As in January, Orion the Hunter dominates the southern sky this month. Rising soon after him in the southeastern sky is his companion for all these millennia, Canis Major, which means “large dog” in Latin.
You couldn’t ask for a more faithful companion. For as long as humans have looked upward, Canis Major has followed Orion as they chase their prey again and again across the face of the night. Perhaps they are chasing Lepus, the Hare, a faint constellation directly beneath Orion’s feet.
Canis Major doesn’t have to worry about playing second fiddle to Orion. His head is formed by Sirius, the most brilliant nighttime star in the sky, marked with an “alpha” on most star maps.
Let’s find him. First, find Orion, which is located almost due south in the early evening. At the center of Orion is a bright, almost horizontal, line of stars that form his belt.
The belt stars point toward the southeast to a very bright star that will be close to the horizon — that’s Sirius, the Dog Star.
Canis Major’s “ears” are stars found directly to the right and left. The star to the right, Mirzam, is called “The Announcer” because it rises just before Sirius and announces its arrival.
Because it is so bright, Sirius scintillates and twinkles like crazy when it is close to the horizon. Such twinkling is caused by turbulence in the earth’s atmosphere. Often Sirius seems to change color and even disappear as the thick layer of atmosphere near the horizon moves the star’s light around and makes it dance.
Sirius is relatively close to our sun as stars go at about 9 light years or roughly 50 trillion miles. To give you an idea of how far away the stars in our galaxy are, even though Sirius is only the fifth closest star to our solar system, it is still over half a million times farther away than our Earth is from the sun. It’s a long trip to the neighbors in the vastness of the cosmos.
Sirius is only twice the sun’s diameter and mass, yet it shines with over twenty times the sun’s brightness.
The ancient Egyptians called Sirius “The Nile Star” because the Nile River flooded about the time that Sirius rose just before dawn at the end of June. With the subsequent flood came the rich Nile soil, upon which the Egyptians depended for their crops. The Nile flooding meant life to the people of Egypt, and the star heralded their good fortune.
Just below Sirius in the sky is a faint, hazy patch just visible to the naked eye if you aren’t observing too close to the streetlights of Columbus. In a pair of binoculars, this faint patch resolves into a beautiful, course cluster of about a dozen stars, marked as M41 on star maps. I’ve always thought of it as Canis Major’s license tag, hanging as it does just below his neck.
Sirius served as a kind of rough weather forecaster for the ancients. During the coldest time of the year, it has always risen at sunset.
During the sweltering heat of late summer, Sirius rises with the sun. The ancients believed that its brilliance added to the heat of the sun, producing what we still call the “dog days” of summer. Thus, the name Sirius means “the scorching one.”
As a result, the ancient Greeks and Romans feared the daytime rising of Canis Major. It meant for them the drought and possible famine that came with the scorching heat of Sirius in late summer. As Virgil writes in the Aeneid, it was believed to be the “burning constellation” that “brings drought and diseases on sickly mortals, rises and saddens the sky with inauspicious light.”
Well, time to go. It’s supposed to be clear tonight, and Sirius is up this time of year. The temperature will plummet to 10 degrees, or so they say, but I will think of the dog days of August, and Sirius will keep me warm.
Tom Burns is director of the Perkins Observatory in Delaware.