Maybe you've heard the expression "dog days" to describe that stretch of summer when it seems like the weather is hotter than wasabi on a jalapeño. But have you ever wondered about the origin of that idiom? No, it has nothing to do with dogs being hot or grilling hot dogs. In fact, you might be surprised to know that the phrase "dog days" is actually as ancient as the stars.
Most folks in the Northern Hemisphere consider the June solstice the start of summer. It's an event that occurs around June 20 or 21 and happens when the sun reaches its highest point in the sky. But meteorologists divide the year into seasons based on the months and temperatures, which allows for better data comparisons.
That means, meteorologically speaking, summer starts June 1 and ends August 31. So, the hottest days of summer – the "dog days" – run from July 3 until August 11 and the phrase is a reference to Sirius, the "dog star," the brightest star in the night sky. Sirius is the fifth-nearest star, 8.7 light-years from Earth. It seems many civilizations noticed this blazing orb in the sky and attributed powers to it.
Ancient Egyptians called Sirius 'Sothis' and noticed that around the time of the summer solstice it would rise at dawn, just as the sun was rising. The presence of these two bright stars took on cultural significance, as the Nile River typically flooded at this time and Egyptians greatly depended on agriculture around the Nile.
Later, the Greeks called the star Seirios (Sirius) or Σείριος, meaning "glowing" or "scorching" in ancient Greek. And they must've been keeping good records. The ancient Greeks noticed there was an approximate 40-day period when Sirius and the sun were both in the sky and believed that to be the cause of extreme heat.
But it was the Romans who came up with the phrase "dies caniculares" or "days of the dog star," to describe those sweltering hot days when you just don't want to move. The Romans also added Sirius to the beautiful constellation Canis Major (the "Greater Dog" in Latin). By the 1500s, "dies caniculares" was trimmed and translated to "dog days," and used throughout the English-speaking world.
Obviously, the heat experienced during summer isn't due to radiation from a star named Sirius, but from the tilt of Earth. During summer in the Northern Hemisphere, the tilt of Earth causes the sun's rays to strike more directly and for longer periods of time. That's why days are hotter and longer.