an eclipse of the sun, which was considered the orbits of Mars, Jupiter, Saturn, and Uraas a most unfortunate omen; and the whole nus, and eclipse them, when the earth interof the Athenian commanders and their men posed between them and the sun; and these were thrown into the greatest consternation. eclipses, in the case of the most distant Such facts should inspire us with gratitude planets, would be of long duration, on account for the advantages we now enjoy, in a land of the shadow being broader in proportion to where science is cultivated, and useful know- the distance. But as such eclipses never ledge disseminated, and where the light of happen, it forms a demonstrative proof that Divine revelation has dispelled the darkness the sun is not less, but greater than the earth. and superstitions of the heathen world.

Every planet and satellite is enlightened by the sun, and, consequently, casts a shadow towards the point of the heavens which is opposite to that luminary. An eclipse, therefore, is a privation of the light of the sun, or of some other heavenly body, by the interposition of another body between it and our sight. Eclipses are either of the sun, or the moon, or of the satellites which accompany some of the planets. In regard to circumstances, they are divided into total, partial, annular, and central. A total eclipse is when the whole face of the luminary is darkened; a partial eclipse is one when only a part of the disk is darkened; an annular eclipse is when the whole is darkened except a ring, or annuJus, which apppears round the dark part like an illuminated border. This can only happen in the case of an eclipse of the sun. In a central eclipse, the centres of the two luminaries and that of the earth are in one and the same right line, as when in an eclipse, the moon passes through the centre of the earth's shadow

Fig. 54.

Fig. 55.

Fig. 56.

SECTION I.

On the Eclipses of the Moon.

The sun, then, being greater than the earth, the shadow of the earth is a cone, which ends in a point at a certain distance from the earth, as represented in fig. 56. This cone reaches to a distance of 840,000 miles from the earth, or about three and a half times as long as the distance of the moon from the earth.

An eclipse of the moon is produced by the interposition of the earth between the sun and moon, and, consequently, it can only happen at the time of full moon, when the moon is in opposition to the sun. As the earth is an opaque body, enlightened by the sun, it will If the moon always moved in the plane cast a shadow towards those portions of space of the ecliptic, she would suffer a total which are opposite to the sun, and if the eclipse, at the time of every full moon, by moon happen to pass through those spaces passing through the centre of the earth's where the shadow falls, she must necessarily shadow. But the moon's orbit is inclined to be eclipsed. The sun and the earth are both the plane of the ecliptic, at an angle of 5° 8', spherical bodies, and, therefore, if they were and coincides with it only in two places, of an equal size, the shadow of the earth called the nodes, or the points where her orwould be cylindrical, as in fig 54, and would bit intersects the ecliptic. Full moon, therecontinue of the same breadth, at all distances fore, may frequently happen without an from the earth, and would extend to an eclipse, as at this period the moon may be equal distance, and might cause an eclipse either to the north or the south of the ecliptic. of the sun to the superior planets. If the sun It has been calculated, that if the mean were less than the earth, the shadow would opposition of the sun and moon, or the full expand, and grow wider the farther it was moon, happen within 7° 47' of the moon's from the earth, as in fig. 55. It would reach node, there must be an eclipse; but if the

H

E

Fig. 57.

D

distance be greater than 13° 21', there cannot are every where of the same magnitude and be an eclipse. Let a &, fig. 57, represent the duration. 3. The moon's diameter is supmoon's orbit, E F the plane of the ecliptic, posed to be divided into twelve equal parts and N the node of the orbit, or point where called digits, and as many of these parts as it cuts the ecliptic; and A B C D four repre- are darkened by the earth's shadow, so many digits is the moon said to be eclipsed. The extent in which the moon is eclipsed above twelve digits shows how far the shadow of the earth is over the body of the moon on that edge to which she is nearest at the middle of the eclipse. 4. The moon, when totally eclips ed, is not invisible, if she be above the horizon and the atmosphere clear; but appears generally of a dusky colour, and somewhat like tarnished copper, especially towards the edges, sentations of the earth's shadow in the being generally more dark about the middle ecliptic. It is obvious that when the shadow of the earth's shadow. Some have supposed is at A, and the moon at 1, there will be no this to arise from the moon's native light, but eclipse, because the moon is too far from the the true cause of her being visible is, the scatnode, and the earth's shadow does not reach tered beams of the sun bent into the earth's her. When the full moon shadow, by passing through the atmosphere. Fig. 58. is nearer to the node, as at The moon is not eclipsed by the earth alone; K, only a part of her disk the atmosphere, by refracting some of the rays passes through the shadow, of the sun, and reflecting others, casts a shadow, when she suffers a partial though not so dark a one as that which arises eclipse. When the full from an opaque body. Although in most lunar moon is at L, she passes eclipses the body of the moon, though obscure, through a portion of the is still visible, yet it has sometimes happened shadow, and is totally e- otherwise. Hevelíus mentions, in his "Seleclipsed. When the moon's nographia," an eclipse of the moon which centre passes through the happened in August, 1647, when he was not centre of the shadow, which able to distinguish the face of the moon even can only happen when she with a good telescope, although the sky was is in the node at N, it is sufficiently clear for him to see stars of the then both a total and a fifth magnitude: but such cases are rare. central eclipse; and such an eclipse is of the longest duration at which time the total darkness continues about two hours.

E

S

The duration of a lunar eclipse depends on the following circumstances:-1. On the largeness of the circle of the earth's shadow, whose diameter may be different at different times; the nearer the moon is to the earth, the larger Fig. 58 represents, in a is that portion of the earth's shadow through different point of view, a which she passes. 2. On the apparent diam total eclipse of the moon. eter of the moon, which may be different, on The circle A B C represents account of her variable distance as she moves the orbit of the moon, in in an elliptical orbit. 3. On the distance which it revolves round the earth. The moon of the moon from her node at the moment of is supposed to be in the node, and in her op- her being full, which will cause her to pass position to the sun-she therefore passes through a greater, or less, portion of the through the centre of the earth's shadow. earth's shadow. Thus when the moon is at K, And as the shadow of the earth is nearly fig. 57, the eclipse will be of comparatively 6000 miles broad at the distance of the moon, small duration; when at L, its duration will and as the moon is only a little more than be much longer, but not so long as when she is 2000 miles in diameter, she must be complete- at N, in the node, when she passes through ly immersed in the shadow of the earth, and must move nearly three times her own diameter before she can emerge from the shadow.

The following facts may be stated in relation to lunar eclipses:-1. An eclipse of the moon always begins on the moon's eastern side, and goes off on her western side. 2. Lunar eclipses are visible in all parts of the earth which have the moon above their horizon, and

the centre of the shadow. 4. On the velocity of the moon's motion across the shadow of the earth, which is swiftest when she is in perigee, or nearest the earth, and duration of a central eclipse will then be shortest. According to these circumstances will be the time of continuance of a lunar eclipse. When the moon is centrally eclipsed, and when she is at her greatest distance from the earth, its duration is three

hours, 57 minutes, 26 seconds, from beginning having the enlightened side towards the sun, to end and when she is at her least distance, and its dark hemisphere wholly turned to3 hours, 37 minutes, 26 seconds. The moon wards the earth, which is its position at new may be totally eclipsed, although she do not moon. It is also supposed to be in its node, pass through the centre of the shadow; but in an exact line between the sun and the in this case the duration of the eclipse will earth. In this situation the shadow of the be shorter than what has now been stated. moon falls upon a certain portion of the earth, In some instances, the continuation of total darkness may amount only to a few minutes, when the moon passes near the extremity of the shadow.

Fig. 59.

SECTION II.

On Eclipses of the Sun.

An eclipse of the sun is caused by the interposition of the body of the moon between the sun and the earth, when she throws a shadow over a certain portion of the earth. This can happen only at the time of new moon, and when the moon is at or near one of her nodes. The eclipses of the sun and moon, though expressed by the same word, are in their nature very different; the sun, in reality, loses nothing of his native lustre in the greatest eclipse, but is all the while diffusing streams of light around him in every direction, and illuminating without intermission all the bodies of the planetary system. Some of these streams, however, are occasionally intercepted in their course towards the earth by the moon coming between the earth and sun, and at that time the dark side of the moon is turned towards the earth. When the moon is eclipsed, she suffers a real diminution of her borrowed light; but when the sun is said to be eclipsed, there is no diminution of his light, and it is in reality an eclipse of the earth by the shadow of the

moon falling upon a certain portion of our and intercepts the rays of the sun, for a little, globe and this shadow would be distinctly from the inhabitants of the earth on whom seen, by an inhabitant of the moon, passing the shadow falls. Part of the cone of the along a certain zone of the earth, like a small, shadow is represented at a b, and it is never dark, circular spot. The moon being much more than about 180 miles in diameter, within smaller than the earth, and having a conical shadow-because she is less than the suncan cover only a part of the earth by her shadow: hence an eclipse of the sun is visible only to a few inhabitants of the earth, whereas an eclipse of the moon is visible to all who are on that hemisphere, where the lunar orb is seen.

The following diagram, fig. 59, will convey a general idea of the nature of an eclipse of the sun. In this figure, s represents the sun; M, the moon; E, the earth; and M N o, the orbit of the moon. The moon is supposed to be in that part of its orbit next the sun,

the limits of which the sun will appear totally eclipsed. But sometimes it happens that the extremity of the one of the moon's shadow falls short of the earth, in which case an annular eclipse of the sun is produced; in which the sun appears like a brilliant ring of light around the dark body of the moon. Besides the dark shadow, there is a penumbra or fainter shadow produced, which is represented at e d; and in all those parts where the penumbra falls, the sun will be only partially eclipsed. Thus, between e and a, the parts of the sun about A B cannot be seen; the rays coming from thence towards c or a being

intercepted by the moon: but the portions of Therefore, in that time, there will be a reguthe sun about & and H will be visible. The lar period of eclipses for many ages. In this nearer any place of the earth-within the period there are 18 Julian years, 11 days, 7 penumbra is to the dark shadow of the hours, 42 minutes, 31 seconds; when the last moon, the greater will the eclipse appear, and day of February in leap years is four times the nearer it is to the outside of the penum- included. Consequently, if to the mean time bra, the smaller will be that portion of the of any eclipse, either of the sun or moon, we sun which is seen eclipsed. To those who add the above period, we shall have the mean live beyond the boundary of the penumbra, time of the return of the same eclipse. Durthe whole disk of the sun will be seen and no ing this period, there happens about 62 eclipeclipse will be visible. Hence it happens, ses, 21 of the moon and 41 of the sun. If, that the sun may be totally eclipsed in Africa then, we wish to know the mean time of an and the southern parts of Asia, and no trace eclipse for any year, we have only to seek in of an eclipse perceived, at the same moment, old almanacs the exact time that any eclipse either in Britain or America. may have happened 18 years before, and add to such time the above-mentioned period.

Periods of Eclipses. It has been found that in 223 mean lunations, after the sun, moon, and nodes have been once in a line of conjunction, they return so nearly to the same state again, that the same node which was in conjunction with the sun and moon, at the beginning of the first of these lunations, will be within less than half a degree of a line of conjunction with the sun and moon again, when the last of these lunations is completed. of the eclipse was preceded by a blood-red

streak of light from his left limb which con- the countenances of my friends wore a horrible tinued about 6 or 7 seconds; then part of the sun's disk appeared all on a sudden brighter than Venus was ever seen in the night, and in that instant gave light and shadow to objects as strong as the moonlight generally does. One of the most minute accounts of the circumstances accompanying a total eclipse of the sun is that which is given by Dr. Stukely, of the eclipse which happened in 1724, in a letter to Dr. Halley; of which the following is an abridgment:

aspect. It was not without an involuntary exclamation of wonder, I looked round me at this moment: I distinguished colours in the sun, but the earth had lost all its blue, and was entirely black. A few rays shot through the clouds for a moment, but immediately afterwards the earth and the sky appeared totally black. It was the most awful sight I had ever beheld in my life. North-west of the point whence the eclipse came on, it was impossible to distinguish in the least degree the earth from the sky, for a breadth of sixty degrees or more. We looked in vain for the town of Amesbury, situate below us; scarcely could we see the ground under our feet. All the change I could perceive during the totality was that the horizon by degrees drew into two parts, light and dark; the northern hemisphere growing still longer, lighter, and broader, and two opposite dark parts uniting into one, and swallowing up the southern enlightened part.

"I chose for my station, Haradon-hill, near Amesbury, east from Stonehenge avenue. In front is that celebrated edifice upon which I knew that the eclipse would be directed. I had the advantage of a very extensive prospect in every direction, being on the loftiest hill in the neighbourhood, and that nearest to the centre of the shadow. I had two men in company who looked through smoked glasses. The sky, though overcast, gave out some straggling rays of the sun that enabled us to see around us. It was half-past five by my watch when they informed me that the eclipse was begun. We watched its progress by the naked eye, as the clouds performed for us the service of coloured glasses. At the moment when the sun was half obscured, a very evident circular rainbow formed at its circumference with perfect colours. As the darkness increased we saw the shepherds on all sides hastening to fold their flocks, for they expected a total eclipse of an hour and a quarter duration. When the sun assumed the appearance of the new moon, the sky was tolerably clear, but it was soon covered with deeper clouds. The rainbow then vanished; the hill grew very dark, and on each side the horizon exhibited a blue tint like that at the close of day. Scarcely had we time to count ten, when Salisbury spire, six miles to the south, was enveloped in darkness. The hill disappeared entirely, and the deepest night spread around us. We lost sight of the sun, whose place till then we had been able to distinguish in the clouds, but whose trace we could now no more discover than if had never existed. It was now 35 minutes past six; sence of the clouds added much to the solemshortly before the sky and the earth had resumed a livid tint; there was also much black diffused through the clouds, so that the whole picture presented an awful aspect that seemed to announce the death of nature.

"We were now involved in a total and palpable darkness. It came on rapidly, but I watched so attentively that I could perceive its progress. It came upon us like a great dark mantle thrown over us. The horses we held by the bridle seemed deeply struck by it, and pressed closely to us with marks of extreme surprise. As well as I could perceive,

"At length, upon the first lucid point appearing in the heavens where the sun was, I could distinguish pretty plainly a rim of light running alongside of us, a good while together, or sweeping by our elbows, from west to east; just then, having good reason to suppose the totality ended, I found it to be full three minutes and a half. The hill-tops then resumed their natural colour, and I saw no horizon at the point previously occupied by the shadow. My companions cried out that they again saw the steep hill towards which they had been looking attentively. Presently we heard the song of the larks hailing the return of light, after the profound and univer sal silence in which every thing had been plunged. The heavens and the earth now appeared of a grayish cast, interspersed with blue, like the morning before sunrise. As soon as the sun appeared, the clouds grew denser, and for some minutes the light did not increase, as happens at a cloudy sunrise. The instant the eclipse became total, till the emersion of the sun, we saw Venus, but no other stars. We perceived at this moment the spire of Salisbury Cathedral. The pre

nity of the spectacle, incomparably superior, in my opinion, to the eclipse of 1715, which I saw perfectly from the top of Boston steeple, when the sky was very clear. There, indeed, I saw the two sides of the shadow, coming from afar, and passing to a great distance be hind us; but this eclipse exhibited great variety, and was more awfully imposing. So deep an impression has this spectacle made on my mind, that I shall long be able to recount all the circumstances of it with as much precision as now."

There have been no total eclipses of the