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<v Speaker 1>Section twenty one of Sun, Moon and Stars Astronomy for Beginners.

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<v Speaker 1>This is a LibriVox recording. All LibriVox recordings are in

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<v Speaker 1>the public domain. For more information or to volunteer, please

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<v Speaker 1>visit LibriVox dot org. Read by Randy Boyam, Sun Moon

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<v Speaker 1>and Stars Astronomy for Beginners by Agnes Gebairn, Section twenty one.

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<v Speaker 1>More about comets and meteors. He commanded and they were

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<v Speaker 1>created Psalms One forty eight five. There have been several

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<v Speaker 1>comets of importance seen in the present century. In eighteen eleven,

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<v Speaker 1>a remarkably fine one appeared. The bright nucleus was only

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<v Speaker 1>about four hundred miles in diameter, but the whole head,

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<v Speaker 1>including the envelope or coma, measured one hundred and twelve

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<v Speaker 1>thousands of miles across, and the enormous tale stretched out

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<v Speaker 1>to a distance of one hundred and twelve millions of miles.

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<v Speaker 1>This comet travels in so long and narrow an ellipse

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<v Speaker 1>that some say his return must not be looked for

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<v Speaker 1>In less than thirty centuries. The Great Comet, which in

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<v Speaker 1>sixteen eighty two received first the name of Halley's Comet,

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<v Speaker 1>appeared last in eighteen thirty five, his return having been

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<v Speaker 1>foretold within three days of its actually taking place. For

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<v Speaker 1>Halley's comet is a member of the Solar system, having

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<v Speaker 1>a yearly journey of seventy six earthly years. He journeys

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<v Speaker 1>nearer to the Sun than Venus and travels farther away

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<v Speaker 1>than Neptune. In eighteen forty three appeared one of the

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<v Speaker 1>brightest comets ever seen, and also one of the nearest

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<v Speaker 1>to the Sun. So close ran his orbit to the

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<v Speaker 1>center of our system that at one time his nucleus

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<v Speaker 1>was only about thirty thousand miles from the Sun's surface.

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<v Speaker 1>When we remember that sun flames often rise to a

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<v Speaker 1>height of more than fifty thousand miles, we shall see

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<v Speaker 1>how terrific must have been the heat endured. In the

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<v Speaker 1>years eighteen fifty eight, eighteen sixty one, and eighteen sixty two,

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<v Speaker 1>three more comets appear, all visible without the help of

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<v Speaker 1>a telescope. Of these three, Donati's comet in eighteen fifty

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<v Speaker 1>eight was far superior to the rest. It was a

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<v Speaker 1>singular fact about the comet of eighteen fifty eight that

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<v Speaker 1>at one time the star Arcturus could be seen shining

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<v Speaker 1>through the densest portion of the tail close to the nucleus. Now,

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<v Speaker 1>although the faintest cloud wreath of Earth would dim, if

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<v Speaker 1>not hide, this star, yet the tail of the comet

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<v Speaker 1>was of so transparent a nature that our turus shone undimmed,

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<v Speaker 1>as if no veil had come between. The exceedingly slight

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<v Speaker 1>and airy texture of a comet's tail could hardly be

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<v Speaker 1>more plainly shown. It was this gauzy appendage to a

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<v Speaker 1>little nucleus which men once thought could destroy our solid

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<v Speaker 1>Earth at a single blow. Yet while taking care not

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<v Speaker 1>to overrate, we must not underrate true the comets are

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<v Speaker 1>delicate and light in structure. One comet in seventeen seventy

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<v Speaker 1>wandered into the very midst of Jupiter's moons, and so

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<v Speaker 1>so small was its weight that it had no power

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<v Speaker 1>whatever so far as has been detected to disturb the

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<v Speaker 1>said moons in their orbits. Jupiter and his moons did

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<v Speaker 1>very seriously disturb the comet, however, and when he came

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<v Speaker 1>out from their midst, though none the worse for his adventure,

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<v Speaker 1>he was forced to travel in an entirely new orbit

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<v Speaker 1>and never managed to get back to his old pathway again.

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<v Speaker 1>But there are comets and comets, some being heavier than others.

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<v Speaker 1>The comet named after Donati, albeit too transparent to hide

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<v Speaker 1>a star, was yet so immense in size that his

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<v Speaker 1>weight was calculated by one astronomer to amount to as

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<v Speaker 1>much as a mass of water forty thousand miles square

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<v Speaker 1>and one hundred nine yards deep. When first noticed, Donati's

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<v Speaker 1>comet had, like all large comets, a bright envelope of

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<v Speaker 1>light round the nucleus. After a while, the one envelope

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<v Speaker 1>grew into three envelopes, and a new tail formed beside

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<v Speaker 1>the principal tail, which for a time was seen to

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<v Speaker 1>bend gracefully into a curve like a splendid plume. A third,

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<v Speaker 1>but much fainter tail also made its appearance, and many

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<v Speaker 1>angry looking jets were poured out from the nucleus. These

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<v Speaker 1>changes took place while the comet was passing through the

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<v Speaker 1>great heat of near neighborhood to the sun. Afterwards, as

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<v Speaker 1>he passed away, he seemed gradually to cool down and

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<v Speaker 1>grow quiet. The singular changes in the appearance of Newton's

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<v Speaker 1>comet have been earlier noticed, no marvel that he did

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<v Speaker 1>undergo some alterations. The tremendous glare and burning heat which

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<v Speaker 1>that comet had to endure in his rush past the

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<v Speaker 1>Sun were more than twenty five thousand times as much

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<v Speaker 1>as the glare and heat of the fiercest tropical noonday

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<v Speaker 1>ever known upon Earth. Can we wonder that he should

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<v Speaker 1>have shown signs of great excitement, that his head should

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<v Speaker 1>have grown larger in his tail longer. It certainly was

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<v Speaker 1>amazing and bewildering that the said tail, over ninety millions

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<v Speaker 1>of miles in length, should, in four days have seemingly

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<v Speaker 1>swept round in an enormous half circle, so as first

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<v Speaker 1>to point in one direction, and then to point in

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<v Speaker 1>just the opposite direction. We are much in the habit

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<v Speaker 1>of speaking about comets as traveling through the heavens with

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<v Speaker 1>their tails streaming behind them. But though this is sometimes

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<v Speaker 1>the case, it is not always so. The tails of

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<v Speaker 1>comets always stream away from the Sun, whether before or

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<v Speaker 1>behind the comet's head, seeming to be a matter of indifference.

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<v Speaker 1>As the comet comes hurrying along his orbit with ever

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<v Speaker 1>increasing speed towards the Sun, the head journeys first and

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<v Speaker 1>the tail follows after. But as the comet rounds the

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<v Speaker 1>loop of his orbit near the Sun, the point nearest

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<v Speaker 1>of all, being called his perihelion. The head always remains

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<v Speaker 1>towards the Sun, while the tail swings or seems to

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<v Speaker 1>swing in a magnificent sweep round, pointing always in the

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<v Speaker 1>direction just away from the Sun. Then, as the comet

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<v Speaker 1>journeys with slackening speed on the other side of his

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<v Speaker 1>orbit towards the distant apilian or farthest point from the Sun,

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<v Speaker 1>he still keeps his head towards the Sun. So at

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<v Speaker 1>this part of his page, in place of the head

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<v Speaker 1>going first and the tail following after, the tail goes

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<v Speaker 1>first and the head follows after. The comet thus appears

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<v Speaker 1>to be moving backwards, or like an engine pushing instead

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<v Speaker 1>of drawing a train, the head seems to be driving

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<v Speaker 1>the tail before it. This careful avoidance of the Sun

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<v Speaker 1>by comet's tails is remarkable. Astronomers speak of the repulsive

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<v Speaker 1>energy with which the Sun sweeps away from his neighborhood

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<v Speaker 1>the light, gauzy appendages of a comet, but the precise

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<v Speaker 1>nature of that repulsion and the mode of its working

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<v Speaker 1>are not yet manifested. It need not be supposed that

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<v Speaker 1>the enormous tails, millions of miles in length, are formed

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<v Speaker 1>of unbroken and connected substance, whirled round like a vast

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<v Speaker 1>kite tail or a stupendous fan, at a perfectly unimaginable speed.

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<v Speaker 1>The actual make of the tails cannot yet be stated

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<v Speaker 1>with certainty, but there is a growing belief among astronomers

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<v Speaker 1>that electricity may have something of a hand in the matter.

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<v Speaker 1>After all, the tail is a mere adjunct to the comet,

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<v Speaker 1>no more needful to a comet's existence than a man's

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<v Speaker 1>beard is to the man's existence. According to ordinary notions,

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<v Speaker 1>a full blown comet consists of the bright central nucleus,

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<v Speaker 1>the surrounding coma, and the far reaching tail. But if

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<v Speaker 1>you have the nucleus, you have the comet, and no

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<v Speaker 1>more as necessary. It is known to be a comet,

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<v Speaker 1>not a star, mainly by the path that it follows

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<v Speaker 1>and the rates of its motions. The coma has been

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<v Speaker 1>described by doctor Huggins as usually a luminous fog surrounding

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<v Speaker 1>the nucleus and gradually shading off from it, and the

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<v Speaker 1>tail as a continuation in a direction opposite to that

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<v Speaker 1>of the sun. Of the luminous fog of the coma,

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<v Speaker 1>except in the Sun's neighborhood. A comet's tail has commonly

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<v Speaker 1>no existence. As the wanderer draws near to our luminary,

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<v Speaker 1>a tail of less or greater dimensions is often developed

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<v Speaker 1>under the increasing heat, and as the wander passes away

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<v Speaker 1>once more, the tail diminishes and disappears. Spectrum analysis declares

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<v Speaker 1>that comets usually consist of carbon, hydrogen, nitrogen, and possibly oxygen,

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<v Speaker 1>all of course in a state of highly heated and

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<v Speaker 1>light emitting gas. To some extent. Also, a comet reflects

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<v Speaker 1>the sunlight. More than this is known of the nature

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<v Speaker 1>of comets through studying the nature of their nearive kin

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<v Speaker 1>meteors and meteorites. Speaking at the same time as just cited,

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<v Speaker 1>doctor Huggins said further, we are thus led to see

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<v Speaker 1>the close physical connection and oneness of origin, if not

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<v Speaker 1>indeed identity, of nature of comets and of these meteors. Now,

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<v Speaker 1>the meteors, on these occasions to great meteor showers are

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<v Speaker 1>too minute to pass through the ordeal of ignition by

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<v Speaker 1>our atmosphere. They are burned up before they reach the Earth.

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<v Speaker 1>But at other times small celestial masses come down to us,

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<v Speaker 1>which there can be little doubt are of the same

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<v Speaker 1>order of bodies and similar in chemical nature. Here again

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<v Speaker 1>spectrum analysis steps in and tells us that in the

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<v Speaker 1>make of celestial metiorite falling to Earth, among diverse other

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<v Speaker 1>substances may be found the hydrogen, carbon and nitrogen, which

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<v Speaker 1>we have already seen to be the main substances in

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<v Speaker 1>the composition of comets. True, a fallen meteorite is small, solid, heavy,

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<v Speaker 1>while a comet is light and hazy. But if the

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<v Speaker 1>materials of which a meteorite is made were rendered gaseous,

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<v Speaker 1>then at once it would be not only much larger,

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<v Speaker 1>but light and hazy. In eighteen sixty seven, a large

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<v Speaker 1>lecture room was entirely lighted by means of gas extracted

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<v Speaker 1>from a meteorite which had arrived from distant space. Not

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<v Speaker 1>only do solid aerolites fall half burned to the ground,

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<v Speaker 1>but even when the meteorites are quite consumed in the air,

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<v Speaker 1>the fine dust remaining still sinks earthward. This fine dust

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<v Speaker 1>has been found upon mountaintops and has been proved by

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<v Speaker 1>close examination to be precisely the same as the material

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<v Speaker 1>of the solid aerolites. It is a wonderful thought that

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<v Speaker 1>we really should have these visitants from the sky colet

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<v Speaker 1>metal or showers of dust coming to us from distant space.

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<v Speaker 1>If the dust of thousands of meteorites is always thus

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<v Speaker 1>falling earthward, one would imagine that it must in time

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<v Speaker 1>add something to the weight of the Earth. And this

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<v Speaker 1>is actually the case. During the last three thousand years,

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<v Speaker 1>no less than one million tons of meteorite dust must,

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<v Speaker 1>according to the calculation, have fallen to Earth out of

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<v Speaker 1>the sky. A million tons is of course a mere

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<v Speaker 1>nothing compared with the size of the world. Still, the

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<v Speaker 1>fact is curious and interesting. Some of the tremendous outbursts

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<v Speaker 1>seen on the surface of the Sun have been described earlier.

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<v Speaker 1>It is believed that in such outbursts, matter is driven

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<v Speaker 1>forth with violence sufficient to send it whirling through space,

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<v Speaker 1>never to return thither. Many meteorites may have had their

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<v Speaker 1>birth thus, whether from our own sun or from stars

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<v Speaker 1>like in nature to our Sun. It has been conjectured

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<v Speaker 1>that some meteorites may also have sprung from earthly volcanoes,

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<v Speaker 1>being shot aloft with impetuous enough to carry them to

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<v Speaker 1>a vast distance, breaking loose perhaps for a while from

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<v Speaker 1>Earth's control, but after lengthened period, it may be accidentally

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<v Speaker 1>traveling earthword once more, a few more words about comet visitors.

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<v Speaker 1>Many comets, as stated earlier, belong to the Solar system,

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<v Speaker 1>but many also come only once, flashing around the Sun

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<v Speaker 1>and rushing away in a new direction, never to return.

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<v Speaker 1>Any comet at about the distance of our Earth from

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<v Speaker 1>the Sun, traveling at a rate of more than twenty

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<v Speaker 1>six miles per second, is beyond solar control. The Sun

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<v Speaker 1>is powerless to hold prisoner so impulsive a vagrant, and

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<v Speaker 1>it must find its way out of the Solar System.

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<v Speaker 1>Comets which come to us from outside our system must

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<v Speaker 1>come from some other system, and the nearest systems known

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<v Speaker 1>are those of the stars. The nearest star of all,

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<v Speaker 1>among those whose distance has yet been measured, is Alpha Centauri.

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<v Speaker 1>A comet passing from Alpha Centauri to our Solar system

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<v Speaker 1>would require millions of years for his journey. But it

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<v Speaker 1>is not impossible that other stars may lie nearer to

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<v Speaker 1>us than Alpha Centauri, although the fact has not yet

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<v Speaker 1>been discovered. End of section twenty one read by Randy Boyam, Denver, Colorado,

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<v Speaker 1>December twenty sixth, twenty twenty four.