DIALOGUE III.

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Pupil.

I recollect, Sir, you mentioned last night, that the planets appear like stars. Our earth is a planet; how can it have the appearance of a star?

Tutor. If you were on the planet Venus, the earth would have as much the appearance of a star as Venus has to us.

Pupil. But Venus appears amongst the fixed stars.

Tutor. Yes. And so would the earth appear from Venus.

Pupil. How can it be?

Tutor. Because, in whatever part of the universe we are, we appear to be in the center of a concave, that is hollow, sphere, where remote objects appear at equal distances from us: so that, whether we are on the planet Venus or on the earth, in this particular the effect will be the same.

Pupil. Then the light we receive from the sun is by reflection conveyed to the other planets.

Tutor. No doubt of it. And our earth appears as a moon to the inhabitants of the moon, and undergoes the various changes of that planet.

Pupil. Have you any proof of this, Sir?

Tutor. Nothing can be clearer; for, on a fine evening, soon after the change of the moon, when the earth appears nearly as a full moon to the moon, and we see a faint streak of light, the whole body of the moon is visible to us.

Pupil. I remember to have seen it.

Tutor. You do?—The earth then will appear there thirteen times as large as the moon does to us; of course it must reflect a strong light on the body of the moon, and it is by that light we see that part of the moon which is turned from the sun.

Pupil. Is the earth, then, only thirteen times as big as the moon?

Tutor. In solidity it is about fifty times as large; but its disc or face is only thirteen times.

Pupil. What is the moon’s distance from the earth?

Tutor. 240 thousand miles, which is about 400 times less than that of the sun.

Pupil. And yet she appears as far distant as the sun.

Tutor. You are now, I hope, convinced of what I said relative to distant objects.

Pupil. I am, Sir: and I suppose the reason of the moon’s appearing as large as the sun, is because she is so much nearer to us.

Tutor. It is so.—For, at a total eclipse of the sun, which happens when the moon is in a right line between the sun and the earth, the sun is obscured from our sight, although his disc is 160 thousand times as large as that of the moon. In like manner would the moon, when at full, be hid by placing your cricket-ball in a line between your eye and her, yet, you know, the ball is not so large as the moon; but being nearer the eye, it is apparently so.

Pupil. This is very clear. But——

Tutor. I conjecture you were going to ask me to explain the nature of eclipses.

Pupil. That was certainly my intention, Sir.

Tutor. There are other things you must be made acquainted with before you will be able to comprehend it, and which I will endeavour to make you understand before we enter on the subject.

Pupil. Whenever you please, Sir.

Tutor. You have taken a view of the earth from the planet Venus.—Suppose I transport you to one of the planets belonging to another system; what description do you think you should give of it?

Pupil. I must consider. What I now call a star would be a sun. The planets of that system I should see as I now do those belonging to ours: our sun would be a star; and the earth, with all the other planets, would be invisible.

Tutor. Very well, Sir. Can you then find it difficult to conceive that all the stars are as far from each other in unbounded space as our sun is from the nearest star?

Pupil. It is hard to conceive: but when I consider that wherever I am, every remote object appears at an equal distance from me, the difficulty vanishes.

Tutor. That you might form some idea of the immense distance of the fixed stars, you must recollect, I mentioned the time a cannon-ball would be in reaching the nearest of them.

Pupil. I do, Sir. More than 1,868,000 years.

Tutor. You have an excellent memory. I suppose then you know the distance of the earth from the sun?

Pupil. Yes, Sir. I wrote it down; and, it made so strong an impression on my memory, that I believe I shall never forget it.—95 millions of miles.

Tutor. Now, suppose the earth to be in that part of its orbit which is nearest to the star, it would be 95 millions of miles nearer to it than the sun is.

Pupil. Certainly.

Tutor. And, in the opposite side of its orbit, as much farther from the star.

Pupil. Without doubt.

Tutor. Then you find that the earth is 190 millions of miles nearer to the star at one time of the year than it is at another; and yet the magnitude of the star does not appear the least altered, nor is its distance affected by it.

Pupil. A proof of its amazing distance.—I was going to ask a silly question.

Tutor. What is it? perhaps not so simple as you may imagine.

Pupil. Whether the most conspicuous stars are not supposed to be the nearest to us?

Tutor. Undoubtedly.—And are called stars of the first magnitude; the next in splendor, stars of the second magnitude; and so on to the sixth magnitude; and those beyond, which are not visible to the naked eye, are called telescopic stars.

Pupil. The distance of the telescopic stars must be great indeed, beyond all conception.

Tutor. You judge rightly; and their numbers are beyond all computation. Doctor Herschell says, he has not a doubt but that the broad circle in the heavens, called the Milky Way, is a most extensive stratum of stars, he having discovered in it many thousands. Besides, some stars appear to him double, others treble, &c. not that they are really so, but are stars at different distances from us, which appear nearly in a right line.

“As in the milky-way a shining white
“O’erflows the heav’ns with one continued light,
“That not a single star can shew his rays,
“Whilst jointly all promote the common blaze.”

Pupil. I have heard of numbering the stars; but that, I find, is impossible.

Tutor. If you mean that immense host of stars I have been describing, it is impossible; but, though in a clear winter’s night, without moonshine, they seem to be innumerable, which is owing to their strong sparkling, and our looking at them in a confused manner; yet when the whole firmament is divided as it has been done by the ancients, the number that can be seen at a time, by the naked eye, is not above a thousand.

Pupil. Pray, Sir, how did the ancients divide the firmament?

Tutor. I would willingly answer your question; but, as I find I shall not have time to give you that information I wish, I shall postpone it till I see you to-morrow evening.


                                                                                                                                                                                                                                                                                                           

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