860

HISTORY OF PHYSICAL ASTRONOMY.

appears to describe a great circle in tlie heavens, inclined at an incon-siderable angle to the ecliptic. Hence it happens that, when the time ofher conjunction with the sun coincides with her passage across theecliptic, she appears projected upon the region of the heavens occupied bythe sun, and, being an opaque body, conceals the whole or a portion of hisdisk from a spectator on the earth. The defect of light arising from thiscause is what is termed a solar eclipse. It is manifest that the mooncannot wholly intercept the solar light, unless during conjunction herapparent diameter exceed the apparent diameter of the sun. This condi-tion, however, is not always fulfilled, for, as the moon revolves in an orbitof considerable eccentricity, her distance from the earth varies in asensible degree, and hence arises a corresponding variation in her apparentdiameter, causing it sometimes to exceed the apparent diameter of thesun, and other times to fall short of it. Hence, when the centres ofthe sun and moon are projected upon the same point in the heavens,and the apparent diameter of the moon at the same time exceeds thatof the sun, the solar disk will be entirely concealed from the view ofthe spectator, and a total eclipse will be the consequence. If, however,while all other circumstances are alike, the apparent diameter of the moonbe less than that of the sun, the solar disk will not wholly cease to bevisible, but the opaque body of the moon will appear projected centrallyon it, leaving a narrow ring of light exposed to view. In this case, thereoccurs the interesting phenomenon of an annular eclipse. When themoon does not pass centrally over the sun, but merely overlaps a segmentof his disk, a portion of his light only is prevented from reaching theplace where the spectator is situate, and a partial eclipse then ensues.

During a total eclipse of the sun it is manifest that the place on thesurface of the earth which happens from this cause to be obscured, is thensituated within the moon’s shadow. When only a portion of the solar lightis intercepted by the moon as in the case of an annular or partial eclipse,the place to which the eclipse is visible is situate beyond the limits of thelunar shadow in a partially-obscured region termed the penumbra.

Since the moon in passing between the sun and the earth intercepts the so-lar light, either wholly or partially from some place on the surface of the latterbody, so when, in the course of her synodic revolution, she arrives in the op-posite point of her orbit, the earth being now interposed between her and thesun, she is deprived in her turn of the solar light, and the phenomenon ofa lunar eclipse is the consequence. It is to be remarked that in all lunareclipses, the solar rays are supposed to be tvhoUy intercepted from theobscured surface, those cases in which a partial deprivation of light onlyoccurs, being left out of consideration, on account of the difficulty of deter-mining the exact instant when the obscuration begins or ends. Hence alunar eclipse corresponds to a total eclipse of the sun in regard to its specificcharacter, the surface obscured being, in each case, actually immersed inthe shadow of the eclipsing body.

If the moon revolved in the plane of the ecliptic, she would necessarilypass between the sun and the earth at every conjunction, while again atevery opposition, the earth would be interposed between the sun and moon.Hence a solar eclipse would take place at every new moon, and a lunareclipse at every full moon. The lunar orbit being, however, inclined at anangle of about 5° to the plane of the ecliptic, an eclipse, whether solar orlunar, can only occur, when the moon during conjunction or opposition isnear either of the nodes of her orbit. Astronomers have found by calcula-