Astro Bob: Enjoy an extra helping of the 'snowcrust' moon

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

A pretty little corona (not the viral kind!) encircles the waxing moon this weekend. The March full moon will also be a supermoon — a closer-than-usual full moon — and appear slightly larger and brighter. (Bob King, for the News Tribune)

It’s been fun watching the moon fill out and brighten this week. Last night, it coaxed me outside for a ski in the woods. Thanks to its brilliance I didn’t kill myself on the icy trails. Of course, I carry a backup headlamp just in case things look dicey. Which they did on a few occasions. The stars added a touch of light, too. Especially Sirius, the brightest star, and Arcturus, now shining in the eastern sky after 8 o’clock. A fun observation: the handle of the Big Dipper neatly conformed to the shape of some of the tree trunks as I skittered along.

Last night’s waxing gibbous moon photographed with a cell phone through a 10-inch telescope magnifying 76x. (Bob King for the News Tribune)

On Monday, March 9 the moon will be full. The Full Worm Moon. We are a long way from seeing worms in Duluth, Minn. but if you live farther south they’ve already wriggled into view. Here, the Ojibwe call the March full moon Onaabani-giizis or the Snowcrust Moon. A perfect fit. During the day the snow softens under the climbing sun then hardens overnight. Occasionally it firms up like cement the next morning, and you can walk on top of it without fear of punching holes and falling in.

The moment of greatest fullmoonness occurs at 12:47 p.m. (Central Time) March 9 when the sun, Earth and moon (in that order) will be most nearly in line. If they were exactly in a row we’d see an lunar eclipse, but the moon passes a few degrees north of Earth’s shadow and avoids eclipse this time around. But not in June, July and November. During those months the moon will cross through the outer shadow called the penumbra and we’ll get to a penumbral eclipse.


Sunday night’s (March 8) moon rises in Leo the lion about an hour before sunset. This view shows it during late twilight an hour after sunset. (Stellarium)

Tonight’s moon will be 99.2% illuminated — close enough to appear full with the naked eye — and Monday night a smidge fuller at 99.6 percent. Two full moons for the price of one. Last night the moon shone near Leo the lion’s brightest star, Regulus. Tonight it’s in the lion’s tail and rises about an hour before sunset. On Monday night (March 9) it will rise shortly after sunset. Because the eastern sky will be getting dark at that time on Monday the moon will be much more obvious.

Watch for Monday night’s full moon to rise with Earth’s shadow, the gray band topped by the “Belt of Venus” seen over Lake Superior in Duluth in this photo. (Bob King, for the News Tribune)

The moon rises Monday at the same time as Earth’s shadow, a purplish, gray band near the horizon that extends across the entire eastern sky for a short time after sunset. A pink glow called the Belt of Venus fringes the top of the shadow — watch for it. What you’re seeing is the planet’s shadow projected onto the atmosphere, but it’s only visible for about 20 minutes until it blends into the encroaching darkness.

The apparent diameter of Earth’s dark inner shadow at the moon’s distance is only about 2.6 times as wide as the moon, making for a small target in the sky. Because the moon’s orbit is tilted with respect to the Earth’s, it usually misses the target during most full moons, the reason we don’t see a lunar eclipse each month. (Bob King, for the News Tribune)


You might wonder why the moon, plainly visible within the shadow, isn’t eclipsed. Standing on the planet, the shadow is gigantic from our perspective, the reason it circles around half the horizon. But at the moon’s distance of a quarter-million miles, it narrows into a circle (spherical objects cast circular shadows) only a few degrees across. Unless the moon “hits the bullseye” it misses Earth’s shadow. When it does we score an eclipse!

To watch the moonrise, check your local rising time and then find a spot with an open view to the east. I like to bring binoculars and a camera, even if it’s just my mobile phone. The best time to capture the moon so it doesn’t look like a bright, featureless disk, is within 15-20 minutes of sundown when the lingering daylight balances with the brightness of the lunar disk. A half-hour after sunset may still make for pretty scenes with the moon, but the exposure required to show the landscape will “blow out” the dark lunar markings. Similarly, if you expose for the moon, so you’ll lots of lunar detail, but the landscape will be nearly black.

The rayed crater Tycho stands out well in this photo taken with my old iPhone 5e and a 10-inch scope at around 150x several nights ago. (Bob King, for the News Tribune)

I like to photograph the moon in my telescope with a cellphone. I center the moon in the field of view and then hover the camera lens over the eyepiece and snap away. Phone cameras show remarkable details when paired a telescope. Take yours out of the attic and give it a try.

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