About once a month this winter, we can watch the Moon journey across a field of stars termed the “Winter Hexagon.”
Some of the most amazing things about watching the skies are the chances we get to see the universe in action. Even if individual objects seem steady and unchanging, we can pick up on the delicate rhythms and subtle dances between them.
How the Moon Crosses the Winter Hexagon
At the start of November, we can watch this dance in the context of the Winter Hexagon, as winter’s celebrated stars file back into the night, like old friends arriving at an end-of-the-year party.
While that happens, there’s a little extra magic. Not only do we see the brilliant lights of Orion, Gemini, and Taurus again, but we also see the Moon dancing through these constellations.
The Winter Hexagon, you may remember, is a giant asterism of six first-magnitude stars, which ride across the night sky from mid-fall until the last of them fades into the western dusk in the spring. It's a fun party trick to run counterclockwise around the Hexagon and rattle off the names of all its stars: Capella, Castor, Pollux, Procyon, Sirius, Rigel and Aldebaran. (Note that Castor isn't quite a first-magnitude star; still, it’s nearly as bright as Pollux. If we add it in, it gives the names of the Hexagon's stars a pleasing, bouncy rhythm when you say them aloud. Give it a try; you’ll see.)
As the Moon orbits Earth, it rises about 50 minutes later each day. We see that daily motion as an eastward jump of about 13° relative to the stars, and the Moon’s phases change as it goes. The Hexagon is enormous, though! It’s nearly 45° across So, even with those big jumps, it takes about four nights to jog from one side to the other from the time the Moon crosses into it near Taurus, the Bull. As the week goes on, our nearest neighbor finally crosses out of the hexagon near Castor & Pollux, the twins of Gemini.
At the beginning of the trip, the Moon huddles up close to Aldebaran at the top of the “V”-shaped Hyades cluster, with the Pleiades cluster (M45) a short distance away. As the nights go on, we’ll see the Moon cross Taurus, cruise right over Orion’s head, and then glide through Gemini. By the time the Moon steps out of the Hexagon near Pollux, we’ll see a changed Moon. Watching the Moon’s move through its phases as it sings with those stars always puts a smile on my face.
Watch the Dance This Winter
That’s only part of the fun. The chorus of this song repeats throughout the winter:
- November 2-6, 2020
- November 29 - December 3, 2020
- December 27-31, 2020
- January 23-27, 2021
- February 19-24, 2021
- March 19-23, 2021
- April 15-19, 2021
But although the cycle repeats, it’s never quite the same. Since the stars rise four minutes earlier every night, they’re a little higher in the sky and a little farther toward west at the same time as the night before. These tiny changes add up over the months. Meanwhile, the Moon moves farther to the east relative the stars as it orbits Earth. This means the Moon catches up with the Hexagon a little sooner every month than it did the month before.
Let’s keep an eye on this: Every month, try to remember where you were facing the last time around. Notice how you’ve turned slightly toward the right. We can look back at what time of night it was when we looked last, and maybe remember where we were in our lives.
Little by little, we’ll see the whole asterism sweep across the sky toward the west. As it does, the Moon’s phases become younger and thinner each time it meets the Hexagon. That’s because the Hexagon’s western march brings it closer in the sky to the setting Sun — where the Moon’s younger phases occur.
The sky will brighten a bit every night once we pass the solstice on December 21st. Before long, we’ll be complaining about the heat as the last of these stars disappears into the twilight again. Come April, the Moon will catch up with Aldebaran as a thin waxing crescent.
By the time the Hexagon’s stars have crossed the sky, the Moon will have caught up with them almost two weeks earlier in its cycle — it meets Aldebaran just past full in November and just past new in April.
All the while, things change here on Earth, too. In November, when this all starts, the last orange and brown leaves fall. Then, after the stars have guided us through the winter, tiny green leaves come back again in April, just as the Hexagon exits the sky. The asterism itself changes, too, almost rolling across the sky. Even in May, the last of the stars still hang on, making a broad, stately arc that stretches across the western horizon.
It’s easy to think that the skies never change, but we can watch the cycles happen right before our eyes. There’s magic and music out there. I hope you’ll grab someone you love and have a look.