Speaker 1: Section twenty one of Sun, Moon and Stars Astronomy for Beginners.
Speaker 1: This is a LibriVox recording. All LibriVox recordings are in
Speaker 1: the public domain. For more information or to volunteer, please
Speaker 1: visit LibriVox dot org. Read by Randy Boyam, Sun Moon
Speaker 1: and Stars Astronomy for Beginners by Agnes Gebairn, Section twenty one.
Speaker 1: More about comets and meteors. He commanded and they were
Speaker 1: created Psalms One forty eight five. There have been several
Speaker 1: comets of importance seen in the present century. In eighteen eleven,
Speaker 1: a remarkably fine one appeared. The bright nucleus was only
Speaker 1: about four hundred miles in diameter, but the whole head,
Speaker 1: including the envelope or coma, measured one hundred and twelve
Speaker 1: thousands of miles across, and the enormous tale stretched out
Speaker 1: to a distance of one hundred and twelve millions of miles.
Speaker 1: This comet travels in so long and narrow an ellipse
Speaker 1: that some say his return must not be looked for
Speaker 1: In less than thirty centuries. The Great Comet, which in
Speaker 1: sixteen eighty two received first the name of Halley's Comet,
Speaker 1: appeared last in eighteen thirty five, his return having been
Speaker 1: foretold within three days of its actually taking place. For
Speaker 1: Halley's comet is a member of the Solar system, having
Speaker 1: a yearly journey of seventy six earthly years. He journeys
Speaker 1: nearer to the Sun than Venus and travels farther away
Speaker 1: than Neptune. In eighteen forty three appeared one of the
Speaker 1: brightest comets ever seen, and also one of the nearest
Speaker 1: to the Sun. So close ran his orbit to the
Speaker 1: center of our system that at one time his nucleus
Speaker 1: was only about thirty thousand miles from the Sun's surface.
Speaker 1: When we remember that sun flames often rise to a
Speaker 1: height of more than fifty thousand miles, we shall see
Speaker 1: how terrific must have been the heat endured. In the
Speaker 1: years eighteen fifty eight, eighteen sixty one, and eighteen sixty two,
Speaker 1: three more comets appear, all visible without the help of
Speaker 1: a telescope. Of these three, Donati's comet in eighteen fifty
Speaker 1: eight was far superior to the rest. It was a
Speaker 1: singular fact about the comet of eighteen fifty eight that
Speaker 1: at one time the star Arcturus could be seen shining
Speaker 1: through the densest portion of the tail close to the nucleus. Now,
Speaker 1: although the faintest cloud wreath of Earth would dim, if
Speaker 1: not hide, this star, yet the tail of the comet
Speaker 1: was of so transparent a nature that our turus shone undimmed,
Speaker 1: as if no veil had come between. The exceedingly slight
Speaker 1: and airy texture of a comet's tail could hardly be
Speaker 1: more plainly shown. It was this gauzy appendage to a
Speaker 1: little nucleus which men once thought could destroy our solid
Speaker 1: Earth at a single blow. Yet while taking care not
Speaker 1: to overrate, we must not underrate true the comets are
Speaker 1: delicate and light in structure. One comet in seventeen seventy
Speaker 1: wandered into the very midst of Jupiter's moons, and so
Speaker 1: so small was its weight that it had no power
Speaker 1: whatever so far as has been detected to disturb the
Speaker 1: said moons in their orbits. Jupiter and his moons did
Speaker 1: very seriously disturb the comet, however, and when he came
Speaker 1: out from their midst, though none the worse for his adventure,
Speaker 1: he was forced to travel in an entirely new orbit
Speaker 1: and never managed to get back to his old pathway again.
Speaker 1: But there are comets and comets, some being heavier than others.
Speaker 1: The comet named after Donati, albeit too transparent to hide
Speaker 1: a star, was yet so immense in size that his
Speaker 1: weight was calculated by one astronomer to amount to as
Speaker 1: much as a mass of water forty thousand miles square
Speaker 1: and one hundred nine yards deep. When first noticed, Donati's
Speaker 1: comet had, like all large comets, a bright envelope of
Speaker 1: light round the nucleus. After a while, the one envelope
Speaker 1: grew into three envelopes, and a new tail formed beside
Speaker 1: the principal tail, which for a time was seen to
Speaker 1: bend gracefully into a curve like a splendid plume. A third,
Speaker 1: but much fainter tail also made its appearance, and many
Speaker 1: angry looking jets were poured out from the nucleus. These
Speaker 1: changes took place while the comet was passing through the
Speaker 1: great heat of near neighborhood to the sun. Afterwards, as
Speaker 1: he passed away, he seemed gradually to cool down and
Speaker 1: grow quiet. The singular changes in the appearance of Newton's
Speaker 1: comet have been earlier noticed, no marvel that he did
Speaker 1: undergo some alterations. The tremendous glare and burning heat which
Speaker 1: that comet had to endure in his rush past the
Speaker 1: Sun were more than twenty five thousand times as much
Speaker 1: as the glare and heat of the fiercest tropical noonday
Speaker 1: ever known upon Earth. Can we wonder that he should
Speaker 1: have shown signs of great excitement, that his head should
Speaker 1: have grown larger in his tail longer. It certainly was
Speaker 1: amazing and bewildering that the said tail, over ninety millions
Speaker 1: of miles in length, should, in four days have seemingly
Speaker 1: swept round in an enormous half circle, so as first
Speaker 1: to point in one direction, and then to point in
Speaker 1: just the opposite direction. We are much in the habit
Speaker 1: of speaking about comets as traveling through the heavens with
Speaker 1: their tails streaming behind them. But though this is sometimes
Speaker 1: the case, it is not always so. The tails of
Speaker 1: comets always stream away from the Sun, whether before or
Speaker 1: behind the comet's head, seeming to be a matter of indifference.
Speaker 1: As the comet comes hurrying along his orbit with ever
Speaker 1: increasing speed towards the Sun, the head journeys first and
Speaker 1: the tail follows after. But as the comet rounds the
Speaker 1: loop of his orbit near the Sun, the point nearest
Speaker 1: of all, being called his perihelion. The head always remains
Speaker 1: towards the Sun, while the tail swings or seems to
Speaker 1: swing in a magnificent sweep round, pointing always in the
Speaker 1: direction just away from the Sun. Then, as the comet
Speaker 1: journeys with slackening speed on the other side of his
Speaker 1: orbit towards the distant apilian or farthest point from the Sun,
Speaker 1: he still keeps his head towards the Sun. So at
Speaker 1: this part of his page, in place of the head
Speaker 1: going first and the tail following after, the tail goes
Speaker 1: first and the head follows after. The comet thus appears
Speaker 1: to be moving backwards, or like an engine pushing instead
Speaker 1: of drawing a train, the head seems to be driving
Speaker 1: the tail before it. This careful avoidance of the Sun
Speaker 1: by comet's tails is remarkable. Astronomers speak of the repulsive
Speaker 1: energy with which the Sun sweeps away from his neighborhood
Speaker 1: the light, gauzy appendages of a comet, but the precise
Speaker 1: nature of that repulsion and the mode of its working
Speaker 1: are not yet manifested. It need not be supposed that
Speaker 1: the enormous tails, millions of miles in length, are formed
Speaker 1: of unbroken and connected substance, whirled round like a vast
Speaker 1: kite tail or a stupendous fan, at a perfectly unimaginable speed.
Speaker 1: The actual make of the tails cannot yet be stated
Speaker 1: with certainty, but there is a growing belief among astronomers
Speaker 1: that electricity may have something of a hand in the matter.
Speaker 1: After all, the tail is a mere adjunct to the comet,
Speaker 1: no more needful to a comet's existence than a man's
Speaker 1: beard is to the man's existence. According to ordinary notions,
Speaker 1: a full blown comet consists of the bright central nucleus,
Speaker 1: the surrounding coma, and the far reaching tail. But if
Speaker 1: you have the nucleus, you have the comet, and no
Speaker 1: more as necessary. It is known to be a comet,
Speaker 1: not a star, mainly by the path that it follows
Speaker 1: and the rates of its motions. The coma has been
Speaker 1: described by doctor Huggins as usually a luminous fog surrounding
Speaker 1: the nucleus and gradually shading off from it, and the
Speaker 1: tail as a continuation in a direction opposite to that
Speaker 1: of the sun. Of the luminous fog of the coma,
Speaker 1: except in the Sun's neighborhood. A comet's tail has commonly
Speaker 1: no existence. As the wanderer draws near to our luminary,
Speaker 1: a tail of less or greater dimensions is often developed
Speaker 1: under the increasing heat, and as the wander passes away
Speaker 1: once more, the tail diminishes and disappears. Spectrum analysis declares
Speaker 1: that comets usually consist of carbon, hydrogen, nitrogen, and possibly oxygen,
Speaker 1: all of course in a state of highly heated and
Speaker 1: light emitting gas. To some extent. Also, a comet reflects
Speaker 1: the sunlight. More than this is known of the nature
Speaker 1: of comets through studying the nature of their nearive kin
Speaker 1: meteors and meteorites. Speaking at the same time as just cited,
Speaker 1: doctor Huggins said further, we are thus led to see
Speaker 1: the close physical connection and oneness of origin, if not
Speaker 1: indeed identity, of nature of comets and of these meteors. Now,
Speaker 1: the meteors, on these occasions to great meteor showers are
Speaker 1: too minute to pass through the ordeal of ignition by
Speaker 1: our atmosphere. They are burned up before they reach the Earth.
Speaker 1: But at other times small celestial masses come down to us,
Speaker 1: which there can be little doubt are of the same
Speaker 1: order of bodies and similar in chemical nature. Here again
Speaker 1: spectrum analysis steps in and tells us that in the
Speaker 1: make of celestial metiorite falling to Earth, among diverse other
Speaker 1: substances may be found the hydrogen, carbon and nitrogen, which
Speaker 1: we have already seen to be the main substances in
Speaker 1: the composition of comets. True, a fallen meteorite is small, solid, heavy,
Speaker 1: while a comet is light and hazy. But if the
Speaker 1: materials of which a meteorite is made were rendered gaseous,
Speaker 1: then at once it would be not only much larger,
Speaker 1: but light and hazy. In eighteen sixty seven, a large
Speaker 1: lecture room was entirely lighted by means of gas extracted
Speaker 1: from a meteorite which had arrived from distant space. Not
Speaker 1: only do solid aerolites fall half burned to the ground,
Speaker 1: but even when the meteorites are quite consumed in the air,
Speaker 1: the fine dust remaining still sinks earthward. This fine dust
Speaker 1: has been found upon mountaintops and has been proved by
Speaker 1: close examination to be precisely the same as the material
Speaker 1: of the solid aerolites. It is a wonderful thought that
Speaker 1: we really should have these visitants from the sky colet
Speaker 1: metal or showers of dust coming to us from distant space.
Speaker 1: If the dust of thousands of meteorites is always thus
Speaker 1: falling earthward, one would imagine that it must in time
Speaker 1: add something to the weight of the Earth. And this
Speaker 1: is actually the case. During the last three thousand years,
Speaker 1: no less than one million tons of meteorite dust must,
Speaker 1: according to the calculation, have fallen to Earth out of
Speaker 1: the sky. A million tons is of course a mere
Speaker 1: nothing compared with the size of the world. Still, the
Speaker 1: fact is curious and interesting. Some of the tremendous outbursts
Speaker 1: seen on the surface of the Sun have been described earlier.
Speaker 1: It is believed that in such outbursts, matter is driven
Speaker 1: forth with violence sufficient to send it whirling through space,
Speaker 1: never to return thither. Many meteorites may have had their
Speaker 1: birth thus, whether from our own sun or from stars
Speaker 1: like in nature to our Sun. It has been conjectured
Speaker 1: that some meteorites may also have sprung from earthly volcanoes,
Speaker 1: being shot aloft with impetuous enough to carry them to
Speaker 1: a vast distance, breaking loose perhaps for a while from
Speaker 1: Earth's control, but after lengthened period, it may be accidentally
Speaker 1: traveling earthword once more, a few more words about comet visitors.
Speaker 1: Many comets, as stated earlier, belong to the Solar system,
Speaker 1: but many also come only once, flashing around the Sun
Speaker 1: and rushing away in a new direction, never to return.
Speaker 1: Any comet at about the distance of our Earth from
Speaker 1: the Sun, traveling at a rate of more than twenty
Speaker 1: six miles per second, is beyond solar control. The Sun
Speaker 1: is powerless to hold prisoner so impulsive a vagrant, and
Speaker 1: it must find its way out of the Solar System.
Speaker 1: Comets which come to us from outside our system must
Speaker 1: come from some other system, and the nearest systems known
Speaker 1: are those of the stars. The nearest star of all,
Speaker 1: among those whose distance has yet been measured, is Alpha Centauri.
Speaker 1: A comet passing from Alpha Centauri to our Solar system
Speaker 1: would require millions of years for his journey. But it
Speaker 1: is not impossible that other stars may lie nearer to
Speaker 1: us than Alpha Centauri, although the fact has not yet
Speaker 1: been discovered. End of section twenty one read by Randy Boyam, Denver, Colorado,
Speaker 1: December twenty sixth, twenty twenty four.