Entire US will fall into shadow as eclipse passes, with darkest path, or totality, contained in 70-mile (113km) ribbon from Oregon to South Carolina
The sun, moon and Earth will line up perfectly in the cosmos on 21 August, turning day into night for a few wondrous minutes, its path crossing the US from sea to shining sea for the first time in nearly a century.
Never will a total solar eclipse be so heavily viewed and studied or celebrated.
Were going to be looking at this event with unprecedented eyes, promises Alex Young, a solar physicist who is coordinating Nasas education and public outreach.
And the party planning is at full tilt from Oregon to South Carolina. Eclipse fests, StarFests, SolarFests, SolFests, Darkening of the SunFests, MoonshadowFests, EclipseCons, Eclipse Encounters and Star Parties are planned along the long but narrow path of totality, where the moon completely blots out the sun.
Vineyards, breweries, museums, parks, universities, stadiums and just about everybody is getting into the act.
The Astronomical League for amateur astronomers is holing up at Casper, Wyoming. Minor league baseball teams will halt play for eclipse delays in Salem, Oregon, and elsewhere. By a cosmic quirk of the calendar, the Little Green Men Days festival will be in full swing in Kelly, Kentucky, as will the American Atheists annual convention in North Charleston, South Carolina.
And where better to fill up on eclipse T-shirts and safety glasses and eclipse burgers than the Eclipse Kitchen in Makanda, Illinois.
Scientists are also going gaga. This is a really amazing chance to just open the publics eyes to wonder, says Montana State Universitys Angela Des Jardins, a physicist in charge of a Nasa eclipse ballooning project. The student-launched, high-altitude balloons will beam back live video of the eclipse along the route.
Satellites and ground telescopes will also aim at the sun and at the moons shadow cutting a swath 60 to 70 miles wide (97 to 113km) across the land. Astronauts will do the same with cameras aboard the International Space Station. Ships and planes will also catch the action.
Its going to be hard to beat, frankly, says Thomas Zurbuchen, head of Nasas science mission office.
At the same time, researchers and the just plain curious will watch how animals and plants react as darkness falls. It will resemble twilight and the temperature will drop 10 to 15 degrees.
Expect four hours of pageantry, from the time the sun begins to be eclipsed by the moon near Lincoln City, Oregon, until the time the moons shadow vanishes near Charleston, South Carolina.
The total eclipse will last just 90 minutes as the lunar shadow sweeps coast to coast at more than 1,500mph (2,400kph) beginning about 1.15pm EDT and ending at 2.49pm EDT. The suns crown, the normally invisible outer atmosphere known as the corona, will shine like a halo.
These take-your-breath-away eclipses usually occur in the middle of the ocean somewhere, or near the sparsely populated top or bottom of the world. But the US is in the bulls-eye this time.
It will be the first total solar eclipse in 99 years to cross coast-to-coast and the first to pass through any part of the lower 48 states in 38 years.
Nasas meteor guru, Bill Cooke, was in Washington state for that one in 1979. This time, hes headed to his sisters farm in eastern Tennessee. It is the most weird, creepy, awe-inspiring astronomical event you will experience, he says.
In all, 14 states (two of them barely), 21 national park locations and seven national historic trails will be in the path.
Darkness will last just under two minutes in Oregon, gradually expanding to a maximum two minutes and 44 seconds in Shawnee national forest in southernmost Illinois, almost into Kentucky, then dwindling to 2 1/2 minutes in South Carolina. Staring at the sun with unprotected eyes is always dangerous, except during the few minutes of totality. But eye protection is needed during the partial eclipse before and after.
With an estimated 200 million people living within a days drive of the path, huge crowds are expected.
A partial eclipse will extend up through Canada and down through Central America and the top of South America.