There are actually two types of shadows: the umbra is that part of the shadow where all sunlight is completely blocked out and takes the shape of a dark, slender cone. It is surrounded by the penumbra, a lighter, funnel-shaped shadow from which sunlight is partially obscured.
During a total solar eclipse, the moon casts its umbra upon Earth's surface; that shadow can sweep a third of the way around the Earth in just a few hours. Those who are fortunate enough to be positioned in the direct path of the umbra will see the sun's disk diminish into a crescent as the moon's dark shadow rushes toward them across the landscape.
During the brief period of totality, when the sun is completely covered, the beautiful corona – the tenuous outer atmosphere of the sun – is revealed. Totality may last as long as 7 minutes 31 seconds, though most total eclipses are usually much shorter. On the average a total eclipse occurs somewhere on Earth about every 18 months.