Meteors

Meteors are small bodies travelling in vast numbers, and in various directions, through space. Our earth continually encounters them in its orbital path, and they are then revealed to our observation as aërolites, fireballs, and shooting or falling stars. Every night, if the sky be clear, some may be observed, on the average five to seven every hour, while on certain occasions they are so numerous as to present the spectacle of a perfect rain of fire. Besides those visible to the eye, there are numbers unseen, some of which are occasionally noted in the course of telescopic observation. The total number encountered by the earth in one day has been estimated by Professor Newton, of Yale College, United States, at 7,500,000. Their total mass, however, he estimates at only 100 tons, so that individually they must in general be exceedingly minute. They dissipate, however, a quantity of dust in the upper regions of the air, which in its slow descent and fall upon the earth is easily detected by proper means. Our air in this case acts as a shield, so that, instead of frequent showers of stones descending with deadly force, we have this quiet falling of impalpable dust. Our conclusions regarding meteors are reached by a proper interpretation of various phenomena, long considered as having no mutual connection, but now grouped coherently under one simple explanation. In order to appreciate the reasoning which has led to this result, it will be convenient to consider first the observed facts regarding (1) aërolites, (2) fireballs, and (3) shooting-stars.

The first group, aërolites, includes all stony or metallic masses actually falling to the earth from the sky. They have been classed as (1) aërosiderites, or siderites, chiefly consisting of meteoric iron; (2) aërosiderolites, or siderolites, conglomerates of stone and iron; (3) aërolites, almost entirely consisting of stone. The common title aërolites embraces, however, all kinds. The descent of such bodies, though rare, has occurred with greater frequency than would be imagined. The British Museum alone has specimens of more than three hundred, of which nearly two hundred were seen to fall. Some sacred stones, as the black stone worshipped at Emesa in Syria, the holy Kaaba of Mecca, and the great stone of the pyramid of Cholula in Mexico, owed their sanctity to a report, probably true, that they had fallen from heaven. It has been suggested that the earliest image of Diana of the Ephesians, which 'fell down from Jupiter,' had taken the place of an actual meteorite. Livy mentions the falling of a shower of stones on the Alban Mount near Rome, about 654 B.C. A Chinese catalogue records the fall of an aërolite on January 14, 616 B.C., which broke several chariots and killed ten men. Plutarch and Pliny mention a great stone, as large as a wagon, the latter says, and of a burnt colour, the fall of which, at Ægospotamos on the Hellespont about 467 B.C., is recorded in the Parian Chronicle. In 1492 A.D., 'on Wednesday, November 7,' a stone weighing 260 lb. was seen to fall near Ensisleim in Alsace: part of it is still preserved in the village church there. In 1510 about 1200 stones, one weighing 120 lb., another 60 lb., fell near Padua in Italy. We are told that the Emperor Jehangir caused a sword to be forged from a mass of meteoric iron which fell at Jullunder in the Punjab in 1620. On November 27, 1627, the astronomer Gassendi witnessed the fall of a stone weighing 59 lb. at Mount Vasier in Provence. At Wold Cottage, Yorkshire, December 13, 1795, a ploughman saw a stone of 56 lb. weight fall near him in a field. But the most interesting of such modern observations was made on April 26, 1803, near L'Aigle, in Normandy. About 1 P.M. a brilliant fireball was seen traversing the air at great speed. A violent explosion followed, apparently proceeding from a small and lofty cloud, followed by a shower of thousands of stones, one 8 lb. in weight. A large meteorite exploded with prodigious noise over Madrid on 10th February 1896. On April 20, 1876, a mass of meteoric iron more than 7 lb. weight fell at Rowton in Shropshire, accompanied also by an explosion. On September 4, 1887, a large aërolite fell at Krasnoslobodsk, in the government of Penza. It was accompanied by a loud explosion, and in it (as in some others) were found crystals having all the chemical properties of the diamond. In nearly every one of these and other cases are noticed the following features—(1) a noise, often an explosion; (2) cloud or smoke; (3) partial fusion of the mass or masses, especially on the surface. These indicate that the aërolite by some means is brought to a very high temperature, at least above the melting-point of iron, which often causes it to burst into fragments. Pieces of one which fell in India in 1861, though picked up miles apart, were found by Maskelyne to fit together into one whole, the fractures coinciding. This high temperature, on the surface of the mass, would easily be produced by the compression and friction of the air in the case of a body moving with sufficient velocity. There is no observed connection between aërolites and volcanoes, nor can volcanic agency account for their velocity, and so this simple explanation of aërial friction is now universally accepted. A sufficient velocity is at once guaranteed when we consider aërolites as simply fireballs whose mass and course are such as to bring them entirely through our atmosphere into contact with the earth. Meteoric iron is also alloyed with nickel, cobalt, manganese, magnesium, copper, carbon, and tin, in a manner in which it is not yet found alloyed in terrestrial minerals; and this also points to its cosmical origin. Altogether twenty-four of the terrestrial chemical elements have been found in aërolites—viz. oxygen, hydrogen, chlorine, sulphur, phosphorus, carbon, silicon, iron, nickel, cobalt, magnesium, chromium, manganese, copper, tin, antimony, aluminium, calcium, potassium, sodium, lithium, titanium, arsenic, and vanadium. No new element not found on earth has been found in them.

The second class of meteors form fireballs, which appear as brilliantly luminous bodies, traversing the sky, often with noise, and always with great velocity. Aërolites before their fall have often been seen as fireballs, and the substantial unity of the two classes is now almost universally accepted. Fireballs, then, are regarded as aërolites whose mass and course are such that they escape actual contact with the earth. They are much more numerous than aërolites, and are of great variety in velocity, size, and brilliance. On August 18, 1783, one of great size traversed the air over Europe, from Shetland to Rome, at a height of 50 miles and with a speed of 30 miles per second, giving off a greater light than the full moon. More recently, on November 17, 1887, a splendid specimen, seen first over the Irish Sea, crossed westwards over Ireland, at a height of probably about 20 miles, and disappeared above the Atlantic. Many hundreds of such, though usually less brilliant, have been observed. Arago enumerates 813. More are constantly being seen. Their height is obtained by comparison of observations at stations widely separated, and from it and their observed speed the actual velocity is computed. From a careful comparison of many observations made by a committee of the British Association it appears that in general they appear at a height of between 20 and 130 miles, and have a velocity of between 17 and 80 miles per second, with an average of 34.4 miles per second. Their actual size has been enormously overestimated, at 12,000 to 100 feet in diameter. The effects of irradiation and the luminous gases discharged during their course no doubt give them an apparent diameter enormously greater than the reality. It is probable that in most cases they are much smaller than aërolites. They generally leave behind them in their track a luminous train or 'tail' which sometimes disappears at once, and at other times persists for some minutes after the fireball itself disappears. These 'tails' are variously coloured, according probably to the different chemical constitution of the 'heads.'

That these bodies originate altogether beyond our earth is evident from several considerations. First, no sufficient terrestrial cause has been assigned. It has never been shown that volcanic explosions can communicate to ejected masses the necessary velocity. No proof has been advanced of the theory that aërolites and fireballs are condensed in the atmosphere itself. There is no volcanic activity on the moon, which might project such masses beyond the influence of her feeble gravity so as to enable them to fall upon our earth. Even if there were such activity in our satellite, the velocity of projection required is so great as to place such a cause outside consideration. Secondly, no good reason can be advanced against the theory of cosmical origin. That numerous masses, of various sizes, are in motion through interplanetary space is not in itself improbable, and is established by the investigation of the paths and velocities of shooting-stars. Thirdly, the velocity of fireballs, averaging 34.4 miles per second, is only comparable with such velocities as that of the earth in its orbit, which is 18.2 miles per second, or of Sirius (see STARS) in its orbit, and those of other planets and stars. It is a velocity not on the terrestrial but on the cosmical scale. Fourthly, there is no special line to be drawn between fireballs and meteors, luminous bodies of all degrees of size between the smallest meteor and the fireball having been observed. It is in fact sometimes a matter of doubt to the observer to which class he should relegate an observed example. To regard all as of common extra-terrestrial origin is then reasonable, and this view is now adopted almost universally.

We are then led onwards to the consideration of shooting-stars, as both the most numerous class of these appearances, and that class by observing which a satisfactory explanation of them all has ultimately been reached. On any fine night a watcher who is careful and patient for a sufficient time will see some of these, but occasionally they are much more numerous. On these occasions they are noted as originating all in one or more distinctly marked parts of the sky. From their point of origin they appear to radiate, and if it be overhead, and the meteors very numerous, the appearance is like an 'umbrella of fire' above the earth. But this point may not be overhead. It may even be below the horizon. In the latter case the meteors appear to come up over the horizon like rockets and ascend into the sky. This 'radiant,' as it is technically called, remains fixed among the stars, so that if at the beginning of an observation it be overhead, it will perhaps be below the horizon before the observer ceases his work. It is either named from the constellation in which it is placed, or indicated by its north polar distance and declination on the sphere of the heavens. Meteors from more than one radiant are frequently passing at the same time, but usually each radiant sends forth a particular kind. Leonids (i.e. the meteors whose radiant is in Leo), or the famous November meteors, are bright and swift, leaving very durable tracks of light. The Taurids (from constellation Taurus) give us many fireballs. Other radiants give meteors of special tints, or more or less disposed to giving off sparks in their course, so that each radiant is evidently the source of a family of meteors, whose characteristics are recognised at each period of activity.

Such a radiating motion implies that the meteors from one radiant move all in parallel courses, the curvature and radiation of their tracks being due to perspective and to projection on the sphere which the eye naturally assumes as the background of all celestial appearances. On the occasion of a meteoric shower the earth, therefore, is passing through a crowd of small bodies, themselves in motion, meeting or passing it on a definite track. We have then to ask what is the form of this meteor track—whence come and whither go the meteors we encounter in such numbers. Usually there is a tolerably definite time, recurring annually, during which a radiant is active. This was the first broad fact impressed upon observers. Although at such yearly periods the number of meteors may be very large or very small, there are at least a few almost always seen. From this it was early seen that certain parts of space, through which the earth passed every year, were occupied, at the date of such passage, by meteors travelling past with planetary velocities. That the meteors, as well as the earth, were in orbital movement round the sun, was soon noted (in 1834) by Professor Olmsted of Yale. He considered that the November meteors (or Leonids) revolved in a narrow ellipse in a period of about 182 days, and that each November the earth in its orbit passed across the outer end of this ellipse, encountering there what meteors might be in that part of their path. This theory, however, though possible in perhaps one case, could hardly be applied to the great number of meteor tracks which the earth crosses, as it is exceedingly improbable that so many meteor orbits would just touch the earth's orbit at their aphelion.

It was proposed, then, to regard the meteors as travelling in a ring round the sun, which ring the earth crossed in two parts of its annual track in August and November. Both these theories regarded the meteors as gathered into a cloud or swarm at one particular part of their orbit. When the earth chanced to cross the place of intersection at the same time as the main swarm of meteors, then a vivid display was produced, but a difference in period between the earth and main swarm caused such meetings to take place only at long intervals. Meteors, however, being distributed all along the meteor track, the earth encountered some at least in August and November every year.

This investigation received its impetus from the great display of Leonids in 1833, chiefly noted in America, and for some time remained the 'text-book explanation.' Professor H. A. Newton of Yale, showed, however, in 1864 that other great Leonid displays had taken place on twelve occasions between 902 A.D. and 1833, separated by periods of either 33·24 years or multiples of that number. He therefore predicted a grand display on November 13-14, 1866, which was duly observed. But the date of the earliest display in 902 A.D. was October 13 (o.s.), so that it was evident that the earth encountered the main swarm of Leonids about three days later in each century. From these facts Professor Newton deduced for the meteors an elliptic orbit, with a period of 354·57 days. Other explanations were possible, and that given by Schiaparelli of Milan in 1866 finally satisfied all the conditions. He treated the Leonids as revolving round the sun in a period of 33½ years, the earth passing their orbit every year, but only encountering the main swarm when it also was passing the point of intersection. He also noted a remarkable coincidence between this orbit and that of Tempel's comet seen in 1866. In fact, they were identical, within the errors of calculation. Other similar cases were soon discovered. The Lyraids of April 20 move in the track of a comet of 1861; Biela's comet agrees with the Andromeda meteors of November 28; the August Perseids agree with the bright comet of 1862; and now more than seventy such cases of agreement are known, which led Professor Tait of Edinburgh to publish the theory now generally accepted which regards comets as consisting of meteoric swarms (see COMET). Lockyer in 1887 showed by experiment that the fragments of fallen meteors, glowing in a very rare atmosphere given off by themselves when heated, give spectra closely resembling those of comets. It has also been shown by the same observer that what are practically the spectra of nebulae can be obtained from the same source. So that he regards the feeble meteors of our nights as the material of nebulae and stars—as the earliest known form of matter (see STARS). This assumes that our meteoric swarms are either remnants of the original material of the Solar System (q.v.), or portions of the greater swarms of which all space is full, which have been drawn within our solar system by planetary influence. Leverrier has shown that this latter explanation probably applies to the August and November meteors already referred to, and that the planet Uranus has most likely captured these bodies and added them to our system. The action of gravity would tend to draw out a meteor swarm so that it would gradually spread backwards and forwards until finally it would be distributed all along its track and form a closed elliptic ring. As, then, the August meteors form such a ring, while the November Leonids are a marked swarm, Leverrier concluded that the former had entered our system through the action of Uranus much earlier than the latter.

Some hundreds of 'radiants' are now known, a few of which we name, and the dates on which they are active: (1) The Lyraids, April 19-20; (2) the Pegasids, August 10; (3) the Perseids, August 9-11; (4) the Aurigids, September and October; (5) the Orionids, October and November; (6) the Taurids, November 1-15; (7) the Leonids, November 13-14.

For further information readers may consult Arago's Pop. Astronomy (French edition only), The Report of the Brit. Assoc. Committee on Meteors, Chambers's Descriptive Astronomy, or Herschel's Outlines of Astronomy.