Orion the Hunter claims the southern sky as his own during evening hours in early February, but the times are changing. Nipping at his heels is the Dog Star Sirius and the rest of the stars that comprise the constellation Canis Major the Greater Dog. ‘Greater’ because there’s also a ‘lesser’ dog in the neighborhood called Canis Minor.
Truthfully, this constellation’s so small, it looks more like a dog bone treat for the bigger Canis.
Open your door and look outside at 8 o’clock and you can’t miss Orion’s three belt stars. If you shoot a line downward through the belt toward the southern horizon, you’ll soon arrive at scintillating Sirius, flashing like a silent firecracker in the turbulent air common at lower elevations.
Dangling below Sirius are some half dozen stars in the shape of a musical triangle. These form the legs, tail and head of Canis Major. Connect the dots the next clear night and you’ll see they do form the likeness of a dog jumping at your feet.
Canis Major has been described in myth as Orion’s hunting companion and also as Laelops, “a dog so swift that no prey could outrun it”, according to Ian Ridpath, author of Star Tales. The Big Dog doesn’t have to look far for a snack – just beneath Orion, Lepus the Hare munches contentedly in the stellar garden.
It wasn’t too many weeks ago that you had to stay up late to catch the canine constellation, but now I see it’s nudging Orion from its top spot by 9:30 p.m. As the Earth spins, stars appear to rise in the east, reach their greatest height when due south and set in the west. By month’s end, Canis Major will be due south around 8 p.m. and Orion will have taken a hike to the west.
If the nightly east-west drift of stars is due to Earth’s rotation, what causes the seasonal drift of the stars? Why doesn’t Orion always stay in the same place week after week, month after month? Blame it on Copernicus. He made the claim, outrageous for his time, that the Earth moved around the sun. Throughout antiquity and into the Middle Ages people thought that if the Earth moved, every time you jumped off the ground, the planet would rush away and leave you behind. Since that didn’t happen, it was obvious the Earth must stand still.
Few understood that EVERYTHING – the Earth, the jumper, birds and atmosphere – were all moving at the same speed and so appeared to be at rest relative to each other. The same thing happens when you’re flying at 550 mph in a plane. Once the plane has reached a constant speed, you’re hardly aware you’re moving. And since you, your laptop and that ginger ale are all traveling at 550 mph, they don’t go flying around the plane. This habit of things to stay put as long as they’re all moving at the same speed is called inertia.
OK, back to the Big Dog. As the Earth orbits the sun, our perspective on the nighttime sky changes over the weeks and months. At 10 p.m. in mid-January Orion stands straight up in the southern sky, but at 10 p.m in February, he’s been replaced by Canis Major. Come 10 p.m. in April, Leo the Lion will be high in the south and Orion will have set in the west.
As the weeks and months go by, we peer into a different direction of sky just as a runner sees different groups of fans as she runs the 1500-meter on a race track. Like many good things in skywatching, the cycle repeats anew every year.