LECTURE IV.

Of the Astronomy of the Earth, as the apparent centre of the starry sphere.

We now come to the consideration of what may be termed the Astronomy of the Earth.

When, during a cloudless night, we raise our eyes above the Earth, we seem to be covered by a canopy studded with luminous bodies, all of which appear to be equally distant from us. But as some of these bodies move, and pass over others which seem never to alter their relative stations, it is certain that all are not equally distant.

Nevertheless, all parts of the heavens seem to be at the same distance from us; and as we know that the stars do not merely form a canopy over our heads, but that the same appearance is observed from every part of our globe; so the situation of the Earth appears to be in the centre of an immense hollow sphere: which, from the distribution of the stars over its interior, is called the sphere of the fixed stars.

The ancients, having this appearance perpetually presented to them, and viewing the apparent periodical returns of the Sun and the planetary bodies, it is not to be wondered, that in the infancy of science, and long before the invention of telescopes, they should have considered the Earth to be in reality, what we now

know it is only in appearance,—the centre of the universe; and that they should have believed that the Sun and all the planets move around it. And though we are assured of the fallacy of this appearance and this doctrine, our seeming to be the centre, compels us to assent to it, in some sort, from motives of convenience. Indeed a large and very interesting part of our knowledge of astronomy is derived from it. It is to this appearance that all astronomical references are made; that we are indebted for our knowledge of the true measurement of the Earth, and the distances of one place from another on its surface. Therefore the view that we are about to take may be said to be opticul. That which we have already taken, but of which only the results have been noticed, is founded on mathematical demonstration.

The optical or apparent view, involves many seeming contradictions, not to say absurdities. The Earth seems fixed and immovable as a centre, and when its bulk is separately considered, seems enormously large; but when considered in reference to the whole system, it is a mere point, perpetually in motion. All the luminous bodies of the great starry sphere seem to be at the same distance from us; we know that some of them are nearer than others, and the astronomer tells us that even the fixed stars differ greatly in respect of distance from us. Which of us standing on the sea-shore can correctly guess how far a distant vessel is from us? How much more difficult must it be to imagine, how impossible is it for us to ascertain, the true distances and sizes of bodies so prodigiously distant from us as some of the fixed stars are, when the following fact is considered. The mean distance of the earth from the sun is 95 millions

of miles; the diameter of the earth's orbit is double this, or 190 millions of miles. Six months hence, therefore, we shall be in a straight line 190 millions of miles from where we now are. Yet if we were now to take the precise distance of any two of the fixed stars from each other by means of the best and most accurate instruments, and were again to take it six months hence, our change of place would scarcely show any perceptible difference in the relative situations of the two fixed stars, even though there is reason to believe that some of them are at a distance from us immensely greater than others. The utmost difference that our change of place has been found to make amounts to one second of a degree, or, a 1,296,000th part of the great circle of the starry sphere; so unimportant are 190 millions of miles, when compared with the distance of the fixed stars from the earth.

If therefore we were to depend on our sight for determining the distances or motions of the heavenly bodies, we should delude ourselves: it is on the astronomer, the mathematician we must depend. Nevertheless, we must accommodate ourselves to our situation; which, being apparently, though not really, in the centre of the universe, is therefore most unfavorable to a correct notion of the movements of the celestial bodies; and hence we are compelled to mingle up the true with the apparent. We must follow the astronomer, and in discoursing of astronomy as it belongs to us the inhabitants of the earth, we must talk of many ideal objects as though they really existed; of circles in the heavens and circles on the earth, crossing each other in different directions, and divisible into minute parts. For, by this means alone shall we be able to fix relative situations to

remote objects, so as to define their positions and explain their phenomena. The astronomy of the Earth, therefore, is in some degree hypothetical, but it is justified by the observations of all ages, and by the calculations of the mathematician, from every point of the universe from which they can be made. It is to the results that we must look; and of the beauty and harmony of these, we have already spoken at large.

An inhabitant of the Earth, who knows nothing of the real motions of the planets, finding himself at rest, with the sun and stars in appearance revolving around him, would not at once admit the truth of the astronomer's discoveries. What, he would say, am I to believe that the earth moves around the sun, and at the rate of 68,000 miles an hour?-the thing is impossible. If the earth moved, I should feel the motion; and if it were true, as I am told, that it moves from west to east, I should feel a perpetual and strong wind in the contrary direction, from east to west; besides, if the progress of the earth were thus rapid, the ball I throw up perpendicularly from my hand, would not return to it, but would be left far behind. To the first of these objections it may be replied, that we are not sensible of the motion of a boat on smooth water; which produces the same delusive effect, in regard to objects on land, that the daily motion of the earth round its axis, does, in regard to the stars, the houses and trees seem to move and not the boat. As to the probability of our feeling a continual east wind if we moved with rapidity from west to east, it may be replied that the atmosphere of the earth, being attracted by it, and belonging to it, partakes of its motion. And as to the ball being left behind, instead of returning to the hand, if we moved so

rapidly, it may be answered by asserting, without fear of contradiction, that the ball partakes of the motion of the hand which throws it up, and of the person throwing it up. If a person in rapid motion, say on the top of a mail coach, were to throw up an orange from his hand, it would partake of the motion of the coach on which he sits, and instead of being left behind, and eventually falling on the spot from whence it arose, it would form a figure in the air, somewhat resembling the letter V turned upside down. These are some of the most important objections against the rapid motion of the earth, amongst which there is not one that may not readily be refuted.

But we may do more than this, we may shew the absurdity of the contrary doctrine, which supposes the earth to be at rest, and the sun and stars to move round it, and our argument may be taken from the inconceivable velocity with which the sun and stars must move, if they really performed that revolution.

The most patient and careful observations of astronomers concur in shewing that the distance of the moon from the earth is about 240,000 miles, and that if the earth were at rest, the moon would daily pass round it in a circle of about 1,500,000 miles, which determines that her velocity would in that case be 63,000 miles an hour, or 1,050 miles a minute. This is very considerable; yet is but small, when compared with the rate at which the sun and fixed stars must move, if they really moved round the earth. The sun is known to be at a much greater distance from us than the moon is; and the most minute observations shew that he cannot be at a less distance from the earth, than 400 times the distance of the moon. If therefore, the sun moved round