Skywatchers in North America will be treated to the fourth and final eclipse of the year on Thursday.
The partial solar eclipse - which occurs when a new moon hides part of the Sun - will be widely visible from Canada and the US, as well as from parts of Mexico.
The event starts to unfold near the Kamchatka Peninsula in far eastern Russia at around 19:37 GMT.
In the eastern half of the US, the event occurs near sunset (local time).
The best views will be in the north-west US and northern Canada, especially Prince of Wales Island.
But New England and the Canadian Maritime provinces will miss out.
Prof Jay Pasachoff, from Williams College in Massachusetts, US, said that between 45% and 64% of the Sun's diameter would be covered when the eclipse is viewed from different parts of the US.
Viewing an eclipse without eye protection risks permanent eye damage or blindness.
So skywatchers have been urged to protect their eyes with special filtered glasses - normal sunglasses are not good enough.
'The sun is so bright that even through ordinary sunglasses you can damage your eyes if you stare at it,' said Prof Pasachoff.
'The special solar filters that are available, which are made of a black polymer, block out all but about a thousandth of a per cent of the sun's brightness, while ordinary sunglasses would dim the sun by only a relatively small bit, even in the visible, while allowing almost all the hazardous infrared to come through.'
Another option is to project an image of the Sun on to a surface. This can be done by piercing a hole a quarter of an inch wide in a piece of cardboard and using the gap to project an image onto the ground or on a wall, with the sun behind you.
The point of greatest eclipse occurs at 21:44 GMT in Canada's Nunavut Territory near Prince of Wales Island.
There have been two solar and two lunar eclipses this year.