CHARLESTON: Millions of sky-gazers staked out viewing spots across the United States Monday to see the Sun briefly disappear behind the Moon, in the first total solar eclipse in 99 years to cast a shadow on the entire continent.

The "total" part of the eclipse, when the Moon moves between the Earth and Sun and blocks all of the Sun's light, starts at 1716 GMT over the Pacific coast of Oregon and ends about 90 minutes later over Charleston.

The total eclipse will carve a 70-mile wide path of darkness over 14 states.

Experts warn that looking directly at an eclipse can cause permanent eye damage.

"The damage can really be permanent and right smack in the center of their vision," said Vincent Jerome Giovinazzo, director of ophthalmology at Staten Island University Hospital, Northwell Health.

The only safe time to look at it is for those within the path of totality - and only during the brief moments when the Sun is completely blocked.

Everyone else should use proper solar eyeglasses, which are far darker than regular ones, or make a pinhole projector to see the eclipse while avoiding the glare of the Sun.

Some of the clearest views were expected along the West Coast and in the Midwest.

For those unable to witness it in person, NASA was counting down the minutes until it begins a live broadcast of the event at 1545 GMT.

Scientists plan to study the eclipse to learn more about the super-hot corona, or outer edge of the Sun.

Astronauts orbiting the Earth aboard the International Space Station are also planning to document the eclipse, and will get to see it three times.