Page:The New International Encyclopædia 1st ed. v. 06.djvu/722

* ECLIPSE. 6S0 ECLIPSE. earth are, or arc nearly, equal to the value, above stated, the moon's shadow extends to the earth and beyond it. Should the shadow in these eir- cuinsta'ntes fall upon the earth, there will be a total eclipse of the sun in all places within it or over which it moves. If h (Fig. 3) be the moon. T the earth, and abi. the moon's shadow cast by the sun, there will be a total eclipse of the Sim at every point that is completely within the portion ob'of the eartli's surface. Again, the smallest value of the length of the moon's shadow may be shown to be about 5S semi-diam- eters of the earth, and the greatest distance of Fig. 3. the moon from the earth is about 64 semi- diameters. Suppose the moon interposed be- tween the earth and sun when these values concur, it is dear that the moon's shadow will fall short of the earth. In this case, the sun cannot be altogether hid from any point of the earth's surface: but this case, or one approxi- mate to it. is that in which there will occur an annular eclipse. In Fig. 4, suppose O to be the JEl Fig. 1. npc^ of the shadow which falls short of the earth, and eoueeive the cone of the shadow pro- duced earthward beyond O into a second cone Ord; then from every point within the section cd of the earth's surface, the moon will be seen projected as a black disk on the middle of the disk of the sun, the portion unobscurcd form- ing a ring or annulus of light. While in the two cases just described the eclipse is total or regular at places within nh (Fig. 3) or cd (Fig. 4U respectively, it will be partial at other places: the moon will appear projected against a portiim of the sun's dislc, making a cir- cular indentation. To ascertain the ])laces at which the eclipse will be partial, we have merely to form the cone of the penunibra of the moon's shadow in the manner explained in treating of lunar eclipses: at all places on the earth's surface within that cone there will be a partial eclipse. . simple calculation shows what is the observed fact, that the cone of the penumbra is not nearly large enough to em- brace the whole of the face of the earth di- reet^-d to the sun: in other words, solar eclipses are not universal, like those of the moon, i.e. they are not seen from all ])laces that have tWfc sun above their horizon at the time of the eclipse, which is the reason that though they are of more frequent occurrence than lunar eclipses, the latter are more frequently seen by the public, and therefore commonly supposed to occur more frequently. There are" certain appearances, at- tending an eclipse of the sun, when it is total, that are verj' remarkable. The darkening of the orb of day, more particularly when it is unlocked for, is calculated to impress a spectator with va"ue terror; even when expected, it hlls the mind with awe, as a demonstration of the forces and motions of the mechanism of the universe. The sudden darkness, too, is impressive from its strangeness as much as from occurring by day; it resembles neither the darkness of night nor the gloom of twilight. Stars and planets appear, and'lill animals are dismayed by the gloomy as- pect of nature. There is one important phenomenon attending total eclipses of the sun. which is always seen and the cause of which cannot be said to be as yet fully understood. As long as the total eclipse lasts, there appears round the sun and moon a luminous corona (q.v.), while at its base, and projecting beyond the dark edge of the moon, appear verv brilliant prominences, gen- erally of a red color. These prominences are found to be constant attendants on eclipses, and methods have been invented for rendering them visible at any time without the interposition of the moon. (See Sun.) The spectroscope shows that they consist mainly of hydrogen gas in an incandescent state. The prominences are sometimes seen to shoot up like llames. in wild fantastic shapes, with incredible velocity, and tc the height of hundreds of thousands of miles. PuEDicTios OF Soi..K Eci.iPSES. The period of IS .Julian vears 11 days, referred to in treating of the prediction of lunar eclipses, applies equally to solar eclipses: but tlic ancients, who under- stood that fact, could (ind no law of recurrence of solar eclipses within that period so as to pre- dict them. The reason of the failure is obvious; for thouirh solar eclipses recur in a fixed order within the cycle. they are not visible at the same places on their ' recurrence as when first observed. By modern methods, however, eclipses of tlic sun 'may be predicted, with all their circumstances of time and places of obsenation, with the most perfect certainty. At the time of a solar celi]>se. the sun and moon are in con- junction; they arc also in or near the same node: and no eclipse can haiipcn if they are further than 1S° from the node, or if the hilitnde of the moon, viewed from the earth, exceeds the sum of the apparent scmi-diamcters of the sun and moon. ^^ hen within these limits, it is a problem of numbers and of si)herical trigonome- try to ascertain whether an eclipse will actually occur and what its circumstances will be. The number of eclipses of the sun and moon together in a year cannot be less than two— in which case both are solar— or more th:in seven, five solar and two lunar, or four solar and three lunar: but total solar eclipses are extremely in- frequent in any one place, compared with the .actual frequency of their occurrence. Thus total eclipses were visible somewhere in the I'nited States during the nineteenth century only in the vears 1800. 1S34. 1800, ISfiO, 1878. 1-S80, 1S80,' 1000: and in the present century-, such eclipses will be visilile in the years 1918, 1023, 1025, 1945, 1954, 1970. 1984, and 1094. For a very complete list of eclipse dates, consult: Op- polzer. Canon der Fitistcrnisse (Vienna, 1887); Newcomb, "On the Keeurrence of Solar Eclipses, with Tables of Eclipses from n.c. 700 to ..D. 2300." in Astrovomiral Papers (Washington, 1882). See Solar System; Si'n.