Page:The New International Encyclopædia 1st ed. v. 06.djvu/720

* ECLIPSE. 628 ECLIPSE. sake, fail, from ix, ek, out + el-irtiv, leipein, to leave). JSoinetiiiies one or other of the self- luminous lieuvonly bodies is shut off from our vieAv temporarily by the interposition ol some other body between it and the earth. This is called an eclipse when the object temporarily obscured is the sun, and an occultation (q.v. ) when it is a star. Non-luminous bodies, which shine only by rcflectinjr solar li>;ht, may be 'ecliiised' by having the light cut off by a body passing l)etween themselves and the sun; and they nuiy be "occulted' by being concealed behind a body passing between themselves and the earth. The term "transit' is applied to the partial con- cealment of the sun by the passage across hia face of Venus or Jlercury, and to the similar phenomena of- the passages of Jupiter's satellites across his disk. The causes of eclipse as here suggested are so simple and fajniliar that it is ditlicult for us to imagine how deeply eclipses affected men's minds before the dawn of astro- nomical science. At Rome, at one time, it was blasphemy, and punished by law, to talk publicly of their being due to natural causes. According to some, Luna, when in eclipse, was in the pains of labor; according to others, she was suffering from the arts of wicked magicians. The Chinese imagine eclipses to be caused by great dragons trying to devour the sun and moon, and accord- ingly they beat drums and brass kettles to ter- rify the monsters into letting go their prey. Solar eclipses can occur only at the time of new moon, when the sun and moon are in con- junction on the same side of the earth. In a partial eclipse, the sun's disk suddenly loses its circular form; it becomes indented on one side, the indcntatit>n slowly increasing for some time, and then diminishing until it disappears alto- gether. In a total eclipse, the indentation goes on increasing till the whole orb disappears for a time; after a short interval, the sun rea])pears again, passing through the same phases of obscuration in an inverse order. In an annular eclipse, the whole sun is obscured except a ring or annulus. Lunar eclipses, on the other hand, always occur at full moon, or when the sun and moon are in opposition, and are caused by the moon passing through the earth's shadow. Such eclipses are sometimes partial and sometimes total, but never annular, and in their general phases they resemble those of the sun. The dviration of an eclipse is the time of its con- tinuance, or tlic interval between immersion and emersion. Immersion, or incidence, of an eclipse is the moment when part of the luminary begins to be obscured ; emersion is the time when the luminary begins to reappear or emerge from the shadow. When the size of an eclipse is men- tioned, the part of the luminary obscured is ob- tained by dividing its diameter into twelve digits; and tlic eclipse is said to be of so many digits, according to the number of them contained in that part of the diameter which is obscured. Eclipses of thk ^Moo.n. It has been said that these are caused by the moon passing through the earth's shadow. Before this explanation can be accepted, it must be shown that that shadow e.xtends as far as the moon. This is easily done. Supposing the earth to have no atmosphere, then the shadow is the cone marked in shade in Fig. 1, in which T and S are the centres of the earth and sun. The cone's apex is at O; and the question is, whether the distance OT from the apex to the earth's centre exceeds the moon's distance from the earth. Drawing TB, SA, from the centres of the earth and sun. respectively, perpendicular to the line OBA, tangent to both spheres, and the line TC parallel to the line OBA, we have from the similar triangles 0TB TSC, the proportion, OT : TB : : TS : SO. Now, we know that TS, the (mean) distance of the sun, is equal to al)out •24.000 times TB; also, FlQ. 1. from the construction, AC = TB ; and we know that SA =112 times I'B ; whence it follows that SC =111 times TB. The above proportion then gives OT = 21U times TB, since -JJi'" =216 nearly. But the moon's average distance is only 60 times TB (the earth's radius). Hence it appears that the length of the earth's shadow is almost four times the average distance of the moon, and that the moon can enter it. Further, it is clear that, should it do so, it may be totally obscured; for it must enter at a point much nearer T than half the distance OT, which is 108 times TB : and everywhere within that distance, it might be shown, the breadth of the shadow is much greater than the moon's disk. But one consideration now remains to be stated to com- plete the proof of the theory of lunar eclipses. It was mentioned that they only occur at full moon, and we know that to be the only time when the earth is between the sun and moon, and so has a chance of throw'ing her shadow upon it. Why they do not occur every full motm will be explained in treating of the prediction of eclipses. In the foregoing explanation, we proceeded on the assumption that the earth has no atmos])here. If the assumi)tion were correct, the earth's shadow would be darker and narrower than it is, and the phenomena of eclipses more sudden. The effect of atmospheric refraction (see Refrac- tion) is to bend the rays which fall on the atmosphere in toward the axis of the cone of the earth's shadow, those which jiass through the lowest strata of the air being refracted through an angle of 1° 8', and converging to a point in the line OT (Fig. 1) considerably nearer the earth than the point where the moon crosses her shadow. As a result of this, the moon never enters that part of the shadow which is com- pletely dark; thus, she never loses her light en- tirely. I)Ut appears of a distinct reddisli color Fia. 2. resembling tarnished copper — an appearance caused by the atmospheric refraction in the same way as the ruddy color of the clouds at sunset. There is another reason why the phenomena of a lunar eclipse are Icis striking than might he expected from the explanation given relative to Fig. 1. Every shadow cast by the sun's rays