inentioned. Mercury is nearest to the fun, Venus next, the Earth next, Mars next, Jupiter next, and Saturn is at the greatest distance of all.

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

CHA P. II.

DISTANCES OF THE PLANETS FROM THE SUN.

'HE diftances of the planets from the fun may eafily 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 thefe parts; that of Venus feven; 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 reafon it is, that the three first are called fuperior, 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 Axed star of the first magnitude, called Regulus, or the Lion's Heart.

But

But to exprefs the diftances of the planets from the fun, 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 fun, is not a perfect circle, but a figure called an ellipfis; which, though somewhat resembling a circle, is longer than it is broad. Hence the fame planet is not always at the fame diftance from the fun, and the mean distance of it is that which is exactly between its greatest and least distance.

DIFFERENCE BETWEEN A PLANET AND A FIXED STAR.

THE planets appear at first sight like fixed ftars; but, upon a more accurate view, they

may easily be distinguished from them. B 2

1. By their

never

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

Mercury can never be seen by the naked eye, on account of his nearness to the fun, in the splendor of whose beams he is totally abforbed. The only way of obferving him is in his paffage over the fun, when he appears like a black spot on its furface. Venus is fometimes our evening, and fometimes our morning ftar. Mars and Saturn may be eafily known by their deep red colour. And Jupi ter is diftinguishable from the fixed stars by the largenefs of his fize, and the brightness of his colour, which is fo great, that it will fometimes illuminate a thin cloud in the fame manner as the moon.

CHA P. IV.

ON THE MOTION OF THE PLANETS.

ALL the planets, in different stated periods of

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

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

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

The earth, upon which we live, was long confidered as one large extenfive plane.

The heavens, above it, in which the fun, moon, and ftars, appeared to move daily from east to west, were conceived to be at no great distance from it, and to be only defigned 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 fea, we may be out of fight of land, when the land is near enough to be vifible, if it were not hid from our eye by the convexity of the water. In this cafe, the tops of hills, cliffs, fteeples, towers, &c. first appear to our view, next the buildings, and last of all the fhore; which can proceed from nothing else but the roundness of the earth, whereby the lower objects are longer hid from the fight, than thofe which are higher.

2. When we ftand upon the shore, the highest part of a fhip is vifible at the greatest distance. If a fhip be going from us out to fea, we shall continue to fee the mast, after the hull or body of the ship disappears, and the top of the maft will be feen longeft. But if the furface of the fea were quite flat, every part of an object would be equally vifible; and not the higheft, but the largest part of an object, would be vifible at the greatest distance, so that we should be able to fee the hulk of a fhip farther off than the maft. But this is contrary to experience; confequently the earth is round.

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