We often associate the Milky Way with summer nights, but it's also present in winter. Many people don't get to see the "colder half" because it's not as bright, and chilly temperatures make us think twice about nighttime outings. But since we're all pretty much used to the cold by February, now seems like a good time to leave the nest.
As the Earth orbits the sun we face in different directions along the way just like a kid sees the carnival scenery change as she circles round and round on a carousel. At nightfall during the summer months, we face toward the central regions of the galaxy where the stars are more richly concentrated. In winter, we face the opposite direction toward the edge, where the stars thin out, and the galaxy gives way to a gaping emptiness lit only by the occasional rogue sun.
What exactly is the Milky Way? There are two definitions. The one we're most familiar with is the hazy band of light that cuts an angled path across the constellations. To the unaided eye it looks like fog or mist. Only through binoculars or a telescope does its true nature as a carpet of stars become apparent.
The Milky Way is also the name of the entire galaxy, the one we call home. The foggy band, along with every star in the sky including the sun, are part of it. Additionally, it contains billions of planets and hundreds of star clusters and glowing stellar nurseries called nebulae all packaged into an enormous, flattened disk about 100,000 light-years across.
Viewed from the side, the Milky Way looks something like a pizza but with a denser bulge of stars at its center (extra sausage?). If we could see the galaxy from above we'd be able to admire its multiple spiral arms that unwind from the bulge like octopus arms.
Because the solar system orbits around the center of the galaxy inside the disk we can't distinguish the arms directly. To map their shapes and distances astronomers use optical, infrared and radio telescopes equipped with spectrographs and cameras to determine the motions of stars and gas clouds across the Milky Way.
The solar system is located about 27,000 light-years or halfway between the galaxy's center and edge inside the Orion Arm, a minor spiral feature about 3,500 light-years wide and 10,000 light-years long. When we look directly through the disk of the galaxy, where the stars are most concentrated, they pile up across the light-years to form the familiar Milky Way band.
The summer Milky Way band is comprised of stars within the spiral arms that wrap between us and the center. In winter, we basically turn our back on the center and face outward, where the stars thin out with distance. For this reason the Milky Way looks fainter and not as broad.
Despite appearing as separate, isolated bands, the summer and winter Milky Ways are actually halves of a great circle. If we could depart the Earth and go for a spacewalk, say on a journey to the moon, we'd discover that the two bands join together to form a complete circle of hazy light, brighter and wider in the direction of the center and thinner and fainter in the anti-center direction. We can never see the complete circle from the Earth because it's cut off by the horizon. In space there is no horizon. How liberating!
The next two weeks, from Jan. 31 to Feb. 14, are ideal for seeing the winter half. Best viewing time is from 8 p.m. to 11 p.m. You only need a clear night and a dark sky. To help you find a good sky nearest your home, go to this Light Pollution Map and use your mouse's drag and scroll functions to locate and zoom in on your city. Heavily light-polluted areas are colored red, purple and yellow. Darker regions are blue. Find a location within a reasonable distance and take flight on the next clear night.
If you're uncomfortable going alone bring a friend. Once you've arrived at a suitable spot (for me, that's usually a dirt road), allow your eyes 10-15 minutes to adapt to the darkness.. Wow! What a view, right? The section from Sirius through Orion and Gemini is probably the most obvious. Above Gemini, the Milky Way narrows and fades as it climbs to the top of the sky. If you turn around and face northwest you'll see that it brightens again in the W of Cassiopeia on its way to the Northern Cross at the western horizon.
Breathe in the cold air, lean back and fall into the galaxy's thrall.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.