Thursday, February 21, 2008

Two Dog Night

As Orion the Hunter strides purposefully across the winter sky, he is not alone. With him are his two faithful hunting dogs, represented by the constellations Canis Major (Big Dog) and Canis Minor (Little Dog).

Courtesy of Linda Hall Library of Science, Engineering and Technology

I sometimes wonder which breeds the ancient Greeks envisioned these two dogs to be when they memorialized them in the sky. The star atlas illustrations of the 17th and 18th centuries aren’t consistent on this point and tend to cast Canis Major as a bulldog. I’m not buying it. The bulldog most likely originated in early England, and it became well known throughout Europe primarily as a fighting dog.

Canis Major in Hyginus star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

I prefer the woodcut print in Hyginus’s 1482 star atlas, Poeticon Astronomicon. Now that dog could be a Greek hunting hound--a Cretan Hound, to be exact. Slender and swift like a greyhound, this carefully cultivated Greek breed has been known since antiquity as a champion hare hunter. And let’s not forget who scampers in the sky just ahead of the Big Dog, at Orion’s feet. You betcha: Lepus the Hare.

Courtesy of Linda Hall Library of Science, Engineering and Technology

Canis Minor is a small, scrappy underdog of a constellation. The depictions in the well-known Bayer and Flamsteed star atlases are a bit unsatisfactory; their Little Dogs look like coiffed, pampered pets.

Canis Minor in Bayer's star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

Canis Minor in Flamsteed's star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

Instead, I picture Canis Minor as terrier-like: energetic and feisty, with a natural hunting instinct for small game. In my opinion, another venerable Greek breed fits the bill: the Alopekis. Its name, meaning small and foxlike, appears to suit this compact, resourceful dog. Now this is a pooch with the stamina to keep up with both Orion and a fleet-footed Cretan Hound.

Come on outside, and let’s shake paws with these two admirable canines.

1) You’ll need to face south, so if you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your right shoulder to the west, and you’ll be facing approximately south.

2) Wait at least one hour after sunset to begin observing, so your sky is nice and dark. The waning (shrinking) Moon will be big and bright this week as it is just past Full Moon. However, it will rise later each night, so your dark-sky observing window after sunset will increase as the week goes on. Plus, even with the Moon in the sky, you should still be able to see the bright stars of Canis Major and Minor.

3) Locate Orion the Hunter by spotting his hourglass-shaped asterism (recognizable star pattern). One hour after sunset, Orion is nearly due south. He hangs halfway to two-thirds of the way up from the southern horizon toward the zenith (the point directly above your head). Now locate Orion’s Belt, a diagonal line of three evenly-spaced stars cinching the middle of the hourglass.

4) The stellar landmark that helps us find Canis Major is blue-white Sirius, the brightest star in the night sky. Because Sirius (pronounced SEER ee us) follows Orion across the sky, ancient peoples called it the Dog, and it has come to be known as the Dog Star. Within Canis Major, it marks the location of the dog’s snout.

Find Sirius by drawing an imaginary line through the three stars of Orion’s Belt, moving from upper right to lower left, and continuing on until you come to a very bright star. This is spectacular Sirius. As star distances go, Sirius is considered quite close to Earth. At only 8.6 light years away, it is the fifth closest star to us in the Milky Way. It’s the combination of its proximity to Earth and its extreme luminosity that makes Sirius appear so bright.

Sirius is from the Greek word for scorching. Since Sirius joins the Sun in the daytime sky during July and August, the hot days of late summer have been known for centuries as the dog days. (Modern culture sidebar: remember the wonderfully quirky 1975 film “Dog Day Afternoon,” set on a hot summer day in Brooklyn?)

5) Now that you’ve found Sirius, you can find the Upside Down Y asterism that includes four of the five brightest stars in Canis Major. Sirius marks the bottom of the Y. So look below Sirius, toward the southern horizon, and you should spot three fairly bright stars that form the top of the Y.

Wezen (pronounced WEZZ en), the star at the fork of the Y, is a yellow supergiant. Its name is from the Arabic word for weight. The star at the bottom of the left fork is Aludra (pronounced al UDE rah), a blue-white supergiant whose name means virginity. The end star of the right fork is Adhara, Arabic for the virgins. Adhara (pronounced ad HAR ah) is a blue giant radiating a tremendous amount of ultraviolet light. It’s been said that if our eyes could see in ultraviolet, Adhara would be the brightest star in the sky!

The other bright star in Canis Major is not in the Upside Down Y asterism. It can be found just to the right (west) of bright Sirius. It is called Mirzam, which means announcer, perhaps a reference to its rising before Sirius and heralding its arrival. Mirzam (pronounced MERE zam) is a blue giant star.

Regrettably, observers in the far northern latitudes may have difficulty seeing the entire Y asterism as it doggy paddles in atmospheric murk near the horizon. When near the horizon, even bright Sirius can be affected by atmosphere, often twinkling furiously in a kaleidoscope of colors.

Winter Triangle

6) To find the little dog Canis Minor, first locate Betelgeuse, the red supergiant star at the upper left of the Orion hourglass. Draw an imaginary straight line connecting Betelgeuse and Sirius. Now, look for a bright star left (east) of Betelgeuse that, when connected to both Betelgeuse and Sirius with straight lines, would complete an equilateral triangle. Found it? Great! You have located Procyon, the brightest star in Canis Minor and the eighth brightest star in the night sky. Plus, the equilateral triangle you inscribed is the asterism called the Winter Triangle.

Procyon’s name is from the Greek for before the dog, a reference to its rising just before Sirius. Procyon (pronounced PRO see ahn) is a yellow-white subgiant star. Can you see its slightly golden color? Now compare it to brilliant, bluish Sirius.

The only other notable star in the Little Dog is Gomeisa, Arabic for little bleary-eyed one. Gomeisa (pronounced go MAY sah) is a rapidly spinning blue-white star. Look for it just northwest of Procyon.

7) If you’re observing under a dark sky, try to spot the winter Milky Way winding between the two dogs. I like to think of it as a cool, clear stream they might need to splash through a few times on their galactic romp with their master.

Milky Way marked with swirling gray band