- Video report by ITV News Science Correspondent Alok Jha
Tens of thousands of people have travelled to a sleepy town in the American west, to witness a rare celestial event - a total solar eclipse.
However there is a risk the eclipse chasers may be disappointed on Monday, as meteorologists are forecasting cloudy weather.
The population of Casper in Wyoming has doubled over the weekend, with 50,000 people descending on the town which has been pinpointed as the best place in the world to experience the phenomenon.
Clouds are the biggest threat to eclipse watchers, and while it is not easy to forecast, skies are not likely to be completely clear, from Wyoming to western Missouri.
"It's going to be twitchy for everybody from Nebraska eastward," said retired Canadian meteorologist Jay Anderson, who has been forecasting eclipse weather since 1978.
Anderson, who runs the Eclipsophile weather website, said people east of Idaho and west of the Appalachian Mountains may have to drive a bit to find clear skies, but with a little effort they can find them.
Officials and eclipse watchers have fretted, though, about roads being too clogged as the eclipse approaches.
The full eclipse will cover a 70-mile-wide area from Oregon to South Carolina. The moon will pass between Earth and the sun for America's first coast-to-coast total solar eclipse since 1918.
When this happens, the classic image appears of a black dot surrounded by a feathery red ring, the sun's corona.
A partial eclipse will be visible outside the path, extending up to Canada and down to the top of South America.
The UK will only get a partial eclipse on Monday, with around 4% of the sun being blocked out by the moon.
The phenomenon will last roughly 40 minutes with the mid-point occurring at different times around the country.