Late July and early August usually are the hottest, sultriest weeks of the summer for much of the northern hemisphere. This stretch of sizzling temperatures has been referred to for centuries as "the dog days of summer," but most folks use this phrase without really knowing what it means or where it came from.
The term "dog days" can be traced back many centuries to the early Greek civilization. These careful sky watchers noticed that the sun, on its slow annual journey through the constellations of the Zodiac, passes the brightest star in our nighttime sky, Sirius, on July 22 of every year. For 20 days on either side of that date, the two brightest stars visible from Earth -- the sun and Sirius -- are up together in our daytime sky. Great thinkers of that era thought the extra light and energy of Sirius in our daytime sky between July 3 and August 11 made the days unusually hot and miserable.
During the dog days of summer, Sirius cannot be observed because of the blinding solar glare, but after Aug. 11, early risers once again can spot Sirius, rising just before the sun. That marks the end of the dog days. Sirius also can be seen twinkling brightly on cold winter nights not far from the belt of Orion.
Perhaps Sirius is better known by its nickname, the Dog Star, because it marks the nose of Canis Major, Orion's big hunting dog pictured in the sky. So, the phrase "dog days of summer" refers to the 40-day stretch when both the sun and the Dog Star, Sirius, are in our daytime sky at the same time.
Although the Dog Star is unusually bright, it is not bright enough to affect our temperatures here on Earth. The sun appears nearly 14-million times brighter to us than Sirius does, but that is only because the sun is so much closer to us. The Sun is eight light minutes from Earth, but Sirius is eight light years from Earth! In truth, Sirius is the more luminous star. If the sun were plucked from the center of our solar system and replaced by Sirius, Sirius would burn 23 times brighter in our sky. The seasonal variations in temperature that we experience on Earth are due almost entirely to the 23.5-degree tilt of the Earth on its axis.
Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published on the Web sites of CNN.com, NASA's "Astronomy Picture of the Day," Spaceweather.com, Space.com, Discover.com, and in Sky & Telescope, Astronomy, Night Sky, Discover, and WeatherWise magazines.