GENERAL APPEARANCE OF THE HEAVENS.

LET us suppose a man to be totally ignorant of Astronomy, and to turn his attention from events occurring upon earth, to those which are presented by the heavens. He sees a brilliant and glowing body, the Sun, rise in the east, at from four to eight o'clock in the morning, (according to the season of the year.) This body gradually attains a considerable altitude in the heavens, and continues to rise until noon;-it then gradually descends, and in its descent, bends towards the west, where it sinks, (apparently below the earth,) some time between four and eight o'clock in the afternoon. The spectator now loses sight of the glorious orb, which does not again become visible for several hours; and when it does again appear, it is not at the point where it escaped from view, but at the opposite side of the heavens, namely, at the point where it first appeared on the preceding day. On watching the progress of the sun, he finds that the path, before noticed, is again travelled over by the luminary, which becomes lost to his view, as before, in the west. A third and a fourth day the same phenomenon occurs; and the question naturally suggests itself to the observer's mind, Is it the same cheering and dazzling visiter which I see day after day, or are they different bodies, resembling one another, and following one after another?" Were he to confine his attention to the occur

86

rences of only a few days, he would, perhaps, think the latter supposition to be correct, viz., that they were different bodies which thus appeared to him day after day. But if he were to continue his observations for weeks, months, years, or the greater part of his life, and to find that the daily appearance of such a body still continued, he could hardly fail to conclude that it was the same body which thus so frequently attracted his attention and admiration. "But how?" (he might say,) "do I not see this ruddy disc of light dip into the ground in the west, and appear to me again in the east on the following morning: what becomes of it in the meanwhile?" To answer this question, or to obtain the means of answering it, he would, perhaps, trace more particularly the path which the sun followed from morning to evening, which he would find to be a semicircle, or a curve not differing much from it. If, likewise, he were to take note of the time occupied in these occurrences, he would soon find the time that the sun is above the surface of the earth to be, on an average, equal to the time that it is invisible.

From these two facts, it would be by no means unreasonable to arrive at the conclusion, that the sun moves in a circle; and that, while he is invisible to us, he is passing round the earth, in a direction from west to east, at which latter point he arrives at the moment that the observer sees him rise in the morning.

Here a difficulty arises, which it may well be supposed, would perplex the uninitiated observer. What can be meant by going round the earth, if the earth be, as it seems to our eyes, a flat and extended surface of ground, or water, which seems to touch the sky at the farthest points which the eye can reach :-how then can the sun pass round this? There can scarcely be a more reasonable question, so long as we judge merely by what meets the eye; but, if we extend our observations to certain appearances on the ocean, we begin to see proof that the surface of the water is not quite flat. On land, we have not the means of making a similar observation, because of the intervention of mountains and valleys. The proof that the surface of water is not quite flat, is derived from the following circumstance. If we are at sea, on a clear day, and watch the approach of a vessel from a distance, we find that the masts are visible sooner than the hull: we first see the top of the masts: then the whole height of the masts gradually comes into view; and finally, we see the hull. These different appearances of the ship, as it approaches towards the observer, are represented in the following figure. This could not occur if the earth were perfectly flat, because then the hull and masts would come into view at the same time.

It matters not what part of the world be chosen as the place of experiment; the result is almost exactly the same every where, and the amount of the bending of the surface of the ocean may be illustrated thus:-If we had a piece of string four miles long, and were to apply both ends to the surface of still water, and could possibly draw the string into a perfectly straight line, the middle of the string would be about sixteen inches below the surface of the water; thus showing that the water is not quite flat. Thus might be supposed to arise the first conjecture that the earth is round, like a ball; a fact which was clearly proved by Captain Cook, who was the first to perform a voyage completely round the earth, about the year 1769. He left a given spot, and arrived again at that spot by an opposite course. There is an abundance of proof, derived from other sources, that the earth is a globe; but those will open upon us more plainly, as we advance.

We have hitherto endeavoured to trace what would probably be the feelings and opinions of one who, without previous instruction in Astronomy, should note the appearances presented by the sun. But that golden orb is not seen to be a solitary inhabitant of the sky: another luminous body about as large in apparent size as the sun, would soon attract the notice of the gazer. He would see it rise in the east, soar aloft, and then sink in the west. After the lapse of a few hours it would again appear in the east, attain a height from which

.... With a boundless tide

Of silver radiance, trembling round the world,

it would again descend, to sink as before in the west. The train of reasoning which would lead the observer to conclude that the sun revolved round the earth, would lead him to a similar conclusion regarding the moon. But here the re semblance terminates. The sun always presents a perfect circle to the eye of the observer day after day, and month after month; but a few evenings suffice to show that such is not the case with the moon: at one time a crescent only is seen, whose hollow side is towards the left of the observer; at another time the hollow side is towards the right: the crescent sometimes enlarges to a perfect circle; and at others it contracts from a circle to a crescent. If, then, the moon be a ball or globular body, shedding light upon the earth, there is great difficulty in conceiving what can occasion the change in its apparent shape; but, if we were to consider the moon to be an opaque or non-luminous body, we should find the means of explaining the change in its apparent figure, by supposing that the sun shines upon the moon, and that it is only by the reflection of the sun's light from the moon that the moon becomes visible to us. If we place a large round ball on a table, in a place where the sun is shining, we shall see the ball more or less illuminated, according to our position with regard to the ball and the sun: in one position, the ball will appear to be equally divided by a boundary line, into two semicircles, one illuminated and the other in the shade: in another position, the illuminated portion will be only a crescent, all the rest being shaded: from another point again it will all appear illuminated, except a small crescent of shade. If the observer watch the relative positions of the sun and moon, he will see that the shape of the bright part of the moon depends on its relative position in respect of the sun; just as the bright portion of the illuminated ball depended on the position in which he viewed it.

We should in this way find the means of accounting for the change in the shape of the moon. We shall, by and bye, show that these suppositions are really true. But another point would attract the notice of the observer, independently of the change in apparent form. If the two brilliant bodies, the sun and moon, were observed to be near each other on a certain day when the latter appeared as a thin crescent to the left of the sun, they would be seen the next day further removed, and on the following day, at a still greater distance from each other: if, therefore, the motion of the moon round the earth be admitted, it is necessary to suppose that motion to be slower than the sun's motion round the earth.

But in addition to the glowing light of the sun and moon, a glittering assemblage of smaller bodies meets the eye of the observer: a crowd of little spangles adorns the sky when the sun has left it, and softens the dreary darkness which results from his absence. These stars are seen to resemble the sun and moon in the circumstance of rising in or near the east, attaining a certain altitude, and setting in the west; from which circumstance the observer infers

that the stars, like the sun and moon, revolve round the earth. But, by a careful attention to different stars, he would find that the same remark does not apply to all. Those which rise exactly in the east, set exactly in the west; those which rise to the south of east, set south of west; and many which rise somewhat north of east, set somewhat north of west; but in looking northward he sees stars which appear neither to rise nor to set, but which perform a complete circle round a particular point of the heavens. For example, there are seven stars, which, to most persons who pay any attention to the appearance of the heavens, are known under the name of the Great Bear. These stars never rise or set to an inhabitant of London. If they become invisible, it is either because clouds obscure them, or the superior brilliancy of the sun drowns their comparatively feeble light. The same may be said of five conspicuous stars known by the name of Cassiopeia's Chair.

If we watch the progress of these stars, and others in their vicinity, we shall find that they describe a circle round a star called the Pole-star. The position of this star we shall find to be nearly this: if we suppose the distance from the north point of our horizon (which is the line in which the surface of the earth appears to touch the sky) to the zenith (which is the point immediately over our heads) to be divided into five equal parts, then at about the height of three of those parts from the earth, will be seen the Pole

star.

All this would seem to show to an observer, that the Pole-star is the end of an axis round which all the stars revolve, and that the reason why we cannot see the whole circular path of any stars except of those in the vicinity of the Pole-star, is, that they pass round under the earth during a part of their journey, and are therefore concealed from our view. Those stars which are further removed from the Pole-star describe a larger circle than those which are nearer, while the Pole-star is almost stationary. This is exactly what takes place when we see a wheel turn round; the axle remains in one spot, but any particular point on the outer edge of the wheel describes a larger circle than that which is described by any point between the axle and the circumference.

If we consider the Pole-star to form one end of the axle or axis round which the stars revolve, then it is obvious that there must be an opposite end of the same axis, in the contrary direction. If then we turn towards the south, with the expectation of seeing such a point, we shall find that it will not be realized; for none of the stars in that quarter are seen to describe circles; for they all rise and set. Still, however, the semicircles or portions of semicircles, commonly called arcs, which they describe, appear to have a common centre, which is some distance below the horizon; and this centre may be considered as the southern end of the axis before spoken of.

If we suppose the observer to have arrived at this amount of information respecting the stars, he will be prepared to notice the uniformity of the positions of the stars with respect to one another. The Great Bear, for instance, whether it be under or over the Pole-star, or at the right or the left of it, will always have its seven principal stars at the same relative distances from each other. The changes which take place in the distance of the sun from the moon, would lead to the opinion that these revolve round the earth in unequal times; and by similar reasoning, the constant maintenance of the same distance between any two stars, would seem to imply that the stars revolve in equal times.

The seeming myriads of stars which present themselves to the notice of the observer all appear to follow this rule, of remaining at the same relative distances from one another, with very few exceptions. These exceptions, are, indeed, so few, that a constant watching of the same part of the heavens for a considerable period, would be necessary to determine that a star had actually changed its place relatively to other stars. There have, however, been discovered at different times ten stars, more or less brilliant, which change their relative distances from one another, and from other stars. These we know by the name of Planets; and by a careful attention to their movements, it is seen that each one travels in a curved path among the other stars, and returns again nearly to the point from whence it set out.

At intervals, again, star-like bodies of another order present themselves, whose progress among the other stars is more rapid than that of the planets; and which at a period more or less brief, vanish altogether from the view

of the observer, and do not return till after a long absence.
To such bodies we give the name of Comets.
Such then are the equally sublime and facinating ap
pearances which present themselves to the eye of an
observer, when that eye is directed towards the heavens.
That the invigorating and fructifying warmth and light
shed by the sun,-the serene, quiet light of the moon,-
and the diamond-like glittering of the stars,—should invite
men to a study of the laws which, under their Divine
Creator, govern the motions of such exquisite globes of
light, is what we are not only prepared to expect, but fancy
that we should feel disappointment in finding it otherwise.
Man is not, by nature, the cold heartless being who can let
such beauties pass unheeded; and if he approach the study
with the humility which true self-knowledge is calculated
to engender, he becomes more and more able to appreciate
the surpassing grandeur and power of the great Being who
made and who rules all. Well, indeed, may we direct our
attention to the phenomena which we have briefly de-
scribed, and to which Milton so exquisitely alludes in the
following lines:-
First in his East the glorious lamp was seen,
Regent of day, and all th' horizon round
Invested with bright rays, jocund to run

His longitude through Heaven's high road; the gray
Dawn, and the Pleiades* before him danced,
Shedding sweet influence: less bright the moon,
But opposite in levell'd West was set

His mirror, with full face borrowing her light
From him; for other light she needed none
In that aspect, and still that distance keeps
Till night; then in the East her turn she shines,
Revolved on Heaven's great axle, and her reign
With thousand lesser lights dividual holds,
With thousand thousand stars, that then appear'd
Spangling the hemisphere: then first adorn'd
With their bright luminaries that set and rose,
Glad evening and glad morn crown'd the fourth day.
Par. Lost, b. vii.

THEORIES TO EXPLAIN THE MOTIONS OF THE HEAVENLY

BODIES.

IN very early ages, before Europe occupied a page in the history of nations, the phenomena of the heavens were studied with great attention by several nations of the East. The Chaldeans, the Indians, the Chinese, and the Egyptians, have all left evidences of the industry and ingenuity with which their observations were conducted. They constructed observatories,-invented instruments for observing and measuring with accuracy,-separated the stars into different groups, called Constellations, for the facility of finding any particular star,-gave particular names to most of the moving stars or planets, and noted the period which each took to move through its apparent path in the heavens; and, in many other ways, the ancients helped to lay the foundation of that mass of astronomical knowledge which the men of later ages have brought to more maturity.

Various opinions were formed respecting the motions of the sun, moon, and stars of all kinds, both with reference to one another, and also to the earth; but the first theory which had attained a name and an importance in the early ages of the world, was that of Ptolemy, a distinguished Egyptian astronomer, who lived about one hundred and thirty years after the birth of Christ. He conceived that the various bodies which had been distinguished by the appellation of "the heavenly host," were disposed in the order represented in the annexed diagram.

He supposed, according to the popular opinion, that the Earth was fixed as the centre of the universe, and that the Sun, Moon, Planets, and Stars, revolved round it in the following order; namely-the Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn; the Moon being the nearest, and so on; exterior to all of which, he supposed that a great concave sphere in which all the stars were fixed, kept on revolving round the earth. From the early history of Astronomy, we learn that before the time of Ptolemy, it had been conjectured by some, that the earth passed round the sun, and not the sun round the earth; but the difficulty of believing a statement so contrary to appearances and to the evidence of one's physical senses, led to the rejection of this opinion; and although it was afterwards found to be

Seven small stars clustered together in the constellation Taurus. These stars rise with the sun about the time of Spring, and our poet, in this passage, intimates the old and common opinion that the Creation took place in the Spring.

correct, yet nearly two thousand years elapsed before such a theory was generally admitted by philosophers.

After Ptolemy had promulgated the theory which bears his name, he found that there were certain difficulties which followed from the adoption of it. He conceived that the seven bodies mentioned before which revolved round the earth, moved in the same general direction from West to East. But on watching the progress of some of the planets, he found that they did not appear to travel uniformly round the earth, but seemed to have, at certain times, a retrograde or opposite motion, with reference to the other planets; while at other times they seemed to be stationary. To account for this, he was obliged to suppose that those planets did not revolve in a perfect circle round the earth, but that they described a peculiar path called au epicy cloid, of the nature of which some idea may be formed from the following illustration:-if we had a large coach wheel, and by any contrivance could make a smaller wheel roll round the outside of it, at its circumference, then, any particular point on the small wheel would describe that curve which is called an epicycloid: the axle of the small wheel would describe a perfect circle, but it is easy to see that any point on the edge would not describe a circle. Ptolemy was obliged to multiply these epicycloids to a most perplexing extent, in order to account for the various appearances of the planets. It is to this circumstance that Milton alluded, when he spoke of the shifts and difficulties which beset the progress of those who build their opinions on a wrong foundation :

When they come to model Heaven

And calculate the stars, how they will wield
The mighty frame; how build, unbuild, contrive,
To save appearances; how gird the sphere
With centric and eccentric scribbled o'er,

Cycle and epicycle, orb in orb.Par. Lost, b. viii. There is nothing which more beautifully shows the power and force of truth, than the embarrassments which retard the progress of those who do not take truth for their guide. In religion, in morals, in science, he whose steps are guided by the light of truth, can arrive, by a short and pleasant path, at results which others can scarcely obtain by a complex and wearying track. Thus Ptolemy was forced to assume the existence of much unwieldly machinery in the scheme of the heavens, in order to account for those motions which are most simple and beautiful; this was because he placed (in his own imagination,) the earth in the midst of the heavenly bodies, all of which were made to revolve round it.

Great as were the difficulties which attended the adoption of the Ptolemaic system, it yet retained its ground, with some slight modifications, until the time of Nicholas

Copernicus, an eminent astronomer, who was born at Thorn, in Polish Prussia, in 1473. This distinguished individual perceived the unreasonable results which follow from the theory of Ptolemy. That all the planets, the sun, the moon, and all the stars and comets, should revolve round the earth, seemed to him much more unnatural and complex than that the earth should revolve on its own axis, and move in an orbit round the sun. All the appearances of the heavenly bodies can be explained with much greater ease and simplicity by the latter supposition than by the former; and Copernicus was thus led, after the study of forty years, to the adoption of a theory which had been advocated by Pythagoras and Thales of Greece, five or six hundred years before the time of Ptolemy.

The frontispiece to this number is a representation of the solar system, according to the theory of Copernicus. In this system, the sun, the glorious source of light and heat to us, is placed in the centre. Round him the planets revolve in the following order :-Mercury, Venus, Earth, Mars, Jupiter, and Saturn. Since the time of Copernicus, five more planets have been discovered, namely, Uranus, Pallas, Vesta, Juno, and Ceres. In order to account for the phenomena presented by the moon, Copernicus assumed, (what has since been confirmed,) that the moon has a twofold motion,-round the earth and round the sun: a small circle, therefore, surrounds the earth, which circle represents the path of the moon round the earth, while the earth and the accompanying moon together revolve round the sun. The frontispiece of course represents the Copernican system with the addition of the five planets since discovered. The circles represent the orbits of the several planets; that is, the paths in which they travel in their progress round the sun. The distances of these circles from the centre at which the sun is placed, could not conveniently be in the same proportion one to another, as the real distance of the planets is respectively from the sun, because the innermost orbits would be too small to be conveniently seen. The real proportions, however, which exist between the distances of the planets from the sun, admit of being easily understood. For instance, the earth, (as we shall hereafter explain more fully) is about ninety-five millions of miles from the sun. If now we call that distance 1, the distances of the other ten planets from the sun, will be represented by the following numbers, with sufficient nearness for our present purpose. Mercury Venus Earth Mars. Vesta Juno.

24 21

3 Ceres Pallas Jupiter Saturn Uranus

2

51/

9/

19

could give, would be, "Because it is the nature of things to fall, when unsupported." This vague explanation did not satisfy the sagacious Newton. He set himself to consider the nature of the occurrence which had attracted his notice, and of others similar to it, and he finished by framing that train of opinions which, under the name of the Theory of Gravitation, laid the foundation of all correct knowledge of the motions of the heavenly bodies, and which is now received by all who deserve the name of philosophers.

All intellectual eye, our solar round

First gazing through, he by the blended power
Of gravitation and projection saw

The whole in silent harmony revolve.-THOMSON. We shall here find it convenient to consider the nature of this force of gravitation, and will endeavour to do so in the simplest way possible. Every particle of matter in the creation has a tendency to attract, or draw towards itself, every other particle, however distant it may be. We know not what this attraction is; we can therefore only judge of it by its effects. This attraction is of different degrees of force according to the size or density of the attracting body. A small loadstone or magnet will take up a little key, whereas a larger magnet will take up a larger key: so it is with attraction; a lump of lead weighing one pound only, attracts a distant body with half the force which is exerted by another piece of lead weighing two pounds. Again, the force of this invisible agency is subject to variation with respect to the distance between the attracting and attracted bodies. If a mass of lead be the attracting body, and two equal-sized bodies be at some distance from it, the one which is the nearer of the two will be attracted with greater force than that which is further removed. The diminution of attractive power from increase of distance is much more rapid than from the lessening of the size of the attracting body. For instance, if any mass of matter attract another mass with such force as to make it move through twenty feet in a second of time; then, if the first mass be removed to double its former distance, the strength of the attraction will be so much weakened, that the second body will move only five feet in a second, or with one-fourth of its former rapidity.

These simple principles, when applied to such immense bodies as the planets, produce results of a gigantic kind. It may be asked, "If a pound of lead will attract a piece of cork more strongly at the distance of one than of two feet, how is it that we cannot see the proofs of such a fact in practice?" The reason is, that the earth itself forms such an enormous mass of matter that it completely neutralizes the effects of the comparatively small bodies at its surface. In strict truth, if an apple be suspended from the branch of a tree, it tends to attract the earth upwards to meet it; but the attraction of the earth for the apple is so incalculably more powerful, that the moment the apple is loosened from the tree, the earth draws it down to meet it, and that is what occurs when we say that the apple falls. Suppose that, instead of an apple, a mass of lead weighing a thousand pounds, were for a monient suspended in the air; the lead would tend to attract the earth upwards towards itself with much greater force than the apple had done; but still the earth's attraction would so completely overpower the lead, that it would be drawn down to the earth; while the latter would rise to meet it by a quantity, wholly inappreciable by mortal sense, though not by computation.

But let us now suppose a body placed in the universe, equal in size to the earth, but at some distance from it. Here we perceive that there is no reason why the earth should attract the other body more strongly than the latter attracts the earth; accordingly, if they were both free to move, they would naturally approach towards each other, each one moving through half the distance which separated them. If, therefore, we were to suppose that the universe contained many such bodies, each as large as the earth, each one would attract all the others, with a force varying only as the several distances varied. But if one body were very much larger than any of the others, it would attract each of the others more powerfully than itself could be attracted; and would not, therefore, have to move through so great a distance to meet any of the other bodies, as they would have to move through, in order to meet it. All this admits of being impressed upon the mind with tolerable clearness, so long as we consider the bodies to be in the first instance stationary, and then receiving an impulse. But how can we explain the curved path which each of the heavenly bodies describes in its progress through

space; and why do not all the planets get close together by virtue of the attraction which draws them one to another? These questions we cannot answer without a previous attention to other particulars, which we must shortly explain. If we stand at the top of a high tower, and throw a stone forward to the ground, we shall find that the stone will not proceed on in a straight line, but that it will soon assume a bending path and approach towards the earth. The direction in which this bending occurs, is such as to bring the stone to the earth in a more perpendicular direction than it had when it set out. Now the reason why the stone is thus forced to change the direction in which it first began to move, is, because the earth attracts it and hastens its descent. If we suppose it possible that a hole could be bored through the earth from side to side, we should find the mass of matter would be more accumulated in the direction which passes through the centre of the earth than in any other direction; and as bodies attract other bodies in proportion to their mass of matter, we see reason to believe that a falling stone is attracted more powerfully towards the central direction of the earth, than towards any other part. When, therefore, the stone has left the hand, it gradually tends to a direction perpendicular to the surface of the earth, or it gets into a direction which, if continued, would lead to the centre of the earth.

Now if there were no such force as gravitation, the stone would proceed in the same straight line in which it was first propelled. If it were thrown upward from the surface of the earth, it would continue to travel upwards, without ever again descending to the earth. Here we should remember that there is in nature no such thing as up and down; but that up means simply a departure from the centre of the earth, down an approaching towards that centre.

We all know with what amazing velocity a cannon-ball moves, after it has left the mouth of the cannon; yet it soon begins to decline towards the earth, whether it be originally shot from the cannon in a horizontal or in an upward direction. But still the curvature of its path towards the earth is much less rapid than in the case of the stone, because the latter moved so much more slowly. The mortar in St. James's Park can propel a bomb to a distance of about four miles, before it will fall to the earth; whereas a stone thrown by a very powerful arm would certainly fall to the ground within a few hundred yards. Suppose, now, that it were possible to increase to an indefinite extent the velocity with which a cannon-ball would move; say that it should travel ten or twenty miles before it touched the ground; a greater velocity would carry it one hundred miles before it fell: and we may go on in the same train to any extent we please, always bearin mind that, the quicker the body moves, the greater distance will it travel before it falls to the ground. Now, if we assume such a velocity that the ball would travel twenty-five thousand miles before it fell to the earth, we shall arrive at a very curious result. The diameter of the earth is almost eight thousand miles, which gives, for the circumference, about twenty-five thousand miles. The ball would, therefore, have gone completely round the earth before it fell to the ground. If the velocity were still greater it would not reach the ground at the completion of this circuit, but would go on to describe part of another revolution round the earth. When this condition is once attained, the ball might, by a due increase of the projectile force, continue to revolve for ever about the earth, and we should thus have a cast-iron satellite moving around us.

Let not the reader smile at the absurdity of supposing a cannon-ball to travel twenty-five thousand miles; for it will lead us to important results. If we suppose that the sun were stationary in the universe, and that a body very much smaller than the sun were to be projected with immense velocity in a direction at right angles to a line joining the sun and the other body, then the latter (which would proceed in a straight line if the sun were not present) is drawn by the sun into a curved path, the concave or hollow side of which is always towards the sun. If the velocity with which this body were propelled were below a certain limit, it would move in a spiral which would gradually end at the sun himself, to which, therefore, the body would fall: if the velocity were beyond a certain limit, the body would describe a spiral round the sun, but gradually receding from it, and the body would continue through infinite ages to recede farther and farther from the sun: but if a certain velocity, of a corresponding ratio with the attractive power of the sun, were imparted to this body in the first instance,