Get ready for this weekend’s total lunar eclipse

The moon dips in and out of the Earth's shadow during the Feb. 20, 2008 lunar eclipse. At totality (center), it glows orange from sunlight spilling through our atmosphere along the circumference of the Earth. Photos: Bob King

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.

Use this map to determine how much of the eclipse will be visible from your town. Click the diagram to visit Larry Koehn's excellent Shadow & Substance website for more helpful eclipse diagrams and animations. Credit: Larry Koehn

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.

A total lunar eclipse occurs when the moon passes into Earth's shadow. The outer part of the shadow, which creates only a subtle shading on the moon is the penumbra. The dark inner portion, tinted red from sunlight refracted around the Earth by our atmosphere, is called the umbra. Credit: Sagredo

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.

About 40% of the moon will be in shadow shortly before moonset in Duluth, Minn. Saturday morning. This view shows the sky facing northwest at 7:15 a.m. or about 1/2 hour before sunrise in twilight. Maps made with Stellarium

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.

Farther west in San Francisco, Calif. skywatchers will see a totally eclipsed moon low in the west-northwest sky around 6:15 a.m.

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.

This detailed view shows each phase of the upcoming eclipse. Only the early portion before totality will be visible from the Midwest. Total eclipse begins at 8:06 a.m. when the moon is fully within the umbra. Add one hour for Eastern time, subtract one hour for Mountain and two hours for Pacific time. Credit: NASA with my own additions

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.

18 thoughts on “Get ready for this weekend’s total lunar eclipse

  1. Saw P-G in all it’s wasted glory last evening.
    It was movin’ that’s forsure.

    So Bob I got an 8in Dob. Just arrived today! Thanks for all the inspiration on here.

  2. Hi , what was up with the moon last night ? Im a night dweller I left my home from @ 1:50 a.m and the moon was on top and it was white and small on my way back it was really low ,huge and had like an orange color this was around 3:20am im in the los angeles,ca area. it was interesting to me can you explain ty :)

    • SpaceLover,
      What you saw often happens when the moon is near rising or setting. The moon is orange because the thicker, dustier air near the horizon filters out the cooler colors like blue and green from moonlight leaving it orange or even red. The same happens the sun when it’s near rising or setting. As for the size – that’s an illusion caused by how we perceive distant objects when they’re near the horizon. To find out more, click on the moon illusion link in the paragraph at the end of today’s blog at http://www.astrobob.areavoices.com

  3. Hey Bob, great article! Off topic but I was hoping you will keep us up to date with your thoughts on all the latest happenings with Voyager 1 as it heads towards the heliopause and closer to interstellar space. What an amazing journey!

  4. Bob, I always think the moon looks bigger on rising than it does at the zenith, but I was asking you if you thought it looked CLOSER rather than bigger. To me, when it’s low on the horizon, it looks much nearer, as it appears to be ‘at the end of the road’, within reach, so to speak. The overhead sky appears to be farther away than the sky at the horizon, so the moon when located there, also appears to me to be farther away.

    • Carol,
      OK – well, let’s see. I equated closer and bigger. It doesn’t really look closer to me at the horizon unless again it’s in a setting with trees and homes. I distinctly recall a few times seeing the waning gibbous moon rising and setting and it appearing very distant to me those nights.

      • Well, equating ‘closer’ and ‘bigger’ is one of the problems with the standard explanation of the moon illusion. Traditionally, the explanation went that because of the Ponzo effect, the horizon moon looked much bigger because the brain was compensating for it being was ‘farther’ away than the zenith moon. I’ve never liked that explanation, and have always thought the opposite – that the horizon moon, because it seems bigger, looks ‘nearer’ not further away. Something like 90% of people also see it that way! It seems that newer explanations are taking that figure into consideration and giving less emphasis to the Ponzo effect.

        • Carol,
          I totally agree with you that a ‘bigger moon’ looks ‘closer’ but would argue that perception of the ‘large’ moon also depends – at least for me – on whether it’s seen in the wide open horizon vs. a more cluttered one.

    • Hi Les,
      Doing good. I’m happy that NASA extended the Mercury MESSENGER mission for an additional year into 2013. The more data and photos the better! I’m also intrigued that we’ve found the planet was once more volcanically active and that sulfur-containing minerals are found in its crust, making it a different world than the moon despite similarities in external appearance.

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