(CNN) -- A total solar eclipse was sweeping across Earth Friday, providing a rare sight for people in the upper Northern Hemisphere.
The eclipse began mid Friday morning in the Northern Hemisphere.
The eclipse will be visible in parts of Canada, northern Greenland, the Arctic, central Russia, Mongolia, and China. It will move across the planet in a narrow path that begins in Canada's northern territory of Nunavut and ends in northern China's Silk Road region at sunset, according to NASA.
A solar eclipse happens when the moon passes directly between Earth and the sun. When the moon's shadow falls on Earth, people within that shadow see the moon block a portion of the sun's light.
The moon's shadow has two parts, an umbra and a penumbra. The umbra is the main part of the shadow which appears to black out the sun. The penumbra is the moon's faint "outer" shadow -- the outer ring of the main shadow.
During a total solar eclipse, the moon appears to cover all of the sun for observers located in the moon's umbral shadow. Those viewing the eclipse from the moon's penumbral shadow see the moon cover only a portion of the sun.
At the moment of "totality," when the moon's shadow totally obscures the sun, the sun's outer atmosphere -- the solar corona -- becomes visible. NASA says this seldom-seen sight -- visible for about two minutes at most -- is coveted by experienced eclipse watchers and an awe-inspiring vision for first-time viewers. Watch the eclipse »
The solar corona extends farther than 620,000 miles from the sun's visible surface and reaches temperatures up to 2 million degrees. iReport.com: Witnesses cheer as sky grows dark
Total solar eclipses happen once or twice a year somewhere in the world but can still be a special experience for viewers.
"Just to say how special it is, in the whole universe we might be among the few beings that have the chance of seeing something like that," said Michael Khan, a mission analyst at the European Space Agency. "The fact that we have a moon and that the moon passes in front of our sun, and that the apparent size of our moon is approximately equal to the apparent size of the sun -- all these factors combine to create the solar eclipse phenomenon."
Those viewing any part of the eclipse should never look directly at it, experts warn. People can use a dense plastic foil made specifically for viewing eclipses, available at opticians or astronomy shops, or they can mount telescopes or binoculars and project the image of the sun on to cardboard.
CNN's Melissa Gray contributed to this report.
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