"Running like a road through the constellations" (_see_ p. 105).]

CHAPTER IX

COMETS

Great comets are almost always unexpected visitors. There is only one great comet that we know has been seen more than once, and expect with reasonable certainty to see again. This is Halley's comet, which has been returning to a near approach to the sun at somewhat irregular intervals of seventy-five to seventy-eight years during the last centuries: indeed, it is possible that it was this comet that was coincident with the invasion of England by William the Conqueror.

There are other small comets that are also regular inhabitants of the solar system; but, as with Halley's comet, so with these, two circ.u.mstances are to be borne in mind. First, that each successive revolution round the sun involves an increasing degradation of their brightness, since there is a manifest waste of their material at each near approach to the sun; until at length the comet is seen no more, not because it has left the warm precincts of the sun for the outer darkness, but because it has spent its substance. Halley's comet was not as brilliant or as impressive in 1835 as it was in 1759: in 1910 it may have become degraded to an appearance of quite the second order.

Next, we have no knowledge, no evidence, that any of these comets have always been members of the solar family. Some of them, indeed, we know were adopted into it by the influence of one or other of the greater planets: Ura.n.u.s we know is responsible for the introduction of one, Jupiter of a considerable number. The vast majority of comets, great or small, seem to blunder into the solar system anyhow, anywhere, from any direction: they come within the attractive influence of the sun; obey his laws whilst within that influence; make one close approach to him, pa.s.sing rapidly across our sky; and then depart in an orbit which will never bring them to his neighbourhood again. Some chance of direction, some compelling influence of a planet that it may have approached, so modified the path of Halley's comet when it first entered the solar system, that it has remained a member ever since, and may so remain until it has ceased to be a comet at all.

It follows, therefore, that, as to the number of great comets that may be seen in any age, we can scarcely even apply the laws of probability.

During the last couple of thousand years, since chronicles have been abundant, we know that many great comets have been seen. We may suppose, therefore, that during the preceding age, that in which the Scriptures were written, there were also many great comets seen, but we do not know. And most emphatically we are not able to say, from our knowledge of comets themselves and of their motions, that in the days of this or that writer a comet was flaming in the sky.

If a comet had been observed in those ages we might not recognize the description of it. Thus in the fourth year of the 101st Olympiad, the Greeks were startled by a celestial portent. They did not draw fine distinctions, and posterity might have remained ignorant that the terrifying object was possibly a comet, had not Aristotle, who saw it as a boy at Stagira, left a rather more scientifically worded description of it. It flared up from the sunset sky with a narrow definite tail running "like a road through the constellations." In recent times the great comet of 1843 may be mentioned as having exactly such an appearance.

So we cannot expect to find in the Scriptures definite and precise descriptions that we can recognize as those of comets. At the most we may find some expressions, some descriptions, that to us may seem appropriate to the forms and appearances of these objects, and we may therefore infer that the appearance of a comet may have suggested these descriptions or expressions.

The head of a great comet is brilliant, sometimes starlike. But its tail often takes on the most impressive appearance. Donati's comet, in 1858, a.s.sumed the most varied shapes--sometimes its tail was broad, with one bright and curving edge, the other fainter and finer, the whole making up a stupendous semi-circular blade-like object. Later, the tail was shaped like a scimitar, and later again, it a.s.sumed a duplex form.

Though the bulk of comets is huge, they contain extraordinarily little substance. Their heads must contain some solid matter, but it is probably in the form of a loose aggregation of stones enveloped in vaporous material. There is some reason to suppose that comets are apt to shed some of these stones as they travel along their paths, for the orbits of the meteors that cause some of our greatest "star showers" are coincident with the paths of comets that have been observed.

But it is not only by shedding its loose stones that a comet diminishes its bulk; it loses also through its tail. As the comet gets close to the sun its head becomes heated, and throws off concentric envelopes, much of which consists of matter in an extremely fine state of division. Now it has been shown that the radiations of the sun have the power of repelling matter, whilst the sun itself attracts by its gravitational force. But there is a difference in the action of the two forces. The light-pressure varies with the surface of the particle upon which it is exercised; the gravitational attraction varies with the ma.s.s or volume.

If we consider the behaviour of very small particles, it follows that the attraction due to gravitation (depending on the volume of the particle) will diminish more rapidly than the repulsion due to light-pressure (depending on the surface of the particle), as we decrease continually the size of the particle, since its volume diminishes more rapidly than its surface. A limit therefore will be reached below which the repulsion will become greater than the attraction. Thus for particles less than the 1/25000 part of an inch in diameter the repulsion of the sun is greater than its attraction.

Particles in the outer envelope of the comet below this size will be driven away in a continuous stream, and will form that thin, luminous fog which we see as the comet's tail.

We cannot tell whether such objects as these were present to the mind of Joel when he spoke of "blood and fire and pillars of smoke"; possibly these metaphors are better explained by a sand- or thunder-storm, especially when we consider that the Hebrew expression for the "pillars of smoke" indicates a resemblance to a palm-tree, as in the spreading out of the head of a sand- or thunder-cloud in the sky. The suggestion has been made,--following the closing lines of _Paradise Lost_ (for Milton is responsible for many of our interpretations of Scripture)

"High in front advanced, The brandished sword of G.o.d before them blazed, Fierce as a comet,"

--that a comet was indeed the "flaming sword which turned every way, to keep the way of the tree of life." There is less improbability in the suggestion made by several writers that, when the pestilence wasted Jerusalem, and David offered up the sacrifice of intercession in the threshing floor of Ornan the Jebusite, the king may have seen, in the scimitar-like tail of a comet such as Donati's, G.o.d's "minister,"--"a flame of fire,"--"the angel of the Lord stand between the earth and the heaven, having a drawn sword in his hand stretched out over Jerusalem."

The late R. A. Proctor describes the wanderings of a comet thus:--

"A comet is seen in the far distant depths of s.p.a.ce as a faint and scarcely discernible speck. It draws nearer and nearer with continually increasing velocity, growing continually larger and brighter. Faster and faster it rushes on until it makes its nearest approach to our sun, and then, sweeping round him, it begins its long return voyage into infinite s.p.a.ce. As it recedes it grows fainter and fainter, until at length it pa.s.ses beyond the range of the most powerful telescopes made by man, and is seen no more. It has been seen for the first and last time by the generation of men to whom it has displayed its glories. It has been seen for the first and last time by the race of man itself."[108:1]

"These are . . . wandering stars, to whom is reserved the blackness of darkness for ever."

FOOTNOTES:

[108:1] R. A. Proctor, _The Expanse of Heaven_, p. 134.

[Ill.u.s.tration: FALL OF AN AEROLITE.

"There fell a great star from heaven, burning as it were a lamp." (_see_ p. 116).]

CHAPTER X

METEORS

Great meteorites--"aerolites" as they are called--are like great comets, chance visitors to our world. Now and then they come, but we cannot foretell their coming. Such an aerolite exploded some fifteen miles above Madrid at about 9{h} 29{m}, on the morning of February 10, 1896:--

"A vivid glare of blinding light was followed in 1-1/2 minutes by a loud report, the concussion being such as not merely to create a panic, but to break many windows, and in some cases to shake down part.i.tions. The sky was clear, and the sun shining brightly, when a white cloud, bordered with red, was seen rushing from south-west to north-east, leaving behind it a train of fine white dust. A red-tinted cloud was long visible in the east."

Many fragments were picked up, and a.n.a.lyzed, and, like other aerolites, were found to consist of materials already known on the earth. The outer crust showed the signs of fire,--the meteoric stone had been fused and ignited by its very rapid rush through the air--but the interior was entirely unaffected by the heat. The manner in which the elements were combined is somewhat peculiar to aerolites; the nearest terrestrial affinity of the minerals aggregated in them, is to be found in the volcanic products from great depths. Thus aerolites seem to be broken-up fragments from the interior parts of globes like our own. They do not come from our own volcanoes, for the velocities with which they entered our atmosphere prove their cosmical origin. Had our atmosphere not entangled them, many, circuiting the sun in a parabolic or hyperbolic curve, would have escaped for ever from our system. The swift motions, which they had on entering our atmosphere, are considerably greater on the average than those of comets, and probably their true home is not in our solar system, but in interstellar s.p.a.ce.

The aerolites that reach the surface are not always exploded into very small fragments, but every now and then quite large ma.s.ses remain intact. Most of these are stony; some have bits of iron scattered through them; others are almost pure iron, or with a little nickel alloy, or have pockets in them laden with stone. There are hundreds of accounts of the falls of aerolites during the past 2,500 years. The Greeks and Romans considered them as celestial omens, and kept some of them in temples. One at Mecca is revered by the faithful Mohammedans, and Jehangir, the great Mogul, is said to have had a sword forged from an iron aerolite which fell in 1620 in the Panjab. Diana of Ephesus stood on a shapeless block which, tradition says, was a meteoric stone, and reference may perhaps be found to this in the speech of the town-clerk of the city to appease the riot stirred up against St. Paul by Demetrius the silversmith and his companions:--

"Ye men of Ephesus, what man is there who knoweth not how that the city of the Ephesians is temple-keeper of the great Diana, and of the image which fell down from Jupiter?"

Aerolites come singly and unexpectedly, falling actually to earth on land or sea. "Shooting stars" come usually in battalions. They travel together in swarms, and the earth may meet the same swarm again and again. They are smaller than aerolites, probably mere particles of dust, and for the most part are entirely consumed in our upper atmosphere, so that they do not actually reach the earth. The swarms travel along paths that resemble cometary orbits; they are very elongated ellipses, inclined at all angles to the plane of the ecliptic. Indeed, several of the orbits are actually those of known comets, and it is generally held that these meteorites or "shooting stars" are the _debris_ that a comet sheds on its journey.

We can never see the same "shooting star" twice; its visibility implies its dissolution, for it is only as it is entrapped and burnt up in our atmosphere that we see it, or can see it. Its companions in a great meteoric swarm, are, however, as the sand on the sea-sh.o.r.e, and we recognize them as members of the same swarm by their agreement in direction and date. The swarms move in a closed orbit, and it is where this...o...b..t intersects that of the earth that we get a great "star shower," if both earth and swarm are present together at the intersection. If the swarm is drawn out, so that many meteorites are scattered throughout the whole circuit of its...o...b..t, then we get a "shower" every year. If the meteor swarm is more condensed, so as to form a cl.u.s.ter, then the "shower" only comes when the "gem of the ring,"

as it is termed, is at the intersection of the orbits, and the earth is there too.

Such a conjunction may present the most impressive spectacle that the heavens can afford. The Leonid meteor shower is, perhaps, the most famous. It has been seen at intervals of about thirty-three years, since early in the tenth century. When Ibrahim ben Ahmed lay dying, in the year 902 A.D., it was recorded that "an infinite number of stars were seen during the night, scattering themselves like rain to the right and left, and that year was known as the year of stars." When the earth encountered the same system in 1202 A.D. the Mohammedan record runs that "on the night of Sat.u.r.day, on the last day of Muharram, stars shot hither and thither in the heavens, eastward and westward, and flew against one another, like a scattering swarm of locusts, to the right and left." There are not records of all the returns of this meteoric swarm between the thirteenth century and the eighteenth, but when the earth encountered it in 1799, Humboldt reported that "from the beginning of the phenomenon there was not a s.p.a.ce in the firmament equal in extent to three diameters of the moon that was not filled every instant with bolides and falling stars;" and Mr. Andrew Ellicott, an agent of the United States, cruising off the coast of Florida, watched this same meteoric display, and made the drawing reproduced on the opposite page.

In 1833 a planter in South Carolina wrote of a return of this same system, "Never did rain fall much thicker than the meteors fell towards the earth; east, west, north, south, it was the same." In 1866 the shower was again heavy and brilliant, but at the end of the nineteenth century, when the swarm should have returned, the display was meagre and ineffective.

[Ill.u.s.tration: METEORIC SHOWER OF 1799, NOVEMBER 12.

Seen off Cape Florida, by Mr. Andrew Ellicott.]

The Leonid system of meteorites did not always move in a closed orbit round our sun. Tracing back their records and history, we find that in A.D. 126 the swarm pa.s.sed close to Ura.n.u.s, and probably at that time the planet captured them for the sun. But we cannot doubt that some such similar sight as they have afforded us suggested the imagery employed by the Apostle St. John when he wrote, "The stars of heaven fell unto the earth, even as a fig-tree casteth her untimely figs, when she is shaken of a mighty wind. And the heavens departed as a scroll when it is rolled together."

And the prophet Isaiah used a very similar figure--

"All the host of heaven shall be dissolved, and the heavens shall be rolled together as a scroll: and all their host shall fall down, as the leaf falleth off from the vine, and as a falling fig from the fig-tree."

Whilst the simile of a great aerolite is that employed by St. John in his description of the star "Wormwood"--

"The third angel sounded, and there fell a great star from heaven, burning as it were a lamp, and it fell upon the third part of the rivers, and upon the fountains of waters."

St. Jude's simile of the "wandering stars, to whom is reserved the blackness of darkness for ever," may have been drawn from meteors rather than from comets. But, as has been seen, the two cla.s.ses of objects are closely connected.

The word "meteor" is sometimes used for any unusual light seen in the sky. The Zodiacal Light, the pale conical beam seen after sunset in the west in the spring, and before sunrise in the east in the autumn, and known to the Arabs as the "False Dawn," does not appear to be mentioned in Scripture. Some commentators wrongly consider that the expression, "the eyelids of the morning," occurring twice in the Book of Job, is intended to describe it, but the metaphor does not in the least apply.

The Aurora Borealis, on the other hand, seldom though it is seen on an impressive scale in Palestine, seems clearly indicated in one pa.s.sage.