A Text-Book of Astronomy Part 18

[Ill.u.s.tration: FIG. 102.--Some famous comets.]

Note in Fig. 102 the great variety of aspect presented by some of the more famous comets, which are here represented upon a very small scale.

Fig. 103 is from a photograph of one of the faint comets of the year 1893, which appears here as a rather feeble streak of light amid the stars which are scattered over the background of the picture. An apparently detached portion of this comet is shown at the extreme left of the picture, looking almost like another independent comet. The clean, straight line running diagonally across the picture is the flash of a bright meteor that chanced to pa.s.s within the range of the camera while the comet was being photographed.

A more striking representation of a moderately bright telescopic comet is contained in Figs. 104 and 105, which present two different views of the same comet, showing a considerable change in its appearance. A striking feature of Fig. 105 is the star images, which are here drawn out into short lines all parallel with each other. During the exposure of 2h. 20m. required to imprint this picture upon the photographic plate, the comet was continually changing its position among the stars on account of its...o...b..tal motion, and the plate was therefore moved from time to time, so as to follow the comet and make its image always fall at the same place. Hence the plate was continually s.h.i.+fted relative to the stars whose images, drawn out into lines, show the direction in which the plate was moved--i. e., the direction in which the comet was moving across the sky. The same effect is shown in the other photographs, but less conspicuously than here on account of their shorter exposure times.

These pictures all show that one end of the comet is brighter and apparently more dense than the other, and it is customary to call this bright part the _head_ of the comet, while the brushlike appendage that streams away from it is called the comet's _tail_.

[Ill.u.s.tration: FIG. 103.--Brooks's comet, November 13, 1893. BARNARD.]

160. THE PARTS OF A COMET.--It is not every comet that has a tail, though all the large ones do, and in Fig. 103 the detached piece of cometary matter at the left of the picture represents very well the appearance of a tailless comet, a rather large but not very bright star of a fuzzy or hairy appearance. The word comet means long-haired or hairy star. Something of this vagueness of outline is found in all comets, whose exact boundaries are hard to define, instead of being sharp and clean-cut like those of a planet or satellite. Often, however, there is found in the head of a comet a much more solid appearing part, like the round white ball at the center of Fig. 106, which is called the nucleus of the comet, and appears to be in some sort the center from which its activities radiate. As shown in Figs. 106 and 107, the nucleus is sometimes surrounded by what are called envelopes, which have the appearance of successive wrappings or halos placed about it, and odd, spurlike projections, called jets, are sometimes found in connection with the envelopes or in place of them. These figures also show what is quite a common characteristic of large comets, a dark streak running down the axis of the tail, showing that the tail is hollow, a mere sh.e.l.l surrounding empty s.p.a.ce.

[Ill.u.s.tration: FIG. 104.--Swift's comet, April 17, 1892.--BARNARD.]

The amount of detail shown in Figs. 106 and 107 is, however, quite exceptional, and the ordinary comet is much more like Fig. 103 or 104.

Even a great comet when it first appears is not unlike the detached fragment in Fig. 103, a faint and roundish patch of foggy light which grows through successive stages to its maximum estate, developing a tail, nucleus, envelopes, etc., only to lose them again as it shrinks and finally disappears.

[Ill.u.s.tration: FIG. 105.--Swift's comet, April 24, 1892.--BARNARD.]

161. THE ORBITS OF COMETS.--It will be remembered that Newton found, as a theoretical consequence of the law of gravitation, that a body moving under the influence of the sun's attraction might have as its...o...b..t any one of the conic sections, ellipse, parabola, or hyperbola, and among the 400 and more comet orbits which have been determined every one of these orbit forms appears, but curiously enough there is not a hyperbola among them which, if drawn upon paper, could be distinguished by the unaided eye from a parabola, and the ellipses are all so long and narrow, not one of them being so nearly round as is the most eccentric planet orbit, that astronomers are accustomed to look upon the parabola as being the normal type of comet orbit, and to regard a comet whose motion differs much from a parabola as being abnormal and calling for some special explanation.

The fact that comet orbits are parabolas, or differ but little from them, explains at once the temporary character and speedy disappearance of these bodies. They are visitors to the solar system and visible for only a short time, because the parabola in which they travel is not a closed curve, and the comet, having pa.s.sed once along that portion of it near the earth and the sun, moves off along a path which ever thereafter takes it farther and farther away, beyond the limit of visibility. The development of the comet during the time it is visible, the growth and disappearance of tail, nucleus, etc., depend upon its changing distance from the sun, the highest development and most complex structure being presented when it is nearest to the sun.

[Ill.u.s.tration: FIG. 106.--Head of Coggia's comet, July 13, 1874.--TROUVELOT.]

Fig. 108 shows the path of the Great Comet of 1882 during the period in which it was seen, from September 3, 1882, to May 26, 1883. These dates--IX, 3, and V, 26--are marked in the figure opposite the parts of the orbit in which the comet stood at those times. Similarly, the positions of the earth in its...o...b..t at the beginning of September, October, November, etc., are marked by the Roman numerals IX, X, XI, etc. The line _S V_ shows the direction from the sun to the vernal equinox, and _S_ O is the line along which the plane of the comet's...o...b..t intersects the plane of the earth's...o...b..t--i. e., it is the line of nodes of the comet orbit. Since the comet approached the sun from the south side of the ecliptic, all of its...o...b..t, save the little segment which falls to the left of _S_ O, lies below (south) of the plane of the earth's...o...b..t, and the part which would be hidden if this plane were opaque is represented by a broken line.

[Ill.u.s.tration: FIG. 107.--Head of Donati's comet, September 30, October 2, 1858.--BOND.]

162. ELEMENTS OF A COMET's...o...b..T.--There is a theorem of geometry to the effect that through any three points not in the same straight line one circle, and only one, can be drawn. Corresponding to this there is a theorem of celestial mechanics, that through any three positions of a comet one conic section, and only one, can be pa.s.sed along which the comet can move in accordance with the law of gravitation. This conic section is, of course, its...o...b..t, and at the discovery of a comet astronomers always hasten to observe its position in the sky on different nights in order to obtain the three positions (right ascensions and declinations) necessary for determining the particular orbit in which it moves. The circle, to which reference was made above, is completely ascertained and defined when we know its radius and the position of its center. A parabola is not so simply defined, and five numbers, called the _elements_ of its...o...b..t, are required to fix accurately a comet's path around the sun. Two of these relate to the position of the line of nodes and the angle which the orbit plane makes with the plane of the ecliptic; a third fixes the direction of the axis of the orbit in its plane, and the remaining two, which are of more interest to us, are the date at which the comet makes its nearest approach to the sun (_perihelion pa.s.sage_) and its distance from the sun at that date (_perihelion distance_). The date, September 17th, placed near the center of Fig. 108, is the former of these elements, while the latter, which is too small to be accurately measured here, may be found from Fig. 109 to be 0.82 of the sun's diameter, or, in terms of the earth's distance from the sun, 0.008.

[Ill.u.s.tration: FIG. 108.--Orbits of the earth and the Great Comet of 1882.]

Fig. 109 shows on a large scale the shape of that part of the orbit near the sun and gives the successive positions of the comet, at intervals of 2/10 of a day, on September 16th and 17th, showing that in less than 10 hours--17.0 to 17.4--the comet swung around the sun through an angle of more than 240. When at its perihelion it was moving with a velocity of 300 miles per second! This very unusual velocity was due to the comet's extraordinarily close approach to the sun. The earth's velocity in its...o...b..t is only 19 miles per second, and the velocity of any comet at any distance from the sun, provided its...o...b..t is a parabola, may be found by dividing this number by the square root of half the comet's distance--e. g., 300 miles per second equals 19 v 0.004.

[Ill.u.s.tration: FIG. 109.--Motion of the Great Comet of 1883 in pa.s.sing around the sun.]

Most of the visible comets have their perihelion distances included between 1/3 and 4/3 of the earth's distance from the sun, but occasionally one is found, like the second comet of 1885, whose nearest approach to the sun lies far outside the earth's...o...b..t, in this case halfway out to the orbit of Jupiter; but such a comet must be a very large one in order to be seen at all from the earth. There is, however, some reason for believing that the number of comets which move around the sun without ever coming inside the orbit of Jupiter, or even that of Saturn, is much larger than the number of those which come close enough to be discovered from the earth. In any case we are reminded of Kepler's saying, that comets in the sky are as plentiful as fishes in the sea, which seems to be very little exaggerated when we consider that, according to Kleiber, out of all the comets which enter the solar system probably not more than 2 or 3 per cent are ever discovered.

[Ill.u.s.tration: FIG. 110.--The Great Comet of 1843.]

163. DIMENSIONS OF COMETS.--The comet whose orbit is shown in Figs. 108 and 109 is the finest and largest that has appeared in recent years. Its tail, which at its maximum extent would have more than bridged the s.p.a.ce between sun and earth (100,000,000 miles), is made very much too short in Fig. 109, but when at its best was probably not inferior to that of the Great Comet of 1843, shown in Fig. 110. As we shall see later, there is a peculiar and special relations.h.i.+p between these two comets.

The head of the comet of 1882 was not especially large--about twice the diameter of the ball of Saturn--but its nucleus, according to an estimate made by Dr. Elkin when it was very near perihelion, was as large as the moon. The head of the comet shown in Fig. 107 was too large to be put in the s.p.a.ce between the earth and the moon, and the Great Comet of 1811 had a head considerably larger than the sun itself. From these colossal sizes down to the smallest shred just visible in the telescope, comets of all dimensions may be found, but the smaller the comet the less the chance of its being discovered, and a comet as small as the earth would probably go un.o.bserved unless it approached very close to us.

164. THE Ma.s.s OF A COMET.--There is no known case in which the ma.s.s of a comet has ever been measured, yet nothing about them is more sure than that they are bodies with ma.s.s which is attracted by the sun and the planets, and which in its turn attracts both sun and planets and produces perturbations in their motion. These perturbations are, however, too small to be measured, although the corresponding perturbations in the comet's motion are sometimes enormous, and since these mutual perturbations are proportional to the ma.s.ses of comet and planet, we are forced to say that, by comparison with even such small bodies as the moon or Mercury, the ma.s.s of a comet is utterly insignificant, certainly not as great as a ten-thousandth part of the ma.s.s of the earth. In the case of the Great Comet of 1882, if we leave its hundred million miles of tail out of account and suppose the entire ma.s.s condensed into its head, we find by a little computation that the average density of the head under these circ.u.mstances must have been less than 1/1500 of the density of air. In ordinary laboratory practice this would be called a pretty good vacuum. A striking observation made on September 17, 1882, goes to confirm the very small density of this comet. It is shown in Fig. 109 that early on that day the comet crossed the line joining earth and sun, and therefore pa.s.sed in transit over the sun's disk. Two observers at the Cape of Good Hope saw the comet approach the sun, and followed it with their telescopes until the nucleus actually reached the edge of the sun and disappeared, behind it as they supposed, for no trace of the comet, not even its nucleus, could be seen against the sun, although it was carefully looked for. Now, the figure shows that the comet pa.s.sed between the earth and sun, and its densest parts were therefore too attenuated to cut off any perceptible fraction of the sun's rays. In other cases stars have been seen through the head of a comet, s.h.i.+ning apparently with undimmed l.u.s.ter, although in some cases they seem to have been slightly refracted out of their true positions.

165. METEORS.--Before proceeding further with the study of comets it is well to turn aside and consider their humbler relatives, the shooting stars. On some clear evening, when the moon is absent from the sky, watch the heavens for an hour and count the meteors visible during that time. Note their paths, the part of the sky where they appear and where they disappear, their brightness, and whether they all move with equal swiftness. Out of such simple observations with the unaided eye there has grown a large and important branch of astronomical science, some parts of which we shall briefly summarize here.

A particular meteor is a local phenomenon seen over only a small part of the earth's surface, although occasionally a very big and bright one may travel and be visible over a considerable territory. Such a one in December, 1876, swept over the United States from Kansas to Pennsylvania, and was seen from eleven different States. But the ordinary shooting star is much less conspicuous, and, as we know from simultaneous observations made at neighboring places, it makes its appearance at a height of some 75 miles above the earth's surface, occupies something like a second in moving over its path, and then disappears at a height of about 50 miles or more, although occasionally a big one comes down to the very surface of the earth with force sufficient to bury itself in the ground, from which it may be dug up, handled, weighed, and turned over to the chemist to be a.n.a.lyzed. The pieces thus found show that the big meteors, at least, are ma.s.ses of stone or mineral; iron is quite commonly found in them, as are a considerable number of other terrestrial substances combined in rather peculiar ways. But no chemical element not found on the earth has ever been discovered in a meteor.

166. NATURE OF METEORS.--The swiftness with which the meteors sweep down shows that they must come from outside the earth, for even half their velocity, if given to them by some terrestrial volcano or other explosive agent, would send them completely away from the earth never to return. We must therefore look upon them as so many projectiles, bullets, fired against the earth from some outside source and arrested in their motion by the earth's atmosphere, which serves as a cus.h.i.+on to protect the ground from the bombardment which would otherwise prove in the highest degree dangerous to both property and life. The speed of the meteor is checked by the resistance which the atmosphere offers to its motion, and the energy represented by that speed is transformed into heat, which in less than a second raises the meteor and the surrounding air to incandescence, melts the meteor either wholly or in part, and usually destroys its ident.i.ty, leaving only an impalpable dust, which cools off as it settles slowly through the lower atmosphere to the ground. The heating effect of the air's resistance is proportional to the square of the meteor's velocity, and even at such a moderate speed as 1 mile per second the effect upon the meteor is the same as if it stood still in a bath of red-hot air. Now, the actual velocity of meteors through the air is often 30 or 40 times as great as this, and the corresponding effect of the air in raising its temperature is more than 1,000 times that of red heat. Small wonder that the meteor is brought to lively incandescence and consumed even in a fraction of a second.

167. THE NUMBER OF METEORS.--A single observer may expect to see in the evening hours about one meteor every 10 minutes on the average, although, of course, in this respect much irregularity may occur. Later in the night they become more frequent, and after 2 A. M. there are about three times as many to be seen as in the evening hours. But no one person can keep a watch upon the whole sky, high and low, in front and behind, and experience shows that by increasing the number of observers and a.s.signing to each a particular part of the sky, the total number of meteors counted may be increased about five-fold. So, too, the observers at any one place can keep an effective watch upon only those meteors which come into the earth's atmosphere within some moderate distance of their station, say 50 or 100 miles, and to watch every part of that atmosphere would require a large number of stations, estimated at something more than 10,000, scattered systematically over the whole face of the earth. If we piece together the several numbers above considered, taking 14 as a fair average of the hourly number of meteors to be seen by a single observer at all hours of the night, we shall find for the total number of meteors encountered by the earth in 24 hours, 14 5 10,000 24 = 16,800,000. Without laying too much stress upon this particular number, we may fairly say that the meteors picked up by the earth every day are to be reckoned by millions, and since they come at all seasons of the year, we shall have to admit that the region through which the earth moves, instead of being empty s.p.a.ce, is really a dust cloud, each individual particle of dust being a prospective meteor.

On the average these individual particles are very small and very far apart; a cloud of silver dimes each about 250 miles from its nearest neighbor is perhaps a fair representation of their average ma.s.s and distance from each other, but, of course, great variations are to be expected both in the size and in the frequency of the particles. There must be great numbers of them that are too small to make shooting stars visible to the naked eye, and such are occasionally seen darting by chance across the field of view of a telescope.

168. THE ZODIACAL LIGHT is an effect probably due to the reflection of sunlight from the myriads of these tiny meteors which occupy the s.p.a.ce inside the earth's...o...b..t. It is a faint and diffuse stream of light, something like the Milky Way, which may be seen in the early evening or morning stretching up from the sunrise or sunset point of the horizon along the ecliptic and following its course for many degrees, possibly around the entire circ.u.mference of the sky. It may be seen at any season of the year, although it shows to the best advantage in spring evenings and autumn mornings. Look for it.

169. GREAT METEORS.--But there are other meteors, veritable fireb.a.l.l.s in appearance, far more conspicuous and imposing than the ordinary shooting star. Such a one exploded over the city of Madrid, Spain, on the morning of February 10, 1896, giving in broad sunlight "a brilliant flash which was followed ninety seconds later by a succession of terrific noises like the discharge of a battery of artillery." Fig. 111 shows a large meteor which was seen in California in the early evening of July 27, 1894, and which left behind it a luminous trail or cloud visible for more than half an hour.

Not infrequently large meteors are found traveling together, two or three or more in company, making their appearance simultaneously as did the California meteor of October 22, 1896, which is described as triple, the trio following one another like a train of cars, and Arago cites an instance, from the year 1830, where within a short s.p.a.ce of time some forty brilliant meteors crossed the sky, all moving in the same direction with a whistling noise and displaying in their flight all the colors of the rainbow.

The ma.s.s of great meteors such as these must be measured in hundreds if not thousands of pounds, and stories are current, although not very well authenticated, of even larger ones, many tons in weight, having been found partially buried in the ground. Of meteors which have been actually seen to fall from the sky, the largest single fragment recovered weighs about 500 pounds, but it is only a fragment of the original meteor, which must have been much more ma.s.sive before it was broken up by collision with the atmosphere.

[Ill.u.s.tration: FIG. 111.--The California meteor of July 27, 1894.]

170. THE VELOCITY OF METEORS.--Every meteor, big or little, is subject to the law of gravitation, and before it encounters the earth must be moving in some kind of orbit having the sun at its focus, the particular species of orbit--ellipse, parabola, hyperbola--depending upon the velocity and direction of its motion. Now, the direction in which a meteor is moving can be determined without serious difficulty from observations of its apparent path across the sky made by two or more observers, but the velocity can not be so readily found, since the meteors go too fast for any ordinary process of timing. But by photographing one of them two or three times on the same plate, with an interval of only a tenth of a second between exposures, Dr. Elkin has succeeded in showing, in a few cases, that their velocities varied from 20 to 25 miles per second, and must have been considerably greater than this before the meteors encountered the earth's atmosphere. This is a greater velocity than that of the earth in its...o...b..t, 19 miles per second, as might have been antic.i.p.ated, since the mere fact that meteors can be seen at all in the evening hours shows that some of them at least must travel considerably faster than the earth, for, counting in the direction of the earth's motion, the region of sunset and evening is always on the rear side of the earth, and meteors in order to strike this region must overtake it by their swifter motion. We have here, in fact, the reason why meteors are especially abundant in the morning hours; at this time the observer is on the front side of the earth which catches swift and slow meteors alike, while the rear is pelted only by the swifter ones which follow it.

A comparison of the relative number of morning and evening meteors makes it probable that the average meteor moves, relative to the sun, with a velocity of about 26 miles per second, which is very approximately the average velocity of comets when they are at the earth's distance from the sun. Astronomers, therefore, consider meteors as well as comets to have the parabola and the elongated ellipse as their characteristic orbits.

171. METEOR SHOWERS--THE RADIANT.--There is evident among meteors a distinct tendency for individuals, to the number of hundreds or even hundreds of millions, to travel together in flocks or swarms, all going the same way in orbits almost exactly alike. This gregarious tendency is made manifest not only by the fact that from time to time there are unusually abundant meteoric displays, but also by a striking peculiarity of their behavior at such times. The meteors all seem to come from a particular part of the heavens, as if here were a hole in the sky through which they were introduced, and from which they flow away in every direction, even those which do not visibly start from this place having paths among the stars which, if prolonged backward, would pa.s.s through it. The cause of this appearance may be understood from Fig.

112, which represents a group of meteors moving together along parallel paths toward an observer at _D_. Traveling unseen above the earth until they encounter the upper strata of its atmosphere, they here become incandescent and speed on in parallel paths, _1_, _2_, _3_, _4_, _5_, _6_, which, as seen by the observer, are projected back against the sky into luminous streaks that, as is shown by the arrowheads, _b_, _c_, _d_, all seem to radiate from the point _a_--i. e., from the point in the sky whose direction from the observer is parallel to the paths of the meteors.

[Ill.u.s.tration: FIG. 112.--Explanation of the radiant of a meteoric shower.--DENNING.]

Such a display is called a meteor shower, and the point _a_ is called its radiant. Note how those meteors which appear near the radiant all have short paths, while those remote from it in the sky have longer ones. Query: As the night wears on and the stars s.h.i.+ft toward the west, will the radiant share in their motion or will it be left behind? Would the luminous part of the path of any of these meteors pa.s.s across the radiant from one side to the other? Is such a crossing of the radiant possible under any circ.u.mstances? Fig. 113 shows how the meteor paths are grouped around the radiant of a strongly marked shower. Select from it the meteors which do not belong to this shower.

[Ill.u.s.tration: FIG. 113.--The radiant of a meteoric shower, showing also the paths of three meteors which do not belong to this shower.--DENNING.]

Many hundreds of these radiants have been observed in the sky, each of which represents an orbit along which a group of meteors moves, and the relation of one of these orbits to that of the earth is shown in Fig.

114. The orbit of the meteors is an ellipse extending out beyond the orbit of Ura.n.u.s, but so eccentric that a part of it comes inside the orbit of the earth, and the figure shows only that part of it which lies nearest the sun. The Roman numerals which are placed along the earth's...o...b..t show the position of the earth at the beginning of the tenth month, eleventh month, etc. The meteors flow along their orbit in a long procession, whose direction of motion is indicated by the arrow heads, and the earth, coming in the opposite direction, plunges into this stream and receives the meteor shower when it reaches the intersection of the two orbits. The long arrow at the left of the figure represents the direction of motion of another meteor shower which encounters the earth at this point.

[Ill.u.s.tration: FIG. 114.--The orbits of the earth and the November meteors.]

Can you determine from the figure answers to the following questions? On what day of the year will the earth meet each of these showers? Will the radiant points of the showers lie above or below the plane of the earth's...o...b..t? Will these meteors strike the front or the rear of the earth? Can they be seen in the evening hours?

From many of the radiants year after year, upon the same day or week in each year, there comes a swarm of shooting stars, showing that there must be a continuous procession of meteors moving along this...o...b..t, so that some are always ready to strike the earth whenever it reaches the intersection of its...o...b..t with theirs. Such is the explanation of the shower which appears each year in the first half of August, and whose meteors are sometimes called Perseids, because their radiant lies in the constellation Perseus, and a similar explanation holds for all the star showers which are repeated year after year.

172. THE LEONIDS.--There is, however, a kind of star shower, of which the Leonids (radiant in Leo) is the most conspicuous type, in which the shower, although repeated from year to year, is much more striking in some years than in others. Thus, to quote from the historian: "In 1833 the shower was well observed along the whole eastern coast of North America from the Gulf of Mexico to Halifax. The meteors were most numerous at about 5 A. M. on November 13th, and the rising sun could not blot out all traces of the phenomena, for large meteors were seen now and then in full daylight. Within the scope that the eye could contain, more than twenty could be seen at a time shooting in every direction.

Not a cloud obscured the broad expanse, and millions of meteors sped their way across in every point of the compa.s.s. Their coruscations were bright, gleaming, and incessant, and they fell thick as the flakes in the early snows of December." But, so far as is known, none of them reached the ground. An illiterate man on the following day remarked: "The stars continued to fall until none were left. I am anxious to see how the heavens will appear this evening, for I believe we shall see no more stars."

An eyewitness in the Southern States thus describes the effect of this shower upon the plantation negroes: "Upward of a hundred lay prostrate upon the ground, some speechless and some with the bitterest cries, but with their hands upraised, imploring G.o.d to save the world and them. The scene was truly awful, for never did rain fall much thicker than the meteors fell toward the earth--east, west, north, and south it was the same." In the preceding year a similar but feebler shower from the same radiant created much alarm in France, and through the old historic records its repet.i.tions may be traced back at intervals of 33 or 34 years, although with many interruptions, to October 12, 902, O. S., when "an immense number of falling stars were seen to spread themselves over the face of the sky like rain."

Such a star shower differs from the one repeated every year chiefly in the fact that its meteors, instead of being drawn out into a long procession, are mainly cl.u.s.tered in a single flock which may be long enough to require two or three or four years to pa.s.s a given point of its...o...b..t, but which is far from extending entirely around it, so that meteors from this source are abundant only in those years in which the flock is at or near the intersection of its...o...b..t with that of the earth. The fact that the Leonid shower is repeated at intervals of 33 or 34 years (it appeared in 1799, 1832-'33, 1866-'67) shows that this is the "periodic time" in its...o...b..t, which latter must of course be an ellipse, and presumably a long and narrow one. It is this...o...b..t which is shown in Fig. 114, and the student should note in this figure that if the meteor stream at the point where it cuts through the plane of the earth's...o...b..t were either nearer to or farther from the sun than is the earth there could be no shower; the earth and the meteors would pa.s.s by without a collision. Now, the meteors in their motion are subject to perturbations, particularly by the large planets Jupiter, Saturn, and Ura.n.u.s, which slightly change the meteor orbit, and it seems certain that the changes thus produced will sometimes thrust the swarm inside or outside the orbit of the earth, and thus cause a failure of the shower at times when it is expected. The meteors were due at the crossing of the orbits in November, 1899 and 1900, and, although a few were then seen, the shower was far from being a brilliant one, and its failure was doubtless caused by the outer planets, which switched the meteors aside from the path in which they had been moving for a century. Whether they will be again switched back so as to produce future showers is at the present time uncertain.

173. CAPTURE OF THE LEONIDS.--But a far more striking effect of perturbations is to be found in Fig. 115, which shows the relation of the Leonid orbit to those of the princ.i.p.al planets, and ill.u.s.trates a curious chapter in the history of the meteor swarm that has been worked out by mathematical a.n.a.lysis, and is probably a pretty good account of what actually befell them. Early in the second century of the Christian era this flock of meteors came down toward the sun from outer s.p.a.ce, moving along a parabolic orbit which would have carried it just inside the orbit of Jupiter, and then have sent it off to return no more. But such was not to be its fate. As it approached the orbit of Ura.n.u.s, in the year 126 A. D., that planet chanced to be very near at hand and perturbed the motion of the meteors to such an extent that the character of their orbit was completely changed into the ellipse shown in the figure, and in this new orbit they have moved from that time to this, permanent instead of transient members of the solar system. The perturbations, however, did not end with the year in which the meteors were captured and annexed to the solar system, but ever since that time Jupiter, Saturn, and Ura.n.u.s have been pulling together upon the orbit, and have gradually turned it around into its present position as shown in the figure, and it is chiefly this s.h.i.+fting of the orbit's position in the thousand years that have elapsed since 902 A. D. that makes the meteor shower now come in November instead of in October as it did then.