You have probably seen photos of the sun in total eclipse such as the one shown above And although it is accurate and may look quite unusual and interesting, such pictures really do not capture the experience at all adequately. Many, probably most, people who have seen them consider it to be by far the most spectacular natural phenomena they have ever witnessed...an “experience of a lifetime.” I have known very experienced world travelers who have been all over the place and seen all sorts of famous and rare sights, man-made and natural, to react to a total eclipse by screaming, yelling OMG!, jumping up and down, running around hugging people, staring with their mouths hanging open, talking about it incessantly for days, etcetera. It is hard to understand from one of the standard eclipse photos why it is quite that impressive, I think. I want to try to explain a little.
The basic explanation is that being in the dark heart of the Moon's shadow involves much more than just seeing a striking and unusual visual in the sky, although that is a major part of it of course. But much else happens, depending somewhat on the location and weather conditions where you view it. Someone said that it is like suddenly being in some sort of CGI of another world or maybe like a drug-induced hallucination that feels (and is) totally real. Here are some of its aspects:
The light coming from the faint halo of gas around the sun (the corona) has a different color than any light you have ever seen before. That gas is at temperatures of a few million degrees, which is far, far hotter than the solar surface (only about 10,000 degrees Fahrenheit, or 5,800 kelvins). To the eye it typically looks like a slightly bluish white, but the strange hue of this illumination makes the colors of everything around you look quite weird and vaguely unreal.
The black of the moon's night side (the black circle inside the halo of white light) is a profoundly dark black, much blacker than the night sky normally is, although it requires you to be in a place with little or no artificial lighting and atmospheric conditions with no haze or dust to really appreciate how black it is. It is also helpful to be well dark-adapted before the totality starts; this requires wearing a completely black/dark eye-mask or blindfold over your eyes for some time up until the totality begins...not terribly convenient in most circumstances.
There often appears to be a beautiful twilight sky all around the horizon for 360 degrees, providing a brilliantly colored " frame" for the eclipse itself. The wind often starts gusting around in a sort of unstable way, blowing first one way and then another, dying and then starting up again. (I don't know why this happens.) Animals may freak out and behave strangely or start acting as if it were night and going to sleep or coming out (if they are nocturnal). People can act pretty oddly, too, as noted above.
The temperature will drop noticeably in a very short time if it is not too humid at your viewing site. In a very dry environment, like on top of a mountain or in a desert, it may drop 10 to 20 degrees F in just a few minutes.
Just as totality starts and ends you can often see other optical effects such as the eclipse “diamond ring,” Bailey's beads and “shadow bands.” These can all add to the spectacle. (The shadow bands are usually difficult to see and are most easily visible on a smooth featureless pale surface, such as a white sheet laid out on the ground or attached to a wall facing the eclipse.) In addition, if you are viewing from a favorable location (such as a high place overlooking a valley or plain), you may see the moon's dark shadow approaching or departing across the terrain at roughly 1,000 miles per hour.
So that's the story of what you can expect—but let me add a few important points about viewing the eclipse:
First and most importantly, you will not see anything particularly spectacular unless you are in the path of totality, where the sun is 100 percent covered by the moon—and you will want to be as close to the center of that path as possible to get the maximum duration of the eclipse (which is just a few minutes at best). The standard astronomical way of describing eclipses is highly misleading in this respect. It simply states the maximum fraction of the sun covered by the moon as seen from some location. So a total eclipse is 100 percent, but over a much wider area of the world, the moon will cover a smaller fraction, say, 95 percent or 80 percent or 40 percent or whatever.
A reasonable person might conclude that an 80 percent eclipse is 80 percent as interesting/spectacular as a 100 percent eclipse. That is completely wrong. As one illustration, it gets about 10,000 times darker when the moon covers the last 1 percent of the sun's surface! Moreover, most of the effects mentioned above occur only during or just before/after totality and not at all outside the path of totality. A total solar eclipse is also nothing like and enormously more spectacular than a total lunar eclipse. I have traveled far across the world to see the former and sometimes not even bothered to step outside my house to see the latter.
Second and also critical to having the experience, despite what you might hear in the media, it is utterly safe to look at the eclipse with your naked/unprotected eyes during the few minutes of totality. Before and after totality, however, when the moon only covers a portion of the sun (as well as when viewing previously mentioned phenomena such as the diamond ring or Bailey’s beads) is when it is necessary to use eye protection, project images of the sun through a pinhole or telescope for indirect viewing or whatever. If you do not use such precautions before and after totality, you risk eye damage. But if you use such eye protections during totality, you will almost certainly see nothing at all. Sometimes public health authorities and/or the media advise using eye protection throughout the eclipse in an effort to err on the side of caution—but if you’re going to do that, there is no point in trying to see the total eclipse at all.
Third, the location from which you view the eclipse can make a big difference. The August 21, 2017, total solar eclipse path goes all the way across the U.S. from the Oregon coast south of Portland to the South Carolina coast at Charleston. Maps are available here.
Fourth, although it is very tempting to try to photograph the eclipse, your pictures are almost certain to be disappointing unless you are an expert photographer using fairly elaborate equipment, and you will almost certainly be distracted from fully appreciating the brief spectacle of totality by the effort to capture a good image. I suggest concentrating on the eclipse and forgetting about photography during totality. There will be lots of excellent photos available online afterwards anyway.
You have to be in the path to see totality. And the closer you are to the blue line in the center of the path as shown on the maps at that link, the longer the totality will last. If it is cloudy where you are, it will get dark but there will be little else to see, and the probability of clouds varies from place to place. The probability of cloud cover along the path of totality is given at this site: Beyond being in the path of totality and having clear skies, the factors that tend to make the eclipse more spectacular are absence of artificial lighting, low dust and haze conditions, low humidity and good, wide views of the surrounding terrain and horizon. Obviously there are also practical considerations such as roads and traffic, a place to stay, food and so forth.
I hope that you get to see it and that I haven’t built it up too much and so cause you to be disappointed as a result. But in my experience, a good view of a total solar eclipse disappoints very few people.