Astro Bob: The Full Cold Moon has no wrinkles

The final full moon of 2020 occurs on Tuesday evening, Dec. 29th. Let it light a path into a better, brighter new year.

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The full moon rises over Lake Superior at Brighton Beach in Duluth, Minn. on Nov. 30, 2020. A full moon shines opposite the sun, rises when the sun sets and shines all night until it set around sunrise the following morning. (Bob King)

The Full Cold Moon will peep over the northeastern horizon Tuesday, Dec. 29, about the same time the sun dips out of view in the southwest. Full moon and sun are always opposite one another in the sky. As one orb sets the other rises. Like runners handing off the baton in a relay race, sun and full moon work in tandem to provide one day a month of nonstop light.

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A full moon is opposite the sun. Sunlight shines from behind, past the Earth and fully illuminates the daylight side of the moon. (Bob King)

At full moon, the Earth stands between the sun and moon. When we turn to put the sun at our backs, sunlight streams from directly behind us out to the moon and lights up its entire face. It's like standing face to face with a friend 10 feet away while another friend directly behind you shines a flashlight in their face. Your friend's face is fully illuminated and without shadows because you're standing in line with the light source.


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The shadowless full moon looks flat and pasty compared to the moon around first quarter phase when sunlight strikes it from the side. Shadows reveal numerous craters and other details. (Bob King)

In the same way, the full moon lacks shadows, which makes the lunar landscape appear flat and two-dimensional. Shadows reveal depth, texture and relief. Without them it's difficult to assess the contours of the moon or for that matter, a face. For that reason aging celebrities on magazine covers are often photographed with direct, straight-on lighting to hide their wrinkles. Similarly, the full moon has no "wrinkles" and appears quite smooth through the telescope compared to the other lunar phases.

Full moons are always opposite the sun, so when the sun is low in the sky, as it is in December, the moon must be high. And when the sun summits the sky in June, the full moon rides low. Last night the nearly-full moon occupied almost the identical spot the sun does on the summer solstice, a high perch in the constellation Taurus the bull directly above Orion.

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Around the time of the summer solstice the sun shines high in the sky in the constellation Taurus. Six months later, when the sun reaches its lowest point in the sky in Sagittarius, the full moon — located opposite the sun — occupies the same spot. You can also see why Orion isn't visible in the summertime. It's too near the sun in the daytime sky. (Stellarium at left, Bob King)

I thought it would be fun to compare the two, so I took a photo around midnight and paired it with a simulation made using Stellarium . Six months ago the sun occupied nearly the same spot the moon does now. Where was the full moon last June? Hunkered down in Sagittarius, the same place the sun is now. The point to remember is that whatever constellation the sun is in now, the full moon will occupy the constellation 180° opposite. In March, when the sun travels through Pisces, the moon lounges in Virgo on the opposite side of the sky.

With snow about and a bright moon there's no need to turn on a porch light when walking out the door. The brilliance of the moon and the light reflecting from the snow are more than enough to find our way. Artificial light has become such an entrenched part of our lives we forget that our eyes are capable of seeing quite well in the dark given the chance.


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Before the moon reached it peak elevation around midnight on Dec. 28 it cast long, dark shadows of these spruce trees across the snow. Moonlight has a special quality different from sunlight. To my eye it appears slightly bluer, and the contrast between shadow and light is stronger. (Bob King)

A headlamp or handheld flashlight focuses our attention on what's ahead of us to the exclusion of everything else. Lose the beam and you'll connect with the totality of the night environment. That's true with or without a moon. The moon just throws more light, making it easier to look further into the distance, see closeup details and sense shades of color. One other thing: If you catch the angle just right, moonlight makes the snow sparkle like glitter, one of my favorite winter night sights.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at .

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