Winter’s Milky Way

February 4th, 2021 – Leo Rising

I’ve been star watching for more than fifty years. But until this week, I had never seen the winter Milky Way – because I had never looked for it. In my mind, the Milky Way was a summer sight: a band of diffuse light rising from the southern horizon until it arches up and over the top of the night sky. Cygnus the Swan flies high above, along that path of light, heading south down towards the brightest clouds of celestial light around Scorpio and Sagittarius just above the southern horizon.

I knew that the Milky Way is how our galaxy appears from inside it. Our galaxy is a vast pinwheel of spiral arms.

It’s between 150,000 and 200,000 light years across but only about 1000 light years thick.

Therefore, stars are most abundant along the plane of our galaxy so that plane glows with the blended light of millions upon millions of stars. But since the “disc” is thin, the glow of this galactic plane forms only a narrow band across the sky, the Milky Way.

If we wanted to somehow jet up out of our galaxy, the quickest way would be straight up and away from the “thin” galactic plane towards the Galactic North Pole. That direction would take us through the space “beneath” the handle of the Big Dipper. Look up there; it’s the quickest way out of the galaxy. Relatively few stars of our galaxy exist in that direction. For astronomers, that direction presents a clearer window to see what lies beyond our galaxy.

The brightest part of the Milky Way will be found facing towards the galactic center. In that direction, we see the bright clouds around Sagittarius and Scorpio. Those celestial clouds are not the center. The center lies far beyond our naked-eye sight. The super-massive black hole at the center of the galaxy is obscured by billions of stars and clouds of gas and dust between it and us. As I follow the Milky Way away from Sagittarius, it gradually dims because the number of stars diminish as I look further away from the center.

All this I already knew. For decades, I have practiced seeing this when I gazed at the Milky Way in its full, late summer glory.

I try going our for a star watch each “moonth”. I pick a clear night, about a week after the full moon, when the moonless evening sky is at its darkest. At sunset, I carry a folding chair and hike out to a place away from most human light. There I simply sit for two to three hours, watching the sun’s light recede beyond the western horizon and watching new constellations appear to rise along the eastern horizon. This “moonth”, Leo was rising. Canis Major with Sirius, the brightest star in the night sky, shone low above the southern horizon. Over Sirius sparkled Orion the Hunter with his three star belt from which hangs his sword. As the night darkened, the brilliance of Orion’s stars kept catching my attention. They were so brilliant against the dark night sky. To the left of Orion were equally bright stars – Procyon in Canis Minor, Castor and Pollux in Gemini, Capella in Auriga. But they didn’t stand out like Orion’s stars did.

Time passed. Darkness deepened but the difference between the radiance of Orion and the stars to its left remained. The contrast between Orion’s stars set in black space was so stark; the stars to the left appeared with less contrast. As I studied this area of less contrast, I noticed it extended upward through Gemini and Auriga, almost like a wall, and then on to Perseus high in the northern sky. That’s when my mind engaged.

During a late summer star watch with very clear sky. I had traced the Milky Way up from Scorpio and Sagittarius through Cygnus almost overhead. Beyond Cygnus, the Milky Way dimmed because my eyes were moving away from our galaxy’s center. But in the clear sky of that night, I could see a very dim Milky Way stretching through Cassiopea to Perseus before completely fading from sight.

Now, tonight, this February star watch, I was noticing Perseus at the top of this band of less contrast between bright stars and dark sky. Perseus connected these observations from opposite times of the year. The connection grew stronger when I remembered a story of Orion, the mighty hunter. In one version, Orion, like too many Greek heroes, fell into hubris (pride so arrogant it offends the gods). He boasted that he could kill any animal on Earth. One of the goddesses sent the lowly, hiding in the dirt scorpion to sting and kill Orion, Later, both Orion and the scorpion were raised into the sky. Out of sympathy for Orion, however, Orion and Scorpio were placed on opposite sides of the sky so that they would never be in the sky at the same time. Orion is opposite Scorpio. Now I see! Scorpio is close to Sagittarius, the direction towards the center. Orion opposite Scorpio would place Orion just off to the side of the galactic plane facing away from the center, where the Milky Way would be dimmest.

In that direction, the Milky Way isn’t bright enough to show up as light but it is “bright” enough to make the black space between visible stars less black. The winter Milky Way is almost invisible, experienced not as a band of light but as a band of less darkness offering less contrast with the stars shining before it. This connection between the summer Milky Way and winter Milky Way formed a celestial embrace that centered me within the vastness of our galaxy. I was delighted for days thereafter.

The next day, I read and confirmed that yes, that band of less contrast from Perseus down through Sirius is part of the Milky Way, The faintest part appears in Auriga which lies in the direction opposite of the galactic center. Down by Orion, the view is starting to move back towards the center and growing more prominent. We of the mid-northern latitudes don’t ever get to see the part of the Milky Way between Sirius and Scorpio. It’s always below our horizon. That view includes the Magellanic Clouds, two small but close galaxies orbiting our galaxy.

Now, throughout the year, I will see the galactic plane of the Milky Way Galaxy. Parts of it will be bright, other parts less dark but it will always be there, a continuous band uniting the summer and winter skies. This omnipresence has two effects. Now, on moonless nights, I will always notice my orientation to our galactic plane so I can start watching how it appears to change through the night and seasons. I will learn to feel the dynamics of this very different frame of reference. The second effect is that this encompassing band, more than anything else in the sky, will remind me that deep space includes beneath me. The band of the Milky Way encircles the Earth. It does not stop at the horizon but continues through the sky underfoot until I see it rising, over there, on the other side.

Space is not out there. We are within it.

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I hope that a book will develop out of my moonthly star watches. (A tentative title for the book is Within the Night Sky.) The book would help people actually see those parts of astronomy that can be observed with the naked eye. Much of it involves shifts in frames of reference. This can be hard to describe and hard to experience. The winter’s Milky Way is an example of this. I’d appreciate feedback on how to write about such things more lucidly.

My third illustration of constellations along the galactic plane depended on a star map that was organized on the galactic coordinate system rather than the celestial coordinate system that most star maps use. Check out this link for a different frame of reference on the night sky. http://shadetreephysics.com/fun/sm-newd.gif