William Frederick Denning

“ The August Perseids have been more frequently observed than any other system of shooting-stars, from the fact that they are visible every year with more or less distinctness, and that, as an annual shower, they can not be surpassed by any other display. The two celebrated streams of November 13th and 27th, occasionally giving rise to showers of great splendor, are periodical in character, though it is extremely probable that a few of their meteors encounter the earth at the regular return of the dates ”

“ It is computed that twenty millions of meteors enter the atmosphere every day and would be visible to unassisted vision in the absence of sunlight, moonlight and clouds, while if telescopic meteors are included the number will be increased twentyfold. Ordinary meteors, in the region of the earth’s orbit, appear to be separated by intervals of about 250 m. ”

“ It also follows that, as some of the meteors are seen annually, they must be scattered around the whole orbit. Travelling at the rate of 26 m. per second, they encounter the earth moving 18 1/4 m. per second in an opposite direction, so that the apparent velocity of the meteors is about 44 m. per second. They radiate from a point within the Sickle of Leo and are termed Leonids. In 1867 the remarkable discovery was made that Tempel’s comet (1866 : I.) revolved in an orbit identical with that of the Leonids. That the comet and meteors have a close physical association seems certain. ”

“ If a ball is thrown up in a thick shower of rain, it will only encounter a few drops. This may be taken as an illustration. The earth, with its diameter of 8,000 miles, can only meet with a few meteors in its rapid flight through a zone exceeding 10,000,000 miles in width. The period of the August meteors is uncertain. Their distribution appears to have been so effectual that the element can not be determined. Some years give plentiful showers, but there have been no decided traces of regularly recurring maxima, as in the case of the Leonids. ”

“ If the meteors of the orbit during that period were condensed in the region of their derivative comet, then we can understand the singular paucity of observations. The earth, as it passed the node, would year after year encounter no meteors until the perihelion approach of the cluster, when possibly the display may have occurred in the daytime, and been of such brief duration as entirely to elude detection. ”

“ Meteors look larger than they are, from the glare and flaming effect due to their momentary combustion. The finer meteors on entering the air only weigh a few hundred or, at most, a few thousand pounds, While the smallest shooting stars visible to the eye may probably be equal in size to coarse grains of sand, and still be large enough to evolve all the light presented by them. ”

“ This is readily explained by the fact that the principal radiant points of the showers are massed together in the eastern region of the sky where the earth's orbital motion is directed, and it is obvious that in the evening hours, when the altitude of many of them is very low, and when others have scarcely appeared above the horizon, their operation is in a great measure restricted, so that only a feeble indication of their displays is perceptible at such a time. ”

“ It may either be interspersed with many smaller meteors in a shower or may be isolated. The latter usually move more slowly and approach rather near to the earth. The ordinary shooting stars vary from the brilliancy of a first- to a sixth-magnitude star. They exhibit a great dissimilarity in paths, motions and colours. The smallest and most numerous class are the telescopic meteors invisible to the naked eye. ”

“ The fact is, there are many separate showers directed from that constellation early and late in August, so that we require some distinguishing titles or symbols to conveniently particularize either of them which it may be necessary to refer to. The method now adopted, of naming the chief periodical showers by the constellations in which their radiant points are situated, is very appropriate; and such displays as the Orionids, Leonids, and Geminids, have become so well known by their titles that it would be unwise and inconsistent to attempt reform. ”

“ But the August meteors recur annually with considerable intensity, and had attracted attention at a very remote epoch, though the phenomenon was not systematically studied until later times. It was reserved for Heis at Aix-la-Chapelle to more thoroughly investigate the meteors of August, for the previous observers, though they had ascertained the fact that the month was notable in this respect, had yet neglected to obtain any important data with regard to the number or directions of the meteors seen. Schmidt also, at Bonn, began assiduously to devote himself to this special line of inquiry. ”

“ There appear to be moving and stationary radiants, contracted and diffused radiants, and long-enduring and brief radiants. The Perseids are visible from about the 11th of July to the 20th of August, the radiant having a daily motion of about 1° R.A. to E.N.E. The Lyrids also vary in the position of their radiant, but the Orionids form a stationary position from about the 9th to the 24th of October. A large proportion of the ordinary feeble showers also appear to be stationary. ”

“ It is, however, not the business of the ordinary gazer to regard such occurrences with more than a passing interest, and he simply watches their progress with a feeling almost amounting to utter indifference. ”

“ Perseids near the Radiant (44° + 57°) , August 8th-18th.
source of compensation for the expenditure of its materials upon the earth. But, though many millions of the atoms are annually consumed in our atmosphere, the effect of the thinning out will be very gradual in making itself appreciable, for, as compared with the vast assemblage which constitutes the main ring, the proportion which encounters the earth is small indeed. As it is enveloped in the stream, comparatively few of the meteors are actually intercepted. By far the greater number pass by untouched. ”

“ A fair comparison can not be instituted between the horary numbers found by observers, unless the observations, from which the values are deduced, are made, in each case, at similar hours of the night; for shooting-stars, though often plentiful after midnight, are comparatively scarce in the evening hours. ”

“ But, in addition to these, there are a large number of radiants scattered over the sky, especially in the eastern quadrant. One of the most notable of these proceeds from the eastern extremity of Aries (44° + 25°) , and supplies some bright meteors in the morning hours; but the most conspicuous shower discovered east of Perseus at this epoch lies in Camelopardus, and in the diagram (Fig. 4) a number of its meteors, falling among the stars of Ursa Major, are reproduced from the catalogues of foreign observers. ”

“ The August Perseids are always best observable in the morning hours, for the radiant point is very low on the horizon soon after dark, and a person who persistently watches it during the night will find, with increasing elevation of the radiant, a corresponding increase in the hourly number of meteors. In 1877, at Bristol, the eastern sky was persistently watched between 9h. 30m. and 14h. 30m., when 354 meteors were seen; and, though the horary
Fig. ”

“ In 1878 two points of departure were manifest from a series of precisely fixed courses at 44° 59° and 421/2° 54°; but in 1879 the weather interrupted observations. The present year, however, afforded an exceptionally favorable opportunity to observers, and the major radiant determined by the writer was at 44° +56°, with the decided traces of a sub-radiant at 45° 60°. In 1878 Major Tupman found the shower double at 46° 57.6° and 38° 56°; and in 1880 he strongly corroborated the results obtained at Bristol, though his observations were mainly confined to the night of August 9th. ”