What is a total solar eclipse?
· 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).
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!)
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 well as circles, but the math - and the animations - are much more complicated!)
Look at the animation below. This is the outline of the shadow of the moon traveling over the surface of the earth at a particular spot. You can see that observer #2 at the centerline is getting to be in the shadow more than observer #1 (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 an observer like #1 - who chose not to be on centerline!
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 indeed some 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, or if this is your first eclipse, then try to get to the centerline. Whatever you do, though, make sure you're IN THE PATH!