Astronomy for Everybody/Part 5/Chapter 2

Every reader of this book must frequently have seen what is familiarly called a "shooting star"—an object like a star, which darts through the heavens a greater or less distance, and then disappears. These objects are, in astronomy, called by the generic name of meteors. They are of every degree of brightness, but the brighter they may be, the more rarely they appear. One who is out much at night will seldom pass a year without seeing such a meteor of striking brilliancy. Once or twice in a lifetime he will see one that illuminates the whole sky with its light.

On almost any clear night in the year a watcher may see three or four or even more meteors in the course of an hour. Sometimes, however, they are vastly more numerous, for example, between the tenth and fifteenth of August, more and brighter ones than usual will be seen. On a number of occasions in history they have coursed the heavens in such numbers as to fill the beholders with surprise and terror. There were remarkable cases of this kind in 1799 and 1833. In the latter year, especially, the negroes of the South were so terrified that the recollection of the phenomenon is brought down by tradition to the present day.

The cause of meteors was unknown until after the beginning of the nineteenth century. It is now, however, well made out. Besides the known objects of the solar system—planets, satellites, and comets—there are, coursing through space, and revolving around the sun, countless millions of particles, or minute collections of matter, too small to be seen with the most powerful telescope. Quite likely the greater number of these objects are scarcely larger than pebbles, or even grains of sand. The earth, in its course around the sun, is continually encountering them. One in the line of motion of the earth may have a velocity amounting to many miles a second; perhaps ten, twenty, thirty, or even forty. Meeting the atmosphere with this immense velocity causes the body to be immediately heated to so high a temperature that its substance dissolves away with a brilliant effusion of light no matter how solid it may be. What we see is the course of a particle thus burning away as it darts through the rare regions of the upper atmosphere.

Of course, a meteor will appear brighter and last longer the larger and solider it is. Sometimes it is so large and solid that it comes within a few miles of the earth before being finally melted and dissolved away. Then, the people in the region over which it is passing, see a remarkably bright meteor. In such a case it frequently happens that in a few minutes after the meteor has passed a loud explosion, like the firing of a cannon, is heard coming from the region through which it passed. ​This arises from the concussion of the air compressed by the rapid flight.

In rare cases the mass is so large that it reaches the earth without being melted or evaporated. Then we have the fall of a meteoric stone, as it is called, which commonly occurs several times a year in some part or another of the world. There is at least one case on record in which a man was killed by the fall of such a body. When these stones are dug up they are found to be composed mostly of iron. Specimens of them are kept in our museums, where they may be examined by anyone who wishes to see them. Some remarkable ones are found at the Smithsonian Institution, Washington, D. C.

How these objects originated we cannot say, and even a guess on the subject would be hazardous. When found they bear marks on their surface of having been melted; this, however, is a natural result of their passage through the air, by which the surface is always heated far above the melting point.

The greatest discovery of our times on the subject of meteors is connected with the meteoric showers already referred to, which occur at certain seasons of the year. The most remarkable of these occur in November, and the meteors of the shower are called Leonides, because their lines of apparent motion all diverge from the constellation Leo. It was found by historical research on the subject that this shower had recurred at intervals of about one third of a century for at least thirteen hundred ​years. The earliest account is the following from an Arabian writer:

In the year 599, on the last day of Moharren, stars shot hither and thither, and flew against each other like a swarm of locusts; people were thrown into consternation and made supplication to the Most High; there was never the like seen except on the coming of the messenger of God; on whom be benediction and peace.

The first well-described shower of this class occurred on November 12, 1799. It was seen by Humboldt, then on the Andes. He seems to have considered it as a very remarkable display, but made no exact investigation as to its cause.

The next recurrence was in 1833, which seems to have been the most remarkable one ever observed. The astronomer Olbers suggested from this that the shower had a period of thirty-four years, and predicted a possible return in 1867, which actually appeared in 1866. In 1866 and 1867 the observations were more carefully made than ever before, and led to the remarkable astronomical discovery, just alluded to, that of the relation between meteors and comets. To explain this we must define the radiant point of meteors.

It is found that if, during a meteoric shower, we mark the course of each meteor by a line on the celestial sphere, and continue these lines backward, we shall find them all to meet at a certain point in the heavens. In the case of the November meteors this point is in the constellation Leo; in the August meteors it is in Perseus. It is called the radiant point of the shower. The lines in which the ​meteors move are the same as if they were all shot out from this one point, but it must not be supposed that the meteors are actually seen at this point; they may begin to show themselves at any distance from it less than ninety degrees; but when they are seen they are moving from the point. This shows that the meteors are all moving in parallel lines when they encounter our atmosphere. The radiant point is what, in perspective, is called the vanishing point.

The period of the November meteors, thirty-three years, being known, and the exact position of the radiant point determined, it became possible to calculate the orbit of these objects. This was done by Leverrier soon after the shower of 1866. Now it happened that, in December, 1865, a comet appeared which passed its perihelion in January, 1866. Careful study of its motion showed that its period was about thirty-three years. This orbit was computed by Oppolzer, who published it without noticing its resemblance to that of the meteors. Then it was noticed by Schiaparelli that there was an almost perfect resemblance between the orbit of Oppolzer's comet and the Leverrier orbit of the November meteors. So near together were they that no doubt could be felt that the two orbits were identical. The evident fact was that the bodies which produced these November meteors were following the comet in its orbit. It was therefore concluded that these objects had originally formed part of the comet and had gradually separated from it. When a ​comet is disintegrated in the manner described in the last chapter, those portions of its mass which are not completely dissipated continue to revolve around the sun as minute particles, which get gradually separated from each other in consequence of there being no sufficient bond of attraction, but they still follow each other in line in nearly the same orbit.

The same thing was found to be true of the August meteors. They are found to move in an orbit very near to that of a comet observed in 1862. The period of this comet could not be exactly determined, but it is supposed to be between one and two hundred years.

The third remarkable case of this kind occurred in 1872. We have already spoken of the disappearance of Biela's comet. It happens that the orbit of this body nearly intersected that of the earth at the point which the latter passes toward the end of November. From the observed period of this comet it should have passed this point about the first of September, 1872, between two and three months before the passage of the earth through the same point. From the analogy of the other cases it was therefore judged that there would be a meteoric shower on the evening of November 27, 1872, and that the radiant point would be in the constellation Andromeda. This prediction was fulfilled in every respect. The Andromedes, as these meteors are called, now recur with great regularity.

There are now some disappointing circumstances to narrate. The comet of 1866 should have reappeared sometime during the years 1898-1900, but it was not ​seen. Probably it was missed, not because of its complete disintegration, but because it happened to pass its perihelion at a time when the earth was too far away to admit of the comet being visible. But, what is still more curious is that the meteors themselves, a shower of which was expected in 1899-1900, did not reappear in great numbers at either date. The probable reason for this is that the swarm was deflected from its course by the attraction of the planets, which continually changes the orbit of every object of this kind.

The general conclusion is that the countless thousands of comets which in time past have coursed around the sun, leave behind minute fragments of their mass, which follow in their orbits like stragglers from an army, and that, when the earth encounters a swarm of these fragments a meteoric shower is produced. But it is still an open question whether all these meteoric particles can be fragments of comets, with the probabilities in favor of a negative answer. If we are to accept the conclusions drawn by Professor Elkin from recent photographs of meteors, the velocities of these bodies sometimes exceed the parabolic limit described in the last chapter. If this be so, they must be wanderers through the infinite stellar spaces, having no connection with our system.

This is a very soft, faint light, surrounding the sun, extending out to about the orbit of the earth, and lying nearly in the plane of the ecliptic. In tropical latitudes it may be seen on any clear evening about an hour or ​less after sunset. In our latitudes it is best seen in the spring, when, about an hour and a half after sunset, it may always be seen in the west and southwest, extending upward toward the Pleiades. It is best seen at this

season because, lying in the plane of the ecliptic, it makes a greater angle with the horizon then than at other seasons. In autumn it may be seen in the morning before daybreak, rising from the east and extending toward the south.

It is said that in regions where the atmosphere is clearer than with us, it may be seen all night, spanning the heavens like a complete circle. If so, the light is so ​faint as to elude ordinary vision, and this continuity does not seem to be well established.

But there is associated with it a phenomena which is still one of the mysteries of astronomy. In the heavens, immediately opposite the sun, there is always a faint light, to which the term Gegenschein is applied. This is a German word, of which the best English equivalent is counter-glow. The light is so faint that it can be seen only under the most favourable conditions. When it falls in the Milky Way the light of that body is sufficient to drown it out, as is that of the moon, if the latter is above the horizon.

It passes through the Milky Way in June and December of each year, and can therefore not be seen during these months. Nor is it likely to be seen during the first part of January or July. At other times it must be looked for when the sun is considerably below the horizon, the sky perfectly clear and the moon not in sight. It may then be seen as an extremely faint impression of light, to which no exact outline can be assigned. The observer will find it by sweeping his eye over the region of the spot exactly opposite the sun.

There can be little doubt that the zodiacal light is caused by the reflection of the light of the sun from a swarm of very minute bodies, perhaps in the nature of meteors, continually revolving around it. We might naturally attribute the Gegenschein to the same cause, but the question would then arise why it is only seen opposite the sun. It has been suggested that possibly the earth has a tail, like a comet, and that the Gegen ​schein is simply this tail seen endwise. This is not an impossibility, but there is no proof that it is true.

Facts are now being discovered, and physical theories developed, the ultimate outcome of which may be an explanation of a number of mysterious phenomena associated with the earth and the universe. These phenomena are presented by the corona of the sun, the tails of comets, the aurora, terrestrial magnetism and its variations, nebulae, the Gegenschein, and the zodiacal light. The theories in question belong rather to the physicist than the astronomer, and the writer does not feel competent to explain them fully in their latest form, nor to define where established facts end and speculation begins. He must therefore limit himself to a few points.

First in order we have a pressure exerted by light, which was pointed out by Maxwell thirty years ago, but which seems to have been very generally overlooked, by astronomers at least. This principle was deduced by Maxwell from the electro-magnetic theory of light, and may be stated as follows:

When a pencil of light impinges perpendicularly on an opaque object, it produces a pressure upon the surface of that object, determined by the condition that if the object were set in motion with the velocity of light, and the force against it were kept up, the power required to keep up the pressure would be equal to that carried by the ray of light.

Another way of expressing the principle is this: Sup​posing the rays of light to be parallel, the work done by the pressure upon a surface moving through any length of the pencil is equal to the energy of the light contained in that length.

By the aid of this principle and a knowledge of the heat or energy contained in the rays of the sun, it is possible to calculate the pressure in question. It is found to be too slight to be detected by any ordinary mode of measurement. The great difficulty arises from the fact that, if the experiment is not tried in a vacuum, the pressure will be confused with that exerted by the surrounding air. A vacuum so nearly perfect that the slight residuum of air still contained within it shall not exert a force comparable with the light has not yet been attained. Our conclusion must therefore depend on observations made on minute particles contained in the celestial spaces; and we cannot ascend into these spaces to make the experiments, nor can we send matter up there to be experimented upon. All we can do is to observe matter already at hand. Here, then, is a wide gap which we cannot bridge over in practice.

The other element in the case is the discovery that particles smaller than atoms, called corpuscles or ions are thrown off with high velocity from intensely heated bodies. The sun being such a body, it follows that such ions must be shot out from it.

On Maxwell's theory, the explanation of a comet's tail is simple in the extreme. Being in the vacuum of celestial space, the matter of the comet evaporates on the side next to the sun, and, there being no pressure to hin​der its expansion, it begins by flying off in all directions, especially toward the sun. It condenses into very minute particles, which are acted upon by the sun's rays and thus tlirown in the direction away from the sun. That the tail of the comet was produced by a repulsion like this has been evident ever since observations were made, but not until Maxwell's law was understood could any explanation be given of the seeming repulsion of the matter of the tail by the sun.

The explanations of the other phenomena we have mentioned are not yet so simple and satisfactory that they may be clearly stated in a short space. The reader who is interested in the subject must therefore be referred to special papers and treatises.^[1]