Preparing For Monday Night’s Lunar Eclipse – Part I

During a total lunar eclipse, the moon passes through the outer part of Earth's shadow, called the penumbra, and then into the dark umbra. When fully immersed in the umbra, the moon is in total eclipse or in totality. Credit: Sagredo

We’ve got this little thing happening Monday night-Tuesday morning that I thought you’d like to know more about – a total eclipse of the moon, the first visible across the Americas since February 2008. Eclipses of both the moon and sun are uncommon enough that each is special and not be missed. Lunar eclipses always and only happen at full moon, when the sun, Earth and moon are lined up in that order.

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, lo and behold, 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.

The amazing photo montage at left combines an image of Earth, taken by the Apollo astronauts, and one by the Japanese lunar probe Kaguya during a lunar eclipse in 2009. A person standing on the moon during a total lunar eclipse would witness the Earth eclipsing the sun. The pink ring around the Earth is our atmosphere aglow with the combined light of all the sunrises and sunsets on the planet. At right, the fully eclipsed moon takes on their color. Credit: JAXA (left), Jim Fakatselis

Why red? Picture yourself standing on the moon looking back at Earth during totality. From this perspective, the Earth eclipses the sun (above) with sunlight grazing the edge of the planet at a very shallow angle. This is exactly how we see the sun at sunset or sunrise when it grazes or nearly touches the horizon. Because we’re looking back at Earth from the moon, we get to see a full 360 circle of sunrises and sunsets around the entire planet. Light from this fiery sunlit rim falls into the umbra and colors the moon a ghostly red. Fabulous, isn’t it?

The Earth's shadow takes a big bite out of the moon during the partial phases before totality. Credit: NASA

The moon’s color changes as the eclipse progresses – especially as it nears totality – makes for a very special sight. At the same time our planet’s shadow devours the moon by degrees, look around at the landscape and the sky. Everything is getting darker. Before the beginning of the eclipse, the snow was bright and shadows sharp and the number of stars visible in the sky minimal. During totality, the light that once lit your path vanishes, and the sky returns to its starry grandeur. I guarantee the transition will blow you away.

You don’t need any optical aid to view a lunar eclipse, though a telescope or pair of binoculars will allow you to see subtle color variations across the moon’s face and spot stars right up to the moon’s edge, something otherwise impossible to do during full moon.

This diagram shows the moon at different times during the eclipse. The penumbra or outer shadow is pale because it contains a mix of shadow and sunlight. You notice a slight penumbral shading about a half hour before the partial eclipse begins at 12:33 a.m. That's when the umbra, the dark inner shadow, takes it first bite of lunar cheese. Totality occurs when the moon is immersed in the umbra and lasts 72 minutes. Credit: Adapted from F. Espenak, NASA/GSFC

The eclipse will be visible across the entire Western Hemisphere with the moon high in the sky during totality. I realize the hour is painful – the partial phases begin after midnight for the Midwest. Totality, the part you really want to see, stars at 1:41 a.m. Tuesday morning. Yikes! It’s at times like these that I trot out my buddy Glenn’s take on celestial spectacles: “Bobby, there’s plenty of time to sleep when you’re dead.” His candor has helped me forge on through more than a few late nights, and if the weather cooperates, will do so again.

Tomorrow in Part II of our guide, we’ll take a closer look at what to look for during all phases of the eclipse and check out weather prospects for the Duluth region and across the country. See you then.