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involved in the shadow of Jupiter, and cannot see it for 8 minutes after its emersion.

The rays of light emitted from the sun while he is in any place do not arrive at our eyes, for 8 minutes after he has left that place.

The beginnings and endings of the eclipses of Jupiter's satellites are almost instantaneous, and therefore are very proper for determining the longitudes of places on the earth. The eclipses of the first are the most proper for this purpose, because of their frequency, and its greater velocity. As they are the same to every spectator on that side of the earth that is turned to Jupiter, if two persons observe the beginning or end of the same eclipse, called the immersion or emersion, and note the precise times of their observations, the difference of these times converted into degrees, at the rate of one hour for every fifteen degrees, will give the difference of longitude between the places of observation.

When Jupiter is near to his conjunction with the sun, he and his satellites are concealed in the rays of the sun. From the time of his appearing after his conjunction till near his opposition, his shadow being projected to the right hand of his body, the immersions of the satellites are visible on the west of the planet, when they fall into his shadow. From the opposition to the conjunction, his shadow being projected to the left hand, the emersions are visible on the east side of Jupiter, when they emerge out of his shadow on that side.

The beginning and end of the same eclipse of the first satellite can never be seen by us, because one side of the shadow of Jupiter is always hid behind his body, at the distance where the first satellite passes

through it. Nor can they be both seen in the second, unless the satellite be near to its limits, or 90° from its node, and Jupiter at the same time be near his quadrature with the sun, and near his perihelion. As to the third, when Jupiter is more than 46° from his conjunction with, or opposition to, the sun, both the beginnings and ends of the eclipse may be seen on the same side of Jupiter; as they are also of the fourth, when Jupiter is 24° from his opposition or conjunction. When Jupiter is in quadrature with the sun, his shadow is then most exposed to our view, and the exterior satellites pass through so small a section of it, that both the immersions and emersions are seen either before or after the occultation behind the body of Jupiter. When the fourth satellite is about 52° from its node, the inclination of its orbit is such, as to make it pass either above or below the shadow, so that it will escape being eclipsed for two years together, which happens twice in every revolution of Jupiter, and therefore for two years in every six there are no eclipses of this satellite.

THE MOON.

THE moon accompanies the earth in its revolution round the sun, at the same time revolving round the earth as the center of her motion, and finishing her period in 27d 7h 43', moving at the rate of 2290 miles every hour in her orbit. She turns round her axis in the same time that she spends in going round the earth, which is the reason that she constantly turns the same face towards the earth, so that her day and night is as long as her revolution. As her axis is almost perpendicular to her orbit, she has scarcely any change of seasons. Yet the lunarians may determine the length

of their year and of ours, which is the same, by observing when either of our poles begins to be illuminated.

As she has no light inherent, and only shines by light from the sun reflected to us, she exhibits all her different phases, according as more or less of her enlightened side is turned towards us. When she is between us and the sun, her enlightened side is turned away from us, and therefore she is invisible. When she has advanced in her orbit towards the east, a little segment of her enlightened surface is turned towards us, and therefore she exhibits the appearance of a section of two circles, less than the area of a semi-circle, until she has arrived at 90° from the sun, when the segment of her enlightened surface appears to be a complete semi-circle. From thence more and more of her enlightened face is gradually turned towards us, until she come to her opposition, when she shines with a full face. From thence to her change again she gradually assumes the same appearances, which she exhibited in the first half of her revolution, constantly turning her illuminated disk more and more from the earth, until she become invisible again in her change. All these phases of the moon may be exemplified by the sun shining on a round stone on the top of a gate. Place yourself in such a position that the lower limb of the moon may appear to touch the upper side of the stone, while the sun shines upon it, and the enlightened part of the stone, that is then turned to the eye, will appear precisely similar to that of the moon.

One half of the inhabitants of the moon have no absolute darkness at all; for during the absence of the sun, our earth appears to them under all the different phases of the moon to us, waxing and weaning, being

full to them when she is new to us, and vice versâ: and affording them thirteen times as much light, as the moon affords to us. It is owing to the light reflected from the earth, that the unilluminated part of the moon is visible to us for a few days after her change. The inhabitants of the other half of the moon can never see our earth, unless they travel round to the side that is exposed to our view, and therefore they have no moon-light in the absence of the sun, which continues for a fortnight together. From the center of that hemisphere that is next to us, the earth appears in their zenith continually, neither rising nor setting, but turning round on her axis, once in 24 hours, and thereby affording to them a complete and regular dial for the measure of their time, by means of the spots on her surface. From the parts which appear to us as the verge of her disk, the earth appears in their opposite horizon, without rising or setting, except as they travel towards the center of the disk, by which means the earth will appear higher and higher until she come to their zenith. Thus the lunarians can, from the altitude of the earth to them, easily determine both their longitude and latitude, with sufficient precision.

From the light observed to surround the moon in a total eclipse of the sun, some think that she is furnished with a rare and transparent atmosphere. But others are of opinion, from the sudden occultation of the fixed stars by the moon, without any previous diminution of their light, that she is destitute of an atmosphere, unless it be exceedingly rare: so that the stars will appear at noon day to her inhabitants, when they turn their backs to the sun. It is owing to our átmosphere, that the heavens appear bright to us, so as to obscure the stars: as we can see them through a

telescope or from the bottom of a deep pit, in the day time, when the glare arising from the refracted light of the sun is thereby removed. If she have no atmosphere she can have no twilight, but must suffer an immediate transition from noonday light to midnight darkness. Nor can she have storms and rains as we have.

Her face, when viewed through a good telescope, appears to be exceedingly rough and uneven, diversified with high mountains and deep cavities, which are easily distinguished from each other, by the different projections of their shadows, from the different positions of the sun, and from the irregular and indented boundary of light and darkness; which if her surface were smooth and even should appear to be a straight line at her quadratures, and a regular elliptic curve at every other time. But it is quite otherwise, and although her limb appears to be a well-defined circle, this is owing to the mountains rising behind the cavities, and by their oblique positions to us, preventing us from discovering the irregularity of the boundary of light and darkness, near the limb; while we can easily see it, in any other position. She revolves round the earth in an orbit inclined to the plane of the ecliptic in a variable angle, which is about 5° 18' at a mean, being sometimes more and sometimes less. This causes her to appear to be not quite round when she is full, near her greatest distance from her node; a small deficiency of light being discovered about her south pole, when she is full in the upper part of her orbit, and a similar deficiency about her north pole, when she is full in the southern part of her orbit; because we can at these times see farther than what is enlightened by the sun on one side, and not so far on the other. Hence it is that she will appear to nod back

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