On Monday, August 21, 2017 a total solar eclipse will be visible to anyone standing within a 60 to 70 mile-wide Path of Totality stretching across the United States.
As the moon's shadow will entirely block out the sun only for those within that narrow track, eclipse-chasers in parts of Oregon, Idaho, Wyoming, Nebraska, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina, and South Carolina will have the best views of the solar corona. Clear skies allowing, of course.
It's the first time in 99 years that a total solar eclipse will sweep across the entire continental United States — and the event will only last about two-and-a-half hours from beginning to end.
Most of the event (and all of it, if you're outside the Path of Totality) is a partial eclipse, which begins when the moon's shadow intersects with the sun before slowly moving across it. When the moon covers the sun completely, after about 90 minutes, this is what's known as Totality.
It's these few minutes that are so precious — and what 10 million-plus people are traveling from all over the world to see.
You can find out exactly what you will experience from where you plan to be on August 21, 2017 by going to the Eclipse Megamovie 2017 Simulator. Eclipse cartographers have been hard at work, too: Xavier Jubier's Google Map and Michael Zeiler of GreatAmericanEclipse.com's handy web app are also worth investigating.
After Totality is over, the whole thing goes into reverse, as the moon slowly moves away from the sun. As a result, spectators will get to see another partial eclipse before the celestial event wraps.
First Contact: 0 to 80 Minutes
A total solar eclipse begins with a long partial eclipse phase lasting around one hour. The edge of the moon touches the sun, and takes a bite that slowly gets bigger. Everyone needs solar eclipse safety glasses to watch this phase.
The moon will gradually cover the sun over the next 80 minutes or so, and when it's about 90 percent covered, the light will begin to fade dramatically. The result? An eerie, silvery atmosphere. Tiny crescent suns may be scattered on the ground beneath trees. Shadows become sharper, and it begins to get cooler.
How to see shadow snakes
Just before Totality, dark shadow snakes, or bands, can sometimes be seen rippling across flat surfaces like sand, the sides of vehicles, and snow. It's a little-understood phenomenon, but shadow bands may have something to do with crescent sunlight being refracted through the Earth's atmosphere. Most people won't see them during this eclipse, but keep an eye out.
How to see Baily’s beads
Just seconds before the sun gets almost completely covered by the moon, beads of the only remaining visible sunlight will be filtered through the mountains and valleys of the moon. Now is the time to remove your solar eclipse glasses.
How to see a diamond ring
Baily's beads recede as the very last rays of sunlight create a brief 'diamond ring' flash around the Moon. The Moon now covers the sun 100 percent, and the silvery twilight recedes to darkness. That signals the start of Second Contact.
Second Contact and Totality: 80 to 82 Minutes
As the sun, the moon, and Earth line-up precisely with each other, the mighty solar corona is visible. It appears suddenly, as if the result of a pushed button. It might look like a hole has opened up in the sky.
How to see the solar corona and prominences
You must remove your solar eclipse glasses to see this, which is surely one of nature's most spectacular sights. The corona appears as wispy fingers of white light swaying in space. Some have likened it to a white flower, or cotton candy smeared around the moon. If you have binoculars, look around the edges for bright red solar flares — called prominences — on the surface of the sun. Look around you briefly, even behind you, and see how the colors differ across the sky.
How to see stars and planets during Totality
Most obvious during Totality will be the bright planet Venus on the right of the sun and moon, though Jupiter may also be visible on the far left. Mars and Mercury are also close to the eclipse, though they may be difficult to see with the naked eye. Regulus — the brightest star in the constellation of Leo — will be a little to the left of the sun and the moon, and visible to those using binoculars or a small telescope.
Third Contact: 82 to 160 Minutes
As the moon moves away from the sun, the entire spectacle goes into reverse. Bailey’s beads appear to the top-right until another diamond ring flashes for a split second. Suddenly, the sun has returned. Put your solar eclipse glasses on again for this phase. You still have approximately 80 minutes of a partial eclipse to enjoy.
The moon will continue to edge away from the sun, and it will appear to kiss the sun's edge farewell as it resumes its more conventional 28-day orbit of Earth. The spectacle is over, and everyone asks the same question: when is the next eclipse?
What will happen if it's cloudy?
If it's cloudy where you are, Baily's beads, shadow bands, and even Totality may not be visible — but it will get incredibly dark. A deep partial eclipse will be great fun too, despite their being no Totality. As a trade-off, those who witness Totality will not experience such dramatic darkness.