It is claimed up to 250 million people across the US will watch the first total eclipse span the country, coast to coast, in nearly 100 years.
Millions of Americans are gathering along a stretch from Oregon to South Carolina to watch the spectacle.
Southern-most Illinois will have the longest period of darkness at two minutes and 44 seconds.
It is expected to be the most observed and most photographed eclipse in history.
There will also be a partial eclipse of the sun over Ireland.
Just before sunset, the moon will appear to take a "bite" out of the sun in a phenomenon lasting roughly 40 minutes.
It will be visible from beginning to end in Ireland until sunset.
There hasn not been one visible in Ireland since 1727 nor will there be one until 2090.
Due to the partial eclipse occurring near sunset, there is unlikely to be an observable reduction in light.
It will begin in Oregon around 6.15pm (Irish time) - and plunge much of North America into darkness for almost three minutes.
People have flocked to Madras in the north-western state to see it.
Paul has been there since Thursday with his four-year-old daughter - and explains why.
"We're in the path of totality, we live in Portland (Oregan), so we'll just drive over so she can experience it.
"The last one in Oregan was in 1918, so it's a once in a lifetime thing."
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.