moving still to the eastward, the crescent becomes more full; and, when the Moon comes to the meridian, about six in the evening, she has again the appearance of a bright semicircle: advancing still to the eastward, she becomes fuller on the east side; at last, in about 29 days, she is again opposite to the sun, and again full. It frequently happens that the Moon is eclipsed when at the full; and that the sun is eclipsed some time between the disappearance of the Moon in the morning on the west side of the sun, and her appearance in the evening on the east side of the sun. The nature of these phenomena will be more fully considered, when we come to treat particularly of eclipses.

In every revolution of the Moon about the earth, she turns once round upon her axis, and therefore always presents to us the same face; and as, during her course round the earth, the sun enlightens successively every part of her globe only once, consequently she has but one day in all that time, and her day and night together are as long as our lunar month. As we see only one side of the Moon, we are therefore invisible to the inhabitants on the opposite side, without they take a journey to that side which is next to us; for which purpose some of them must travel more than 1500 miles.

As the Moon illuminates the earth by a light reflected from the sun, she is reciprocally enlightened, but in a much greater degree by the earth, for the surface is above thirteen times greater than that of the Moon; and, therefore, supposing their power of

reflecting light to be equal, the earth will reflect thirteen times more light on the Moon than she receives from it. When it is what we call New Moon, we shall appear as a Full Moon to the Lunarians; as it increases in light to us, ours will decrease to them; in a word, our earth will exhibit to them the same phases as she does to us.

We have already observed, that from one half of the Moon the earth is never seen; from the middle of the other half, it is always seen over head, turning round almost thirty times as quick as the Moon does. To her inhabitants the earth seems to be the largest body in the universe; about thirteen times as large to them, as she does to us. As the earth turns round its axis, the several continents and islands appear to the Lunarians as so many spots of different forms; by these spots they may determine the time of the earth's diurnal motion; by these spots, they may perhaps measure their time; they cannot have a better dial.

OF THE SUPERIOR PLANETS.

Mars, Jupiter, Saturn, and the Georgium Sidus, are called superior planets, because they are higher in the system, or farther from the centre of it than the earth is.

They exhibit several phenomena, which are very different from those of Mercury and Venus; among other things, they come to our meridian both at noon and midnight, and are never seen crossing the sun's disc.

OF MARS.

Mars is the least bright and elegant of all the planets; its orbit lies between that of the Earth and Jupiter; but distant from both. He appears of

very

adusky reddish hue; from the dulness of his appearance, many have conjectured that he is encompassed with a thick cloudy atmosphere; his light is not near so bright as that of Venus, though he is sometimes nearly equal to her in size.

Mars, which appears so inconsiderable in the heavens, is 5,309 miles in diameter. Its distance from the sun is 146,000,000 miles. It goes round the sun in one year, 321 days, 23 hours; moving at the rate of 55,287 miles per hour. It revolves round its a is in about 24 hours, 40 minutes. To an inhabitant in Mars, the sun would appear one-third less in diameter than it does to us. Its apparent diameter, as viewed at a mean distance from the earth, in 30 seconds.

Mars, when in opposition to the sun, is five times nearer to us than when in conjunction. This has a very visible effect on the appearance of the planet, causing him to appear much larger at some periods than at others.

The analogy between Mars and the earth is by far the greatest in the whole solar system; their diurnal motion is nearly the same; the obliquities of their respective ecliptics not very different. Of all the superior planets, that of Mars is by far the nearest like the earth; nor will the Martial year appear so

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dissimilar to our's, when we compare it with the long duration of the years of Jupiter, Saturn, and the Georgium Sidus. It probably has a considerable atmosphere; for besides the permanent spots on its surface, Dr. Herschel has often perceived occasional changes of partial bright belts, and also once a darkish one in a pretty high latitude; alterations which we can attribute to no other cause, than the variable disposition of clouds and vapours floating in the atmosphere of the planet.

A spectator in Mars will rarely, if ever, see Mercury, except when he sees it passing over the sun's disc. Venus will appear to him at about the same distance from the sun, as Mercury appears to us. The earth will appear about the size of Venus, and never above 48 degrees from the sun; and will be, by turns, a morning and evening star to the inhabitants of Mars. It appears, from the most accurate observations, that Mars is a spheroid, or flatted sphere, the equatorial diameter to the polar being in the proportion of about 131 to 127; and there is reason to suppose that all the planets are of this figure.

OF JUPITER. 2

Jupiter is situated still further in the system, revolving round the sun between Mars and Saturn. It is the largest of all the planets, and easily distinguished from them by his peculiar magnitude and light. To the naked eye it appears almost as large as Venus, but not altogether so bright.

Jupiter revolves round its axis in nine hours, 56 minutes; its revolution in its orbit to the same point of the ecliptic, is 11 years, 314 days, 10 hours. The disproportion of Jupiter to the earth, in size, is very great; viewing him in the heavens, we consider him as small in magnitude; whereas he is in reality 90,228 miles in diameter; his distance from the sun is 494,750,000 miles; he moves at the rate of rather more than 29,083 miles per hour. Its apparent diameter, as seen at a mean distance from the earth, is 39′′.

To an eye placed in Jupiter, the sun would not be a fifth part of the size he appears to us, and his disc be 25 times less. Though Jupiter be the largest of all the planets, yet its revolution round its axis is the swiftest. The polar axis is shorter than the equatorial one, and his axis perpendicular to the plane of his orbit.

Jupiter, when in opposition to the sun, is much nearer the earth, than when he is in conjunction with him; at those times he appears also larger, and more luminous than at other times.

In Jupiter, the days and nights are of an equal length, each being about five hours long. We have already observed, that the axis of his diurnal rotation is nearly at right angles to the plane of his annual one; and consequently there can be scarce any difference in seasons: and here, as far as we may reason from analogy, we may discover the footsteps. of wisdom; for, if the axis of this planet were inclined by any considerable number of degrees, just