Every July, people talk about the "dog days of summer" and imagine an exhausted golden retriever sprawled across a porch. But that is not where the expression comes from. The real story begins in the night sky.

The phrase traces back to Sirius, the brightest star visible from Earth. It is known as the Dog Star because it is the most prominent star in the constellation Canis Major, the Great Dog. During part of the summer, Sirius rises and sets at nearly the same time as the sun.

To the ancient Greeks and Romans, that seemed more than a coincidence. They reasoned that if the brilliant Dog Star was rising alongside the sun, it must be adding its own heat to the season, creating the year's hottest and most oppressive days. They called this period the "dog days," after the Dog Star, not after any four-legged animal.

Sirius contributes nothing to Earth's summer heat. July and August are hottest because Earth's tilted axis allows the Northern Hemisphere to absorb more solar energy, while the land and oceans gradually warm over the course of the season.

The ancient explanation was wrong, but the name endured. More than two thousand years later, we are still using a phrase inspired by a star that people once believed was making summer hotter.