Whew! It’s hot in Wonderopolis today. In fact, it feels like the dog days of summer have arrived. But what exactly does that mean?

Many people believe the phrase “dog days of summer” stems from the fact that dogs tend to be a bit on the lazy side during the hottest days of summer. Of course, who can blame them? With that much fur, dogs that exercise during the hot days of summer can overheat easily.

However, the phrase doesn’t stem from lazy dogs lying around on hot and humid days. Instead, to find the answer, we only need to look to the summer sky.

The ancient Romans called the hottest, most humid days of summer “diēs caniculārēs” or “dog days.” The name came about because they associated the hottest days of summer with the star Sirius. Sirius was known as the “Dog Star” because it was the brightest star in the constellation Canis Major (Large Dog). Sirius also happens to be the brightest star in the night sky.

Sirius is so bright that the ancient Romans thought it radiated extra heat toward Earth. During the summer, when Sirius rises and sets with the Sun, they thought Sirius added heat to the Sun’s heat to cause hotter summer temperatures.

For the ancient Romans, the dog days of summer occurred from about July 24 to around August 24. Over time, though, the constellations have drifted somewhat. Today, The Old Farmer’s Almanac lists the traditional timing of the dog days of summer as being July 3 until August 11.

Although the dog days of summer are usually the hottest, they don’t have anything to do with either dogs or the star Sirius. Instead, the tilt of the Earth explains why these days tend to be the summer’s hottest.

During summer in the Northern Hemisphere, the tilt of the Earth causes Sun's light to hit the Northern Hemisphere at a more direct angle, and for a longer period of time throughout the day. This means longer, hotter days during the summer.

This explains why the dog days of summer are from late January to early March in the Southern Hemisphere. The actual dates of the dog days of summer will vary by location, depending upon latitude and climate.

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