It will be a total eclipse in a small swath from the West Coast to the East Coast and a partial eclipse for the entire North American continent. What you see will vary greatly depending on your location.
The eclipse will spread from Oregon around midmorning (West Coast time) to the South Carolina coast about 90 minutes later, about 2:45 p.m. ET. The relatively small 70-mile-wide path will provide the greatest show. Within this area, a total eclipse will occur as the moon aligns with the sun to an umbra. In this shadow region known as totality, the sun will be 100% covered, and it will become dark.
Scientists like Kelly Beatty, a senior editor with Sky and Telescope magazine who has witnessed a dozen total eclipses, advise that you want to be in that path.
"This is one of those cases where being close isn't good enough," he said.
Dave Jones, CEO of StormCenter Communications, whose group is helping states in the path prepare for the eclipse, agrees. "Is 99% coverage good enough? My simple answer is no. Only in totality can you take off your protective glasses and watch with the naked eye. Some people call it a life-changing experience or even spiritual."
Beatty added, "only in totality can you witness the corona, the hot gases that surround the sun. The stars come out; the temperature drops; you see a bright glow around the horizon, and clouds in the distance can blink out. People are thrilled by the senses around them. When you see it, it's overwhelming."
An estimated 12 million people are lucky enough to have a front row seat, living within that narrow path of totality. The largest cities include Nashville, Tennessee; Greenville, Columbia and Charleston, South Carolina; the north side of Kansas City; and the south side of St. Louis. Here, you will have the best show right in your backyard.
But NASA estimates that 200 million Americans live within a day's drive of totality, and many of them are expected to head to where the show is best.
Gridlock traffic is expected
Traffic could become a big issue. Matt Hiebert, assistant director of communications for the Missouri Department of Transportation and the man who is helping coordinate the message for the states in the path of the eclipse, says tens of millions could make the drive.
"In Missouri, we expect anywhere between 300,000 and 1.2 million out-of-state guests driving in. We're preparing for the bigger number. Missouri alone has 107 events planned."
The estimates are highest for South Carolina, where over 2 million visitors are expected.
Hiebert advises travelers to use common sense. "Find a location and arrive early and plan on staying later." He expects the worst traffic to occur immediately after the eclipse, when everyone leaves at the same time.
"Have a full tank of gas, and pack food and water. Don't take pictures or wear your protective glasses while driving. Don't stop along the side of the road, and above all, watch out for pedestrians. Lots of people are going to be looking up at the sky and not paying attention."
Small towns in the path of totality will blossom into metropolises. Glendo, Wyoming, southeast of Casper, has a population of 200 but is expecting upwards of 100,000 visitors on eclipse day.
Some communities have turned to Kickstarter to help fund what could be a strain on their budgets for things as simple as portable toilets.
Jones jokingly calls it "the day of national gridlock outside of Washington" and likens it to the experience of a full-blown hurricane evacuation. "In Hurricane Floyd in 1999, you had millions evacuating. It was taking as long as 15 hours to get from Myrtle Beach to Columbia, South Carolina."
Beatty predicts that "Some rural communities will look like Woodstock did in the 60's."
What will it look like in my city?
If school or work keeps you from making it into the path of totality, there will still be a show; it just won't be as vivid.
The problem is that sunlight is extremely bright. It seems logical that if 70% of the sun is blocked by the moon, it would appear like dusk or even twilight, but that is not the case. The percentage doesn't directly correspond to how much darkness you see.
Even at 99% blocked, the sun is still 10,000 times brighter than it would be in totality. Beatty says that if you were walking around New York City on August 21 and were unaware that the eclipse was happening, you may not even notice. "At around 80%, or around 200 miles from the path of totality, you will begin to notice something is going on."
Certainly, in cities like Atlanta, Boise and Portland, Oregon, that are all above 95%, the daylight will appear dim, and the temperatures will drop. Luckily, many cities are above 80% and will see some effects, such as Cleveland, Detroit, Pittsburgh, Denver, Washington, Salt Lake City and Indianapolis.
The weather factor
So what will be the absolute best location for viewing the eclipse? Beatty put it simply: "Wherever it is clear."
Weather will have an enormous impact on what you see. That is especially true in totality, where the rare sight of the sun's corona will be visible only if clouds do not shroud it.
It will still turn dark, but the show won't be as good, and an interesting phenomenon could develop. You could call it reverse storm chasing: Hundreds of thousands of people may chase sunshine, not storms, which could cause gridlock.
"What if it's cloudy on one end of the state and sunny on the other? Law enforcement actually has a plan in place in case hundreds of thousands jump on the road and try and get to where it is clear," Hiebert said.
The National Weather Service
-- which crunched the numbers for August 21 -- shows that coastal sections of Oregon and South Carolina have the greatest threat of clouds, along with areas east of the Mississippi River, while the Intermountain West has the best chance of clear skies.
Tens of millions are expected to see this once-in-a-lifetime celestial event. Hopefully, Mother Nature will cooperate.