Dog days of summer: something to bark about?

As we finish up the second month of summer, one might be hearing (or uttering) the phrase "the dog days of summer." So, is this just a saying about the summer weather, or is there some astronomy behind the saying?

Here in the Northern Hemisphere, the "dog days of summer" usually run from mid-July to mid-September and often relate to the hottest part of the summer season. The hot weather tends to lead to inactivity. These dog days have also been seen as a time when it is too hot for humans to be outside and even dogs would only go out to quickly do their business and come back in. Thus, only dogs would be seen in this type of hot summer weather.

However, the phrase is actually tied in with astronomy. The Sirius star, also known as the Dog Star, is in close proximity to the sun during this time of the year. The star rises and sets with the sun. The Sirius star is the brightest star in the sky this time of the year. In ancient times, sky watchers thought this star added heat to Earth, thus creating the hottest days of the summer season.

More from a Web page at Cornell University:

The name "dog star" came from the ancient Egyptians who called Sirius the Dog Star after their god Osiris, whose head in pictograms resembled that of a dog. In Egypt, and in ancient Rome, Sirius was in conjunction with the sun in the summer (i.e., it was up in the sky at the same time as the sun) and ancient Egyptians and Romans argued that it was responsible for the summer heat by adding its heat to the heat from the sun.

They called the period of time from 20 days before to 20 days after the conjunction "the dog days of summer" because it coincidentally fell at the time of year when it was very hot.

The exact time of conjunction changes with the precession of the equinoxes so that now the conjunction of Sirius with the sun is a little earlier in the Northern summer than it was during Roman times, and as time passes it will move out of the summer season altogether (note: the conjunction is in the Southern winter (both now and in Roman times), so ancient civilizations in the Southern Hemisphere could not have come up with this myth).

Posted by Jeff Rae on Aug 23, 2011 at 7:03 AM