OF ECLIPSES.
When any one of the heavenly bodies is obscured or darkened by the shadow of another falling upon it, or by the interpo sition of any body, it is said to be eclipsed. The eclipses of the sun and moon are the most striking of any. They were formerly considered as ominous, and have often excited the dread and apprehension of the vulgar ; but the improvement of science has shewn that they have no con nection with future events ; that they de pend upon regular and invariable causes, and may be calculated and foretold with the greatest certainty. As the earth is an opaque body, enlightened only by the sun, it will cast a shadow towards that side which is farthest from the sun. If the sun and earth were of the same size, this shadow would be cylindrical, and would extend to an infinite distance : but as the sun is much larger than the earth, the shadow of the latter must he conical, or end in a point, (see fig. 6.) On the sides of this conical shadow, there is a diverging shadow, the density of which decreases in proportion as it recedes from the sides of the former conical shadow : this is called the penumbra. As the moon revolves round the earth sufficiently near to pass through the shadow of the earth, an eclipse must always take place when these three are all in one straight line. An eclipse of the moon can never happen but at the time of full moon ; but, on ac count of the inclination of the moon's or bit to that of the earth, an eclipse cannot take place every full moon. When the moon passes entirely through the earth's shadow, the eclipse it total ; but when only a part of it passes through the sha dow, the eclipse is partial. The quantity of the moon's disc which is eclipsed (and the same thing is to be understood of that of the sun in a solar eclipse) is expressed by twelfth parts, called digits ; that is, the disc is supposed to be divided by twelve parallel lines : then, if half the disc is eclipsed, the quantity of the eclipse is said to be six digits. When the diame ter of the shadow through which the moon must pass is greater than the dia meter of the moon, the quantity of the eclipse is said to be more than 12 digits ; thus, if the diameter of the moon 'is to that of the shadow as four to five, then the eclipse is said to be fifteen digits. The duration of a lunar eclipse is vari ous, it sometimes lasts two or three hours.
The eclipses of the sun are owing to a different cause than those of the moon. They are occasioned by the moon's com ing directly between us and the sun, and therefore obstructing our view of it. When the moon happens to be in conjunc tion with the sun, or between the sun and the earth, viz. at the time of the new moons, the shadow of the moon falls up on the surface of the earth ; hence, pro perly speaking, such eclipses should be called eclipses of the earth. But the whole disc of the earth cannot be involv ed in the shadow of the moon, because the moon is much smaller than the earth, and the shadow of the moon is conical. Thus, in Plate III. fig. 1, the rays of the sun, S, being intercepted by the moon, L, form the conical shadow C D G, which, falling upon the surface of the earth, en tirely deprives that portion of it upon which it falls of the sun's light, and of course the inhabitants of that part of the earth will have a total eclipse of the sun. Beyond the dense conical shadow, C D G, there is a diverging half shadow, or pe numbra, C D E F, which is occasioned by the moon's intercepting only a pprt of the sun's rays from those places whit-a fall within this penumbral cone, and are out of the dense shadow. Thus, from the part of the earth Z the portion Y Y B of the sun only can be seen ; consequently, the inhabitants of that part will have a partial eclipse. As the moon is not al ways at the same distance from the earth, it sometimes happens that the conical dense shadow does not reach the earth, as in fig. 2, and only the penumbral sha dow falls upon it, the eclipse consequently is partial to every part of the earth. Those who are at the centre of the penumbra will lose sight of the centre of the sun by the interposition of the moon's body, which, subtending a small er angle than the sun, will not entirely cover its surface, so that there will be a ring of light all round. The eclipse is then said to be annular. The satellites, or moons, are often eclipsed by the pla nets to which they belong. The eclipses of Jupiter's moons, as we have already observed, are very useful in ascertaining the longitude. When any of the planet ary bodies disappear, by another coming before it, it is called an occultation. The occultations of the fixed stars by the moon are of great importance also in determin ing the longitudes of places.