Telescopic Work for Starlight Evenings

Identity of Meteors and Comets.—The mystery concerning these fugitive objects and their vagaries of appearance was not always to remain concealed. Denison Olmsted had, in his work on ‘The Mechanism of the Heavens,’ published in 1850, stated that the constitution of the body to which the meteors of 1833 belonged bore “a strong analogy to comets.” Reichenbach, in 1858, wrote a paper in which it was sought to prove that a comet is a swarm of meteorites. Prof. Kirkwood, in 1861, also concluded that “meteors and meteoric rings are the debris of ancient but now disintegrated comets, whose matter has become distributed around their orbits.” But it remained for Schiaparelli, of Milan, in 1866, to demonstrate the identity of meteoric and cometary systems. Others had reasoned up to it, and observers had amassed many useful observations bearing on the subject; but absolute proof was wanting until Schiaparelli supplied it. He computed elements for a well-known shower of meteors occurring on August 10th, and found the orbit presented a very close resemblance to that of Comet III. 1862; and he detected a similar analogy between the November meteors and Comet I. 1866. The orbit of the April meteors was afterwards shown by Galle and Weiss to agree with the path of Comet I. 1861; and a meteor-shower occurring at the end of November was found to coincide with Biela’s Comet. Facts like these could not be disproved. Comets were thenceforth known to be the parents—the derivative source—of meteors. Thus two important classes of objects became as one, the differences observed being merely those of aspect due to the variable conditions under which they were presented. The great meteor-shower of November was found to be the dispersed materials of Tempel’s Comet of 1866 seen in detail and from a near standpoint. Every meteoric display was known to be the visible effects of the collision of the Earth with a comet or with the great stream of planetary fragments describing a cometary orbit.

Aerolites.—Meteors enter our atmosphere with such great velocity that the friction induced by their impact is sufficient to destroy them by combustion. They rarely approach the Earth’s surface within 15 miles. Occasionally, however, a slow-moving meteor of large size, and formed of a very compact substance, will penetrate entirely through the air-strata and fall upon the Earth’s surface. Many instances of the kind have been recorded, and a few of these are quoted below:—

1478 B.C. The Parian chronicle records that an aerolite or thunder-stone fell in the island of Crete. This appears to be the earliest stone-fall described in history.

654 B.C. A shower of stones descended near Rome.

465 B.C. A stone, surrounded with fire, fell in Thrace. This stone is referred to by several ancient writers. It was termed the “Mother of the Gods” and is said to have fallen at the feet of the poet Pindar.

52 B.C. A shower of iron descended at Lucania, in the time of Crassus.

1492 A.D. A stone weighing 262 lb. fell at Ensisheim, in Alsace.

1642. A stone of 4 lb. fell near Woodbridge, in Suffolk.

1795, Dec. 13. A stone of 56 lb. fell at Wold Cottage, Thwing, Yorkshire.

1860, July 14. A shower of aerolites fell at Dhurmsala, in India. A tremendous detonation attended their descent, and the natives became greatly alarmed. They supposed the stones to have been thrown by some of their deities from the summit of the Himalayas, and many of them were preserved as objects of religious veneration.

1864, May 14. A very large meteor was observed in France. At Montauban and the neighbourhood deafening explosions occurred, and showers of stones fell near the villages of Orgueil and Nohic.

1876, April 20. A piece of iron weighing 7-3/4 lb. fell at Rowton, Shropshire.

1881, March 14. A stone weighing 3 lb. 8-1/4 oz. fell at Middlesborough, Yorkshire, on a part of the North-Eastern Railway Company’s branch line. The descent of the aerolite was witnessed by an inspector and three platelayers, who were working about fifty yards distant. At first they became aware of a whizzing or rushing noise in the air, immediately followed by the sudden blow of a body striking the ground near. The hole, 11 inches deep, which the stone made was found directly after, and the stone was extracted.

Many other examples might be given, but the above will be sufficient for our purpose. Records of this nature were discredited in former times; but more modern researches have long since placed their reality beyond all question. The fall of stones from the sky is no longer regarded as a mere legendary tale, but as one of the well-assured operations of nature.

Meteoric stones and irons have been classified according to the ingredients of their composition. Those in which iron is found in considerable amount are termed siderites, those containing an admixture of iron and stone, siderolites, and those consisting almost entirely of stone are known as aerolites. The siderite which fell in Shropshire on April 20, 1876, forms only the seventh recorded instance where a mass of meteoric iron has been actually seen to fall.

Fireballs.—The table on p. 268 gives the dates, heights, &c. of fifteen fireballs observed during the last quarter of a century.

Fireballs are sometimes detonating, though more often silent. The fireball of Nov. 23, 1877, gave a sound like salvoes of artillery, and doors and windows were shaken violently. At Chester the noise of its explosion was compared to loud but distant thunder. Lieut.-Col. Tupman says that “thunder, to be loud, must be within five miles; hence it appears that the violence of the explosion must have been at least a hundred times greater than a peal of thunder, the intensity of sound-waves diminishing as the square of the distance.” “The explosion of a 13-inch bomb-shell, consisting of some 200 lb. of iron, would not have produced a sound of one hundredth part of the intensity of the meteor-explosion.” This fireball must therefore have been an object of considerable mass before its dissolution; and it is fortunate that such bodies are usually destroyed by the effects of combustion before they reach the Earth’s surface.

These phenomena exhibit many varieties of appearance.

Sometimes there is no visible explosion; the bright nucleus slowly dies out until reduced to a faint spark before final disappearance. Several outbursts of light are often noted; and a curious halting motion has been observed in regard to large slow-moving meteors. I have occasionally remarked a succession of four brilliant flashes given by individual fireballs. These flashes, though sometimes of startling intensity, are somewhat different to the transient vividness of lightning; they come more softly, and remind one forcibly of moonlight breaking suddenly from the clear intervals in passing clouds.

Fireballs differ vastly from shooting-stars in point of size; but their origin is identical. The August meteor-shower yields the smallest shooting-stars and the largest type of fireballs. The great display of meteors on Nov. 27, 1885, not only presented us with large and small members, but it also furnished us with a siderite or piece of iron, presumably from Biela’s Comet. This fell at Mazapil, Mexico; and as considerable interest is attached to the case, I quote a part of the discoverer’s statement:—

“It was at about 9 o’clock on the night of November 27th, when I went out to the corral to feed certain horses: suddenly I heard a loud sizzing noise, exactly as though something red-hot was being plunged into cold water; and almost instantly there followed a somewhat loud thud. At once the corral was covered with a phosphorescent light; while suspended in the air were small luminous sparks, as though from a rocket.... A number of people came running towards me; and when we had recovered from our fright we saw the light disappear, and bringing lanterns to look for the cause found a hole in the ground, and in it a ball of light. We retired to a distance, fearing it would explode and harm us. Looking up to the sky, we saw from time to time exhalations of stars, which soon went out without noise. We returned after a little, and found in the hole a hot stone which we could barely handle; this, on the next day, we saw looked like a piece of iron. All night it rained stars; but we saw none fall to the ground, as they all seemed to be extinguished while yet very high up.”

This is the first observed instance in which a meteorite has actually reached the Earth’s surface during the progress of a star-shower. If its identity with the meteors of Biela’s Comet is admitted, then all classes of meteoric phenomena would appear to have a community of origin.

Differences of Motion.—Great differences are observed in the velocity of meteors. An observer may notice all varieties on the same night of observation. Some will move very slowly, others shoot quickly across the sky. These differences are occasioned by the astronomical conditions affecting the position of the meteor-orbit relatively to the motion of the Earth. Thus the meteors of Nov. 13 move with great velocity (44 miles per second), because they come directly from that part of the heavens towards which the Earth is moving; hence the orbital speed of the Earth (18½ miles per second) and meteors (26 miles per second) is combined in the observed effects. But in the case of the meteor-shower of Nov. 27 the motions are extremely slow (about 10 miles per second), as the Earth and the meteors are travelling nearly parallel in the same direction, and the latter have to overtake the Earth.

Nomenclature of Meteor-Systems.—It is customary to name the showers after the constellation from which the meteors appear to diverge. Thus the meteors of April 20 are called Lyrids, the radiant being in Lyra; the meteors of August 10 are termed Perseids, the point of emanation being in Perseus. The two great streams of November are known as the Leonids (13th) and Andromedes (27th). Several showers are often visible in the same constellation; and when it is desired to name these according to the above system, it is necessary to add the approximate star to distinguish them. Thus, in August there are showers of μ Perseids, ε Perseids, and α Perseids, in addition to the well-known Perseids of August 10.

Meteor-Storms.—On Nov. 12, 1799, Humboldt, at Cumana, in South America, saw “thousands of bolides and falling stars succeed each other during four hours.” On Nov. 12, 1833, this shower recurred, and was witnessed with magnificent effect in America. One observer stated that between 4 and 6 A.M. (Nov. 13) about 1000 meteors per minute might have been counted! Another display occurred on Nov. 13, 1866, and on this occasion 8485 meteors were enumerated by several observers at Greenwich. A different system gave us a brilliant exhibition on Nov. 27, 1872, when 33,000 meteors were counted by Denza and his assistants at Moncalieri, in Italy, between the hours of 5^h 50^m and 10^h 30^m P.M. A repetition of this phenomenon occurred on Nov. 27, 1885, when the same observers counted nearly 40,000 meteors between 6^h and 10^h P.M.

Heights of Meteors.—Usually the height of meteors at their first appearance is less than 90 miles, and at disappearance more than 40 miles. From a comparison of a large number of computations I derived the following average values:—

But if fireballs and the smaller shooting-stars are separated I find the usual heights at disappearance are:—fireballs, 30 miles; shooting-stars, 54 miles. Fireballs therefore approach much nearer to the Earth’s surface before disruption than the ordinary falling stars.

A very slight acquaintance with trigonometry will enable anyone to compute the real path of a meteor if two or more observations, made at distant stations, are available for the purpose. The observed courses of the meteor should be marked upon a celestial globe, and extended backwards to the point where they mutually intersect; this will be the radiant-point. The globe having been set for the time and latitude, the apparent tracks should also be prolonged in a forward direction until they meet the horizon, this will indicate the Earth-points, or azimuths of the place where the meteor would have been precipitated on the Earth had it been enabled to continue its flight so far. The azimuths and altitudes of the beginning and end of the path, and the azimuths of the Earth-point should then be read off, and by means of a reliable map and a protractor their points of intersection over the Earth’s surface may be readily found by lines drawn from the two places of observation. From the spot where the Earth-points intersect a straight line should also be drawn in the direction of the radiant, and it is along this line the meteor’s motion was directed. The coordinates of the observed points of appearance and disappearance of the meteor, at the two stations, would intersect this line at identical points were the observations perfectly accurate, but this is rarely the case. The distance between the observer’s station and the places over which the meteor began and ended is easily derived from the map, and the height of the object may be found by adding the logarithm of the distance to the log. of the tangent of the altitude. Thus, if the end of a meteor is witnessed from London in azimuth 130° W. of S. (alt. 25°), and from Bristol in azimuth 216° W. of S. (alt. 30°) the place of intersection on the map will be at Warwick, so that the meteor must have disappeared when vertically over this city. London is distant from Warwick about 86 miles, and from Bristol 70 miles, and the resulting height of the meteor is:—

so that the observations accord very closely in fixing the height at a little exceeding 40 miles at disappearance, but a slight correction is necessary to allow for the Earth’s curvature. There are other methods of computing the heights, one of which is explained by Prof. A. S. Herschel in a paper entitled “Height of a Meteor” (‘Monthly Notices,’ vol. xxv. p. 251).

Meteoric Observations.—A large number of meteor-showers still await discovery, and there are features even in connection with the best known streams which remain to be elucidated. Such doubts as now exist are only to be cleared away by assiduous observation made with the utmost accuracy possible both of the directions and durations of meteors.

This attractive field of investigation has certainly been neglected in recent years, and the reason of this may perhaps be found in the complications inseparable from it, in the need of great patience and scrupulous care in observation, and the necessity of gaining experience before the observer can feel a reliance on his work, and draw safe conclusions. Meteors are so fugitive, so diverse and erratic in their apparitions, as to be quite beyond the scope of instrumental refinements. They must necessarily be observed under many disadvantages. Positions have to be fixed from very hurried and often imperfect impressions. But these drawbacks, formidable as they at first appear, may be severally overcome by practice, by careful regard for the conditions under which meteors are displayed, and the marked differences of aspect induced by these conditions. When the observer has acquired a practical knowledge he will proceed with confidence in his work, and avoid many of the difficulties surrounding it.

In recording meteor-tracks for the purpose of discovering the radiant-points, the chief feature in which precision is essential is the direction of flight. A perfectly straight wand, held in the hand for the purpose, should be projected upon the path of every meteor directly it is seen, and then when the eye has quickly noted the position and slope relatively to the fixed stars near, it should be reproduced on the chart or celestial globe. The time, mag., estimated duration, and details of appearance should be registered in a tabular form, with the R.A. and Dec. of the beginning-point and probable radiant. The end-point and length of path may be left until next day, in order to save valuable time. The wand is a great assistance to the eye in retaining the approximate directions and noting the places. If a meteor belongs to the slow, trained class, or if it belongs to the swift, streak-leaving order, the path may be very accurately noted, for the wand can be adjusted to its direction before the meteor or its visible offcome has died away. In the case of short, quick meteors, devoid of either streaks or trains, and generally shooting from radiants at high altitudes, they are more difficult to secure, as they vanish before one may turn, and the observer must rely upon the mere impression he received. But even these succumb to experience, and will be found to resolve themselves into a number of sharply defined radiants scarcely less certain than the positions derived from the streaked or trained meteors.

These positions are only to be fixed by the exercise of much cautious discrimination on the part of the observer, for the direction of the flight is not sufficient, alone, to indicate it. The visible aspect of the meteor has to be equally considered, for the place of its radiant imparts certain peculiarities to it which are rarely to be mistaken. First, the astronomical position of the radiant. If the radiant is at, or within 50° of, the Earth’s apex (a point 90° preceding the Sun along the ecliptic, and towards which the Earth’s motion is directed) the meteors generally leave streaks, especially the brighter ones, and move with great speed. They are usually white, exhibiting a high degree of incandescence. If the radiant is near the anti-apex or anywhere in the anti-apex half-sphere the meteors are streakless, they travel slowly or very slowly, and often leave trails of yellowish sparks. Bearing these facts in mind the region may be assigned in which any radiant is situated, if not the exact position of the radiant itself. If, say, on Aug. 10, at midnight a swift, streaked meteor is seen shooting from the Pleiades towards Aldebaran, just risen, the radiant is either in Musca, Triangulum, or Andromeda. But if the meteor is slow, with a train, then we must go further back in the direction of its flight, and seek the radiant in the S. or S.W. sky. If the motion is very slow, the radiant may be as far away as Aquila. Second, the sensible position of the radiant. A low radiant yields long-pathed meteors, characterized by slowness of speed and a flaky appearance either of the streaks or trains. A radiant near the zenith gives short, darting meteors, with rather dense streaks or trains. These nearly vertical meteors have a less extensive range of atmosphere to penetrate than the horizontal meteors, which are sometimes abnormally long. In the case of brilliant meteors, however, the paths occasionally extend over considerable arcs though the radiant may be high. Third, the position of the radiant relatively to the path of a meteor. If a meteor is close to its radiant its track is usually slow, and appears greatly foreshortened by the effects of perspective. It is travelling (approaching) nearly in the line of sight, and the streak or offcome of sparks is especially dense because it is seen through its entire depth; and the nucleus in such a case has a brushy diffused appearance. Such meteors often traverse sinuous, or curved paths of 2°, 3°, or 4°, and they are readily distinguishable from other meteors far from the radiants to which they belong.

A good method of tabulating meteor-tracks is that adopted by Lieut.-Col. Tupman in his catalogue published by the British Association in 1874. I have adopted the same form, and herewith append a copy of my register of a few isolated bright meteors observed in the autumn of 1890:—