Americans gazed in wonder through telescopes, cameras and protective glasses as the moon blotted out the midday sun in the first full-blown solar eclipse to sweep the US from coast to coast in nearly a century.
It promised to be the most observed and photographed eclipse in history, with millions staking out prime viewing spots and settling into garden chairs to watch, especially along the path of totality - the line of shadow created when the sun is completely obscured.
The shadow - a corridor just 60 to 70 miles wide - came ashore in Oregon and then began racing diagonally across the continent to South Carolina, with darkness lasting only about two to three minutes in any one spot.
"The show has just begun, people! What a gorgeous day! Isn't this great, people?" Jim Todd, a director at the Oregon Museum of Science and Industry, told a crowd of thousands in Salem, Oregon, as the moon seemed to take an ever-bigger bite out of the sun and the temperature soon dropped noticeably.
With 200 million people within a day's drive from the path of totality, towns and parks braced for monumental crowds.
Clear skies beckoned along most of the route, to the relief of those who feared cloud cover would spoil this once-in-a-lifetime moment.
"It's like nothing else you will ever see or ever do," said veteran eclipse-watcher Mike O'Leary, of San Diego, who set up his camera along with hundreds of other amateur astronomers gathered in Casper, Wyoming.
"It can be religious. It makes you feel insignificant, like you're just a speck in the whole scheme of things."
Astronomers were giddy with excitement. A solar eclipse is considered one of the grandest of cosmic spectacles.
Nasa solar physicist Alex Young said the last time people had a connection like this to the heavens was during man's first flight to the moon, on Apollo 8 in 1968.
The first, famous Earthrise photo came from that mission and, like this eclipse, showed us "we are part of something bigger".
With half hour to go before totality, Nasa's acting administrator, Robert Lightfoot, enjoyed the moon's "first bites out of the sun" from a plane flying over the Oregon coast and declared it "just an incredible view".
"I'm about to fight this man for a window seat," Mr Lightfoot said, referring to a fellow Nasa scientist.
The Earth, moon and sun line up perfectly every one to three years, briefly turning day into night for a sliver of the planet.
But these sights normally are in no man's land, like the vast Pacific or Earth's poles.
The moon has not thrown this much shade at the US since 1918, during the country's last coast-to-coast total eclipse.
In fact, the US mainland has not seen a total solar eclipse since 1979 - and even then, only five states in the north-west experienced total darkness.
"It's really, really, really, really awesome," said nine-year-old Cami Smith as she watched the fully eclipsed sun from a lane near her grandfather's home at Beverly Beach, Oregon.
Scientists said the total eclipse would cast a shadow that would race 2,600 miles through 14 states, entering near Lincoln City, Oregon, moving diagonally across the heartland over Casper, Wyoming,
Nice video of the #Eclipse hereAugust 21, 2017
Carbondale, Illinois, and Nashville, Tennessee, and then exiting near Charleston, South Carolina.
Shawnee National Forest in southern Illinois was in line to see the longest stretch of darkness: two minutes and 44 seconds.
All of North America was on track to get at least a partial eclipse, along with Central America and the top of South America.
The next total solar eclipse in the US will be in 2024. The next coast-to-coast one will not be until 2045.