Skywatchers in the United States are preparing for the spectacular sight of a total solar eclipse.
The Moon is set to pass in front of the Sun, casting a deep shadow that will sweep over the nation from Oregon in the west to South Carolina in the east.
It is the first such event since 1918 where the path of darkness crosses both the Pacific and Atlantic coasts.
Indeed, it is the first total solar eclipse to make landfall exclusively in the US since independence in 1776.
People lucky enough to be directly in the path of deepest shadow (“totality”) – and blessed with a clear sky – will witness our star’s light blocked out for up to two minutes and 40 seconds.
Those who stand off to the side will experience a partial eclipse, which on this occasion will encompass all of North America and northern parts of South America.
There are even parts of Western Europe, including the UK and Ireland, that will snatch a sight of the Moon’s disc taking a bite out of the Sun just as it sets.
Wherever people watch the drama unfold, they are urged to take care. Looking directly at the Sun with the naked eye can be dangerous.
Many commentators believe Monday’s eclipse will prove to be the most observed, most photographed, and best documented such event in human history.
It will certainly challenge the numbers that saw the 2009 eclipse that swept across India and China.
The US, of course, has excellent transport connections, and this will help many people get into a good position.
As it is, more than 12 million people live in the 115km-wide (70 miles) path of totality. Nearly four times that many live within a two-hour’s drive, and over 200 million live within a day’s drive.
State and local authorities have been preparing for Monday as if they were about to confront the fallout from some natural disaster.
“The eclipse path in the most part avoids the big cities,” said Angela Specks from the American Astronomical Society’s eclipse taskforce. “It skims part of Kansas City and St Louis and then Nashville. But there aren’t any really big cities on the path, and so you’ve got lot and lots of small towns, some of which don’t have hotels. And they’re going to be inundated with people,” she told BBC News.
Carbondale in Illinois has billed itself as the “Eclipse Crossroads of America” because it is in the path of darkness both on Monday and when the next US eclipse occurs in 2024.
Given this status, city authorities anticipate a huge influx of visitors. Its regular population numbers 26,000 people, but for Monday more than 60,000 extra car parking spaces have been organised.
Many skywatchers will be waiting until as late as possible before deciding where to go, based on up-to-date weather forecasts. Many of those who planned ahead will have consulted historical weather data.
Ireland, Northern Ireland, northern England and Scotland see a full partial eclipse. In Belfast, for example, this partial starts at 19:37 BST and ends at 20:23 BST.
Southern England sees the moon start to take a chunk out of the Sun just as it goes over the horizon. Skywatchers will want to make for high ground and pray that any cloud clears just at the right moment.