Total Solar Eclipse 2017 - Information for Educators

Total Solar Eclipse 2017
Monday, 21 Aug 2017

The Basics:
What is a total solar eclipse?

Here is perhaps the greatest eclipse photograph ever created.
It is a composite of many many exposures combined digitally by the master Miloslav Druckmüller.
It is honestly as close as a picture can be to the full effect of what you see (though you don't see so many stars!).
Please note how this doesn't look like ANY other eclipse photo you've likely seen!

There is a lot of good information out there, that we can't really add a lot to.
Digest the info on the sites below, and you'll understand quite a bit about eclipses!

· Here is a great animation to show you how the moon's shadow traces out a path along the earth's surface. With this, you can see why (a) the path on the earth's surface that you have to be in is so small, and (b) why you have to be in that path in order to see the whole sun blocked by the moon (a TOTAL solar eclipse).
· Here is a page that has a narrative description of what an eclipse is.
· Another wonderful site with descriptions and animations.
· Yet another site with a quick but nice description of what you see during an eclipse.
· A cool animation of the moon's shadow racing across the earth in 2006, as seen from a satellite in orbit!
· Another site from South Africa with lots of good eclipse information!
· Fred Espenak is truly 'Mr. Eclipse', and here is his site for beginners!
· Here's a site with a little more technical bent.
· A great site by Xavier Jubier
· And a wonderful article in Nature by noted eclipse chaser Jay Pasachoff

And there are a lot more great sites out there!

OK, so now you know what causes an eclipse, and you're ready to go to the maps to see if you can see totality from where you live. (If not, then please get on down the road and find a spot that's within the shadow!)

But first, some answers to questions you may have (that we haven't been able to find good animations of on the Internet):

I've looked at the maps, and read the narrative of the path's travels over the United States. If the path of totality goes over all these cities from Oregon to South Carolina, then why doesn't every place in the path get the same amount of totality?

We've put together an animation (greatly exaggerated!) to explain this -- go here to see it. There are a couple of things going on. First, as the shadow passes over the earth, the earth's curvature causes observers farther away from the midpoint of the eclipse path (near Hopkinsville, KY for the 2017 eclipse) to be farther away from the moon. This makes the moon look smaller, so it takes less time for it to pass across the face of the sun. The other way to look at this is that these observers are in a part of the shadow cone where its cross-section is smaller. Again, less shadow to be in. But second, the shadow moves across the earth faster as we get closer to the ends of the path. These two effects combine to produce less totality at the centerline (and at other corresponding points within the path) nearer the path's ends.

I know why I have to be in the path of totality to see a total eclipse, but I don't get why I need to be near the centerline.

You will want to see as much totality as possible - every second counts when you're confronted with this magnificent a sight! The reason you need to be near the centerline to get as much totality as possible is simple geometry. The shadow is an oval, but we can get away with calling it a circle for the sake of discussion. (The same statements apply to ellipses as circles, but the math - and the animations - are much more complicated!)

Look at the animation below. This is the shadow of the moon traveling over the surface of the earth at a particular spot. You can see that the observer at the centerline is getting to be in the shadow more than the observer closer to the edge of the path, simply because the diameter of the circle is longer than the chord of the circle that passes over the non-centralized eclipse chaser!

True, this effect is rather slight until you get right to the edge (where the time of totality falls off dramatically), but it will still account for more than a few seconds of time lost in the shadow!

Now after having said all that, there are good reasons for wanting to be close to the edge - you get beautiful effects during the entire eclipse (although it is shorter). In fact, we will be setting up some experiments using viewers stationed along the path edges. But if all you want to do is look, then try to get to the centerline. Whatever you do, though, make sure you're IN THE PATH!