ECLIPSES.

Lunar and Solar Eclipses.-Their Causes.-Shadow of the Earth.-And Moon.-Magnitude of Eclipses. When they can happen.-Central Solar Eclipse.-Great Solar Eclipse described by Halley.-Ecliptic Limits.

ECLIPSES.

Of all the occasional astrononomical phenomena, those which have attracted most popular attention are LUNAR and SOLAR ECLIPSES. We shall on the present occasion explain the principal circumstances attending them.

When a luminous body, radiating light in all directions around it, throws these rays upon an opaque body, that body prevents a portion of the rays from penetrating into the space behind it. That portion of the space from which the light is thus excluded by the interposition of the opaque body, is called in astronomy the SHADOW of that body.

The shape, magnitude, and extent, of the shadow of an opaque body, will depend partly on the shape and magnitude of the opaque body itself, and partly on that of the body from which the light proceeds.

In the cases before us, the form of the bodies are globes. If the globe of the SUN were equal in magnitude to the globe of the earth, the shadow of the latter would be a cylinder, the base of which would be equal to a great circle of the earth, and such shadow would be interminable, since its sides would be parallel. This will be evident by an inspection of the annexed figure, 1, in which S. represents the sun, and E. the earth; the rays S. E. forming the sides of the shadow, being parallel, could never meet, and consequently the shadow would be infinite, since light can never penetrate into the space between them. If, on the other hand, the sun were a globe less in magnitude than the earth, then the shadow of the latter would have diverging sides as represented in the annexed figure, 2, which would widen as they proceed from the earth, and would be interminable; but the sun having in reality a diameter about one hundred and twelve times greater than that of the earth, the rays which proceed from the upper and lower limb of the sun, and which touch the earth at a and b, fig. 3, will converge to certain point at f, behind the earth, and will form a conical space, whose base will be at a b, and whose apex will be at f. From the space enclosed by this cone the light of the sun is entirely excluded, and it is therefore properly the shadow of the earth. But there is also a certain space behind the earth from which the sun's light is only partially excluded, and which

forms what is called the earth's penumbra. The ray m a, fig. 4, from the top of the sun's disk passes to the point f, while the ray n a from the lowest point of the sun's disk passes to the point c. The space between a f and a c will be partially illuminated by the sun. If a spectator were placed anywhere in that space, he would see a portion of the upper limb of the sun, and would see more of it the nearer he might be to c, and less of it the nearer he might be to f.

As he would see the sun, he would of course receive a portion of its light. Thus that part of the space included between a f, and a c, which is near a f, receives light from a small portion of the upper limb of the sun, while that part which is near a c receives light from nearly the whole of the sun; and in short, proceeding from a f to a c, the light received from the sun will be gradually increased.

Fig. 4.

In like manner, the ray m b proceeding from the upper limb of the sun and continued to d, will include between it and the ray bf a space which is only partially illuminated, and will be subject to the same observations as we have made respecting the space between a f and a c.

When any object which receives its light from the sun passes between the lines a c and b d, it will be either wholly or partially deprived of the sun's light. If it be outside the limits bf and a f, it will be only partially obscured; but if it be within these limits, it will be altogether darkened.

The length of the line a f being incomparably less than the distance of any body in the universe from the earth except the moon, but being on the contrary considerably greater than the distance of the moon, it follows that the only body in the system which can be deprived of light by the earth's shadow is the moon, and that whenever that object is in opposition to the sun, and at the same time so near the ecliptic as to be included between the lines ac and b d, it will

be partially deprived of the sun's light; but if it be so much nearer as to be included between the lines a fand bf, it will be wholly deprived of the sun's light. Thus the causes of a partial or total eclipse of the moon are explained.

If the plane of the moon's orbit coincided with that of the ecliptic, the moon would pass behind the centre of the earth in the direction of the line Eƒ forming the axis of the shadow, every revolution, and consequently there would be a total lunar eclipse every month; but as the moon's orbit is inclined at an angle of five degrees to the plane of the ecliptic, the distance of the moon from that plane is greater than the distance of lines of a c and b d from Ef, except when the moon is near to that point where its orbit crosses the ecliptic, which is called the moon's node.

No lunar eclipses happen, therefore, except when either of the moon's nodes is nearly in opposition to the sun,

When a lunar eclipse does happen, the moon will first enter the penumbra at a c, and will be very slightly obscured. As it approaches a f, it is more and more deprived of the sun's light, until finally it enters the shadow af b, where it is altogether obscured. At the end of the eclipse, as it must pass through the penumbra, it will recover the sun's light by slow degrees.

The length of the line E ƒ being about 800,000 miles, and the distance of the moon from the earth being less than 250,000, the moon when it passes through the shadow will be about 500,000 miles within the point f, and will consequently pass through the shadow at a part of considerable breadth.

In expressing the magnitude of the eclipse, whether of the sun or of the moon, it is customary to suppose the diameters of these bodies divided into twelye equal parts, called digits, and the magnitude of the eclipse is expressed by stating the proportion of the diameter of the disk which is obscured. Thus when half the disk is obscured, we say that the eclipse measures six digits, and so on.

From what has been stated, it is evident that an eclipse of the moon will not be affected in its appearance by the position of the observer on the surface of the earth. Wherever he may be, the eclipse will appear to him the same; but if it should happen that while the moon is passing through the shadow, the person desirous to observe it is in a portion of the earth which at that time is turned toward the sun, the eclipse will, of course, be invisible to him. In short, it will only be visible from that hemisphere of the earth that is turned from the sun at the time of its occurrence.

The moon, like the earth, receiving the sun's light, projects behind it a conical shadow and a diverging penumbra: if this shadow or penumbra fall upon any portion of the earth's surface, they will deprive such portion wholly or partially of the sun's light, and there will be a solar eclipse of a corresponding species. When the moon is between the sun and earth, the length of its shadow is about equal to its distance from the earth, and consequently the point of the shadow would just reach the surface of the earth; but as the moon's distance is subject to a slight variation, it sometimes happens that the length of the moon's shadow is a little more and sometimes a little less than its distance from the earth. If the length of the shadow be greater than its distance from the earth, then the shadow will cover a small portion of the earth's surface, to all places within which there will be a total solar eclipse. The circumstances affecting a solar eclipse are represented in the annexed figure, where S is the centre of the sun's disk, W is its upper limb, and V its lower limb; c d is the moon, and e the point of its shadow; d h and cg are the sides of its penumbra, and a b is the portion of the earth on which the penumbra falls. An observer placed between e and g, will see the upper limb of the sun only, the lower limb being