Lunar Eclipse Alert!!!
Observing > Next week, on Wednesday the 20th, we are all invited to a celestial phenomenon for the whole family. At about 6 in the evening, in the eastern skies, our Moon will rise full but not like it usually does. There is a twist tonight. As it rises it will be crawling into our shadow in an event we call a total lunar eclipse.
The timing of this particular eclipse is perfect for those of us in southern California, and we won't get it this good again for a couple years. We essentially get to see the whole show at a reasonable hour. Now we may miss part of the first act, but that's OK; the opening act is a snoozer. Here is a synopsis of our play.
Every month the Moon makes one trip around our planet. During that circuit it manages to get itself on the side of the earth opposite the sun. When it does this it risks going through our huge shadow. Because the Moon's orbit is slightly tilted, it doesn't always manage to hit the shadow, but it will on the 20th, and thus we have ourselves an eclipse.
The whole process takes several hours, which will give you some time not only to relax and enjoy it, but some time also to observe some other cool science things.
When the moon first peeks over the eastern horizon, it will already be fully into what is called the earth's "penumbra." The penumbra is just the partial shadow an object lays down. You have unwittingly experienced it yourself. As the sun rises in the morning from behind a distant hill or mountain, we do not see the whole sun yet. When it first shows itself, when all we see is part of it, we are in the partial shadow - the penumbra - of the earth. The sun is neither completely hidden from us, nor is it blazing in all its full glory.
This is what the Moon is experiencing when an eclipse begins. For someone on the Moon, the sun appears to be moving behind the earth. But a penumbra is still so bright, most of us would never notice it.
It isn't until the Moon moves completely into the full shadow, the umbra, that the show really gets underway. This will have just begun as we see the Moon rising Wednesday. You will see it maybe a third covered in the dark umbral shadow.
Notice at this time, too, after the sun sets, how the whole eastern horizon seems to be lifting up in a purple haze. That curved semi-darkness is the earth's shadow, the same shadow the Moon is moving into.
Over the next hour or so, until about 7, watch how the shadow crosses the face of the Moon, darkening it. Notice also how the shape of the shadow is curved. It is always curved. This fact led ancient Greeks to believe the earth was a sphere, the only shape that always has a curved shadow.
The darkness will last for about an hour, it takes that long for it to pass through. It is at this time you might notice that the Moon takes on a reddish appearance. That would be because our own atmosphere bends the light from the sun, like a lens, toward the Moon. The only wavelengths from the sun long enough to make it through all our atmosphere and get the free trip to our satellite are from the red end of the spectrum. More poetically, all the world's sunsets are bleeding towards our Moon.
Just before 8 PM, the Moon will have reached the other side of our shadow. The umbra will begin to give way to the penumbra, all the main action having been completed. By about 9 PM the umbra is gone; by 10 PM the nearly invisible penumbra will fade by and the entire show is over.
This will be the last time to see a total lunar eclipse at a reasonable hour for a couple years. You and your family, or school class, or scout troupe might want to make it a nice evening of observation of one of the heaven's free dramas.
Posted by Mark Ritter at 2008.02.10 04:03 PM | Comments (0)
