Astro Bob: Mars transforms Winter Hexagon into Winter Heptagon
The familiar Winter Hexagon just grew a new side!
Bright patterns of easily recognizable stars that aren't official constellations are called asterisms. Even casual skywatchers are familiar with them. Examples include the Big Dipper, Orion's Belt and the Great Square of Pegasus. So is the Summer Triangle, which we visited in a recent column. Let's now set our sights on winter's most famous asterism, the Winter Hexagon.
I've described the Hexagon in previous posts, but it's worth revisiting because the figure so dominates the sky from late December through February. It also happens to be undergoing a mid-life crisis right now, popping out an additional side to become a seven-side heptagon. How so? Blame on that upstart planet Mars.
The Red Planet had been tucked inside the Hexagon in eastern Taurus up until this week. As you're well aware, planets are always on the move. Mars has been traveling westward in retrograde motion for some time and now resides above the orange-red star Aldebaran in western Taurus. Freed of its six-sided cage, the Red Planet now contributes an additional plank to the familiar figure, making it into a heptagon.
We'll get to enjoy this geometrical flight of fancy for the next six weeks. In fact, it will look even more heptagon-ish in early January as Mars continues to press west. On Jan. 12, the planet puts on the brakes and reverses course, turning back to the east. And a little more than a month later on Valentine's Day, it burrows back into the Winter Hexagon, and the Heptagon will be no more.
Nearly all of winter's brightest stars occupy the vertices of the Hexagon, which spans 66 degrees from top to bottom (Sirius to Capella) and 46 degrees from side to side (Procyon to Aldebaran). If you make your fist into a ball and hold it vertically against the sky, that's equal to 6 1/2 by 4 1/2 fists!
Sirius glimmers at the bottom, low in the southern sky, while Capella shines from nearly overhead. Each star is at least as bright as first magnitude and heads up a different constellation, one of the reasons the asterism is so useful — find all six suns, and you've got six easy entry points into each star group. For a localized map showing each constellation in greater detail, check out skyandtelescope.org/interactive-sky-chart .
Sirius is brightest, followed by Capella, Rigel, Procyon, Aldebaran and Pollux. Sirius is the alpha star in Canis Major the Big Dog; Procyon in Canis Minor the Small Dog; Pollux in Gemini the Twins, Capella in Auriga the Charioteer; Aldebaran in Taurus the Bull and Rigel in Orion the Hunter. Only Betelgeuse stands off by itself. To include it in the gang, skywatchers have reworked the Hexagon into a giant letter "G."
Another great thing about the Winter Hexagon is that all of its stars are visible even in bright moonlight, so you can see it anytime during winter up through mid-spring. Right now, all its stars are up around 9 o'clock, but by mid-January the pattern rises a full hour earlier.
I wasn't particularly good at geometry in high school. I loved learning about all the different types of polygons but recall that geometrical proofs taxed my young brain. One thing I can promise — the Hexagon-Heptagon will make you a whiz at night-sky observational geometry in no time.