Your eclipse guide (with the things not in many eclipse guides)
I will be heading to western Idaho this weekend to watch my sixth total Eclipse. That makes me a mid-grade eclipse chaser, so let me tell you some important things you need to know, which are not in some of the other eclipse guides out there. For good general sites look at places like NASA's Eclipse Guide which has nice maps or this map.
Totality is everything
The difference between a total solar eclipse and a partial one -- even a 98% partial one -- is literally night and day. It's like the difference between sex and holding hands. They are really two different things with a similar sounding name. And a lunar eclipse is again something vastly different. This does not mean a high-partial eclipse is not an interesting thing, but the total eclipse is by far the most spectacular natural phenomenon visible on this planet. Beyond the Grand Canyon, Yosemite, Norway, etc. So if you can get to totality, get there. Do not think you are seeing the eclipse if you don't get into the zone of totality.
People debate about how total it should be
Many people seek to get close to the centerline of the eclipse. This provides the longest eclipse for your area. You will only lose a modest number of seconds if you are within 15 miles of the centerline, so you don't have to get exactly there, and in fact it may be too crowded there.
On the other hand there are those who deliberately get close to the edge, giving up 30-40% of their eclipse time in order to see more "edge effects." Near the edge, the edge effects are longer and a bit more spectacular. In particular the diamond ring will be a fair bit longer, and you may see more prominences and chromosphere for longer. If this is your first eclipse, I am not sure you want to get too close to the edge. But try any of the map web sites that will tell you your duration, and get somewhere that has within 30-40 seconds of the centerline time.
You look at the total eclipse with zero eye protection
You've been hearing endless talk about eclipse glasses and how well made they are. Eclipse glasses are only for the boring partial phase. They give you a way to track the progress of the moon while waiting for the main event. Once totality is over, everybody packs up and does not even bother to watch the 2nd half of the partial eclipse, that's how boring the partial part is.
But don't be one of those people who, told about the danger of eclipses, does not watch totality with your bare eyes. In fact, use binoculars in addition to your naked eyes, and perhaps a short look through a telescope -- but not during the diamond rings or any partial phase.
Update: There is a nice large sunspot group that should still be there on Eclipse day, making the partial phase more interesting to those with good eyesight.
In totality you are looking not at the sun, but its amazing atmosphere -- the "corona" -- full of streamers, and many times the size of the sun or moon. You may also see jets of fire coming off the sun, and at the start and end of totality you will see the hot red inner atmosphere of the sun, known as the chromosphere.
If you are crazy enough to be outside the total zone but close to it, you still can't look with your bare eyes at any part of the eclipse.
There are some cool things in a 99% partial eclipse (which you see just before and after totality.)
An eclipse is most glorious in the sky but a lot of other things happen around it. As it gets very close to total you will see the nature of the sunlight change and become quite eerie. Shadows of trees will turn into collections of crescents. About 20-60 seconds before and after totality, if you have a white sheet on the ground, you will see ripples of light waving, like on the bottom of a giant swimming pool. And the shadow. You will see it approach. If you are up on a mountain or in a plane this will be more obvious. It is going at 1,000 to 2,000 miles per hour. You will also see things like animals getting ready to go to bed as though it's night. All leading up to the big event.
There are some things to look at during totality other than the eclipse
Don't spend too much time on them, but unless your eclipse is very short, look around. Notice it looks like sunset in every direction. Notice some of the stars and planets are visible to you. Notice a little about the people around you. Then get back to the eclipse.
The diamond ring is the most spectacular part of the eclipse
This is the tricky part. Just before the partial phase ends or starts again, there is a moment when the sun is 99.9% eclipsed and one or a few tiny spots of sun shine through. We call this the diamond ring, and I have included a picture. It's amazing. It is easy to see it when the eclipse ends, as this is how it ends. Seeing the starting diamond ring is more difficult, as you must time it right. The diamond ring is not perfectly safe to look at -- don't look at it in a telescope -- but it is OK to watch it for just a few seconds. And you should. For me, each time it has been obvious when to look away, but if in doubt, grasp it quickly and then avert your eyes. Or actually, what I do is close one eye and hold out my thumb and cover the bright spot -- and I then can watch the eclipse for another 10-20 seconds, as it is still there. The main reason the diamond ring is dangerous is you may think it's the most amazing thing you've ever seen and not look away when your senses tell you too. Be ready for that.
Traffic might be real bad
Nobody is sure about this. There are eclipses every year or two, but other than a few, they have not taken place in rich areas where 100 million people have cars and live within driving distance. Where they are all on the internet being told about how great the eclipse is. Europe handled it in 1999, but who knows what the traffic will be the morning of the 21st. So if you are serious, sleep in the path the night before, don't expect to drive after sunrise. It might work. 95% it will work. But why risk it?
On the other hand, you and everybody else will be checking online weather forecasts. If your chosen site is going to be cloudy, you will have to move. Traffic may limit your ability. Lucky people in planes will be able to move to airports in the path or even see it from the air.
I chose my site based on the best weather odds, but now, 7 days out, there are no longer odds, but actual forecasts (though of low accuracy this far out.) So if you have not chosen your site, do it from the forecasts, not the odds. Be careful, the cooling effect of an eclipse (10 degree drop) can change the weather and cause clouds.
(Seeing it from the air is spectacular too, though I have not done it, but it lacks some of the grandeur of being on the planet while you see it.)
Don't photograph the eclipse
Really, don't. I mean it. If this is your first eclipse, do not attempt to photograph it. Especially not with your cell phone, except as I tell you below. Nobody has ever made a photo of an eclipse that looks like an eclipse. You won't either. Don't waste your time trying.
Since I am on Eclipse #6 I will photograph it, but I have the serious gear you need to do a decent job. And part of that gear is software to automate all the photos. I am going to set it up, adjust it about 5 minutes before totality and then leave it to the computers. (This means I need a clock drive to track the sun.) I will be watching.
You can do some automated photography, too. Set up a video of you and your friends. The eclipse will be high in the sky so it may not be in it, and it doesn't have to be. Start the video well before totality and leave it be. Consider both fixed exposure video and auto exposure. Time lapses can also be cool -- but perhaps just shoot video and turn that into a time lapse for the partial phases and regular speed for total. Eclipsedroid can also be programmed to take photos at the special times in the eclipse with your phone -- but put your phone in a tripod or clamp phone holder and leave it to do the work.
I took some decent photos of my 4th eclipse and the story of how we got to Enewetak, site of the first hydrogen bomb, is a cool one too. But I mistakenly wasted a lot of my 2nd eclipse photographing it. Don't be me!
I did not photograph my first eclipse in 1991 -- but you can see the T-shirt I designed and sold for it. With that and buttons I paid for my eclipse expedition! Oddly, Corona beer had an eclipse T-shirt but it did not clue into the pun at all.
Don't check it once you are 2 minutes from eclipse. Set it and forget it.
Also note that autoexposure may not be your friend. First, it takes away the great change of "day and night" that an eclipse represents. Secondly, most cameras tend to vastly overexpose the corona. So you get a fuzzy black circle covered by a big overexposed white blob with some streamers off of it. Looks even less like an eclipse than most photos. This is what you will get if you try to take a photo of yourself or friends with the eclipse behind you.
Get an eclipse App
There are some apps for Android like EclipseDroid and Solar Eclipse Timer for iPhone which will read your GPS and give you the eclipse timings for your location. They will send you alerts as the eclipse approaches and even let you catch the first diamond right, which happens at what we call "second contact." (The 2nd diamond ring is obvious.) There is also software for Windows and Mac, often meant to control cameras. (I will be using such programs.)
What if I get clouded out?
It happens, though often you see parts of the eclipse through the clouds, just as you see the full moon through them. But while a total eclipse is the most amazing thing there is to see on the Earth, and a cloudy eclipse is a very poor substitute by comparison, a cloudy eclipse is still a pretty cool thing to see. While I would not travel the globe to see an eclipse under clouds, or an annular eclipse (which is one where the moon is too small to make it total, so it's a very special 99% partial) I would travel a modest distance to see the world get dark in the most curious way. So go, even if it's cloudy. I have heard many people who, never having seen a clear total one, claim they were quite pleased with the cloudy total they saw.