With thrilling cosmic clockwork, the moon began to 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. 

"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 who gathered to witness the phenomenon at an amphitheater in Salem, Oregon, the Associated Press reported.

Some 12 million people live in the path of totality, and many experts expected that number to at least double as veteran eclipse chasers, armchair astronomers and the merely curious flocked to prime viewing areas to catch a glimpse.

"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 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 got underway near Lincoln Beach, Oregon, with the moon's outer shadow, or penumbra, crossing the coast at 9:05 a.m. PDT (12:05 p.m. EDT), marking the start of a partial solar eclipse.

One hour and 11 minutes later, at 10:16 a.m. local time (1:16 p.m. EDT), the dark inner heart of the moon's shadow -- the umbra -- crossed the coast. For the next one minute and 58 seconds for those along the coast, the moon totally blocked out the sun as the umbra, moving at some 2,400 mph, began its race across the heartland of America.

Because of the swiftly-changing geometry and the motions of the Earth and moon as they wheel about in space, the duration of totality increases as the shadow races eastward, lasting an additional four seconds by the time it reached Madras, Oregon, three-and-a-half minutes after landfall.

A town of 6,200 with historically clear skies, Madras was inundated with a veritable flood of visitors.

"They're expecting about a million people to enter the state, a million out-of-towners are supposed to come to the state of Oregon," said CBS News correspondent Jamie Yuccas. "Where we're located in Madras, they're expecting between 100,000 and 200,000 people."

She said the local residents have been "really, really nice and accommodating."

"What the mayor said to me was kind of funny," Yuccas said. "He said 'you know, I think it's going to be one of those situations that you might not get your newspaper, you might not have your daily Starbucks and if that happens, I guess it's a first-world problem, and you're going to have to figure out your own survival skills.'"

She laughed, saying "there are going to be some minor inconveniences, but I actually think they had a pretty good plan together."

Fienberg also was in Madras, leading a tour group for his 13th solar eclipse.

"It's like children, you know, you love them all, you can't have a favorite," he said. "I'm excited for this one. This is he first opportunity I have to actually shoot pictures with a tracking telescope mount and computer controlled camera. I'm usually traveling to far, distant lands where I can't ... bring all that stuff.

The computer is programmed to track the sun and take 300 pictures between the start of the eclipse and its conclusion. "And I'm not going to have to touch it! I'm going to get to look this time instead of spending half my time trying to take picturesmanually," Fienberg said.

From Madras, the moon's shadow swept across Oregon and into Idaho, passing just north of Boise before moving on across Idaho Falls at 11:33 a.m. local time (1:33 p.m. EDT), and Casper, Wyoming, at 11:42 a.m. (1:42 p.m. EDT).