PORTLAND, Oregon - With thrilling cosmic clockwork, the moon will pass in front of the sun Monday, casting a 70-mile-wide shadow that will sweep across the United States from coast to coast, giving millions along the "path of totality" a chance to marvel at one of nature's grandest spectacles, a total eclipse of the sun.
It is the first solar eclipse visible from the United States since 1979 and the first to cross the entire continent in 99 years.
Some 12 million people live in the path of totality, and many experts expect that number to at least double when veteran eclipse chasers, armchair astronomers and the merely curious rush in, possibly at the last minute.
"So instead of being 12 million, we're expecting 20 plus," said Rick Fienberg, a spokesman for the American Astronomical Society.
Weather permitting, and with eye safety in mind -- everyone in the continental United States, Canada, Central America and the northern quarter of South America will enjoy a partial solar eclipse, with the moon blocking some or even most of the sun as the three-hour event unfolds.
But for the millions of residents who live in the 14 states along the path of totality, along with millions more who braved predicted heavy traffic to join them, the sky will darken as the sun is completely obscured, the temperature will drop, bright stars and planets will come out and a 360-degree sunset will be visible around the horizon.
In the seconds before the sun is totally obscured, brilliant shafts of light passing through lunar valleys and chasms around the moon's limb will flicker and flare, a phenomenon known as Baily's Beads, before a brief, final burst of concentrated sunshine giving the sun the appearance of a diamond ring.
And suddenly, that final flare will vanish, the sun will disappear and its outer atmosphere, the normally unseen, super-heated corona, will shine and shimmer with the brightness of a full moon, a crown-like halo stretching away in all directions.
"If you're in the path of totality, it will get dark, it will get cool, you will experience a total eclipse," Fienberg said in a telephone interview from Oregon.
"Of course, the part that's most exciting is actually seeing the corona and seeing the beautiful sunset colors and seeing the stars and planets come out. Hopefully, as many people as possible will see that. Whatever the weather, I suspect this will still go down as the most observed eclipse in history."
Michael Bakich, a senior editor with Astronomy magazine, put it like this:
"Do you know the difference between a partial and a total eclipse? It's the difference between a lightning bug and lightning," he wrote. "Between testing negative and positive with a pregnancy test. Between a paper cut and stepping on a landmine. In other words, there's no comparison.
"I think of it as 'awesome' in the truest sense of the word: able to inspire or generate awe. ... I guarantee that if you stand in the moon's shadow under a clear sky, you'll never forget it. Furthermore, it will stand out as one of the greatest -- if not the greatest -- sights you ever have or ever will behold."
The spectacle begins near Lincoln Beach, Oregon, when the moon's outer shadow, or penumbra crosses the coast at 9:05 a.m. PDT (12:05 p.m. EDT), marking the start of a partial solar eclipse.