The Term dog days has an interesting origin. 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.
“Our Earth is like a spinning top,” said Bradley Schaefer, professor of physics and astronomy at Louisiana State University. “If you toss it onto a table, after it slows down … the pointing direction of the top will slowly go around in circles.” Similarly to a top, “the Earth’s rotation is kind of wobbling around.”“The calendar is fixed according to certain events, but the stars have shifted according to the way that the Earth wobbles,” said Larry Ciupik, astronomer at Adler Planetarium and director of the Doane Observatory. “So in about 50-some years, the sky shifts about one degree.”This means that the dog days of ancient Greece aren’t the dog days of today. What it also means is that several millennia from now, this astrological event won’t even occur during the summer.“In 26,000 years, the dog days would completely move all around the sky,” said Schaefer. “Roughly 13,000 years from now, Sirius will be rising with the sun in mid-winter.”Ah yes, the dog days of winter. When it’s so cold that even the dogs lie around the fire, trying to stay warm.