"Dog days" or the "dog days of summer" generally refer to that time during the summer in the Northern Hemisphere when everyone gets far too hot and sticky. You may have wondered what this expression has to do with dogs. Perhaps they thrive in the weather, or were merely forced to take the summer months after the winter ones were acquired by cats? Well, no. The origins of the expression date back to ancient Greece and Rome.
Ancient Greeks observed that Sirius – which they called the "dog star" – appeared to rise alongside the Sun in late July. They mistakenly believed that the heat from Sirius, which is the brightest star in the night sky, was what caused the heat of the summer months.
The phrase – which has already lost most of its meaning since its creation – will become even more meaningless as time goes on, and the Earth wobbles the constellation out of the Northern Hemisphere's summer skies.
“Our Earth is like a spinning top,” Bradley Schaefer, professor of physics and astronomy at Louisiana State University, told National Geographic. “If you toss it onto a table, after it slows down … the pointing direction of the top will slowly go around in circles.”
Earth's rotation is wobbling too, placing the constellation near the Sun at different times of the year.
“In 26,000 years, the dog days would completely move all around the sky,” Schaefer added. “Roughly 13,000 years from now, Sirius will be rising with the sun in mid-winter.”