dog days of summer explained

The Dog Days of Summer

It’s an expression you probably use year after year: “the dog days of summer.” We often hear about the “Dog Days” of summer, but few know where the expression actually originated. Some think it’s a reference to the hot, sultry days that are “not fit for a dog.”

Others suggest it’s the time of year when the extreme heat drives dogs mad. But where does the term come from? And what does it have to do with dogs? You may be surprised to see it has to do with the stars! Read on.

dog at beach

When Are The Dog Days of Summer?

The “Dog Days” of summer are from July 3 to August 11 each year. They’re usually the hottest and most unbearable days of the season.

According to the Farmer’s Almanac, the phrase is a reference to Sirius, the Dog Star. During the “Dog Days” period, the Sun occupies the same region of the sky as Sirius, the brightest star visible from any part of Earth. Sirius is a part of the constellation called Canis Major, the Greater Dog.

Sirius is colloquially known as the "Dog Star", reflecting its prominence in its constellation, The Canis Major (the Greater Dog).

In astronomical dog days of the summer, Sirius rises and sets with the Sun. On July 23rd, specifically, it is in conjunction with the Sun, and because the star is so bright, the ancient Romans believed it actually gave off heat and added to the Sun’s warmth, accounting for the long stretch of sultry weather. They referred to this time as diēs caniculārēs, or “dog days.”

Thus, the term The Dog Days of Summer came to mean the 20 days before and 20 days after this alignment of Sirius with the Sun—July 3 to August 11 each year, making the dog days bright.

dog Days of Summer

The constellation called Canis Major, the Greater Dog.

Summer Heat is Due to the Earth’s Tilt

While this period usually is the hottest stretch of summer, the heat is not due to any added radiation from Sirius, regardless of its brightness. The rising heat of summer is simply a direct result of the Earth’s tilt.

During summer and late summer in the Northern Hemisphere, the tilt of the Earth causes the Sun’s rays to hit at a more direct angle, and for a longer period of time throughout the day. This means longer, hotter days. Especially on the hottest days in late summer.

The Brightest Star in the Sky

Sirius is the brightest star in the sky, if you don’t count our own Sun. Under the right conditions, especially in the summer months, it can even be seen with the naked eye during the day. Sirius is one star in a group of stars that form the constellation Canis Major, meaning “Greater Dog.” It’s no surprise, then, that the nickname of this big, bold star became “the Dog Star.” 

Canis Major, the Great Dog

This constellation is named for the larger of Orion’s two hunting dogs (the other, Canis Minor, has only two stars).

constellation  canis major, how to find canis major on the sky

To find Canis Major:

  • Imagine a straight line through Orion’s belt. 
  • Move your eyes left (south) until you come to a very bright star—that’s Sirius, the nose of the dog.
  • Look farther south to find a triangle of stars that marks the dog’s hindquarters.

More About Sirius

Sirius is the brightest star in the sky, if you don’t count our own Sun. Under the right conditions, it can even be seen with the naked eye during the day. Sirius is one star in a group of stars that form the constellation Canis Major, meaning “Greater Dog.” It’s no surprise, then, that the nickname of this big, bold star became “the Dog Star.” 

The Dog Star in Ancient Egypt

In ancient Egypt, the Nile River flooded each year, usually beginning in late June. The people welcomed this event, called the Inundation, because the floodwaters brought rich soil needed to grow crops in what was otherwise a desert. 

No one in Egypt knew exactly when the flooding would start, but they noticed a coincidence that gave them a clue: The water began to rise on the days when Sirius (known to them as Sothis) began to rise before the Sun. Sothis and the Inundation became so important to the Egyptians’ survival that they began their new year with the new Moon that followed the star’s first appearance on the eastern horizon.

dog days of summer constellation,

What does science have to do with the dog days of summer?

Even though Sirius is extremely bright, the extra heat is not because of any additional radiation from this star. The earth rotates around the sun and the Earth’s tilted axis is why we experience seasons. The sun is hotter in certain hemispheres because the sun’s rays hit the Earth in a more direct way due to Earth’s tilt. For example, if the North Pole is tilted toward the sun, that translates to hotter days in the Northern Hemisphere. Similarly, if the South Pole is tilted toward the sun, that means it’s winter in the Northern Hemisphere. However, stars shift, the earth wobbles, and the “dog days” known by the ancient Greeks aren’t the same “dog days” of now, and the “dog days” of now won’t be the same as thousands of years from now.

cool dog


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.

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.


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