Part 19 (1/2)
When a suspect is found, the first thing to do is to observe its position accurately with relation to the surrounding stars. Then, if on the next occasion when it is seen the object has moved, the chances are that it is a comet; and a few days' observation will provide material from which the path of the comet in s.p.a.ce can be calculated. By comparing this with the complete lists of comets, now about 700 in number, it is possible to tell whether the comet is a new one, or an old one returning. The total number of comets in the heavens must be very great, and thousands are doubtless pa.s.sing continually undetected, because their light is wholly overpowered by that of the sun. Of those that are known, perhaps one in twelve develops into a naked-eye comet, and in some years six or seven will be discovered. With sufficiently powerful telescopes, there are as a rule not many weeks in the year when no comet is visible. Brilliant naked-eye comets are, however, infrequent.
Comets, except Halley's, generally bear the name of their discoverer, as Donati (1858), and Pons-Brooks (1893). Pons was a very active discoverer of comets in France early in the nineteenth century: he was a doorkeeper at the observatory of Ma.r.s.eilles, and his name is now more famous in astronomy than that of Thulis, then the director of the Observatory, who taught and encouraged him. Messier was another very successful discoverer of comets in France, and in America we have had many: Swift, Brooks, and Barnard the most successful.
How bright a comet will be and how long it will be visible depends upon many conditions. So the comets vary much in these respects. The first comet of 1811 was under observation for nearly a year and a half, the longest on record till Halley's in 1910. In case a comet eludes discovery and observation until it has pa.s.sed its perihelion, or nearest point to the sun, its period of visibility may be reduced to a few weeks only. The brightest comets on record were visible in 1843 and 1882: so brilliant were they that even the effulgence of full daylight did not overpower them. In particular the comet of 1843 was not only excessively bright, but at its nearest approach to the earth its tail swept all the way across the sky from one horizon to the other. It must have looked very much like the straight beam of an enormous searchlight, though very much brighter.
The tails of comets are to the naked eye the most compelling thing about them, and to the ancient peoples they were naturally most terrifying.
Their tails are not only curved, but sometimes curved with varying degrees of curvature, and this circ.u.mstance adds to their weirdness of appearance. If we examine the tail of a comet with a telescope, it vanishes as if there were nothing to it: as indeed one may almost say there is not. Ordinarily, only the head of the comet is of interest in the telescope. When first seen there is usually nothing but the head visible, and that is made up of portions which develop more or less rapidly, presenting a succession of phenomena quite different in different comets.
When first discovered a comet is usually at a great distance from the sun, about the distance of Jupiter; and we see it, not as we do the planets, by sunlight reflected from them, but by the comet's own light.
This is at that time very faint, and nearly all comets at such a distance look alike: small roundish hazy patches of faint, cloudlike light, with very often a concentration toward the center called the nucleus, on the average about 4,000 miles in diameter. Approach toward the sun brightens up the comet more and more, and the nucleus usually becomes very much brighter and more starlike. Then on the sunward side of the nucleus, jetlike streamers or envelopes appear to be thrown off, often as if in parallel curved strata, or concentrically. As they expand and move outward from the nucleus, these envelopes grow fainter and are finally merged in the general nebulosity known as the comet's head, which is anywhere from 30,000 to 100,000 miles in diameter. As a rule, this is an orderly development which can be watched in the telescope from hour to hour and from night to night; but occasionally a cometary visitor is quite a law to itself in development, presenting a fascinating succession of unpredictable surprises.
Then follows the development of the comet's tail, perhaps more striking than anything that has preceded it. Here a genuine repulsion from the sun appears to come into play. It may be an electrical repulsion. Much of the material projected from the comet's nucleus, seems to be driven backward or repelled by the sun, and it is this that goes to form the tail. The particles which form the tail then travel in modified paths which nevertheless can be calculated. The tail is made up of these luminous particles and it expands in s.p.a.ce much in the form of a hollow, horn-shaped cone, the nucleus being near the tip of the horn.
Some comets possess multiple tails with different degrees of curvature, Donati's for example. Usually there is a nearly straight central dark s.p.a.ce, marking the axis of the comet, and following the nucleus. But occasionally this is replaced by a thin light streak very much less in breadth than the diameter of the head. Cometary tails are sometimes 100 million miles in length.
Three different types of cometary tails are recognized. First, the long straight ones, apparently made up of matter repelled by the sun twelve to fifteen times more powerfully than gravitation attracts it. Such particles must be brushed away from the comet's head with a velocity of perhaps five miles a second, and their speed is continually increasing.
Probably these straight tails are due to hydrogen. The second type tails are somewhat curved, or plume-like, and they form the most common type of cometary tail. In them the sun's repulsion is perhaps twice its gravitational attraction, and hydrocarbons in some form appear to be responsible for tails of this character. Then there is a third type, much less often seen, short and quickly curving, probably due to heavier vapors, as of chlorine, or iron, or sodium, in which the repulsive force is only a small fraction of that of gravitation.
Many features of this theory of cometary tails are borne out by examination of their light with the spectroscope, although the investigation is as yet fragmentary. It is evident that the tail of a comet is formed at the expense of the substance of the nucleus and head; so that the matter repelled is forever dissipated through the regions of s.p.a.ce which the comet has traveled. Comets must lose much of their original substance every time they return to perihelion. Comets actually age, therefore, and grow less and less in magnitude of material as well as brightness, until they are at last opaque, nonluminous bodies which it becomes impossible to follow with the telescope.
CHAPTER XLI
WHERE DO COMETS COME FROM?
Where do comets come from? The answer to this question is not yet fully made out. Most likely they have not all had a similar origin, and theories are abundant. Apparently they come into the solar system from outer s.p.a.ce, from any direction whatsoever. The depths of interstellar s.p.a.ce seem to be responsible for most, if not all, of the new ones.
Whether they have come from other stars or stellar systems we cannot say.
While comets are tremendous in size or volume, their ma.s.s or the amount of real substance in them is relatively very slight. We know this by the effect they produce on planets that they pa.s.s near, or rather by the effect that they fail to produce. The earth's atmosphere weighs about one two hundred and fifty thousandth as much as the earth itself, but a comet's entire ma.s.s must be vastly less than this. Even if a comet were to collide with the earth head on, there is little reason to believe that dire catastrophe would ensue. At least twice the earth is known to have pa.s.sed through the tail of a comet, and the only effect noticed was upon the comet itself; its...o...b..t had been modified somewhat by the attraction of the earth. If the comet were a small one, collision with any of the planets would result in absorption and dissipation of the comet into vapor.
The whole of a large comet has perhaps as much ma.s.s or weight as a sphere of iron a hundred miles in diameter. Even this could not wreck the earth, but the effect would depend upon what part of the earth was. .h.i.t. A comet is very thin and tenuous, because its relatively small ma.s.s is distributed through a volume so enormous. So it is probable that the earth's atmosphere could scatter and burn up the invading comet, and we should have only a shower of meteors on an unprecedented scale.
Diffusion of noxious gases through the atmosphere might vitiate it to some extent, though probably not enough to cause the extinction of animal life.
Every comet has an interesting history of its own, almost indeed unique.
One of the smallest comets and the briefest in its period round the sun is known as Encke's comet. It is a telescopic comet with a very short tail, its time of revolution is about three and a half years, and it exhibits a remarkable contraction of volume on approach to the sun.
Biela's comet has a period about twice as long. At one time it pa.s.sed within about 15 million miles of the earth, and somewhere about the year 1840 this comet divided into two distinct comets, which traveled for months side by side, but later separated and both have since completely disappeared. Perhaps the most beautiful of all comets is that discovered by Donati of Florence in 1858. Its coma presented the development of jets and envelopes in remarkable perfection, and its tail was of the secondary or hydrocarbon type, but accompanied by two faint streamer tails, nearly tangential to the main tail and of the hydrogen type.
Donati's comet moves in an ellipse of extraordinary length, and it will not return to the sun for nearly 2,000 years.
The most brilliant comet of the last half century is known as the great comet of 1882. In a clear sky it could readily be seen at midday. On September 17 it pa.s.sed across the disk of the sun and was practically as bright as the surface of the sun itself. The comet had a multiple nucleus and a hydrocarbon tail of the second type, nearly a hundred million miles in length. Doubtless this great comet is a member of what is known as a cometary group, which consists of comets having the same orbit and traveling tandem round the sun. The comets of 1668, 1843, 1880, 1882 and 1887 belong to this particular group, and they all pa.s.s within 300,000 miles of the sun's surface, at a maximum velocity exceeding 300 miles a second. They must therefore invade the regions of the solar corona, the inference being that the corona as well as the comet is composed of exceedingly rare matter.
Photography of comets has developed remarkably within recent years, especially under the deft manipulation of Barnard, whose plates, in particular during his residence at the Lick Observatory on Mount Hamilton, California, show the features of cometary heads and tails in excellent definition. Halley's comet, at the 1910 apparition, was particularly well photographed at many observatories.
The question is often asked, When will the next comet come? If a large bright comet is meant, astronomers cannot tell. At almost any time one may blaze into prominence within only a few days. During the latter half of the last century, bright comets appeared at perihelion at intervals of eight years on the average. Several of the lesser and fainter periodic comets return nearly every year, but they are mostly telescopic, and are rarely seen except by astronomers who are particularly interested in observing them.