Imágenes de páginas
PDF
EPUB

Inentioned. Mercury is nearest to the sun; Venus next, the Earth next, Mars next, Jupiter next, and Saturn is at the greatest distance of all.

Dr, Herschel, the famous German astronomer, lately discovered another planet, to which he gave the name of the Georgium Sidus *.

CHAP. II.

DISTANCES OF THE PLANETS FROM THE SUN.

THE "HE distances of the planets from the sun may

easily be conceived in the following manner. Supposing the distance of the earth from the fun to be divided into ten equal parts, then that of Mercury will be four of these parts; that of Venus seven ; that of Mars fifteen ; that of Jupiter fiftytwo; and that. of Saturn ninety-five. Hence it appears, that the Earth is placed between Mars and Venus, having Mars, Jupiter, and Saturn above her, and Venus and Mercury below her, and for this reason it is, that the three first are called superior, and the two last inferior planets.

* This planet may be seen through a telescope, on the 2d of October, at two o'clock in the morning, in conjunction with that fixed star of the first magnitude, called Regulus, or the Lion's Heart.

. But

But to express the distances of the planets from the sun, in English miles, the distance of Mercury from it is much about 37 millions of miles; of Venus 69 millions of miles; of the Earth 95 millions of miles; of Mars 145 millions of miles; of Jupiter 495 millions of miles; and of Saturn 908 millions of miles,

By these distances, however, are to be understood their mean distances; in order to comprehend which, it must be observed, that the orbit, or path, which a planet describes about the sun, is not a perfect circle, but a figure called an ellipsis; which, though somewhat resembling a circle, is longer than it is broad. Hence the same planet is not always at the same distance from the sun, and the mean distance of it is that which is exactly between its greatest and least distance.

CH A P.

III.

DIFFERENCE BETWEEN A PLANET AND A

FIXED STAR.

THE

HE planets appear at first sight like fixed

stars; but, upon a more accurate view, they may easily be distinguished from them.

1. By their

B2

never

never twinkling, as these last do. 2. By their being feen earliest in the evening, and latest in the morning. And 3. By their changing their position with regard to the fixed stars, and to one another.

Mercury can never be seen by the naked eye, on account of his nearness to the sun, in the splendor of whose beams he is totally absorbed. The only way of observing him is in his passage over the sun, when he appears like a black spot on its surface. Venus is sometimes our evening, and sometimes our morning star. Mars and Saturn may be easily known by their deep red colour. And Jupi.. ter is distinguishable from the fixed stars by the largenefs of his fize, and the brightness of his colour, which is so great, that it will sometimes illuminate a thin cloud in the same manner as the moon.

CH A P. IV.

ON THE MOTION OF THE PLANETS.

ALL the planets, in different stated periods of

time, perform their motion round the sun from west to east, in orbits nearly circular. Mercury performs his revolution in about three years;

Venus in about seven months and a half; the Earth in a yeår; Mars in about two years; Jupiter in twelve ; and Saturn in about thirty years.

If we can form a notion of the inanner in which any one of the planets, suppose our earth, moves round the fun, we can easily conceive the manner in which all the rest do it.

The earth, upon which we live; was long confidered as one large extensive plane. The heavens, above it, in which the fun, moon, and stars, appeared to move daily from east to west, were conceived to be at no great distance from it, and to be only deligned for the use or ornament of our earth. Mankind, however, are now convinced that they live on a globe ; and the spherical figure of the earth may be proved by a variety of arguments : 1. When we are on board a fhip at sea, we may be out of sight of land, when the land is near enough to be visible, if it were not hid from our eye by the convexity of the water. In this case, the tops of hills, cliffs, steeples, towers, &c. first appear to our view, next the buildings, and last of all the shore ; which can proceed from nothing else but the roundness of the earth, whereby the lower objects are longer hid from the fight, than those which are higher.

[blocks in formation]

2. When we stand upon the shore, the highest part of a ship is visible at the greatest distance. If a ship be going from us out to sea, we shall continue to fee the mast, after the hull or body of the ship disappears, and the top of the mast will be feen longest. But if the surface of the fea were quite flat, every part of an object would be equally visible ; and not the highest, but the largest part of an object, would be visible at the greatest distance, so that we should be able to see the hul) of a fhip farther off than the maít. But this is contrary to experience ; consequently the earth is tound.

3. Several navigators have failed quite round the earth ; not in an exact circle, the winding of the fhores preventing them from failing in a direct courfe ; but by failing continually to the westward, they have reached the place from whence they at first departed. This was performed by Magellan, Cavendish, Sir Francis Drake, Lord Anfon, Bougainville, Commodore Byron, the Captains Carteret, Wallis, Cook, and others.

4. Eclipses of the moon, which are occafioned by the shadow of the earth falling on that planet, demonstrate that the earth is of a globular figure; for this shadow is always circular, whatever fituation the earth may be in at that time. Now a

body

« AnteriorContinuar »