Millions of people have have been gathering to see less than three minutes of darkness on Monday, 21 August, during a total eclipse of the Sun.
The rare but dramatic spectacle can be seen along a 68 mile (110 km) wide track that crosses from the west to the east coast of America.
Eclipse fans from all over the world were today converging on that narrow zone, which runs through the states of Oregon, Idaho, Montana, Wyoming, Nebraska, Kansas, Iowa, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina and South Carolina.
The eclipse will be caused by the shadow of the Moon sweeping across the face of the Earth as it blots out the Sun. The shadow will take around two hours to cross the continent, at a speed of around 1,500 mph, as the Moon travels in its orbit.
But the longest anyone will see totality along the eclipse track is just 2 minutes 40 seconds from Kentucky. Despite that, accommodation prices have rocketed from coast to coast with many hotels booked up years ahead.
During totality, the sky becomes dark like night, with the brighter planets and stars visible. The Sun’s ghostly atmosphere, the corona, will come into view, along with any fiery prominences around it.
Then a brilliant flash will signal the end of the event when sunlight floods through a valley on the edge of the Moon as it begins to move off the Sun.
Away from the track of totality, the whole of the USA will see a partial eclipse, where the Moon and Sun do not completely line up. But this cannot compare to the full spectacle of the main event.
For those not in the USA, organisations including NASA will be streaming the total eclipse live on the internet. You can watch it here. Ian Musgrave has compiled a comprehensive list of other live feeds of the event on his Astroblog.
By an amazing coincidence, the Sun and Moon appear the same size in our sky. That’s because although the Sun is 400 times bigger than the Moon, it is also 400 times father away.
Related: What is an eclipse of the Sun?
Related: About the Sun
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