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is in the first of Aries, and of course the descending in the first of Libra. When the nodes are in these points 13° of the Moon's orbit, about the first of Aries, rises in the space of 16 minutes, in the latitude of London, and consequently, when the Moon is in this part of her orbit, the time of her rising will differ only 16 minutes from the time she rose on the preceding evening. When the Moon is in the opposite part of her orbit, or about the signs Virgo and Libra, which make the greatest angle with the horizon at rising, 13° of her orbit will require 1 h. 15' to rise, although it were coincident with the ecliptic; and if the nodes be in the points just mentioned, the same portion of the orbit will require 1 h. 20' to ascend above the horizon of the same place; and so much later will the Moon rise every night for several nights when in this part of her orbit. As the Moon is full in these signs in the months of March and April they may be called vernal full Moons.

Those signs of the ecliptic which rise with the greatest angle, set with the least; and those that rise with the least, set with the greatest. Therefore, the vernal full Moons differ as much in their times of rising, every night, as the autumnal, or harvest, Moons differ in the times of their setting; and they set with as little difference of time as the autumnal ones rise, supposing the full Moons to happen in opposite points of the Moon's orbit, and the nodes to remain in the same point of the ecliptic.

In southern latitudes, the harvest Moons are just as regular as in the northern, because the seasons are contrary; and those parts of the Moon's orbit about Virgo and Libra, where the vernal full Moons happen in northern latitudes, (and the harvest ones in southern latitudes) rise at as small an angle at the same degree of south latitude, as those about Pisces and Aries in north latitude, where the autumnal full Moons take place.

At places near the Equator, this phenomenon does not happen; for every point of the ecliptic, and nearly every point of the Moon's orbit, makes the same angle with the horizon, both at rising and setting, and therefore equal portions of it will rise and set in equal times. As the Moon's nodes make a complete circuit of the ecliptic in 18 years 225 days, it is evident, that when the ascending node is in the first of Aries at any given time, the descending one must be in the same point about 9 years 112 days afterwards; consequently, there will be a regular interval of about 9-years between the most beneficial and least beneficial harvest Moons.

APPARENT SIZE OF THE MOON.

It has been already remarked at page 55, that the apparent size of the Moon is nearly equal to that of the sun; but the apparent size of the Moon is not always the same, for she is often much nearer the earth at one time than another; hence, it is evident, her apparent magnitude must vary, and that it will be greatest when she is nearest the earth. (See page 54.)

But she appears larger when in the horizon than, in the zenith even on the same evening; and yet it may easily be proved, that she is a semi-diameter of the earth, or about 4000 miles, farther from the spectator when she is in the horizon than when she is in the zenith, and consequently ought to appear smaller, which will be found to be really the case if accurately measured.

This apparent increase of magnitude in the horizontal Moon, must therefore be considered as an optical illusion; and may be explained upon the well known principle, that the eye in judging of distant objects is guided entirely by the previous knowledge which the mind has acquired of the intervening objects. Hence arise the erroneous estimates we make of the size of distant objects at sea, of objects below us when viewed from great heights, and of objects highly elevated when viewed from below. Now when the Moon is near the zenith, she is seen precisely in this last situation, of course there is nothing near her, or that can be seen at the same time with which her size can be compared; but the horizontal Moon may be compared with a number of objects whose magnitude is previously known.

That the Moon appears under no greater an angle (or is not larger) in the horizon, than when she is on the meridian, may be proved by the following simple experiment.

Take a large sheet of paper and roll it up in the form of a tube, of such width as just to include the whole of the Moon when she rises; then tie a thread round it to keep it exactly of the same size, and when the Moon comes to the meridian, where she will appear to the naked eye to be much less, look at her again through the same tube, and she will fill it as completely as she did before.

When the Moon is full and in the horizon, she appears of an oval form, with her longest diameter parallel to the horizon. This appearance is occasioned by the refraction of the atmosphere, which is always greatest at the horizon, consequently the lower limb or edge must be more refracted than the upper edge, and therefore these two edges will appear to be brought nearer each other, or the vertical diameter will appear to be shortened; and as the horizontal diameter is very little affected by the refraction, she must appear to have somewhat of an oval shape. The sun is affected in the same manner when in the horizon.

SPOTS, MOUNTAINS, &c. IN THE MOON.

Turn'd to the sun direct, her spotted disk

Shows mountains rise, umbrageous dales descend,
And caverns deep, as optic tube descries. THOMSON.

When the Moon is viewed through a good telescope, her surface appears to be diversified with hills and valleys; but this is most discernable when she is observed a few nights after the change or opposition; for when she is either horned or gibbous, the edge about the confines of the illuminated part is jagged and uneven.

Many celebrated astronomers have delineated maps of the face of the Moon; but the most celebrated are those of Hevelius, Grimaldi, Riccioli, and Cassini; in which the appearance of the Moon is represented in its different states, from new to full, and from full

to new.

The plate which we have given at page 56, represents the face of the Moon as viewed by the most powerful telescopes, the light or illuminated parts being elevated tracts, some of which rise into very high mountains, while the dark parts appear to be perfectly smooth and level. This apparent smoothness in the faint parts, naturally led astronomers to conclude that they were immense collections of water; and the names given to them, by some celebrated astronomers, are founded on this supposition. For Hevelius distinguished them by giving them the names of the seas on the earth; while he distinguished the bright parts by the names of the countries and islands on the earth. But Riccioli and Langreni distinguished both the dark and light spots, by giving them the names of celebrated astronomers and mathematicians, which is now the general manner of distinguishing them.

That the spots which are taken for mountains and valleys are really such, is evident from their shadows. For in all situations in which the Moon is seen from the earth, the elevated parts are constantly found to cast a triangular shadow in a direction from the sun; and on the contrary, the cavities are always dark on the side next the sun, and illuminated on the opposite side, which is quite conformable to what we observe of hills and valleys on the earth. And as the tops of these mountains are considerably elevated above the other parts of the surface; they are often illuminated when they are at a considerable distance from the line which separates the enlightened from the unenlighted part of the disc, and by this means afford us a method of even determining their height.

Previous to the time of Dr. Herschel, some of the lunar mountains were considered to be double the height of any on the earth; but by the observations of that celebrated astronomer, their height is considerably reduced.

For after measuring many of the most conspicuous prominences, he says, "From these observations I believe it is evident, that the height of the lunar mountains is, in general, overrated; and that when we have excepted a few, the generality do not exceed half a mile in their perpendicular elevation."

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As the Moon's surface is diversified by mountains and valleys as well as the earth, some modern astronomers say they have discovered a still greater similarity; namely, that some of these are really volcanoes, emitting fire, as those on the earth do. An appearance of this kind was discovered by Don Ulloa in an eclipse of the sun, which happened on the 24th June, 1778. It was a small bright spot like a star, near the margin of the Moon, which he supposed at the time to be a hole or valley, which permitted the sun's light to shine through it. Succeeding observations have, however, led astronomers to believe, that appearances of this kind are occasioned by the

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eruption of volcanic fire. Dr. Herschel, in particular, has observed several eruptions of this kind, the last of which he has described in the Philosophical Transactions for 1787, as follows: "On April the 19th, at 10h. 6m. I perceived three volcanoes in different places of the dark part of the new Moon. Two of them are either already nearly extinct, or otherwise in a state of going to break out, which perhaps may be decided next lunation. The third shows an actual eruption of fire or luminous matter: its light is much brighter than the nucleus of the Comet which M. Mechain discovered at Paris on the 10th of this month." The following night the Doctor found it burned with greater violence; and by measurement he found that the shining or burning matter must be more than three miles in diameter, of an irregular round figure, and very sharply defined about the edges. The other two volcanoes resembled large faint nebulæ, which appeared to be gradually brighter towards the middle, but no well defined luminous spot could be discovered in them. Herschel adds, "the appearance of what I have called the actual fire, or eruption of a volcano, exactly resembled a small piece of burning charcoal, when it is covered by a very thin coat of white ashes, which frequently adhere to it when it has been some time ignited; and it had a degree of brightness about as strong as that with which a coal would be seen to glow in fair daylight."

Dr.

The appearance which Dr. Herschel here describes so minutely, was also observed at the Royal Observatory of Paris, about six days before, by Dominic Nouet, like a star of the sixth magnitude, the brightness of which occasionally increased by flashes. Other astronomers also saw the same thing, for M. de Villeneuve observed it on the 22d of May, 1787. This volcano is situated in the north-east part of the Moon, about 3' from her edge, towards the spot called Helicon. After considering all the circumstances respecting these appearances which have just been mentioned, we must subscribe to Dr. Herschel's opinion, that volcanoes exist in the Moon as well as the earth.

It has long been a disputed point among astronomers, whether or not the Moon is surrounded by an atmosphere. Those who deny that she is, say that the Moon always appears with the same brightness when our atmosphere is clear; which could not be the case if she were surrounded by an atmosphere like ours, so variable in density, and so often obscured by clouds and vapours.

A second argument is, that when the Moon approaches a star, before she passes between it and the earth, the star neither alters its colour nor its situation, which would be the case if the Moon had an atmosphere, on account of the refraction, which would both alter the colour of the star, and also make it appear to change its place. A third argument is, that as there are no seas or lakes in the Moon, there is, therefore, no atmosphere, as there is no water to be raised up into vapour. But those who contend that the Moon is surrounded by an atmosphere, deny that she always appears of the same brightness, even when our atmosphere appears equally clear. Instances of the contrary are mentioned by Hevelius and some other astronomers,

but it is unnecessary to take any farther notice of them here. In the case of total eclipses of the Moon, it is well known that she exhibits very different appearances, which it is supposed are owing to changes in the state of her atmosphere. It is remarked by Dr. Long, that Newton had shown that the weight of any body on the Moon, is but a third part of the weight of what the same body would be on the earth; from which he concludes that the atmosphere of the Moon is only one-third part as dense as that of the earth, and therefore it is impossible to produce any sensible refraction on the light of a fixed star which may pass through it. Other astronomers assert that they have observed such a refraction; and that Jupiter, Saturn, and the fixed stars had their circular figures changed into an elliptical one, on these occasions.

But although the moon be surrounded by an atmosphere of the same nature as that which surrounds the earth, and to extend as far from her surface; yet no such effect as a gradual diminution of the light of a fixed star could be occasioned by it, at least none, that could be observed by a spectator on the earth. For at the height of 44 miles our atmosphere is so rare, that it is incapable of refracting the rays of light, now this height is only the 180th part of the earth's diameter; but as clouds are never observed higher than 4 miles, it therefore follows that the obscure part of our atmosphere is about the 2000th part of the earth's diameter, and if the Moon's apparent diameter be divided by this number, it will give the angle under which the obscure part of her atmosphere will be seen from the earth, which is not quite one second, a space passed over by the Moon in less than two seconds of time. It can, therefore, scarcely be expected that any obscuration of a star could be observed in so short a time, although it do take place.

As to the argument against a lunar atmosphere drawn from the conclusion, that there are no seas or lakes in the Moon, it proves nothing, because it is not positively known whether there is any

water in the Moon or not.

The question of a lunar atmosphere seems to be at last settled by the numerous and accurate observations of the celebrated Astronomers Shroeter and Piazzi, who have proved as convincingly as the nature of the subject seems to allow, that the Moon has really an atmosphere, though much less dense than ours, and scarcely exceeding in height some of the lunar mountains.

It is remarked by Dr. Brewster, "The mountain scenery of the Moon bears a stronger resemblance to the lowering sublimity and terrific ruggedness of the Alpin regions, than to the tamer inequalities of less elevated countries. Huge masses of rock rise at once from the plains, and raise their peaked summits to an immense height in the air, while projecting craggs spring from their rugged flanks, and threatening the vallies below seem to bid defiance to the laws of gravitation. Around the base of these frightful eminences, are strewed numerous loose and unconnected fragments, which time seems to have detached from their parent mass, and when we examine the rents and ravines which accompany the overhanging cliffs,

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