The full moon rises atop Earth's rising shadow in early spring 2010. The full moon tomorrow morning June 4 will undergo a partial eclipse. Photo: Bob King
Article by astrobob www.astrobob.areavoices.com
All ready for tomorrow morning’s partial lunar eclipse? Earth’s shadow will cover a maximum of 37% of the moon shortly before and after sunrise for the Midwest, South and and Western U.S. You might be scratching your head wondering if it’s worth getting up at 4 or 5 a.m. to see such a wimpy eclipse. I understand your reluctance. We all have our thresholds.
While I might drive a couple hundred miles for a total lunar eclipse, I wouldn’t for a minor partial. Here in Duluth, Minn. only a few percent of the moon will be covered before it sets shortly after sunrise. If skies are clear, I’ll still be out there. Just about any eclipse is rare enough to expend some effort. After all, the next one – a total – won’t happen until April 2014. And honestly, I like the picture in my head of sun, Earth, moon and me all lined up.
The shadow of the planet rises into the sky for an observer facing east as the sun sets in the west. The situation's reversed for the rising sun, with the shadow in the west as the sun rises in the east. Illustration: Bob King
Eclipses are about shadows. We can see the cause of this eclipse anytime it’s clear around sunset. Face opposite the setting sun and you’ll see a long, purple-gray band rising in the eastern sky. That’s the Earth’s shadow. Standing here on the planet, the shadow rises and eventually fills most of the sky by local midnight. We don’t see it at night of course because it’s dark out and the shadow blends into the sky.
The Earth's shadow has two parts, the dark inner umbra and the outer penumbra.The penumbra is not fully dark because a portion of the sun shines into it. Credit: image courtesy of Courtney Seligman
We can see the space station and other satellites disappear into or re-appear out of the shadow on many nights depending on the satellite’s altitude and season of the year.
If we could see the Earth's shadow at the moon's distance it would look like this - a double bullseye covering a surprisingly small patch of sky. Shadow size is approximate. Created with Stellarium
Earth’s shadow stretches out behind the planet to the moon and beyond. At the moon’s average distance of 239,000 miles, Earth’s two-part shadow, consisting of the dark umbra and sun-touched penumbra, covers only about 2.3 degrees of sky or a bit less than five full moons lined up side by side. The distance between the two stars at the end of the bowl of the Big Dipper is twice as wide. That’s a small target.
Times and the moon's path through the outer part of Earth's umbra are shown for tomorrow morning. Add an hour for Eastern time, subtract an hour for Mountain and two hours for Pacific. Credit: Tom Ruen with my own additions
Since the moon’s orbit is inclined to the Earth’s, it easily misses the shadow most full moons. Only occasionally does it hit the mark.
If you’ve been following the transit of Venus updates, you’ll recall that Venus’ inclined orbit is the reason transits are so rare. The same is true with the moon.
I hope you do take a look at tomorrow’s eclipse. Don’t let the routine early risers have all the fun. For more details on the event, check out my earlier blog and this NASA pdf file.
Tycho (button-like crater below and left of center) and its rays dipped in the colors of another partial eclipse on August 16, 2008. Click photo to see more eclipse photos. Thanks and credit to: Pedro Ré
During the eclipse, you’ll see the shadow cover the bottom or southern half of the moon. This is Tycho territory. Tycho, a 53-mile- diameter crater, is crowned with the most spectacular and largest halo of “rays” of any lunar crater. The rays formed when material from the impact that created Tycho rained back down onto the moon’s surface to excavate strand upon strand of fresh secondary craters. Tycho and its rays, normally a blaze of white streaks centered on the brilliant crater at full moon, will be tempered a mellow yellow and orange during eclipse.
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