Two cosmic dogs rule the winter sky — here's how to spot them this week

During the winter season, two celestial dogs — the constellations Canis Major and Canis Minor — are on display in our evening sky.

If we begin to stargaze this week at around 6 p.m. local standard time, just as darkness has fallen, look for a 2nd-magnitude star rising about 30 degrees to the south of that point on the horizon marking due east. Your clenched fist held at arm's length measures approximately 10 degrees, so going roughly "three fists" to the right of east will bring you to this star known as Murzam. An imaginary line drawn through the stars Bellatrix in Orion and Alnitak (the lowest star in Orion's Belt), extended about twice its own length, also leads to Murzam.

This name, based on ancient Arabic, means "roarer" or "announcer." Its appearance on the horizon heralds the rising of the Dog Star Sirius, the brightest of all stars, about 17 minutes later. Both stars are a part of Canis Major, the Big Dog.

Interestingly, another star also serves to announce the appearance of Sirius: Procyon, the brightest star of Canis Minor, the Little Dog. The name Procyon is derived from the ancient Greek, meaning "before the dog" since it precedes Sirius by about 25 minutes as they rise. But later in the night, when they are toward the south, Procyon marches behind its more brilliant companion.

Dog days of summer

It is no wonder that the ancients would watch for Murzam and Procyon to announce the rising of Sirius, for a host of influences on man were attributed to the Dog Star and, by inference, to its constellation. Only the sun, the moon, Venus, Jupiter and occasionally Mercury and Mars, can rival Sirius.

As an example, the celestial dog was thought to aid the sun in causing the intense heat, drought and pestilence of summer. Sirius was said to be primarily responsible for the hot and muggy "dog days" that occur during July and early August for the Northern Hemisphere. Legend has it that because Sirius rises at the same time as the sun during the first half of summer, its brightness adds to the sun's energy, producing additional warmth. As it turns out, its heliacal rising (day of first visibility in the east before sunrise) occurred at the time of the summer solstice some five millennia ago.

Star of the Nile

The ancient Egyptians held a great deal of respect for Sirius. Its heliacal rising would herald the annual flooding of the Nile Valley, the waters re-fertilizing the fields with silt. This event was of such importance to them that it marked the beginning of their year. It, of course, was pure coincidence that for a time the rising of the brightest star in the sky should have coincided with the rising of the Nile River, but certainly the ancient Egyptians did not see it quite that way. After all, here was this brilliant star rising just before the sun, and shortly thereafter, they saw the Nile begin to rise. And this happened year after year, so naturally they concluded that it was the star that had a direct connection to the life-giving flood of their river. They called the star Sopdet, seen as a goddess, often depicted with a star on her head, and built temples oriented to the exact spot on the horizon where it would rise, for they believed that as it rose earlier at each passing morning, it was calling up the waters by its own mystical power and letting it flow across the plain.

Who let the dogs out?

If we wait until around 9:30 p.m., both Canis Major and Canis Minor will be in fine view, roaming across the south-southeast sky. The Big Dog is a rather striking star pattern, yet it is quite overshadowed by the brilliance of Sirius. All through the long winter nights, it scintillates with a dazzling white with a tinge of blue in the southern sky and because of its brilliance, it is always easily recognized. Follow the line of Orion's belt southeastward (toward the lower left), and you will come to this splendid twinkler. Seemingly, it appears as perhaps a stud on the Big Dog's collar or perhaps his tag. He also has a foreleg, and three rather bright stars lower down in a triangle forming his hindquarters, back leg and tail.

The star located in the lower right corner of the triangle is Adhara, which shines at magnitude +1.50 and ranks as the twenty-second brightest star and misses by just a hundredth of a magnitude, the cutoff for first-magnitude classification.

From a few fainter stars, you can perhaps make a head.

There was nothing tame about the dog of the heavens as conceived by the ancients. As late as the 9th century, Canis Major was pictured as a fierce, vicious hunting dog. It was not until 1603 that Johann Bayer (1572-1625) substituted a watchdog in his famous pictorial star Atlas Uranometria and other atlases thereafter followed suit. But the concept of both Canis Major and Canis Minor belonging to Orion appears to be as ancient as the Dog Star itself. The Big Dog is jumping up and down excitedly behind Orion, who is trying to deal with Taurus the Bull, who is charging down on him from his other side. We can only hope that the dog is indeed Orion's and coming to help him, and not to bite him!

As for Canis Minor, the Little Dog, he is certainly smaller, composed of Procyon — the Little Dog Star — and one other noticeable star, so this dog seems to only have a head and a tail. Thus, we have a pair of dogs running on either side of the Milky Way.

Tightly packed companions

Modern astronomers have discovered by a peculiar coincidence, in that each dog star keeps strange company. Both have dim, mysterious companions with unusual characteristics. They are white dwarfs, diminutive as stars go; the companion to Sirius (known as Sirius B or colloquially "The Pup"), for example, is only about the size of our Earth, about 7,500 miles (12,000 km) in diameter, yet contains nearly as much material as our sun. Hence, its average density is exceedingly high — an ordinary glassful of its material would weigh about ten tons if brought to the Earth's surface.

The companion stars were known to exist years before they were observed in a telescope. Both Sirius and Procyon are very near to us (8.6 light-years for Sirius, 11.5 for Procyon). In 1844, German astronomer, mathematician, physicist, and geodesist Friedrich Wilhelm Bessel (1784-1836) found their paths through space to be wavy, instead of straight, and correctly attributed this to the gravitational effects of unseen companions. In 1862, Alvan Clark (1804-1887) was testing the lens of a new 18½-inch refracting telescope, he saw Sirius B for the first time. Procyon's white dwarf companion was not found until 1896 at the Lick Observatory in California.

Personally, I think the discovery of these two amazing objects deserves a round of A-Paws.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, Sky and Telescope and other publications.