By Wendy Langhans
Q. What do the words umbrella, umbral and “take umbrage” have in common?
A. They all derive from the Latin word umbra, which means shade or shadow.
To an astronomer, umbra means “the conical part of the shadow of a celestial body excluding all light from the primary source”. When a lunar eclipse occurs, that shadow is cast by the earth, on the moon, as the earth blocks the light of the sun.
On Saturday, December 10, we will have an opportunity to see a total lunar eclipse. According to this NASA article, the eclipse begins about 4:47 AM, PST and by “6:05 AM Pacific Time, the Moon will be fully engulfed in red light”.
Eclipse begins: 4:47 AM
Totality begins: 6:05 AM
Sunrise: 6:47 AM
Moonset: 6:51 AM
But why is the moon “engulfed in red light? After all, the moon is passing through the earth’s shadow? Aren’t shadows suppose to be dark? The answer has to do with earth’s atmosphere. While the moon is in earth’s shadow (umbra), it is still illuminated by indirect and refracted (bent) sunlight. As described in this ScienceDaily article, “Stronger atmospheric scattering of blue light means that the light that reaches the lunar surface is predominantly red in color so observers on Earth see a Moon that may be brick-colored, rusty, blood red or sometimes dark grey, depending on terrestrial conditions.” Because atmospheric conditions vary – for example, in the amount of volcanic ash – eclipses can vary in their color and light intensity.
Astronomers have adopted a five-point scale, the Danjon scale, to describe the moon’s visual appearance during an eclipse:
“L = 0. Very dark eclipse. Moon almost invisible, especially at mid-totality.
L = 1. Dark Eclipse, gray or brownish in coloration. Details distinguishable only with difficulty.
L = 2. Deep red or rust-colored eclipse. Very dark central shadow, while outer edge of umbra is relatively bright.
L = 3. Brick-red eclipse. Umbral shadow usually has a bright or yellow rim.
L = 4. Very bright copper-red or orange eclipse. Umbral shadow has a bluish, very bright rim.”
Saturday’s eclipse is predicted to be in the L4 range – bright copper-red. But what about that bluish rim? What causes that? Again, it’s the atmosphere – specifically, the ozone layer. According to Dr. Keen in this NASA article, “Light passing through the upper stratosphere penetrates the ozone layer, which absorbs red light and actually makes the passing light ray bluer. This can be seen as a soft blue fringe around the red core of Earth’s shadow.” He says the best time to see the bluish rim on Saturday morning is at the beginning of the eclipse.
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Looks like I’ll be getting up at o’dark hundred on Saturday morning.
I sure hope my photos turn out better than the last time. Maybe I’ll try using a tripod.
(And if you happen to be up and about at that early morning hour, I encourage you to take a pictures of the lunar eclipse and e-mail them to us at firstname.lastname@example.org.)
Upcoming Outdoor Events:
Trail Maintenance Schedule. Come join our volunteers as they help maintain our trails. Contact Steve at email@example.com for time and place.
Wednesday mornings, December 7, 14, 21 & 28.
Saturday mornings, December 10.
Saturday, December 17, 8-10 AM. Holiday Birds at Towsley Canyon. The year end has a long tradition of bird watching and counting. Let’s celebrate our local birds on this easy hike. Beginning birders are welcome. Binoculars optional. Meet at Towsley Canyon’s front parking lot. For a map and directions, click here.
Friday, December 23, 8-10 AM. Get the kids out of the house! Pico Canyon. We have just the right plan to get the kids and visiting family out of the house so you can wrap presents. Send them to the park for a couple of hours of fresh air and cool nature stuff. Meet in the parking area at Pico Canyon (opposite historic Mentryville). For a map and directions, click here.
You can listen to stories like this every Friday morning at 7:10 a.m. on “The SCV Outdoor Report”, brought to you by your hometown radio station KHTS (AM1220) and by the Mountains Recreation and Conservation Authority.
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