Few of us would choose to get up before sunrise on a Saturday morning, but this weekend you might want to make an exception. On Saturday December 10 the full moon will undergo a total eclipse for viewers in the western U.S. and Canada, Alaska, Asia, Australia and eastern Europe. For a worldwide map showing eclipse visibility, click HERE.
Unfortunately, if you live in the eastern United States, the moon will have set by the time it enters Earth’s shadow. Those living in Midwestern states like Kansas and Minnesota will see a partial eclipse before moonset. I’m just a little jealous that my brother Mike in Alaska will see it all unfold in a dark sky before dawn.
For the Duluth, Minnesota region, the first sign that something odd is happening will be around 6:30 a.m., when shading from the Earth’s outer or penumbral shadow dims the moon’s upper edge giving it a blunted appearance. 15 minutes later the dark inner shadow or umbra takes its first bite of lunar cheese and the eclipse is underway.
As twilight swells in the east,the moon drops lower in the west until it sets at around 7:40 a.m., the same time the sun rises. Shortly before setting, about 40% of the moon will be cloaked in shadow. Skywatchers in the Midwest and mountain states would do well to pick a place with a clear view of the northwestern horizon to see as much as possible of the event. To see how the eclipse will appear from your city, use the map below or download a copy of the free sky charting software Stellarium and set time and date to the early hours of Saturday morning.
Eclipses of both the moon and sun are uncommon enough that each is special and not be missed. Lunar eclipses only happen at full moon, when the sun, Earth and moon are lined up in a neat row in that order. Only then does the moon pass behind the Earth and into its shadow.
The Earth casts a shadow into space the same way trees cast shadows on the ground – with a difference. Since the Earth is a sphere, its shadow is circular. When the moon, which orbits the Earth at some 2200 miles per hour, slides into our planet’s shadow, it’s deprived of sunlight. We watch the curved edge of Earth’s shadow slowly bite into the moon’s edge the same way you bite into a cookie. After an hour or so, only a crescent-like slice of sunlit moon remains. As that last bit slips into the dark inner portion of the shadow called the umbra the moon does not disappear! If our planet were airless, it would, but Earth’s atmosphere refracts or bends sunlight into the shadow and paints the moon a coppery red.
We’d get an eclipse every full moon is the moon’s orbit were flat relative to the Earth’s but because it’s slightly tipped, the moon normally passes a small distance above or below the shadow and no eclipse occurs.
Unlike total solar eclipses, which are only visible from a narrow band averaging about a hundred miles wide, a lunar eclipse can be viewed by an entire hemisphere at a time. All you need is for the moon to be up in a dark sky.
At the moon’s distance, the central and darkest portion of the Earth’s shadow, called the umbra, is about three moon diameters across. Since the moon moves eastward in its orbit one moon diameter per hour, a typical total lunar eclipse is a leisurely event lasting about three hours. The moon begins perfectly full and brilliant but is transformed to a deep, dark orange by the time of totality. The color is caused by sunlight bent by Earth’s atmosphere that spills into the otherwise black shadow cone. When the moon is nearly but not squarely in line with sun and Earth, it dips only part-way into the dark umbra for a partial eclipse.
If the weather forecast is on your side, don’t pass up the chance the see this eclipse. No matter what instrument you use – from naked eye to telescope – there’s so much to enjoy, especially if you’re one of the lucky ones to see totality in a dark sky. The transformation from nighttime brilliance to deep darkness in the space of an hour and a half is not to be missed. Depending on the amount of cloud and dust in the atmosphere at the time of the event, colors during totality can range from bright yellow-orange to deep red.
When the moon is completely immersed in shadow, stars that would otherwise be washed out during full moon are visible in binoculars right up to the moon’s edge. What a sight! My favorite moment occurs immediately after totality when the first bit of moon re-emerges from the shadow. The bright edge rimming the red moon gives our satellite a striking three dimensional appearance so different from the familiar flat, shiny disk. While I won’t see this from my home this time around, I hope you’ll have the opportunity.
For the Midwest, the moon during eclipse will be near the horizon and appear unusually large thanks to the full moon illusion, that bit of psychological trickery whereby the moon seems larger near the horizon than overhead. A nice bit of consolation for those of us unable to see the moon during total eclipse.