Millions of Americans are gathering along a stretch from Oregon to South Carolina to watch the spectacle - the first total solar eclipse to sweep coast-to-coast across the US in 99 years.
Southern-most Illinois will see the most darkness at two minutes and 44 seconds.
It is expected to be the most observed and most photographed eclipse in history.
Up to five solar eclipses occur each year, but each one is visible only within a limited band across the Earth's surface where the moon's shadow happens to fall.
Watch it live
The path will cross parts of 12 states: Oregon, Idaho, Wyoming, Nebraska, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina and South Carolina.
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