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Perseids. A meteor track (right) of the Perseid meteor shower. This shower reaches its peak about August the 12th each year, but its meteors are seen for about three weeks around this date. Meteor tracks, also known as falling or shooting stars, are caused by tiny dust grains which enter Earth's atmosphere. The air resistance incandesces the particles making them visible as streaks of light. Meteor showers occur regularly each year when the Earth crosses the orbit of a comet and its debris enters into the atmosphere. The Perseid shower is associated with the Swift-Tuttle comet. The Pleiades star cluster is at bottom left and the California Nebula (red) at upper centre left. (KEYSTONE/SCIENCE PHOTO LIBRARY/REV. RONALD ROYER)