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The phrase “dog days of summer” does not come from dogs’ behavior in hot weather, but from the ancient belief that the star Sirius caused extra heat during mid-July to August. However, we now know that the Earth’s tilt causes the heat. The specific dates of dog days vary, but they last around 40 days. In the 19th century, it was believed that dogs were more likely to contract rabies during this time.
Dog days of summer seem like a natural expression. In hot weather, dogs pant and lie down on the ground as much as possible to avoid the heat. However, the expression does not originate with the behavior of dogs during the summer. It dates further back in time to some erroneous astronomical impressions of the Greeks and Romans.
During the hot season, especially the period from mid-July to August, the star Sirius, part of the constellation Canis Major, rises and sets with the sun. The ancients believed that this rising and setting together produced extra heat for the Earth. This explanation has been used to account for the hot, muggy heat produced during the summer months.
Of course, we now know that while Sirius throws a small amount of light energy in our direction, it’s not responsible for the dog days of summer. The heat is actually caused by the Earth’s tilt relative to the Sun. In the Northern Hemisphere, this slight tilt allows the Earth to receive more direct sunlight starting June 21. In the Southern Hemisphere, dog days actually occur in December, when countries like Australia lean slightly more towards the sun and receive more sunlight.
Another misconception about the dog days of summer is that they occur when the Sun is closest to the Earth. This is actually not true, although it is true that the Earth’s distance from the Sun varies slightly throughout its orbit. Tilt, rather than distance, determines how much direct sunlight the Earth receives, and during the Northern Hemisphere in July, the planet is actually the furthest from the Sun.
The days described as the dog days of summer often have a specific date, although there have been differing opinions as to the actual dates. Some describe the date as between July 3 and August 11, while others suggest July 6 to August 17. Most agree that dog days last 40 days.
The phrase took on a new meaning in the 19th century. It was believed that dogs would contract rabies more often during this time period. A scene in To Kill A Mockingbird suggests this belief continued well into the 20th century, when Atticus Finch shoots a rabid dog. Everyone notes that it’s not the time of year for the rabies to set in as the shooting occurs well before the summer months.
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