Chapter 7

If each whirling world should leave behind it a trail of light to mark its orbit, and our perceptions of form were sufficiently acute, we should see that these curves of light are not exact circles, but a little flattened into an ellipse, with the sun always in one of the foci. Hence each planet is nearer to the sun at one part of its orbit than another; that point is called the perihelion, and the farthest point aphelion. This eccentricity of orbit, or distance of the sun from the centre, is very small. [Page 106] In the case of Venus it is only .007 of the whole, and in no instance is it more than .2, viz., that of Mercury. This makes the sun appear twice as large, bright, and hot as seen and felt on Mercury at its perihelion than at its aphelion. The earth is 3,236,000 miles nearer to the sun in our winter than summer. Hence the summer in the southern hemisphere is more intolerable than in the northern. But this eccentricity is steadily diminishing at a uniform rate, by reason of the perturbing influence of the other planets. In the case of some other planets it is steadily increasing, and, if it were to go on a sufficient time, might cause frightful extremes of temperature; but Lalande has shown that there are limits at which it is said, "Thus far shalt thou go, and no farther." Then a compensative diminution will follow.

Conceive a large globe, to represent the sun, floating in a round pond. The axis will be inclined 7-1/2° to the surface of the water, one side of the equator be 7-1/2° below the surface, and the other side the same distance above. Let the half-submerged earth sail around the sun in an appropriate orbit. The surface of the water will be the plane of the orbit, and the water that reaches out to the shore, where the stars would be set, will be the plane of the ecliptic. It is the plane of the earth's orbit extended to the stars.

The orbits of all the planets do not lie in the same plane, but are differently inclined to the plane of the ecliptic, or the plane of the earth's orbit. Going out from the sun's equator, so as to see all the orbits of the planets on the edge, we should see them inclined to that of the earth, as in Fig. 40.

If the earth, and Saturn, and Pallas were lying in [Page 107] the same direction from the sun, and the outer bodies were to start in a direct line for the sun, they would not collide with the earth on their way; but Saturn would pass 4,000,000 and Pallas 50,000,000 miles over our heads. From this same cause we do not see Venus and Mercury make a transit across the disk of the sun at every revolution.

Fig. 41 shows a view of the orbits of the earth and Venus seen not from the edge but from a position somewhat above. The point E, where Venus crosses the plane of the earth's orbit, is called the ascending node. If the earth were at B when Venus is at E, Venus would be seen on the disk of the sun, making a transit. The same would be true if the earth were at D, and Venus at the descending node F.

This general view of the flying spheres is full of interest. [Page 108] While quivering themselves with thunderous noises, all is silent about them; earthquakes may be struggling on their surfaces, but there is no hint of contention in the quiet of space. They are too distant from one another to exchange signals, except, perhaps, the fleet of asteroids that sail the azure between Mars and Jupiter. Some of these come near together, continuing to fill each other's sky for days with brightness, then one gradually draws ahead. They have all phases for each other--crescent, half, full, and gibbous. These hundreds of bodies fill the realm where they are with inexhaustible variety. Beyond are vast spaces--cold, dark, void of matter, but full of power. Occasionally a little spark of light looms up rapidly into a world so huge that a thousand of our earths could not occupy its vast bulk. It swings its four or eight moons with perfect skill and infinite strength; but they go by and leave the silence unbroken, the darkness unlighted for years. Nevertheless, every part of space is full of power. Nowhere in its wide orbit can a world find a place; at no time in its eons of flight can it find an instant when the sun does not hold it in safety and life.

_The Outlook from the Earth._

If we come in from our wanderings in space and take an outlook from the earth, we shall observe certain movements, easily interpreted now that we know the system, but nearly inexplicable to men who naturally supposed that the earth was the largest, most stable, and central body in the universe.

We see, first of all, sun, moon, and stars rise in the east, mount the heavens, and set in the west. As I [Page 109] revolve in my pivoted study-chair, and see all sides of the room--library, maps, photographs, telescope, and windows--I have no suspicion that it is the room that whirls; but looking out of a car-window in a depot at another car, one cannot tell which is moving, whether it be his car or the other. In regard to the world, we have come to feel its whirl. We have noticed the pyramids of Egypt lifted to hide the sun; the mountains of Hymettus hurled down, so as to disclose the moon that was behind them to the watchers on the Acropolis; and the mighty mountains of Moab removed to reveal the stars of the east. Train the telescope on any star; it must be moved frequently, or the world will roll the instrument away from the object. Suspend a cannon-ball by a fine wire at the equator; set it vibrating north and south, and it swings all day in precisely the same direction. But suspend it directly over the north pole, and set it swinging toward Washington; in six hours after it is swinging toward Rome, in Italy; in twelve hours, toward Siam, in Asia; in nineteen hours, toward the Sandwich Islands; and in twenty-four, toward Washington again, not because it has changed the plane of its vibration, but because the earth has whirled beneath it, and the torsion of the wire has not been sufficient to compel the plane of the original direction to change with the turning of the earth. The law of inertia keeps it moving in the same direction. The same experimental proof of revolution is shown in a proportional degree at any point between the pole and the equator.

But the watchers on the Acropolis do not get turned over so as to see the moon at the same time every night. [Page 110] We turn down our eastern horizon, but we do not find fair Luna at the same moment we did the night before. We are obliged to roll on for some thirty to fifty minutes longer before we find the moon. It must be going in the same direction, and it takes us longer to get round to it than if if it were always in the same spot; so we notice a star near the moon one night--it is 13° west of the moon the next night. The moon is going around the earth from west to east, and if it goes 13° in one day, it will take a little more than twenty-seven days to go the entire circle of 360°.

[Page 111] In our outlook we soon observe that we do not by our revolution come to see the same stars rise at the same hour every night. Orion and the Pleiades, our familiar friends in the winter heavens, are gone from the summer sky. Have they fled, or are we turned from them? This is easily understood from Fig. 42.

When the observer on the earth at A looks into the midnight sky he sees the stars at E; but as the earth passes on to B, he sees those stars at E three minutes sooner every night; and at midnight the stars at F are over his head. Thus in a year, by going around the sun, we have every star of the celestial dome in our midnight sky. We see also how the sun appears among the successive constellations. When we are at A, we see the sun among the stars at G; but as we move toward B, the sun appears to move toward H. If we had observed the sun rise on the 20th of August, 1876, we should have seen it rise a little before Regulus, and a little south of it, in such a relation as circle 1 is to the star in Fig. 43. By sunset the earth had moved enough to make the sun appear to be at circle 2, and by the next morning at circle 3, at which time Regulus would rise before the sun. Thus the earth's motion seems to make the sun traverse a regular circle among the stars once a year: but it is not the sun that moves.

There are certain stars that have such irregular, uncertain, vagarious ways that they were called vagabonds, or planets, by the early astronomers. Here is the path of Jupiter in the year 1866 (Fig. 44). These bodies go forward for awhile, then stop, start aside, then retrograde, [Page 112] and go on again. Some are never seen far from the sun, and others in all parts of the ecliptic.

First see them as they stand to-day, as in Fig. 45. The observer stands on the earth at A. It has rolled over so far that he cannot see the sun; it has set. But Venus is still in sight; Jupiter is 45° behind Venus, and Saturn is seen 90° farther east. When A has rolled a little farther, if he is awake, he will see Mars before he sees the sun; or, in common language, Venus will set after, and Mars rise before the sun. All these bodies at near and far distances seem set in the starry dome, as the different stars seem in Fig. 42, p. 110.

The mysterious movements of advance and retreat are rendered intelligible by Fig. 46. The planet Mercury is at A, and, seen from the earth, B is located at _a_, [Page 113] on the background of the stars it seems to be among. It remains apparently stationary at _a_ for some time, because approaching the earth in nearly a straight line. Passing D to C, it appears to retrograde among the stars to _c_; remains apparently stationary for some time, then, in passing from C to E and A, appears to pass back among the stars to _a_. The progress of the earth, meanwhile, although it greatly retards the apparent motion from A to C, greatly hastens it from C to A.

It is also apparent that Mercury and Venus, seen from the earth, can never appear far from the sun. They must be just behind the sun as evening stars, or just before it as heralds of the morning. Venus is never more than 47° from the sun, and Mercury never more than 30°; indeed, it keeps so near the sun that very few people have ever seen the brilliant sparkler. Observe how much larger the planet appears near the earth in conjunction at D than in opposition at E. Observe also what phases it must present, and how transits sometimes take place.

[Page 114] The movement of a superior planet, one whose orbit is exterior to the earth, is clear from Fig. 47. When the earth is at A and Mars at B, it will appear among the stars at C. When the earth is at D, Mars having moved more slowly to E, will have retrograded to F. It remains there while the earth passes on, in a line nearly straight, from Mars to G; then, as the earth begins to curve around the sun, Mars will appear to retraverse the distance from F to C, and beyond. The farther the superior planet is from the earth the less will be the retrograde movement.

The reader should draw the orbits in proportion, and, remembering the relative speed of each planet, note the movement of each in different parts of their orbits.

To account for these most simple movements, the earlier astronomers invented the most complex and impossible machinery. They thought the earth the centre, and that the sun, moon, and stars were carried about it, as stoves around a person to warm him. They thought these strange movements of the planets were accomplished by mounting them on subsidiary eccentric wheels in the revolving crystal sphere. All that was [Page 115] needed to give them a right conception was a sinking of their world and themselves to an appropriate proportion, and an enlargement of their vision, to take in from an exalted stand-point a view of the simplicity of the perfect plan.

EXPERIMENTS.

Fix a rod, or tube, or telescope pointing at a star in the cast or west, and the earth's revolution will be apparent in a moment, turning the tube away from the star. Point it at stars about the north pole, and those on one side will be found going in an opposite direction from those on the other, and very much slower than those about the equator. Anyone can try the pendulum experiment who has access to some lofty place from which to suspend the ball. It was tried in Bunker Hill Monument a few years ago, and is to be tried in Paris, in the summer of 1879, with a seven-hundred-pound pendulum and a suspending wire seventy yards long. The advance and retrograde movements of planets can be illustrated by two persons walking around a centre and noticing the place where the person appears projected on the wall beyond.

* * * * *

PROCESSION OF STARS AND SOULS.

"I stood upon the open casement, And looked upon the night, And saw the westward-going stars Pass slowly out of sight.

"Slowly the bright procession Went down the gleaming arch, And my soul discerned the music Of the long triumphal march;

"Till the great celestial army, Stretching far beyond the poles, Became the eternal symbol Of the mighty march of souls.

[Page 116] "Onward, forever onward, Red Mars led on his clan; And the moon, like a mailèd maiden, Was riding in the van.

"And some were bright in beauty, And some were faint and small, But these might be, in their great heights, The noblest of them all.

"Downward, forever downward, Behind earth's dusky shore, They passed into the unknown night-- They passed, and were no more.

"No more! Oh, say not so! And downward is not just; For the sight is weak and the sense is dim That looks through heated dust.

"The stars and the mailèd moon, Though they seem to fall and die, Still sweep in their embattled lines An endless reach of sky.

"And though the hills of Death May hide the bright array, The marshalled brotherhood of souls Still keeps its onward way.

"Upward, forever upward, I see their march sublime, And hear the glorious music Of the conquerors of Time.

"And long let me remember That the palest fainting one May to diviner vision be A bright and blazing sun."

THOMAS BUCHANAN READ.

[Page 117] VII.

SHOOTING-STARS, METEORS, AND COMETS.

"The Lord cast down great stones from heaven upon them unto Azekah, and they died."--_Joshua_ x. II.

[Page 118] [Illustration: A SWARM OF METEORS MEETING THE EARTH.

Their orbits are all parallel. Those coming in direct line to the eye appear as stars, having no motion. Those on one side of this line are seen in foreshortened perspective. Those furthest from the centre, other things being equal, appear longest. The centre, called the radiant point, of these November meteors is situated in Leo; that of the August meteors in Perseus. Over fifty such radiant points have been discovered. Over 30,000 meteors have been visible in an hour.]

[Page 119] VII.

_SHOOTING-STARS, METEORS, AND COMETS._

Before particularly considering the larger aggregations of matter called planets or worlds as individuals, it is best to investigate a part of the solar system consisting of smaller collections of matter scattered everywhere through space. They are of various densities, from a cloudlet of rarest gas to solid rock; of various sizes, from a grain's weight to little worlds; of various relations to each other, from independent individuality to related streams millions of miles long. When they become visible they are called shooting-stars, which are evanescent star-points darting through the upper air, leaving for an instant a brilliant train; meteors, sudden lights, having a discernible diameter, passing over a large extent of country, often exploding with violence (Fig. 48), and throwing down upon the earth aerolites; and comets, vast extents of ghostly light, that come we know not whence and go we know not whither. All these forms of matter are governed by the same laws as the worlds, and are an integral part of the solar system--a part of the unity of the universe.

Everyone has seen the so-called shooting-stars. They break out with a sudden brilliancy, shoot a few degrees with quiet speed, and are gone before we can say, "See there!" The cause of their appearance, the [Page 120] conversion of force into heat by their contact with our atmosphere, has been already explained. Other facts remain to be studied. They are found to appear about seventy-three miles above the earth, and to disappear about twenty miles nearer the surface. Their average velocity, thirty-five, sometimes rises to one hundred miles a second. They exhibit different colors, according to their different chemical substances, which are consumed. The number of them to be seen on different nights is exceedingly variable; sometimes not more [Page 121] than five or six an hour, and sometimes so many that a man cannot count those appearing in a small section of sky. This variability is found to be periodic. There are everywhere in space little meteoric masses of matter, from the weight of a grain to a ton, and from the density of gas to rock. The earth meets 7,500,000 little bodies every day--there is collision--the little meteoroid gives out its lightning sign of extinction, and, consumed in fervent heat, drops to the earth as gas or dust. If we add the number light enough to be seen by a telescope, they cannot be less than 400,000,000 a day. Everywhere we go, in a space as large as that occupied by the earth and its atmosphere, there must be at least 13,000 bodies--one in 20,000,000 cubic miles--large enough to make a light visible to the naked eye, and forty times that number capable of revealing themselves to telescopic vision. Professor Peirce is about to publish, as the startling result of his investigations, "that the heat which the earth receives directly from meteors is the same in amount which it receives from the sun by radiation, and that the sun receives five-sixths of its heat from the meteors that fall upon it."

[Page 121] In 1783 Dr. Schmidt was fortunate enough to have a telescopic view of a system of bodies which had turned into meteors. These were two larger bodies followed by several smaller ones, going in parallel lines till they were extinguished. They probably had been revolving about each other as worlds and satellites before entering our atmosphere. It is more than probable that the earth has many such bodies, too small to be visible, revolving around it as moons.

_Aerolites._

Sometimes the bodies are large enough to bear the heat, and the unconsumed centre comes to the earth. [Page 123] Their velocity has been lessened by the resisting air, and the excessive heat diminished. Still, if found soon after their descent, they are too hot to be handled. These are called aerolites or air-stones. There was a fall in Iowa, in February, 1875, from which fragments amounting to five hundred pounds weight were secured. On the evening of December 21st, 1876, a meteor of unusual size and brilliancy passed over the states of Kansas, Missouri, Illinois, Indiana, and Ohio. It was first seen in the western part of Kansas, at an altitude of about sixty miles. In crossing the State of Missouri it began to explode, and this breaking up continued while passing Illinois, Indiana, and Ohio, till it consisted of a large flock of brilliant balls chasing each other across the sky, the number being variously estimated at from twenty to one hundred. It was accompanied by terrific explosions, and was seen along a path of not less than a thousand miles. When first seen in Kansas, it is said to have appeared as large as the full moon, and with a train from twenty-five to one hundred feet long. Another, very similar in appearance and behavior, passed over a part of the same course in February, 1879. At Laigle, France, on April 26th, 1803, about one o'clock in the day, from two to three thousand fell. The largest did not exceed seventeen pounds weight. One fell in Weston, Connecticut, in 1807, weighing two hundred pounds. A very destructive shower is mentioned in the book of Joshua, chap. x. ver. 11.

These bodies are not evenly distributed through space. In some places they are gathered into systems which circle round the sun in orbits as certain as those of the [Page 124] planets. The chain of asteroids is an illustration of meteoric bodies on a large scale. They are hundreds in number--meteors are millions. They have their region of travel, and the sun holds them and the giant Jupiter by the same power. The Power that cares for a world cares for a sparrow. If their orbit so lies that a planet passes through it, and the planet and the meteors are at the point of intersection at the same time, there must be collisions, and the lightning signs of extinction proportioned to the number of little bodies in a given space.

It is demonstrated that the earth encounters more than one hundred such systems of meteoric bodies in a single year. It passes through one on the 10th of August, another on the 11th of November. In a certain part of the first there is an agglomeration of bodies sufficient to become visible as it approaches the sun, and this is known as the comet of 1862; in the second is a similar agglomeration, known as Temple's comet. It is repeating the same thing to say that meteoroids follow in the train of the comets. The probable orbit of the November meteors and the comet of 1866 is an exceedingly elongated ellipse, embracing the orbit of the earth at one end and a portion of the orbit of Uranus at the other (Fig. 51). That of the August meteors and the comet of 1862 embraces the orbit of the earth at one end, and thirty per cent. of the other end is beyond the orbit of Neptune.

In January, 1846, Biela's comet was observed to be divided. At its next return, in 1852, the parts were 1,500,000 miles apart. They could not be found on their periodic returns in 1859, 1865, and 1872; but it [Page 125] should have crossed the earth's orbit early in September, 1872. The earth itself would arrive at the point of crossing two or three months later. If the law of revolution held, we might still expect to find some of the trailing meteoroids of the comet not gone by on our arrival. It was shown that the point of the earth that would strike them would be toward a certain place in the constellation of Andromeda, if the remains of the diluted comet were still there. The prediction was verified in every respect. At the appointed time, place, [Page 126] and direction, the streaming lights were in our sky. That these little bodies belonged to the original comet none can doubt. By the perturbations of planetary attraction, or by different original velocities, a comet may be lengthened into an invisible stream, or an invisible stream agglomerated till it is visible as a comet.

_Comets._