Ian Clarke

We’ve almost made it through the “dog days of summer” at last. Today we use that phrase simply to mean the hottest, laziest part of summer, but the term has astronomical origins. The association was with Sirius, the “dog star,” and it can be traced to both the ancient Egyptians and Greeks. For the Egyptians, the first sight of Sirius before sunrise came soon before the flooding of the Nile. Ancient Greeks believed that when Sirius was too close to the sun to be seen, it was actually adding its light to that of the sun and making the earth hotter, and humans miserable. Needless to say, there is no scientific merit to this, but the idea stuck. There were various methods for figuring the start and end of “dog days,” but for our purposes, let’s say that they end when Sirius can first be seen in the east before sunrise. That will depend on how good you are at spotting stars and how flat your eastern horizon is. Go out about 45 minutes before sunrise. First find Orion the Hunter in the southeast; scan below and to the left of Orion for a bright star, Sirius. If you can see it, you can declare dog days over.

But wait. Perhaps you are thinking, “Don’t we see Sirius and Orion in the winter sky?” Yes, we think of Orion and Canis Major (where Sirius is located) as winter constellations, because that’s when they are prominent for prime-time, evening stargazers. But stars continue to rise through the night. The constellations a night-owl sees at midnight on September 1 will be in the same positions on October 1 two hours earlier, at 10 p.m. The stars rise and set each day because of the rotation of the earth on its axis; they change with the seasons because of the earth’s revolution around the sun.

Ian Clarke is the director of the Hatter Planetarium at Gettysburg College. More information available at www.gettysburg.edu/hatterplanetarium.

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