Friday, April 3, 2015

Tonight's Sky for April 3-4: Full Moon/Total Lunar Eclipse

Tonight and leading into tomorrow morning (hence April 3 and 4's feature), sky watchers in North America will be treated to one of the most spectacular, and rare, of all celestial sights: a 
total lunar eclipse. So, with the eclipse coming within a day, why not start understanding what you will see and why you will see it now?

Total lunar eclipses occur when the Sun, Earth, and Moon fall exactly into line ion that order. Unfortunately, because the Moon orbits the Earth on a slightly tilted axis, the Moon rarely falls into earth's shadow, thus becoming eclipsed. Or perhaps this is a good thing as, if there was an eclipse at every Full Moon, eclipses wouldn't be all that special, would they? Okay, personal opinions aside, every now and then, at a point on its orbit called a “node,” the Moon crosses into the Earth's shadow, thus resulting in an eclipse.
So, what can one expect to see?

First, the eclipse takes place over the course of several hours. First up: the penumbral phase where the Moon moves into the lightest part of Earth's shadow, called the penumbra. At this point, one may or may not notice a slight darkening of the Moon.

See also: Start to finish lunar eclipse gallery.

Next up: the partial stages. In the partial phases, the Moon starts moving into the darkest part of the Earth's shadow, called the umbra. In this phase, the Earth's shadow will start to eat into the corner of the Moon, eventually coming to the point where the Moon looks like a crescent, but at an otherwise impossible angle. In time, more and more of the Moon will disappear into the Earth's black shadow until the entire lunar disc is consumed.

Phase 3: totality. Near the point where the Moon completely disappears into the Earth's shadow, it will begin to take on a very distinctive, reddish color thanks to the scattering of light rays caused by our atmosphere. Basically, the particles in the air scatters all the colors of the visible spectrum, with the exception of the reds, away into space, thus only allowing the red light to fall on the Moon. Totality can last for around an hour, give or take a few minutes either way. For something interesting, compare the number of stars you can see during totality to the number you can see when the Moon is full. Basically, totality is effectively a Moonless sky. For the record, deepest eclipse will take place about 6:30am EDT on the morning of the 8th. Live West of the Eastern Time Zone? Well, subtract an hour for each time zone West you reside.

After totality ends, the Moon will again go through partial phases, becoming more and more exposed as time progresses. In time, the partial stage will end, the second penumbral stage will begin, and the the Moon will eventually go back to normal.

Now, before getting too excited, there are two points to consider with this eclipse . . .

First (and not unusually) not everyone in North America will be privy to the event from start to finish. For anyone living in the Eastern U.S., you will only get to see a partially obscured Moon at moonset. Moving to the Great Plains, a totally eclipsed Moon will be visible at moonset while people living on the West Coast will get to see the Moon start to reemerge from totality as it sets. To see the event in entirety, living in Alaska or Hawaii is a must.

As the second item of note, totality will only last about 5 minutes, making this the shortest totality of a total lunar eclipse in over 100 years. For people familiar with eclipses, the far-rarer total solar variety often last as long if not a few minutes longer! Needless to say, be sure to check your local times !

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