Popular Science Monthly/Volume 18/December 1880/The August Meteors
By W. F. DENNING, F. R. A. S.
THE August shower of meteors forms one of the most attractive and important of the annual phenomena witnessed by astronomers, and the display is awaited every year with considerable interest, not only by a large section of habitual observers, but by many persons who have their attention called to it in a mere casual way by the frequency and brightness of the meteors. For, on the 10th of August, if the night is clear and the moonlight not very strong, a person can not be long in the open before his curiosity is excited by numbers of these "falling stars," which he will notice traveling swiftly athwart the sky, and leaving lines of phosphorescence along their paths. 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. But it serves to while away a leisure hour and to give rise to some curious speculations as to the origin and end of the transient objects which now and again come before his view. The case is different with the scientific observer. He has a practical interest in the phenomenon, and zealously endeavors to record its more remarkable features as they become successively presented, and to watch with increasing diligence its further development in the later hours of the night, remembering that his notes must hereafter have some value in the general comparison of results.
Quetelet's catalogue of observed meteor-showers embraces a large number which obviously belong to the August period, but the majority occurred during the present century. This can not be ascribed to an increasing activity of the meteor-stream. It is at once explained by a greater assiduity of observation, and by the fact that the subject is
Fig. 1.—Broken Streak of a Perseid in Pegasus, August 11th, 11h. 10m.
considered of more importance than formerly. Hence in more recent years the shower has been diligently looked for by many observers; and the result is that we find a large number of records of its displays. In former years it was comparatively neglected. The uncertainty attached to the whole subject rendered it unattractive, for there seemed little likelihood that it would ever become an important branch of astronomy, or yield any valuable results to the patient observer of its nightly displays. Thus we find, among historical records, only a few scattered references to this shower, and we are led, at first, to the inference that it was only rarely visible in consequence of the meteors being slightly dispersed over the orbit in former years. But the irregularities in the dates of its former apparitions may safely be ascribed to other causes than a physical peculiarity of the shower itself. The lack of interest in the subject would cause it frequently to be disregarded. Many of its exhibitions would pass wholly unobserved. Indeed, it would only be described when it recurred with such striking intensity as to force itself upon the attention as a celestial event of considerable interest. Between 811 and 841 it furnished a succession of brilliant displays at the end of July. Then there occurred a break until eighty-three years later, when it several times reappeared with similar splendor. A wide interval of more than three hundred years brings us to the year 1243, when it seems to have been again recognized, after which, until 1709, there is only one other observation of the shower (in 1451). During the last hundred years it has, however, been frequently observed, though many of the recent displays can not be compared with those of ancient times. The intermittent and rare character of the shower, as it existed between the tenth and eighteenth centuries, proves that few returns were of a sufficiently imposing nature to be recorded, and that possibly the conditions were opposed to its appearance. 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.
The entry of this stream into the solar system probably dates back to a very remote antiquity—for there are several circumstances which conspire to prove that such must have been the case, and that it preceded, by many ages, the apparition of the Leonids, Andromedes, and some of the other periodical meteor-showers. The fact that it constitutes an unbroken ring leads to the inference that it must have existed from the earliest times in order to bring about so complete a dispersion of its particles, for on its first introduction, as a comet, to the earth, it is to be assumed that it formed a condensed mass like the Leonids, and only appeared as a meteor-shower when the comet returned to perihelion. A very slight difference in the periodic times of the individual meteors following the nucleus must have eventually distributed them (by its cumulative effects) along the entire orbit. In other words, the original group must have undergone a process of lengthening out, until, at the present day, it consists of a parabolic zone of meteoric pellets, through which the earth passes annually on August 10th. Moreover, the radiant point of the shower often fails to become sharply defined. Several concentric streams of similar meteors appear to diverge from the region about η Persei, and their physical identity is unquestionable. They are merely the deflections or offshoots from the original system which must be greatly disturbed and contorted as the earth annually intersects it. The full effects of these perturbations can hardly be estimated: many of the particles must be diverted into new orbits, and one of the results upon the main stream may be a constant widening out, so that the apparent duration of the shower must go on increasing. It now actively extends over at least eight nights; hence the width must exceed 10,000,000 miles. And some diminution in its intensity must occur at each return, unless there is a
Fig. 2. Perseids near the Radiant (44° + 57°), August 8th-18th.
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. This may possibly be explained by the fact that the period is a long one, and would not become defined until after centuries of research. Comet III, 1862, which shows an exact resemblance of orbit to this system, was computed by Oppolzer to have a period of 121·5 years; and, as there occurred a fine display of the August meteors in 1863, we can not anticipate its periodical return until about 1964, if the calculations are reliable.
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; notwithstanding that they may elude observation in consequence either of moonlight or cloudy weather, which, indeed, generally offers some impediment to success. 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. The particular night in August when the meteors were most plentifully distributed was found to be the 10th, though the numbers were subject to considerable variations in different years. Schmidt, from an average of several years of observations, gave the following as the horary number of falling stars for one observer. His results are compared with a similar average derived by Major Tupman and the writer from observations in 1869-'71 and 1877-'80 respectively:
Schmidt's figures are very small and much below the numbers found in recent years. But the averages in the table are not thoroughly reliable, inasmuch as they are based upon only a few years' observations. A longer series might give a closer comparison, but it is seldom that the results of independent observers agree within small limits. There are differences in vision, modes of observation, and in position, which must obviously affect the numbers to no small degree; and the intermittent character of the meteor-shower itself must give rise to discrepancies which can not at first sight be accounted for. The horary number of meteors on August 10th may vary, according to Heis, from 160 (in 1839) to 24 (in 1867). During the last ten years the writer has found little variation in the intensity of the annual returns when the conditions of weather and moonlight are fully taken into account; and there is no question that some of the variations ascribed to the shower have no real existence, but are to be explained by the differences referred to above.
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. 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. The case is entirely different at a later period of the night, when the constellations in which the several radiant points are situated have ascended high in the sky, and are in fact so placed that they may be seen to the greatest advantage. 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. 3.—Radiant Points east of the Perseids, August 6th-13th.
rate before llh. was only 47, it rose to about 80 during the last half of the watch. Indeed, the number of meteors observed at the end of the watch was more than double the number recorded at the beginning of it. Thus it is apparent that the most favorable time for such observations is in the morning hours, and though it is generally inconvenient for amateurs to extend their vigils thus far, the importance of doing so can not be too strongly insisted on.
A typical feature of the Perseids is to be found in the streaks which frequently mark their course (Fig. 1), and serve an extremely useful purpose in enabling the directions to be registered with great accuracy. The theoretical velocity of these meteors is thirty-eight miles per second, so that they belong to the swiftest class of such bodies, and, as such, would be individually recorded with much difficulty, were it not for the special feature referred to. Their very rapid transient flights would baffle the observer as he stood endeavoring to retain the exact points of beginning and ending; and in the majority of instances he must absolutely fail to get nearer than a mere approximation. Only in cases where the meteors sped from one star to another, or in courses parallel to closely adjoining stars, could the paths be truthfully reproduced on his map. But, fortunately for such investigations, we have no such difficulties to encounter. The phosphorescent line, almost invariably projected on the sky by the nucleus as it rushes along, remains to guide the eye in fixing its position. It is the authentic signature of the meteor gone before, and during the brief span of its endurance the observer knows how to utilize it. It is seldom these streaks last longer than three or four seconds, though in exceptional cases of Perseid fire-balls they have lingered several minutes. The writer found the average 1·8 second from many observations in August, 1880; and the most frequent duration is about two seconds. All the brighter meteors of the shower display them. Mr. Henry Corder, of Writtle, has observed these Perseids with great diligence in recent years, and retained many interesting notes of their peculiarities. Of 910 meteors belonging to this system, which he saw in the years 1871-'79, 526 were accompanied by streaks. These included 158 of the first magnitude, only 15 of which were devoid of streaks; and 243 of the second magnitude, of which 72 were streakless. Among the smaller members the proportion was larger. He found the brightest meteors were generally pale-green, others orange, etc.
The luminous streaks, which are known to be the ordinary characteristic of these shooting-stars, have acquired a special significance from the fact that by their means the radiant point of the shower is capable of being ascertained with remarkable precision. This important element, to be reliably determined, must rest upon a large number of accurately recorded tracts, which intersect (on being prolonged backward) at a well-defined position. Many observers have succeeded in finding this from results of more or less value. Mr. R. P. Greg analyzed all the positions estimated prior to 1876, and gave the average at R. A. 44°, Dec. 56° north; and Major Tupman, from a discussion of his own elaborate observations in the Mediterranean during the years 1869-'71, derived the point 45½° 56°, as the center of 28 sub-radiants. Evidently the two results, being founded on a large number of trustworthy records, and agreeing so closely as they did, showed the true radiant to be situated on the northern limit of Perseus, close to the star Eta of that constellation; and more recent determinations of a similar nature have fully corroborated that as the chief diverging center of the August meteors. Many other contemporary showers have been detected in the same region of the heavens, but the shower of Perseids recurs year after year from its accustomed point.
During the last eleven years the writer at Bristol has awaited the annual returns of this shower, and the aggregate results of observations during the interval between the 6th and 12th of August show that 2,345 meteors have been recorded, of which 1,428 belonged to the
Fig. 4.—Shower from Camelopardus (96° 71°), August 6th-12th.
display of Perseids, and 917 to other minor streams of the same epoch. In 1869 the radiant was judged to be at η Persei; in 1871 at B. Camelopardi; and in 1874 at 44° 581°. The average position found during the last five years has been at 44° 57°; and in the diagram (Fig. 2) a number of paths near this radiant are shown. Some of the meteors appear to be slightly erratic in their directions; but this may be explained either by errors of observation or by a double or diffused radiant point, which must often occasion non-conformity in the observed flights. In 1878 two points of departure were manifest from a series of precisely fixed courses at 44° 59° and 421° 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. At the latter station the radiant apparently advanced among the stars of Perseus, for, while early in the month it was observed at 38° 56°, it had shifted to 49½° 57½° by the 13th. The same peculiarity was noted in 1877, when the following determinations were made:
There is a prominent display of meteors from the star-group χ Persei at the end of July and beginning of August, and it is possible that these showers may belong to the same system of concentric meteor streams. It is certain that this fact of a progressive radiant requires fuller elucidation, and to this end observers should keep the data obtained each night separate. It may also be suggested that the radiant point should be ascertained during each hour of observation, and then, when the series are compared, any displacement must immediately become obvious, and its extent and character well defined by the observations. The meteors from Perseus are so numerous, and the place of divergence so readily denoted by their enduring streaks, that there will be no difiiculty in an investigation of this kind. The last two years' observations have shown how exactly the radiant may be found by carefully conducted researches, and how closely the positions derived by different observers will agree on being compared together:
From these values a mean of 44.8° 56.8° is derived, which is probably very near the truth. There is a secondary shower higher in declination (at about 44½° 60°), but this is merely a branch of the same stream, for the meteors exhibit the same specialties of appearance as those common to the major shower. An apparent diffuseness of the radiant point is often brought about by imperfectly registered tracks, and by allotting the meteors of bordering showers to the radiant of the Perseids, when in fact they belong to evidently distinct families.
A few years ago the writer undertook the investigation of these co-Perseid showers from the large mass of shooting-stars which had been registered at this epoch at foreign observatories, and are contained in the published catalogues of Heis, Schiaparelli (1872), Weiss, and Konkoly, These include many thousands of paths observed during the period from August 6th to 12th, and such of these as were obviously directed from radiant points situated eastward of Perseus were projected on the star-maps prepared by Professor Herschel for the purposes of the Luminous Meteor Committee of the British Association. In all 762 meteors were thus utilized, and they gave distinct evidence of the positions of a number of active streams in Auriga and Camelopardus, some of which were previously observed by Heis, and many of them have been confirmed by the writer during the last five years. The following list embraces the chief radiants thus deduced:
Meteor-Showers east of Perseus, August 6th-l2th.
The relative positions of these showers are depicted in the diagram (Fig. 3), where the more prominent displays of the group are represented by deeper circles than the minor. Some of the latter can not yet be regarded as certainly established, inasmuch as they rest on slender materials.
Heis devoted much attention to the meteors of the August period during more than forty years (1833-'75), and in his extensive "results," published in 1877, gives the following as the chief radiant points for August 9th-llth:
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. This shower, however, escaped the detection of Heis and others, who had been engaged in similar investigations, though it appears to be of more importance than several radiants in its vicinity which have been independently determined by several observers. At the end of July, 1878, the writer noted a few brilliant, slow meteors, from a point at 96° + 72°, and this may have been an early evidence of the radiant which is placed in a region bare of large trees between Telescopium and Polaris. It is just north of the triangle of faint stars (l. p. q. Camelopardi of Bode), east of a line drawn from β Aurigœ to Polaris, and will, no doubt, be frequently reobserved in future years, though the shower of Perseids usually monopolizes attention at the epoch of its annual returns.
There is a shower near η Persei (No. 2), well defined, on August 6th-12th, August 21st-23d, and September 6th-15th. At the latter
Fig. 5.—Shower of Perseids (61° + 36°), max. September 6th, 7th.
epoch it furnishes some fine meteors and constitutes a prominent display. The diagram (Fig. 5) gives the positions of eighty-six paths conforming to this radiant, observed at Bristol, and at several foreign stations in September.
The ordinary designation of Perseids for the special meteor-shower of August 10th is always understood in its individual application, though it must not be supposed that this is the only shower of Perseids visible in that month. 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 with regard to the minor systems, which are becoming very numerous, and require an equally ready mode of expression, there is a great difficulty in avoiding complications.
There are certainly five nearly simultaneous showers of Perseids early in August; and in every month of the year, except May and June, meteors continue to fall from that constellation. If the present mode is adopted of styling them Perseids I, Perseids II, and so on progressively, a good deal of confusion must eventually arise as new systems are discovered; and this classification by Roman numbers, however appropriate it may be in some of its other applications, will have to give way to a more distinguishing means of reference. The name at present only gives indication of the constellation from which the meteors emanate, without regard to the date or approximate place of the radiant, and it seems to me that the difficulty may be obviated by including the nearest fixed star and the epoch with that name. To render the proposal clear, let us take the different streams proceeding from the undermentioned points in Perseus in August: 44° + 56°, 32° + 53°, 61° + 36°, 61° + 48°, 46° + 47°, which may be thus termed:
η Perseids (August 10th).
This is apparently a preferable method to that of Perseids I, II, III, IV, and V, which must occasion endless trouble in references to find what special stream is meant. Moreover, the numbers seem only in fair application when affixed progressively to the successive showers of the year, for it would be hardly consistent to call a radiant visible in Perseus early in January by the designation of, say, "Perseids XXXVIII." Yet this is what we are drifting to, unless a fresh system is introduced to accommodate the rapidly increasing number of meteor-streams.—Popular Science Review.