July 3 - “Dog Days” Begin

Posted on July 3, 2021

This is an update of my post published on July 3, 2010:

The hottest days of the summer are sometimes called the Dog Days. In the Northern Hemisphere, these days are often during July and August, and The Old Farmer’s Almanac (which has been published every year continuously since 1792) lists today as the start date.

(The days that are actually the hottest depends, of course, on the year and the location. In my corner of the world, the hottest days usually fall in August and September and parts of October!)

Why are the hottest days called “dog days”? The ancients watched the heavens carefully for time-keeping and weather-prediction purposes.

They noticed that Sirius, which is the brightest star in the sky other than the Sun, rose just before or at the same time as sunrise during July and August.

The Greeks and Romans blamed Sirius for the extra heat. Since Sirius is the largest star in the constellation Canis Major (Large Dog), the Romans called Sirius the Dog Star, and dubbed the days supposedly affected by this star the Dog Days.

According to Brady’s Clavis Calendarium (1813), the Dog Days were thought of as a time of evil “when the seas boiled, wine turned sour, dogs grew mad, and all creatures became languid, causing to man burning fevers, hysterics, and phrensies.” The Romans sacrificed a brown dog at the beginning of the Dog Days in an effort to appease the anger of the Dog Star and minimize the heat and troubles.

Of course, we know today that Sirius is really a binary (two-star) system more than 8 light years – which is something like 47,000,000,000,000 miles away – and we also know that Sirius doesn’t affect Earth temperatures at all.

Do you know the actual reason that summers are hotter than winters?

Because the Earth is tilted on its axis, each hemisphere faces the Sun more directly during the summer. This means that, in the summer, the days are longer, and also the Sun’s rays hit the Earth more squarely—and both factors make the temperatures go up.

This diagram is specific to the Northern Hemisphere.
But the situation in the Southern Hemisphere is the same –
but with opposite months markings – because So. Hemi.
folks get summer’s direct rays in December and winter’s
indirect rays in June!

Here and here are season simulators that you can use to see the angle of the Sun’s rays during different seasons and at different places. Can you see why people at the North and South pole sometimes have 24-hour days or 24-hour nights?

Some Summer Fun