Astronomy of To-day Part 18

Any one who happens to gaze at the sky for a short time on a clear night is pretty certain to be rewarded with a view of what is popularly known as a "shooting star." Such an object, however, is not a star at all, but has received its appellation from an a.n.a.logy; for the phenomenon gives to the inexperienced in these matters an impression as if one of the many points of light, which glitter in the vaulted heaven, had suddenly become loosened from its place, and was falling towards the earth. In its pa.s.sage across the sky the moving object leaves behind a trail of light which usually lasts for a few moments. Shooting stars, or meteors, as they are technically termed, are for the most part very small bodies, perhaps no larger than peas or pebbles, which, dashing towards our earth from s.p.a.ce beyond, are heated to a white heat, and reduced to powder by the friction resulting from their rapid pa.s.sage into our atmosphere.

This they enter at various degrees of speed, in some cases so great as 45 miles a second. The speed, of course, will depend greatly upon whether the earth and the meteors are rushing towards each other, or whether the latter are merely overtaking the earth. In the first of these cases the meteors will naturally collide with the atmosphere with great force; in the other case they will plainly come into it with much less rapidity. As has been already stated, it is from observations of such bodies that we are enabled to estimate, though very imperfectly, the height at which the air around our globe practically ceases, and this height is imagined to be somewhere about 100 miles. Fortunate, indeed, is it for us that there is a goodly layer of atmosphere over our heads, for, were this not so, these visitors from s.p.a.ce would strike upon the surface of our earth night and day, and render existence still more unendurable than many persons choose to consider it. To what a bombardment must the moon be continually subject, dest.i.tute as she is of such an atmospheric shield!

It is only in the moment of their dissolution that we really learn anything about meteors, for these bodies are much too small to be seen before they enter our atmosphere. The debris arising from their destruction is wafted over the earth, and, settling down eventually upon its surface, goes to augment the acc.u.mulation of that humble domestic commodity which men call dust. This continual addition of material tends, of course, to increase the ma.s.s of the earth, though the effect thus produced will be on an exceedingly small scale.

The total number of meteors moving about in s.p.a.ce must be practically countless. The number which actually dash into the earth's atmosphere during each year is, indeed, very great. Professor Simon Newcomb, the well-known American astronomer, has estimated that, of the latter, those large enough to be seen with the naked eye cannot be in all less than 146,000,000,000 per annum. Ten times more numerous still are thought to be those insignificant ones which are seen to pa.s.s like mere sparks of light across the field of an observer's telescope.

Until comparatively recent times, perhaps up to about a hundred years ago, it was thought that meteors were purely terrestrial phenomena which had their origin in the upper regions of the air. It, however, began to be noticed that at certain periods of the year these moving objects appeared to come from definite areas of the sky. Considerations, therefore, respecting their observed velocities, directions, and alt.i.tudes, gave rise to the theory that they are swarms of small bodies travelling around the sun in elongated elliptical orbits, all along the length of which they are scattered, and that the earth, in its annual revolution, rushing through the midst of such swarms at the same epoch each year, naturally entangles many of them in its atmospheric net.

The dates at which the earth is expected to pa.s.s through the princ.i.p.al meteor-swarms are now pretty well known. These swarms are distinguished from one another by the direction of the sky from which the meteors seem to arrive. Many of the swarms are so wide that the earth takes days, and even weeks, to pa.s.s through them. In some of these swarms, or streams, as they are also called, the meteors are distributed with fair evenness along the entire length of their orbits, so that the earth is greeted with a somewhat similar shower at each yearly encounter. In others, the chief portions are bunched together, so that, in certain years, the display is exceptional (see Fig. 20, p. 269). That part of the heavens from which a shower of meteors is seen to emanate is called the "radiant," or radiant point, because the foreshortened view we get of the streaks of light makes it appear as if they radiated outwards from this point. In observations of these bodies the attention of astronomers is directed to registering the path and speed of each meteor, and to ascertaining the position of the radiant. It is from data such as these that computations concerning the swarms and their orbits are made.

[Ill.u.s.tration: FIG. 20.--Pa.s.sage of the Earth through the thickest portion of a Meteor Swarm. The Earth and the Meteors are here represented as approaching each other from opposite directions.]

For the present state of knowledge concerning meteors, astronomy is largely indebted to the researches of Mr. W.F. Denning, of Bristol, and of the late Professor A.S. Herschel.

During the course of each year the earth encounters a goodly number of meteor-swarms. Three of these, giving rise to fine displays, are very well known--the "Perseids," or August Meteors, and the "Leonids" and "Bielids," which appear in November.

Of the above three the _Leonid_ display is by far the most important, and the high degree of attention paid to it has laid the foundation of meteoric astronomy in much the same way that the study of the fascinating corona has given such an impetus to our knowledge of the sun. The history of this shower of meteors may be traced back as far as A.D. 902, which was known as the "Year of the Stars." It is related that in that year, on the night of October 12th--the shower now comes about a month later--whilst the Moorish King, Ibrahim Ben Ahmed, lay dying before Cosenza, in Calabria, "a mult.i.tude of falling stars scattered themselves across the sky like rain," and the beholders shuddered at what they considered a dread celestial portent. We have, however, little knowledge of the subsequent history of the Leonids until 1698, since which time the maximum shower has appeared with considerable regularity at intervals of about thirty-three years. But it was not until 1799 that they sprang into especial notice. On the 11th November in that year a splendid display was witnessed at c.u.mana, in South America, by the celebrated travellers, Humboldt and Bonpland. Finer still, and surpa.s.sing all displays of the kind ever seen, was that of November 12, 1833, when the meteors fell thick as snowflakes, 240,000 being estimated to have appeared during seven hours. Some of them were even so bright as to be seen in full daylight. The radiant from which the meteors seem to diverge was ascertained to be situated in the head of the constellation of the Lion, or "Sickle of Leo," as it is popularly termed, whence their name--Leonids. It was from a discussion of the observations then made that the American astronomer, Olmsted, concluded that these meteors sprang upon us from interplanetary s.p.a.ce, and were not, as had been hitherto thought, born of our atmosphere. Later on, in 1837, Olbers formulated the theory that the bodies in question travelled around the sun in an elliptical orbit, and at the same time he established the periodicity of the maximum shower.

The periodic time of recurrence of this maximum, namely, about thirty-three years, led to eager expectancy as 1866 drew near. Hopes were then fulfilled, and another splendid display took place, of which Sir Robert Ball, who observed it, has given a graphic description in his _Story of the Heavens_. The display was repeated upon a smaller scale in the two following years. The Leonids were henceforth deemed to hold an anomalous position among meteor swarms. According to theory the earth cut through their orbit at about the same date each year, and so a certain number were then seen to issue from the radiant. But, in addition, after intervals of thirty-three years, as has been seen, an exceptional display always took place; and this state of things was not limited to one year alone, but was repeated at each meeting for about three years running. The further a.s.sumption was, therefore, made that the swarm was much denser in one portion of the orbit than elsewhere,[27] and that this congested part was drawn out to such an extent that the earth could pa.s.s through the crossing place during several annual meetings, and still find it going by like a long procession (see Fig. 20, p. 269).

In accordance with this ascertained period of thirty-three years, the recurrence of the great Leonid shower was timed to take place on the 15th of November 1899. But there was disappointment then, and the displays which occurred during the few years following were not of much importance. A good deal of comment was made at the time, and theories were accordingly put forward to account for the failure of the great shower. The most probable explanation seems to be, that the attraction of one of the larger planets--Jupiter perhaps--has diverted the orbit somewhat from its old position, and the earth does not in consequence cut through the swarm in the same manner as it used to do.

The other November display alluded to takes place between the 23rd and 27th of that month. It is called the _Andromedid_ Shower, because the meteors appear to issue from the direction of the constellation of Andromeda, which at that period of the year is well overhead during the early hours of the night. These meteors are also known by the name of _Bielids_, from a connection which the orbit a.s.signed to them appears to have with that of the well-known comet of Biela.

M. Egenitis, Director of the Observatory of Athens, accords to the Bielids a high antiquity. He traces the shower back to the days of the Emperor Justinian. Theophanes, the Chronicler of that epoch, writing of the famous revolt of Nika in the year A.D. 532, says:--"During the same year a great fall of stars came from the evening till the dawn." M.

Egenitis notes another early reference to these meteors in A.D. 752, during the reign of the Eastern Emperor, Constantine Cop.r.o.nymous.

Writing of that year, Nicephorus, a Patriarch of Constantinople, has as follows:--"All the stars appeared to be detached from the sky, and to fall upon the earth."

The Bielids, however, do not seem to have attracted particular notice until the nineteenth century. Attention first began to be riveted upon them on account of their suspected connection with Biela's comet. It appeared that the same orbit was shared both by that comet and the Bielid swarm. It will be remembered that the comet in question was not seen after its appearance in 1852. Since that date, however, the Bielid shower has shown an increased activity; which was further noticed to be especially great in those years in which the comet, had it still existed, would be due to pa.s.s near the earth.

The third of these great showers to which allusion has above been made, namely, the _Perseids_, strikes the earth about the 10th of August; for which reason it is known on the Continent under the name of the "tears of St. Lawrence," the day in question being sacred to that Saint. This shower is traceable back many centuries, even as far as the year A.D.

811. The name given to these meteors, "Perseids," arises from the fact that their radiant point is situated in the constellation of Perseus.

This shower is, however, not by any means limited to the particular night of August 10th, for meteors belonging to the swarm may be observed to fall in more or less varying quant.i.ties from about July 8th to August 22nd. The Perseid meteors sometimes fall at the rate of about sixty per hour. They are noted for their great rapidity of motion, and their trails besides often persist for a minute or two before being disseminated. Unlike the other well-known showers, the radiants of which are stationary, that of the Perseids shifts each night a little in an easterly direction.

The orbit of the Perseids cuts that of the earth almost perpendicularly.

The bodies are generally supposed to be the result of the disintegration of an ancient comet which travelled in the same orbit. Tuttle's Comet, which pa.s.sed close to the earth in 1862, also belongs to this...o...b..t; and its period of revolution is calculated to be 131 years. The Perseids appear to be disseminated all along this great orbit, for we meet them in considerable quant.i.ties each year. The bodies in question are in general particularly small. The swarm has, however, like most others, a somewhat denser portion, and through this the earth pa.s.sed in 1848. The _aphelion_, or point where the far end of the orbit turns back again towards the sun, is situated right away beyond the path of Neptune, at a distance of forty-eight times that of the earth from the sun. The comet of 1532 also belongs to the Perseid orbit. It revisited the neighbourhood of the earth in 1661, and should have returned in 1789.

But we have no record of it in that year; for which omission the then politically disturbed state of Europe may account. If not already disintegrated, this comet is due to return in 1919.

This supposed connection between comets and meteor-swarms must be also extended to the case of the Leonids. These meteors appear to travel along the same track as Tempel's Comet of 1866.

It is considered that the attractions of the various bodies of the solar system upon a meteor swarm must eventually result in breaking up the "bunched" portion, so that in time the individual meteors should become distributed along the whole length of the orbit. Upon this a.s.sumption the Perseid swarm, in which the meteors are fairly well scattered along its path, should be of greater age than the Leonid. As to the Leonid swarm itself, Le Verrier held that it was first brought into the solar system in A.D. 126, having been captured from outer s.p.a.ce by the gravitative action of the planet Ura.n.u.s.

The acknowledged theory of meteor swarms has naturally given rise to an idea, that the sunlight shining upon such a large collection of particles ought to render a swarm visible before its collision with the earth's atmosphere. Several attempts have therefore been made to search for approaching swarms by photography, but, so far, it appears without success. It has also been proposed, by Mr. W.H.S. Monck, that the stars in those regions from which swarms are due, should be carefully watched, to see if their light exhibits such temporary diminutions as would be likely to arise from the momentary interposition of a cloud of moving particles.

Between ten and fifteen years ago it happened that several well-known observers, employed in telescopic examination of the sun and moon, reported that from time to time they had seen small dark bodies, sometimes singly, sometimes in numbers, in pa.s.sage across the discs of the luminaries. It was concluded that these were meteors moving in s.p.a.ce beyond the atmosphere of the earth. The bodies were called "dark meteors," to emphasise the fact that they were seen in their natural condition, and not in that momentary one in which they had hitherto been always seen; _i.e._ when heated to white heat, and rapidly vaporised, in the course of their pa.s.sage through the upper regions of our air. This "discovery" gave promise of such a.s.sistance to meteor theories, that calculations were made from the directions in which they had been seen to travel, and the speeds at which they had moved, in the hope that some information concerning their orbits might be revealed. But after a while some doubt began to be thrown upon their being really meteors, and eventually an Australian observer solved the mystery. He found that they were merely tiny particles of dust, or of the black coating on the inner part of the tube of the telescope, becoming detached from the sides of the eye-piece and falling across the field of view. He was led to this conclusion by having noted that a gentle tapping of his instrument produced the "dark" bodies in great numbers! Thus the opportunity of observing meteors beyond our atmosphere had once more failed.

_Meteorites_, also known as aerolites and fireb.a.l.l.s, are usually placed in quite a separate category from meteors. They greatly exceed the latter in size, are comparatively rare, and do not appear in any way connected with the various showers of meteors. The friction of their pa.s.sage through the atmosphere causes them to shine with a great light; and if not shattered to pieces by internal explosions, they reach the ground to bury themselves deep in it with a great rushing and noise.

When found by uncivilised peoples, or savages, they are, on account of their celestial origin, usually regarded as objects of wonder and of worship, and thus have arisen many mythological legends and deifications of blackened stones. On the other hand, when they get into the possession of the civilised, they are subjected to careful examinations and tests in chemical laboratories. The bodies are, as a rule, composed of stone, in conjunction with iron, nickel, and such elements as exist in abundance upon our earth; though occasionally specimens are found which are practically pure metal. In the museums of the great capitals of both Continents are to be seen some fine collections of meteorites.

Several countries--Greenland and Mexico, for instance--contain in the soil much meteoric iron, often in ma.s.ses so large as to baffle all attempts at removal. Blocks of this kind have been known to furnish the natives in their vicinity for many years with sources of workable iron.

The largest meteorite in the world is one known as the Anighito meteorite. It was brought to the United States by the explorer Peary, who found it at Cape York in Greenland. He estimates its weight at from 90 to 100 tons. One found in Mexico, called the Bacubirito, comes next, with an estimated weight of 27-1/2 tons. The third in size is the Willamette meteorite, found at Willamette in Oregon in 1902. It measures 10 6-1/2 4-1/2 feet, and weighs about 15-1/2 tons.

[27] The "gem" of the meteor ring, as it has been termed.

CHAPTER XXII

THE STARS

In the foregoing chapters we have dealt at length with those celestial bodies whose nearness to us brings them into our especial notice. The entire room, however, taken up by these bodies, is as a mere point in the immensities of star-filled s.p.a.ce. The sun, too, is but an ordinary star; perhaps quite an insignificant one[28] in comparison with the majority of those which stud that background of sky against which the planets are seen to perform their wandering courses.

Dropping our earth and the solar system behind, let us go afield and explore the depths of s.p.a.ce.

We have seen how, in very early times, men portioned out the great ma.s.s of the so-called "fixed stars" into divisions known as constellations.

The various arrangements, into which the brilliant points of light fell as a result of perspective, were noticed and roughly compared with such forms as were familiar to men upon the earth. Imagination quickly saw in them the semblances of heroes and of mighty fabled beasts; and, around these monstrous shapes, legends were woven, which told how the great deeds done in the misty dawn of historical time had been enshrined by the G.o.ds in the sky as an example and a memorial for men. Though the centuries have long outlived such fantasies, yet the constellation figures and their ancient names have been retained to this day, pretty well unaltered for want of any better arrangement. The Great and Little Bears, Ca.s.siopeia, Perseus, and Andromeda, Orion and the rest, glitter in our night skies just as they did centuries and centuries ago.

Many persons seem to despair of gaining any real knowledge of astronomy, merely because they are not versed in recognising the constellations.

For instance, they will say:--"What is the use of my reading anything about the subject? Why, I believe I couldn't even point out the Great Bear, were I asked to do so!" But if such persons will only consider for a moment that what we call the Great Bear has no existence in fact, they need not be at all disheartened. Could we but view this familiar constellation from a different position in s.p.a.ce, we should perhaps be quite unable to recognise it. Mountain ma.s.ses, for instance, when seen from new directions, are often unrecognisable.

It took, as we have seen, a very long time for men to acknowledge the immense distances of the stars from our earth. Their seeming unchangeableness of position was, as we have seen, largely responsible for the idea that the earth was immovable in s.p.a.ce. It is a wonder that the Copernican system ever gained the day in the face of this apparent fixity of the stars. As time went on, it became indeed necessary to accord to these objects an almost inconceivable distance, in order to account for the fact that they remained apparently quite undisplaced, notwithstanding the journey of millions of miles which the earth was now acknowledged to make each year around the sun. In the face of the gradual and immense improvement in telescopes, this apparent immobility of the stars was, however, not destined to last. The first ascertained displacement of a star, namely that of 61 Cygni, noted by Bessel in the year 1838, definitely proved to men the truth of the Copernican system.

Since then some forty more stars have been found to show similar tiny displacements. We are, therefore, in possession of the fact, that the actual distances of a few out of the great host can be calculated.

To mention some of these. The nearest star to the earth, so far as we yet know, is Alpha Centauri, which is distant from us about 25 billions of miles. The light from this star, travelling at the stupendous rate of about 186,000 miles per second, takes about 4-1/4 years to reach our earth, or, to speak astronomically, Alpha Centauri is about 4-1/4 "light years" distant from us. Sirius--the brightest star in the whole sky--is at twice this distance, _i.e._ about 8-1/2 light years. Vega is about 30 light years distant from us, Capella about 32, and Arcturus about 100.

The displacements, consequent on the earth's movement, have, however, plainly nothing to say to any real movements on the part of the stars themselves. The old idea was that the stars were absolutely fixed; hence arose the term "fixed stars"--a term which, though inaccurate, has not yet been entirely banished from the astronomical vocabulary. But careful observations extending over a number of years have shown slight changes of position among these bodies; and such alterations cannot be ascribed to the revolution of the earth in its...o...b..t, for they appear to take place in every direction. These evidences of movement are known as "proper motions," that is to say, actual motions in s.p.a.ce proper to the stars themselves. Stars which are comparatively near to us show, as a rule, greater proper motions than those which are farther off. It must not, however, be concluded that these proper motions are of any very noticeable amounts. They are, as a matter of fact, merely upon the same apparently minute scale as other changes in the heavens; and would largely remain unnoticed were it not for the great precision of modern astronomical instruments.

One of the swiftest moving of the stars is a star of the sixth magnitude in the constellation of the Great Bear; which is known as "1830 Groombridge," because this was the number a.s.signed to it in a catalogue of stars made by an astronomer of that name. It is popularly known as the "Runaway Star," a name given to it by Professor Newcomb. Its speed is estimated to be at least 138 miles per second. It may be actually moving at a much greater rate, for it is possible that we see its path somewhat foreshortened.

A still greater proper motion--the greatest, in fact, known--is that of an eighth magnitude star in the southern hemisphere, in the constellation of Pictor. Nothing, indeed, better shows the enormous distance of the stars from us, and the consequent inability of even such rapid movements to alter the appearance of the sky during the course of ages, than the fact that it would take more than two centuries for the star in question to change its position in the sky by a s.p.a.ce equal to the apparent diameter of the moon; a statement which is equivalent to saying that, were it possible to see this star with the naked eye, which it is not, at least twenty-five years would have to elapse before one would notice that it had changed its place at all!

Both the stars just mentioned are very faint. That in Pictor is, as has been said, not visible to the naked eye. It appears besides to be a very small body, for Sir David Gill finds a parallax which makes it only as far off from us as Sirius. The Groombridge star, too, is just about the limit of ordinary visibility. It is, indeed, a curious fact that the fainter stars seem, on the average, to be moving more rapidly than the brighter.

Investigations into proper motions lead us to think that every one of the stars must be moving in s.p.a.ce in some particular direction. To take a few of the best known. Sirius and Vega are both approaching our system at a rate of about 10 miles per second, Arcturus at about 5 miles per second, while Capella is receding from us at about 15 miles per second.

Of the twin brethren, Castor and Pollux, Castor is moving away from us at about 4-1/2 miles per second, while Pollux is coming towards us at about 33 miles per second.

Much of our knowledge of proper motions has been obtained indirectly by means of the spectroscope, on the Doppler principle already treated of, by which we are enabled to ascertain whether a source from which light is coming is approaching or receding.

The sun being, after all, a mere star, it will appear only natural for it also to have a proper motion of its own. This is indeed the case; and it is rushing along in s.p.a.ce at a rate of between ten and twelve miles per second, carrying with it its whole family of planets and satellites, of comets and meteors. The direction in which it is advancing is towards a point in the constellation of Lyra, not far from its chief star Vega.

This is shown by the fact that the stars about the region in question appear to be opening out slightly, while those in the contrary portion of the sky appear similarly to be closing together.

Sir William Herschel was the first to discover this motion of the sun through s.p.a.ce; though in the idea that such a movement might take place he seems to have been antic.i.p.ated by Mayer in 1760, by Mich.e.l.l in 1767, and by Lalande in 1776.

A suggestion has been made that our solar system, in its motion through the celestial s.p.a.ces, may occasionally pa.s.s through regions where abnormal magnetic conditions prevail, in consequence of which disturbances may manifest themselves throughout the system at the same instant. Thus the sun may be getting the credit of _producing_ what it merely reacts to in common with the rest of its family. But this suggestion, plausible though it may seem, will not explain why the magnetic disturbances experienced upon our earth show a certain dependence upon such purely local facts, as the period of the sun's rotation, for instance.