Jimmy Westlake: Welcome to the dog days of summer
Late July and early August are often the hottest weeks of the summer for much of the northern hemisphere.
This stretch of sizzling temperatures has been referred to as “the dog days of summer” for centuries, but most folks use the phrase without really knowing what it means or where it comes from, perhaps thinking that the hot weather is only suitable for dogs. I suspect that dogs don’t enjoy the searing heat any more than humans do.
The use of the phrase “dog days” can be traced back over 2,000 years to the early Greek civilization. These careful sky watchers noticed that the sun, on its slow annual journey around the sky, passes the brightest star in our nighttime sky, Sirius, on July 22 each year.
For 20 days on either side of that date, the two brightest stars visible from Earth, the sun and Sirius, shine together in our daytime sky. Great thinkers long ago believed that the extra light and energy provided by star Sirius in our daytime sky between July 3 and Aug. 11 made the days unusually hot.
The star Sirius is better known by its nickname, the Dog Star, because it marks the nose of Orion’s big hunting dog, Canis Major, pictured in the sky. The phrase “dog days of summer” refers to that 40-day stretch when both the sun and the Dog Star, Sirius, are in our daytime sky at the same time.
During the dog days of summer, the star Sirius cannot be observed because it is too close to the sun’s blinding glare, but after Aug. 11, early risers can spot Sirius once again, rising just before the sun does. That marks the end of the dog days.
In the winter months, Sirius can be seen twinkling brightly in our evening sky, not far from the belt of Orion.
Although the Dog Star is unusually bright — twice as bright as the second brightest star, Canopus — it is not bright enough to affect our temperatures here on Earth. The seasonal variations in temperature that we experience are due almost entirely to the 23.5-degree tilt of the Earth on its axis.
The sun shines 12.7-billion times brighter in our sky than Sirius does, but that’s only because the sun is so much closer to us. Light from the sun reaches Earth in only 8.3 minutes, but light from Sirius requires 8.6 years to reach Earth. In truth, Sirius is the higher wattage star.
If the sun were plucked from the center of our solar system and replaced by Sirius, the Dog Star would burn 23 times brighter than the sun in our daytime sky. Now, that would be uncomfortable. Life on Earth would not be possible under those conditions, so, it’s a good thing that the Dog Star shines on us from a safe distance away.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. His “Celestial News” column appears weekly in the Steamboat Today newspaper. Check out Jimmy’s astrophotography website at http://www.jwestlake.com.
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