What's that dark spot on planet Earth? It's the shadow of the Moon. This image of Earth was taken in May 2012 by MTSAT during an annular eclipse of the Sun. The dark spot appears quite unusual as clouds are white and the oceans are blue in this color corrected image. Earthlings residing within the dark spot would see part of the Sun blocked by the Moon and so receive less sunlight than normal. The spot moved across the Earth at nearly 2,000 kilometers per hour, giving many viewers less than two hours to see a partially eclipsed Sun. MTSAT circles the Earth in a geostationary orbit and so took this image from about three Earth-diameters away.
Explanation from: http://apod.nasa.gov/apod/ap120530.html