METEORITICS

scientific discipline concerned with meteors and meteorites. The awe-inspiring noise and lights accompanying some meteoric falls convinced early humans that meteorites came from the gods; accordingly these objects were widely regarded with awe and veneration. This association of meteorites with the miraculous and religious made 18th-century scientists suspicious of their reality. Members of the French Academy, which was then considered the highest scientific authority, were convinced that the fall of stones from heaven was impossible. Keepers of European museums discarded genuine meteorites as shameful relics of a superstitious past. Against this background, the German physicist Ernst Florens Friedrich Chladni began the science of meteoritics in 1794, when he defended the trustworthiness of accounts of falls. A shower of stones that fell in 1803 at L'Aigle, Fr., finally convinced the scientific world of the reality of meteorites. Interest was intensified by the great meteor shower of Nov. 12, 1833, which was visible in North America. Most natural-history museums now have meteorite collections. For many years the only method of observing meteors was with the naked eye. The observer would plot the path of a meteor among the stars on a chart and note its apparent magnitude, the time, and other information. A similar plot of the same meteor made about 60 km (40 miles) away permitted rough estimates to be made of its altitude and the true angle of its path. This data can now be obtained more accurately with photographic or radar techniques, but visual observation continues to provide information on the magnitudes of meteors and serves as a check of instrumental methods. Binoculars and telescopes extend the range of visual observations from the 5th or 6th magnitude, the limit of the unaided eye, to the 12th or 13th. Direct photographs of a meteor's trail (the column of ionized gases formed by the passage of a meteoric body through the upper atmosphere) taken at two points on the Earth's surface several kilometres apart yield the most accurate data for tracing the path of a meteor. Specially designed wide-angle, high-speed cameras are employed. A rapidly rotating shutter in front of the open lens makes it possible to photograph the meteor trail in short segments, from which the velocity and deceleration of a meteor can be computed. Radar is also widely used for meteor detection and observation. Radar pulses (short bursts of radio-frequency energy) emitted from a ground-based transmitter are reflected by a meteor's trail. The distance to the meteor can be determined by measuring the time lapse between the transmitted and reflected radar signals. Its motion toward (or away from) the radar facility can be derived from the rate at which the distance changes, which in turn yields the velocity of the object in the line of sight.