Saturday, August 13, 2011

The Dog Days are Over

Look for Sirius (alpha canis Major) to reappear in the sky just ahead of the Sun.

Is it feeling any cooler outside, yet? Well, if you live in the Northern Hemisphere, you may soon be as we have now officially exited the Dog Days of Summer, the traditional 40-day sizzle that lasts from July 3 to August 11 and that, more often than not, marks the warmest time of the year. So, while we all know that summer's on the wane (sadly, for most). Where did the 'Dog Days' moniker come from?
Thank the Ancient Egyptians.
For the Egyptians, the Star Sirius was very important, so much so that it eventually became identified with the goddess Isis, who brought her husband, Osiris, back from the dead after he was killed by his jealous twin brother, Seth. So why associate a star with a goddess identified with the triumph of life over death? Easy: every year at the same time in early July, Sirius would rise just ahead of the Sun in an event called the helical rising of Sirius. Shortly thereafter, like clockwork, the Nile would flood, depositing a layer of nutrient-rich, good for farming Nile mud over the landscape, thus ensuring a good harvest for the next growing season.
Okay, back to astronomy.
In the time frame from early July to mid August, Sirius was visible very close to the Sun after having disappeared into the twilight glare a few weeks previously. The re-emergence of Sirius as a morning star so close to the Sun led the Egyptians to believe that the bright star lent its heat to the sun in the period from early July to mid August, the hottest time of year.
Of course, we now know that this is all a convenient coincidence and that Sirius has nothing to do with our weather here on Earth thanks to its distance of roughly 8.6 light years. However, for people who love their history and/or trivia, this is why we have the Dog Days. For people who really like to think, consider this: thanks to precession, the Dog Days should fall much later in the year as, unlike in 2500 B.C., Sirius is lost in the glare of the Sun until early August, a full month after it reappeared to the Ancient Egyptians all those centuries ago . . .

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