Economics teachers know stock market mavens and watchers call August the “dog days.” It’s slow time, usually — which puts a piquant point on the market gyrations of the past three weeks.
Why is late August called the “dog days?”
Roman astrologers and astronomers named two constellations in which they thought they saw the outline or framework of dogs. They named one Big Dog, and the other Little Dog — though, in Latin, they became Canis Major and Canis Minor, respectively.
Sirius is in Canis Major. Sirius is the brightest of the true stars (not so bright as Venus, but Venus is a planet). The name “Sirius” came from ancient Egyptians, who named it after their god Osiris; Osiris, of course, had the head of a dog (“Sirius” is a Latin corruption of Osiris, I suppose). So we have a star named after a dog-headed god, in a constellation called the Big Dog.
In ancient times Sirius was in conjunction with the Sun at the height of summer — that means, it rose and set with the Sun. Some ancient astronomers conjectured that the added heat from Sirius made summers hotter. And so, after the Dog Star, Sirius, those days 20 days prior to the conjunction of the Sun and Sirius to 20 days after the conjunction, were called the “Dog Days.”
Of course, none of what the ancients thought they saw in the stars has anything to do with seasons on our planet. It’s the 23-degree tilt of the planet that puts the Northern Hemisphere into a position to get more direct sunlight and more hours of sunlight in the summer. Plus, due to precession of the Earth, the conjunction of Sirius and the Sun comes earlier in the year than it used to. In a few thousand years, the conjunction won’t be in the summer at all. And of course, today (and in ancient times) the conjunction of Sirius and the Sun comes at the depths of winter in the Southern Hemisphere — this is one way we know it’s a Northern Hemisphere story, since the “Dog Days of Winter” doesn’t make much sense.
Knowing constellations is fun, but it’s also useful for U.S. history teachers (“follow the Drinking Gourd”), all teachers who deal with the great explorers and how they navigated the oceans in days before compasses, chronometers, satellites, radios and Global Positioning Systems, and all geography and other teachers who teach the seasons. Repeated surveys show Americans don’t know the Earth circles the Sun once every 365.25 days; surveys show people think summer occurs when the Earth is closest to the Sun (actually, it’s often farthest from the Sun in summer).
Last year I snatched a globe map of the world for a Texas history class, which proved a popular piece to have in the classroom. One day a kid asked about how we get summer, and he didn’t believe me when I said it was the tilt of the planet. I happened to have a flashlight, and with the kid holding the globe and “orbiting,” we turned the lights off and had near total silence in a normally rowdy classroom as we demonstrated how darkness gets the Arctic and Antarctic Circles every year, and how 24-hour sunshine affects the other pole. By the end of class several kids had a new appreciation for the 23-degree tilt built into the classroom globes.
I’d like to do that sometime in a darkened gymnasium or other huge room, using more planets, and a Moon.
But, of course, they were supposed to already have that information when they hit my classroom. So I caught flack for taking time out for the demonstration. Sometimes even teachers have to follow Mark Twain’s advice, and not let schooling get in the way of students’ education.
Finding Canis Major: I hope you purchase a star finder chart — there are good, small devices available for about $8.00 from your local Boy Scout supply shop, or you can find them at good bookstores, and several other places. Teachers should be able to pick out at least part of the Big Dipper (which is really part of Ursa Major, the Big Bear), and that may help.
Generally, finding Canis Major and Sirius is easiest in winter months, if one can find Orion, and go from there. Here is how to find it in March, as described by Angelfire.com:
Canis Major, the “Big Dog”, is prominent in the South on March evenings. Its a very easy constellation to find, because it contains the brightest star in the entire night sky, Sirius. To find Canis Major, use the familiar hourglass shape of Orion the Hunter, which is in the South-Southwest. Look to the lower left of Orion, and dazzling Sirius will immediately catch your eye. The name Sirius comes from the Greek for “scorching”. Also known as the “Dog Star”, Sirius has a tiny companion that is all but impossible to see visually. Sirius B is magnitude 8.44, ten thousand times dimmer than Sirius A at magnitude -1.9. Click here to read a fascinating article about an ancient west African tribe that knew of Sirius B’s existence before it was discovered by scientists!
Millard Fillmore's Bathtub 
Question:
I understand that the zodiac constellation Libra was added to honour Julius Caesar, as a just man with scales. It was taken from Scorpius, and in fact two of its main stars reflect the claws – the Zubens.
My question. What was the arrangement of zodiac constellations before that committee showed up? Were there only 11 constellations of the zodiac, or was Ophiuchus, whose stars above and below the ecliptic were listed by Ptolemy, then the 12th constellation of the zodiac?
Background:
I give astronomy talks on cruise ships See Page 29, SkyNews, September,November 2007 edition.
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