Meteor

METEOR, a term used by the Greeks to denote atmospheric phenomena in general, such as clouds, halos, rainbows, etc., as well as shooting stars (Gr. pterhopa, literally "things in the air," from µETA, beyond, and lcdpav, to lift up). This usage survived even into older English literature. To-day the term meteor is restricted to those cosmical bodies which, entering the earth's at mosphere from without and shooting across the sky, give the ap pearance of a star in rapid motion, often leaving a bright train behind. (See METEORITE.) As indicated by the name, in ancient times meteors were con sidered to be atmospheric in origin and hence not cosmical bod ies. It is true that accounts of the fall of stones from the sky are found in Chinese and classical literature, running back 2,600 years. The Chinese also recorded single brilliant meteors as well as showers of shooting stars. In Europe many of these latter have also been recorded for the past 1,500 or more years. But not until 1803 were contemporary scientists convinced that meteorites came from space, and not until 1833 was it proved that ordinary me teors or shooting stars also had a cosmical origin. From then on they have been considered true astronomical bodies. In ancient and mediaeval times the passage of a brilliant fireball or fall of a meteorite was considered an omen, and viewed with supersti tious dread. Great meteoric showers filled mankind with terrible fear, and in 1833 it is certain that the ignorant thought the end of the world had come. To-day such phenomena are viewed with keen delight and every effort is made to observe them in the interest of science.

Beginning of Systematic Study.

The study of meteors started in 1833. On Nov. 13 of that year there occurred a meteoric shower of the greatest brilliance that was seen from all parts of eastern North America. It was estimated that more than 200,000 shooting stars were seen at one place between midnight and dawn. Many of the meteors were very bright, leaving persistent trains, yet there was no record of one of them having reached the earth.

Several men who observed this shower noted that the meteors seemed all to shoot out or radiate from a certain fixed point, that happened to be in the constellation Leo. This point is known as the radiant, and the meteors as Leonids. It was soon proved that the radiant was merely the direction in space from which the Leonids came, and, as this radiant was in the same position as seen from many widely separated places, the Leonids must come from without our atmosphere. The radiant is an effect of perspec

tive, due to the meteors actually moving in practically parallel straight lines when they meet the earth. An excellent illustration is the effect when the sun's rays are seen shining through spaces between clouds. The rays are parallel, yet they appear to radiate.

Early Theories.

These discoveries were the foundation of meteoric astronomy. It was later found that the Leonids re volved around the sun in a period of 33 years, and a search of ancient documents showed records of brilliant showers coming in October or November extending back to A.D. 902. In 1866 the further notable discovery was made that the orbit of the Leonids was practically coincident with that of Tempel's Comet. A simi lar connection between the Perseid meteors, which come to a maximum in every August, and Tuttle's Comet had just been shown. One case might be a coincidence, two could hardly be. The intimate connection between comets and meteor streams was thus established. Soon after similar connections were found be tween the Lyrids of April and Comet 1861 I, and the Andro medes of late November and Biela's Comet. In recent years a few more have been found, the most notable being those between Halley's Comet and the May Aquarids, and Pons-Winnecke's Comet and a shower that came to a maximum late in June, 1916.

For many years the discovery of these intimate connections led astronomers to believe that most, if not all, meteors were merely the debris of comets, and hence originated in our Solar System. Strong evidence has, however, been accumulating that this view is only partially correct, and that a very large percentage of all classes of meteors must come to us from the depths of space and hence have originated elsewhere. It has been proved that a body, falling from rest at an infinite distance from the sun towards it, would move with a velocity of 26m. per second as it passed the earth's orbit. This is called the parabolic velocity. In general if a meteor has a heliocentric velocity of less than this value, then it originated in our system ; if a greater, then in some other stellar system. The observing of such velocities, with a high degree of accuracy, is a most difficult problem, and the whole future of meteoric astronomy must rest largely upon its successful solution. The difficulty is due to the visible life of the average meteor lasting much less than one second. Velocities in our atmosphere as low as 8m. per second and as high as 5om. per second have been observed.