Large parts of the northern hemisphere are falling into shadow under the path of a total eclipse of the sun, which has begun in Canada's high Arctic, reported BBC.
The eclipse will arc across the surface of Earth as the Moon passes directly between the planet and its star.
From Canada, the eclipse will pass through Greenland to Russia and China.
Russia will see the longest eclipse, for two minutes, 27 seconds, from 1021 GMT - but the UK will only see a partial eclipse, around 1016 GMT.
The partial eclipse will be seen across most of Europe, Asia and north-eastern North America.
Tourists and amateur and professional astronomers have been flocking to towns in the best viewing locations along the path of totality.
In Novosibirsk, Siberia's cultural and scientific capital, more than 5,000 foreign tourists were expected to show up in the city.
China is to experience the eclipse - which is due to end near the ancient imperial city of Xi'an at 1121 GMT - just a week before the opening ceremony of Beijing's Olympic Games.
Chinese TV is to broadcast the eclipse live, with crowds of people gathered along the Silk Road, a fabled trading route through the country's western deserts.
Eclipses were once viewed as unlucky events in China, but the country's media has rebranded the event as "the Olympic eclipse", reports said, hoping for good fortune ahead of the sporting jamboree.
Total solar eclipses usually take place about once every 18 months, and always at new Moon - when the lunar body sits directly between the Sun and the Earth.
However, they do not happen every new Moon. The lunar orbit is slightly tilted to that of our planet and therefore the Moon's shadow often misses the Earth.
The Moon's shadow has two parts: an umbra and a penumbra.
The umbra is the "inner" part of the Moon's shadow, and people inside this zone will witness the full glory of the eclipse.
The penumbra is the Moon's faint "outer" shadow. It will only give surface viewers a partial eclipse.
This will be the case for skywatchers in the UK, for example.
In London, where the Moon's disc takes its biggest bite out of the Sun at 1016 BST (0916 GMT), a maximum of 12% of the star will be blotted out.
Conditions are better further north. In Lerwick in the Shetland Isles, the Moon will obscure as much as 36% of the Sun.
Astronomical groups have reminded the public that viewing the Sun without protective equipment - even in partial eclipse phases - can result in a retinal burn and permanent eye damage.
Viewing the Sun's harsh light should only be done through proper solar telescopes or glasses, or through a pinhole projection system.
Totality is timed to begin at sunrise at 0921 GMT in Queen Maud Gulf off Victoria Island in the territory of Nunavut, Canada.
The instant of greatest eclipse occurs at 1021 GMT close to the Russian city of Nadym.
Totality ends at 1121 GMT near the Chinese city of Xi'an in Shaanxi province.
In all, the Moon's umbra will have travelled along a path approximately 10,200km (6,338 miles) long.