explanation.

“The meaning has been lost,” said Holberg, “but the phrase has lived on.”

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The Hottest Days?

So, did the Greeks get it right? Are the dog days, around when Sirius rises, really the hottest days of the year?

Well, no.

Although July and August are generally known as the hottest months of the year in the Northern Hemisphere, the hottest period can vary from year to year. And depending on your latitude, the astronomical dog days can come at different times.

In Athens, for instance, Sirius will rise around the middle of August this year. But farther south, it’ll happen earlier in the year; farther north, it’ll happen later.

There’s another reason that the dog days don’t correspond neatly with the heat: the stars in Earth’s night sky shift independently of our calendar seasons.

“Our Earth is like a spinning top,” said Bradley Schaefer , professor of physics and astronomy at Louisiana State University . “If you toss it onto a table, after it slows down … the pointing direction of the top will slowly go around in circles.” Similarly to a top, “the Earth’s rotation is kind of wobbling around.”

“The calendar is fixed according to certain events, but the stars have shifted according to the way that the Earth wobbles,” said Larry Ciupik, astronomer at Adler Planetarium and director of the Doane Observatory . “So in about 50-some years, the sky shifts about one degree.”

This means that the dog days of ancient Greece aren’t the dog days of today. What it also means is that several millennia from now, this astrological event won’t even occur during the summer.

“In 26,000 years, the dog days would completely move all around the sky,” said Schaefer. “Roughly 13,000 years from now, Sirius will be rising with the sun in mid-winter.”

Ah yes, the dog days of winter. When it’s so cold that even the dogs lie around the fire, trying to stay warm.