Penumbral eclipses are for those who like the subtler things in life. During a typical lunar eclipse like the one we enjoyed last September, the moon slips at least part-way into Earth’s dark, inner shadow called the umbra, which bites into the lunar disk in dramatic fashion. In a penumbral eclipse, the moon passes through only the outer part of Earth’s shadow, called the penumbra.
Lunar eclipses always happen at full moon, when the sun, Earth and moon are lined up in a row in that order. Because the moon’s orbit is tilted a bit with respect to Earth’s, the moon normally glides a little north or south of the shadow cone and we don’t get an eclipse. When the lineup is precise, the moon moves through the penumbra and right through the umbra for a total eclipse, but when the lineup is close but not exact, it misses the umbra and instead slice through the penumbra.
From inside the umbra, the sun is covered by the bulk of the Earth, making the umbral shadow very dark. The penumbra on the other hand combines a modest amount of shadowing by Earth’s globe along with sunlight from the part of the sun not hidden by Earth as shown in the diagram below.
The mix of sun and shadow is why the outer shadow looks so pale compared to the umbra; penumbral shading isn’t obvious until the moon’s about halfway to the umbra. You know you’re seeing the penumbral shading, when something’s not quite right about the moon’s appearance. Part of it will look lightly “stained” or soiled.
Wednesday’s eclipse will be the only one visible from the western hemisphere this year and takes place during early morning hours over most of the U.S., Canada and Central and South America. From the eastern states, the moon will have set before it reaches deepest into the penumbra and may be barely noticeable; Midwesterners will see the maximum of the eclipse – such as it is – with the moon hovering just above the western horizon 15-20 minutes before sunrise, while those living further west can watch most if not the entire event.
Again, the eclipse will only be slight. You’ll probably begin to notice a slight shading across the bottom of the disk about a half hour into eclipse. At maximum, the shadow will be still be subtle but much more obvious.
En route to the eclipse, the waxing gibbous moon passes close to the bright planet Jupiter in Leo the Lion tonight. If it’s cloudy or you otherwise can’t make the penumbral eclipse Wednesday, enjoy this little cosmic bone. Last night, the moon was surrounded by a wonderful variety of colorful coronas from passing cirrocumulus clouds. You never know what you might see when you look up.
Wishing you clear weather for Wednesday’s shady affair!