Part 2

TUTOR. No. But we know that the sun shines with his own light on all the planets belonging to our system; and from what I have told you, have the greatest reason to believe that the stars shine with their own light: we therefore from analogy conclude, that they are so many suns conveying light and heat to other worlds[11].

[Footnote 11: Dr. Herschell thinks it probable that the sun and fixed stars may be inhabited.]

PUPIL. Are there then other worlds besides this we live in?

TUTOR. Consider.—Has not the earth we inhabit a moon to enlighten it?

PUPIL. Yes, Sir.

TUTOR. And have I not told you that Jupiter, Saturn, and Georgian, have also moons?

PUPIL. This I well remember.

TUTOR. For what purpose then do you suppose those orbs were designed?

PUPIL. Indeed, I cannot tell.

TUTOR. You surely cannot imagine that they were intended for our use, since we knew nothing of them till after the invention of telescopes.

PUPIL. That is what I think no one can suppose.

TUTOR. And do not all the planets enjoy the benefit of the sun in common with us?

PUPIL. Undoubtedly.

TUTOR. Well, then; of what use would the light and heat be which is conveyed to them from the sun; or the light which they receive from their moons if there are no inhabitants?

PUPIL. I know of none.

TUTOR. Can you then have any doubt about their being inhabited?

PUPIL. No, Sir.—But you say that the stars are suns, each of which is the center of a system of planets or worlds.

TUTOR. If you are satisfied that the planets belonging to our system are inhabited, and that the fixed stars are suns, the centers of other systems, what reasonable objection can you have to all the planets in the universe being so?

PUPIL. It is what I cannot comprehend.

TUTOR. It may be so.—But is not the same Almighty Power, who does nothing in vain, as capable of making ten thousand worlds if he pleased, as well as one?

PUPIL. I will not presume to dispute his power; but are we not told that all mankind descended from Adam?

TUTOR. Yes; Moses wrote concerning this earth, he has not made us acquainted with the inhabitants of the other planets: for aught we know they might descend from other Adams.—To-morrow evening, I hope to see you again.

DIALOGUE III.

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.

DIALOGUE IV.

TUTOR.

The ancients, in reducing astronomy to a science, combined the fixed stars into constellations, allowing several stars to make one constellation: and, for the better distinguishing and observing them, they reduced the constellations to the forms of animals, or to the images of some known things, by which means they were enabled to signify to others any particular star they meant to notice. Job mentions two of the constellations, namely, Orion and Pleiades, which shews the study of astronomy to be very ancient.

PUPIL. Pray, Sir, how may I know them?

TUTOR. By studying the use of the cælestial globe, on which they are drawn.

PUPIL. Will you be kind enough to instruct me, Sir?

TUTOR. At some future time I probably may: at present you are not prepared for it.

PUPIL. I am satisfied.—Have you any thing more to remark of the constellations, Sir?

TUTOR. Yes. The situation of the planets, as they are continually changing their places, could not be pointed out without first dividing the stars into constellations: hence, necessity was the mother of invention.

PUPIL. And I think a very ingenious one.—If I may be allowed a comparison, I will suppose the different kingdoms of the world on my dissected map, to represent so many constellations; then, if I hear of London, I know it is in England; if of Paris, in France; of Lisbon, in Portugal; and so on. These I would compare with stars of the first magnitude, being the chief cities of their respective kingdoms; inferior cities, stars of the second magnitude; principal towns of the third, &c.

TUTOR. A very apt comparison indeed. Now if you hear of a traveller setting off from London to Dover, thence to Calais, Paris, Bern, and so on to Rome, you know that he must go through part of England, Flanders, France, Switzerland, and Italy, passing many towns and villages on his way.

PUPIL. That is very evident.

TUTOR. Very well, then; in like manner would the planets, if seen from the sun, be traced from star to star, from constellation to constellation, through their whole periods.

PUPIL. It is not possible to view them from the sun, surely, is it?

TUTOR. No, certainly.

PUPIL. Why then do you say if seen from the sun?

TUTOR. Because it is there only their motions can appear uniform; as seen from the earth they apparently move very irregularly.—Suppose you were in the center of a circular course; and, whilst a horse was going round, you kept your eye on him: cannot you conceive that you should see him run round the course in a regular manner, moving the whole time the same way?

PUPIL. It is not at all difficult to conceive.

TUTOR. Again. Imagine yourself placed at a considerable distance on the outside of the course, where you could see the horse the whole time he was going round, would he appear to move as uniformly as before?

PUPIL. Certainly not: on the opposite side of the course his motion would be the same as when I stood in the center of it; when he was approaching me, I should scarcely see him move; in that part of the course next to me he would move in a direction contrary to what he did at first; and again when going from me, his motion would be scarcely visible.

TUTOR. This I think will give you a tolerable idea of the irregular motion of the inferior planets, as seen from the earth. When farthest from us their motion is said to be direct; when nearest to us retrograde, because they appear to be moving back again; and, when approaching, or going from us, we say they are stationary; because, if then observed in a line with any particular star, they will continue so for a considerable time: now these appearances could not happen if they moved round the earth.

PUPIL. Nothing can be plainer: for if the earth were in the center we should always see them move the same way.

TUTOR. When the planet is nearest to us, that is in a line between us and the sun, we say it is in its inferior conjunction; when farthest from us, and the sun is between us and the planet, in its superior conjunction. But the superior planets have alternately a conjunction and an opposition.

PUPIL. A conjunction, I suppose, when the sun is between the earth and the planet, and an opposition when the earth is between the sun and the planet; that is, when the planet is nearest to us, and appears to be opposite to the sun?

TUTOR. You are right.—Therefore, when in conjunction it rises and sets, nearly with the sun; but in opposition, it rises nearly when the sun sets, and sets when he rises.

PUPIL. Why do you say nearly, Sir?

TUTOR. Because it cannot be exactly, but when the sun, earth, and planet are in a _right_ line, which seldom happens.

PUPIL. How do you account for this, Sir?

TUTOR. At present I fear you will not be able to comprehend what I wish to explain, as I must use a term you are unacquainted with. The reason is, that the planets are very seldom in or near their nodes at their conjunctions or oppositions.

PUPIL. I do not indeed understand what you mean by the word _nodes_.

TUTOR. It will be explained to you in due time, and I shall conclude this evening with a few more remarks relative to the appearance of the planets.

PUPIL. Any thing you please, Sir.

TUTOR. You know that the planets, being opaque bodies, receive their light from the sun; and that only that part which is turned to the sun can be enlightened by him, whilst the opposite side must remain in darkness.

PUPIL. This is self-evident: if I hold my ball to the candle it will have the same effect.

TUTOR. Tell me then how you think they will appear as seen from the earth.

PUPIL. If, when you shewed me Venus, she had not appeared perfectly round, I should say that, both before and after her superior conjunction I should see her nearly with a full face; when stationary, only half enlightened, like the moon at first quarter; because, an equal portion of the dark and bright parts will be turned towards us; the bright part will be decreasing till her inferior conjunction, when the dark side will be turned towards us, and consequently invisible; the light will then increase; and, when she is again stationary, she will appear like the moon at last quarter.

TUTOR. When seen through a telescope she has the different appearances you have mentioned; and when I next see you I will shew you that both Venus and Mercury may sometimes be seen when in their inferior conjunctions; the superior planets always appear with nearly a full face.

PUPIL. How are the planets distinguished from each other?

TUTOR. _Mercury_, from his vicinity to the sun, is seldom seen, being lost in the splendor of the solar brightness. When seen, he emits a very bright white light.

_Venus_, known by the names of the morning and evening star, is the brightest, and to appearance, the largest of all the planets; her light is of a white colour, and so considerable, that in a dusky place she projects a sensible shade. She is visible only for three or four hours in the morning or evening, according as she is before or after the sun.

_Mars_ is the least bright of all the planets. He appears of a dusky reddish hue, and much larger at some periods than at others, according as he is nearer to, or farther from us.

_Jupiter_ is distinguished by his peculiar magnitude and light. To the naked eye he appears almost as large as Venus, but not altogether so bright.

_Saturn_ shines but with a pale feeble light, less bright than Jupiter, though less ruddy than Mars.

_The Georgium Sidus_ cannot be readily perceived without the assistance of a telescope.

DIALOGUE V.

TUTOR.

Before I proceed to explain what I promised you, it is necessary you should be informed that the earth as seen from the sun, in its periodical revolutions, will describe a circle among the stars which astronomers call the _ecliptic_, and sometimes _the sun’s annual path_, because the sun, as seen from the earth, always appears in that line.

PUPIL. Do not all the planets move in the ecliptic?

TUTOR. No.—On account of the obliquity of their orbits, they are, in every revolution, one half of their periods above the ecliptic, and the other half below it.

PUPIL. I think I comprehend your meaning; but shall be obliged to you, Sir, if you can make it clearer to me.

TUTOR. I have here a little design, (Plate II. Fig. 1.) which will answer our purpose: where S represents the sun; ABCD, the orbit of the earth; and EFGH, the orbit of one of the inferior planets, suppose Venus.

PUPIL. Now I understand it perfectly: the half EHG rises above, and the other half EFG sinks below it, from the points EG, which I perceive are in a line with the orbit of the earth. But pray, Sir, have you any name for that dotted line?

TUTOR. Yes, it is called the _line_ of the nodes; and the points EG the _nodes_ of the planet: the latter is called the ascending node, because, when the planet is in G, it is ascending or rising above the orbit of the earth; or, which is the same thing, above the ecliptic: and when in E, it is descending or sinking below it, whence _it_ is called the descending node. But you must remember that the orbits of all the planets do not cross or intersect the ecliptic in the same points; but that their nodes or intersections are at different parts of it.

PUPIL. How can the orbit of the earth and the ecliptic be the same?

TUTOR. They are very different; but being in the same plane, if the orbit of any planet inclines to one it must incline equally to the other.

PUPIL. You will, I fear, Sir, think me very stupid: but I must beg of you to inform me what you mean by a plane?

TUTOR. Any flat surface is a plane. You may therefore suppose the edge of a round tea-table to represent the ecliptic, and a circle within it, drawn from the center of the table, the orbit of the earth: will they not be both in the same plane?

PUPIL. Certainly.

TUTOR. You must not imagine, when I am speaking to you of the plane of the ecliptic, or plane of the earth’s orbit, that it is a visible flat surface, or, in speaking of the orbits of the planets, I mean solid rings.—No. The planets perform their revolutions with the utmost regularity, in unbounded space; and, like a bird thro’ the air, leave no track behind them.

PUPIL. How then are they retained in their orbits?

TUTOR. The question, I confess, is natural, and is what I expected; but I must of necessity postpone it to another opportunity; and shall now fulfil the promise I made of shewing you in what manner the inferior planets may be seen when in their inferior conjunctions. Cast your eye again on the little design I gave you, and consider, if Venus were in her ascending node at G, when the earth is at _b_; or, in her descending node, at E, when the earth is at _a_, what the effect would be.

PUPIL. She would be in a line with the sun.

TUTOR. And, on the sun’s disc, she would appear a dark round spot, passing over it. These appearances, which are called transits, happen very seldom: because she is very seldom in or near her nodes at her inferior conjunctions. There was one in June 1761, one in June 1769; and the next will be in the year 1874. And as Mercury is seen in the same manner, it is a proof that their orbits must be within that of the earth.

PUPIL. I thank you, Sir, and shall be obliged to you to inform me how many constellations the earth pastes over in every revolution?

TUTOR. Twelve, which correspond with the months of the year, and are called the twelve signs of the zodiac.

PUPIL. What is the zodiac?

TUTOR. That part of the heavens which contains the twelve signs, and which you may conceive to be a zone or belt extending eight degrees on each side the ecliptic, in which the planets constantly revolve: so that no planet is ever seen more than eight degrees either north or south, that is above or below the ecliptic.

PUPIL. What am I to understand by a degree?

TUTOR. All circles, whether great or small, are supposed to be divided into 360 equal parts, called degrees, and each degree into 60 equal parts, called minutes: therefore, if I speak of a circle in the heavens, the circumference of the earth, or any other circle, by a degree is meant the 360th part of that circle; and a minute the 60th part of a degree.

PUPIL. What are the names of the twelve signs?

TUTOR. The first is called Aries, which you know signifies a Ram; Taurus, the Bull; Gemini, the Twins; Cancer, the Crab; Leo, the Lion; Virgo, the Virgin; Libra, the Balance; Scorpio, the Scorpion; Sagittarius, the Archer; Capricorn, the Goat; Aquarius, the Water-bearer; and Pisces, the Fishes.

PUPIL. Do you wish me to commit these to memory, Sir?

TUTOR. It is very requisite; but as I know you are fond of verse, you shall hear what Doctor Watts says—

The Ram, the Bull, the heav’nly Twins, And next the Crab the Lion shines, The Virgin, and the Scales: The Scorpion, Archer, and Sea-goat, The Man that holds the Water-pot, And Fish with glitt’ring tails.

PUPIL. I like it much, as it will assist my memory.

TUTOR. As the twelve signs correspond with the months of the year, the earth must pass over nearly one degree every day, one sign every month, and in twelve months complete a whole circle, or 360 degrees; therefore every sign must contain 30 degrees, because 30 multiplied by 12 is equal to 360.

PUPIL. It must be so.

TUTOR. You must remember, that when the earth is in any sign, as seen from the sun, the sun will be in the opposite sign, as seen from the earth: for instance, if the earth be in Aries, the sun will be in Libra; if in Taurus, the sun will be in Scorpio, &c. therefore, as by the earth’s annual motion, the sun _appears_ to move, we always speak of the sun’s, not the earth’s place, in the ecliptic.—You do not seem to understand me?

PUPIL. Not perfectly, Sir.

TUTOR. Take this orange, and put it in the middle of the round table before us, and place an apple on the opposite side next the window: the orange may represent the sun, the apple the earth, and the window the sign Aries. Now go round the table to the apple; look at the orange, and tell me to what part of the room the eye will be directed.

PUPIL. To the part opposite to the window, Sir.

TUTOR. If then you suppose the door, which is opposite to the window, to be the sign Libra, the sun will be in Libra when the earth is in Aries—will it not?

PUPIL. It is very plain.

TUTOR. I shall now give you a table of the signs, their characters, the corresponding months, and the days of the month the sun enters each sign, by means of which, if you reckon a degree for a day, you may find the sun’s place, nearly, for any day in the year.

PUPIL. This will give me much pleasure, and I shall be happy to have it.

THE TABLE.

NORTHERN SIGNS.