PEKKA PARVIAINEN / SCIENCE PHOTO LIBRARY PEKKA PARVIAINEN / SCIENCE PHOTO LIBRARY
Star trails centred on the north polar star (brighter dot at upper centre left) showing two meteor tracks at bottom centre. Meteor tracks, also known as falling or shooting stars, are caused by microscopic dust grains which enter the Earth's atmosphere at speeds between 35 and 95 km per second. 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 Earth's atmosphere. The Perseid shower is one of the most conspicuous. It is observable from the end of July till mid-August and is associated with the periodic comet known as Swift-Tuttle.
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