Everyone knows that the “dog days of summer” occur during
the hottest and muggiest part of the season. Webster
defines “dog days” as...
1 : the period between early July and early September when the hot sultry
weather of summer usually occurs in the northern hemisphere
2 : a period of stagnation or inactivity
But where does the term come from? Why do we call the hot, sultry days of
summer “dog days?”
In ancient times, when the night sky was unobscured by artificial lights and
smog, different groups of peoples in different parts of the world drew images in
the sky by “connecting the dots” of stars. The images drawn were dependent
upon the culture: The Chinese saw different images than the Native Americans,
who saw different pictures than the Europeans. These star pictures are now
called constellations, and the constellations that are now mapped out in the sky
come from our European ancestors.
They saw images of bears, (Ursa Major and Ursa Minor), twins, (Gemini), a
bull, (Taurus), and others, including dogs, (Canis Major and Canis Minor).
The brightest of the stars in Canis Major (the big dog) is Sirius, which also
happens to be the brightest star in the night sky. In fact, it is so bright that
the ancient Romans thought that the earth received heat from it. Look for it in
the southern sky (viewed from northern latitudes) during January.
In the summer, however, Sirius, the “dog star,” rises and sets with the
sun. During late July Sirius is in conjunction with the sun, and the ancients
believed that its heat added to the heat of the sun, creating a stretch of hot
and sultry weather. They named this period of time, from 20 days before the
conjunction to 20 days after, “dog days” after the dog star.
The conjunction of Sirius with the sun varies somewhat with latitude. And the
“precession of the equinoxes” (a gradual drifting of the constellations over
time) means that the constellations today are not in exactly the same place in
the sky as they were in ancient Rome. Today, dog days occur during the period
between July 3 and August 11. Although it is certainly the warmest period of the
summer, the heat is not due to the added radiation from a far-away star,
regardless of its brightness. No, the heat of summer is a direct result of the
earth's tilt.