What's 70 miles wide, 2,500 miles long and capable of ruining 12 million people's vision?

Twilight will fall at midday on Monday, stars will glimmer and birds will roost in an eerie stillness as millions of Americans and visitors witness the first total solar eclipse to traverse the United States in 99 years.

The sight of the moon's shadow passing directly in front of the sun, blotting out all but the halo-like solar corona, may draw the largest live audience for a celestial event in human history.

When those watching via broadcast and online media are factored into the mix, the spectacle will likely smash records.

"It will certainly be the most observed total eclipse in history," astronomer Rick Fienberg of the American Astronomical Society (AAS) said last week.

The eclipse begins its cross-country trajectory over the Pacific Coast of Oregon in late morning. It will reach South Carolina's Atlantic shore some 90 minutes later.

The total eclipse of the sun is considered one of the most spellbinding phenomena in nature, but it rarely occurs over a wide swath of land.

In terms of audience potential, it is hard to top the US, with its mobile and affluent population, even though the direct path is mostly over rural areas, towns and small cities. The largest is Nashville, a city of 660,000.

Even so, the advent of social media and inexpensive high-tech optics have boosted public awareness, assuring what many US experts predict will be unprecedented viewership for the so-called "Great American Eclipse".

Some might take issue with that prediction, citing a solar eclipse visible over parts of India, Nepal, Bangladesh and central China in July 2009. National Geographic estimated 30 million people in Shanghai and Hangzhou alone were in its path that day.

On Monday, the deepest part of the shadow, or umbra, cast by the moon will fall over a 70-mile-wide, 2,500-mile-long "path of totality" traversing 14 states. The 12 million people who live there can view the eclipse at its fullest merely by walking outside and looking up, weather permitting. A partial eclipse will appear throughout North America.

Some 200 million Americans reside within a day's drive of the totality zone, and as many as 7 million, experts say, are expected to converge on towns and campgrounds along the narrow corridor for the event. Many are attending multiday festivals featuring music, yoga and astronomy lectures.

Affordable solar-safe sunglasses produced by the millions are selling so fast, suppliers are running out.

The owner of one leading manufacturer reported price gouging by second-hand dealers who were buying up large supplies and reselling them over the internet at a huge mark-up.

Welsh pop singer Bonnie Tyler is slated to perform her 1983 hit single Total Eclipse of the Heart aboard a cruise liner as the vessel sails into the path of totality from Florida on Monday.

Reuters