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The Professor on Twinkling, from the Harper's

 

Jack was delighted with the idea. Gus differed from him entirely. Joe and I, being girls, pronounced it—horrid.

"Papa wants us to, you know," said May, who always sets us straight.

Jack, who had recently joined one of the college societies, moved that the Professor be cordially supported. "His lecture last week was exceedingly entertaining," he argued. "That he should be so good as to come here and talk to us about his wonderful science in a pleasant familiar way, simply because he's papa's old friend, shows the interest he takes in the family."

"Do hush, Jack."

"My dear sister, I can not. What the Professor has to tell us about the heavenly bodies—"

"I hate the heavenly bodies," growls Gus.

"Is it jealousy, Augustus, because they are allowed to stay out late nights, while you are not?"

"Whatever it is, I agree with him," puts in Joe, who always stands by Gus. "I hate astronomy too."

"Feeling as I do, Josephine, that your knowledge in the science is confined to 'Twinkle, twinkle, little star,' and the fact that 'the moon is made of green cheese,' I am surprised at you."

"What makes them twinkle?" asked May.

The Professor heard this, for he was just coming up stairs.

"What makes them twinkle, May?" and the Professor seated himself in an easy-chair, as if ready to talk.

"I don't know, Sir. Won't you please tell us?"

"Pooh, May, don't bother the Professor with such juvenile questions. He'll think he's intrusted with the charge of the third form in an infant school."

"But," persisted May, "I would like very much to know, and I don't believe you can tell, if you have been to college. Now there!"

Jack was somewhat nonplussed at this, but after a moment's hesitation said, "Well, anyhow, the books I studied never told anything about the stars twinkling, and I don't believe they do twinkle. It's nothing but a baby notion."

All eyes are now intently fixed upon the Professor, who is expected first to settle the fact, and then to account for it.

"You, Jack, who have been to college," he began, "know that all vision or sight is produced by rays falling upon the eyes. These rays may be broken or turned from their natural course—the word astronomers use is refracted. Now the stars are so far away from us that through the largest telescope they are still only points of light. As the rays come down through space there is nothing to break or refract them, but when they reach our atmosphere, there is the tremulous agitation of the air and ascending vapors. By these the rays coming from the tiny points are at intervals turned aside from the narrow space of the pupil of the eye. When the eye is assisted with the wide opening of the object-glass of a telescope no such thing happens. So Jack is right; the stars don't twinkle. When viewed through a telescope, they are found to shine with a steady brightness, and hence the motion is only in appearance. Recent astronomers have little to say about it; but it is due, doubtless, as Sir Isaac Newton explains in his celebrated Principia, to the ascending vapors and tremulous movements of the atmosphere. You have seen how the heated air or gas rising from a stove will sometimes make things behind it tremble and dance. Now if a small candle were burning on the other side of the ascending vapor, its flame, though really steady, would seem to flicker."

"Then, Sir, the stars, being so very far off, appear so very little, and the rays of light they send are disturbed by atmospheric vapor, and thus to the naked eye they twinkle."

"Yes," said the Professor. "The sun and moon, as you know, present broad disks, with light radiating from every point—"

"Please, Professor," interrupted May, "tell us what a 'disk' is. Jack's big words are dreadful to understand; and this, although a little one, seems just as bad."

"I don't wonder I puzzled you, May. We use the word disk for the face or surface of a heavenly body which appears to have some size. You may always stop me when I use a word you don't understand; but when I have once told you, I shall want you to remember; for we can not know much about science unless we learn some of the hard words. I was saying that the sun and moon present broad disks, so that if some of the light is intercepted, the eye does not notice the loss. The same is true also of the planets, which appear large when they are magnified, but not of the stars, owing to their immense distances; and when the impurities in the atmosphere obscure or divert the narrow line of light they send to us, the eye perceives it at once. Some of the stars appear very brilliant through the large telescopes, but the light still seems to proceed from a single point. There are some four or five thousand stars that can be seen without a telescope."

"Why," interrupted Joe, "I thought there were more than anybody could count."

"So there are," replied the Professor, "but the number that can be distinctly perceived by the unassisted eye is found to be comparatively small when they are carefully looked after. On very clear nights the whole sky may seem to glisten when the eye is suddenly turned upward; and there are some portions of it where a confused light comes from a sort of star-cloud, which has received the name of 'Milky Way.' But the stars that can be seen separately are very easily counted. Some persons can see rather more than others, on account of their eye-sight being naturally better, or improved by use. A rough count of the number that could be seen through Herschel's famous telescope made it twenty thousand. The great telescopes more recently made would probably show as many as forty or fifty millions."

"I should think," said May, "that it would be awful tiresome to count so many things just alike, and that the man would often count the same one over and over without knowing it, and would never be sure that he had counted right."

"They are not all alike," said the Professor. "They differ greatly in brightness, and to some extent in color, and in other particulars. They have been divided according to their brilliancy into sixteen classes or magnitudes. The fifteen brightest stars are said to be of the first magnitude, the fifty next of the second, and so on to the sixth, which comprises the faintest stars visible to the unassisted eye. The brightest star of all visible in our latitude is the dogstar, which gives four times as much light as any other. In every age of the world there have been learned men interested enough in the stars to make catalogues of them, giving their magnitudes and their positions."

"I think they must have been very slow and stupid follows," said Gus, "or they would have found something better to do."

At this the Professor laughed.

"I think, Gus, you are hungry, and have your mind on mutton-chops. I shall not talk to you any more this morning; but, after lunch, if you will look in one of the great books in papa's library, which he will point out, you will find pictures of all the great telescopes in the world. The best one in our own country is that at the United States National Observatory at Washington. Without the aid of these wonderful instruments we should never have learned much about the stars."