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Meteors in History

 

An article from the ENCYCLOPAEDIA BRITTANICA, Vol 15, 1951

(This is taken from an old photocopy and some words could not be clearly made out on the edges. Educated guesses have been made for them. Where this has not been possible, question marks have been supplied. The colour emphasis has been added.)


[page] 338

METEOR

[not until] 1803 were contemporary scientists convinced that meteorites came from space, and not until 1833 was it proved that ordinary meteors 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 meteorite was considered an omen, and viewed with superstitious 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. Today 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 the earth's atmosphere. The radiant is an effect of perspective caused by 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.

Cometary Associations - These discoveries were the foundation of meteoric astronomy. It was later found that the Leonids revolved 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 similar 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 between the Lyrids of April and comet 1861 I, and the Andromedes of the November and Biela's comet. Several more have since been found, the most notable being the association of Halley's comet with two streams, the May Aquarids and the October Orionids. By an unusual coincidence the orbit of Halley's comet nearly intersects the earth's orbit at two points; hence, meteors distributed about the orbit may strike the earth either before or after their minimum approach to the sun. Other associations are the great shower of Oct. 9, 1933, in Europe, with the Giacobinionner (??) comet, and a shower late in June 1916, arising from the ns-Winnecke (??) comet.

Of peculiar interest, demonstrating the value of photographic methods in meteoric astronomy, is the association of the Taurid meteors of late October and early November with Encke's comet. The precise trajectories and velocities of several of these meteors have been determined from simultaneous photographs made by two cameras located several miles apart. Each camera is provided with a rotating shutter that breaks the straight line of a photographed meteor into a number of segments spaced a fraction of a second apart. The meteor trajectory is obtained by trigonometry while the velocity is measured by the spacing of the breaks in the trail. After corrections are made for the effect of the earth's attraction and motion, the orbital path of the meteor in space, prior to its collision with the earth, can be ascertained. The Taurid meteors strike the earth's atmosphere at a speed of about 19 mi. per second. The stream apparently broke away from the main body of Encke's comet some thousands of years ago, its orbit having been altered in a known fashion by the attraction of Jupiter. Encke's comet has the smallest known cometary orbit and the shortest period of revolution about the sun, only 3.3 years.

The Leonids. - The Leonids being the best known of all meteor streams, their history will be briefly traced. The follow table gives the dates of appearances of bright showers:

902 Oct. 13 1101 Oct. 17 1602 Oct. 27 1866 Nov. 14
931 Oct. 14 1202 Oct. 19 1698 Nov. 9 1867 Nov. 14
934 Oct. 14 1366 Oct. 23 1799 Nov. 12 1868 Nov. 14
1002 Oct. 14 1533 Oct. 24 1832 Nov. 13 1901 Nov. 15
    1833 Nov. 13 1932 Nov. 15

It is apparent that while the showers certainly come at intervals of about 33 years, still there are long gaps. These may be explained in two ways; first that no record survives because of mere chance, second that the meteor stream missed the earth at these years. A fine return was hoped for in 1899, but very few Leonids were seen. It was, however, proved that the main group of Leonids, which normally would have met the earth in (Nov ??) 1899, had passed near the giant planet Jupiter en route towards the earth, and so had been switched aside somewhat by its attraction. This caused them to miss the earth. By the time the parts of the stream that met the earth in 1901 passed Jupiter's orbit the planet had passed on too far to disturb appreciably these meteors so they appeared in considerable numbers. Indeed a few Leonids are met every November, so some must be scattered all around their orbit, but the dense part that can give a really fine shower had condensed into a relatively small part of the whole circumference.

That the Leonids are not yet uniformly distributed, but are mostly included in a small group, seems to prove that they were comparatively recently turned into their present orbit, possibly by Uranus in A.D. 126. The predicted shower of 1932-34 fell on Nov. 16, 1932, though Jupiter's attraction again reduced the rate of fall to approximately 30 per hour.

Biela's comet suddenly divided into two parts in the winter of 1845-46. On its next return in 1852 the two parts were seen to be about equally bright and were 1,000,000 mi. apart. The comet (??) was not found at the predicted return of 1859 or 1866, but on Nov. 27, 1872, a splendid display of meteors was seen. Calculation showed that these Andromedes, as they are now called, followed the same orbit as the lost comet which had had a period of six and two-thirds years. Another fine display, notable in that during it a piece of meteoric iron fell at Mazapil in Mexico occurred on Nov. 27, 1885, and another lesser one on Nov. 23 (year??). On Nov. 24, 1899, a fair shower was seen, but, from then onwards the earth has never encountered any considerable number of the meteors. Nor has Biela’s comet reappeared. Older records tell of (??) showers that were probably caused by this group on Dec. 6, (??), Dec. 7, 1798, and Dec. 7, 1838. The main groups have either been broken up or switched so far from the earth's orbit that it longer meets them.

Lyrids, Perseids and Others - There is an annual shower of Lyrids, with a radiant near the star Vega, which in most years is inconspicuous, but occasionally abundant. For instance, on August (??) 20, 1803, it furnished a very brilliant display, and a fair one on April 21, 1922. ancient/ annals give nine other great showers corresponding dates, the earliest being in 687 B.C. The best-known stream that is practically sure to give a good annual display is the Perseid. These meteors, which are seen late in July, and through the first half of August, have been extensively observed for the past 100 years. Their maximum comes on Aug. 12, when as many as 120 per hour sometimes may be counted. Definite daily motion of a radiant point was first proved for the Perseids. Many of these meteors leave persistent trains and because of their appearing in the summer for northern observers they may be specially recommended to the amateur.

The Orionids which appear during the last half of every October, and the Geminids which appear the first half of every December, complete the list of really conspicuous meteors. At the maxima of both streams, from 20 to 40 meteors per hour may fall when conditions are favourable. Others are the Quarantids (??) Jan. 1-3; the Eta Aquarids, May 1-11 ' and the (??) Aquarids, July 27-31. The Eta Aquarids, connected with Haley's comet, are the most important of the three.

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