cessively all the forms indicated in Plate XII.* At the vernal equinox the edges of the shadow are straight, as in fig. 1; at and near the summer solstice the outline of the shadow is part of an ellipse, an extremity of whose longer axis lies within the outer edge of the rings; in all intermediate cases the outlines of the shadow are parts of an ellipse of considerable eccentricity. The interval of time in which the shadow changes from the form indicated in one figure to that indicated in the next is about 384 days. It is easy to determine the manner in which the vast shadow of the planet sweeps over the illuminated face of the rings. At sunset, at and near either equinox, the rings are illuminated throughout their visible extent in all latitudes. Near the equator the shadow of the planet rises in the east, as soon as the sun has set,† eclipsing at once the whole breadth of the rings near the horizon; in higher latitudes the shadow rises later, eclipsing first the outer edge of the rings. Later in the Saturnian year the curvature of the shadow shows its effect; the parallel of latitude within which the eclipse commences along the inner edge of the rings passing higher and higher, until it includes all latitudes within which the rings are visible. Near the summer solstice the outer edge of the outer ring is not eclipsed at all. The shadow also rises later and later to midsummer; but as the nights grow shorter and shorter, and as in high latitudes this change takes place at a greater rate than the change in the hour at which the shadow rises, it will happen that, in high latitudes, great parts of the ring are already in shadow when the sun has set. In all latitudes and at all seasons the central line of the shadow crosses the meridian at midnight. At this hour a very small part of the ring is visible, even from points near the equator, near the time of either equinox; but, for about three years, near the time of the summer solstice, the outer edge of the ring is not in shadow at midnight. At this time the system must present a magnificent appearance, as a vast double arch of light, indented by a broad elliptical shadow. Owing * The point of view in these figures is supposed to lie in the axis produced of the planet; the lines aA A', L L', M M', TT and E E', correspond to the lines similarly lettered in fig. 3, Plate XI.; and the circles n n'n", a a'a", I l'l", m m'm", t t't" and e e'e" to the lines similarly lettered in fig. 2, Plate XI. † Owing to refraction the shadow doubtless rises before sunset, just as the eclipsed moon is sometimes visible while the sun is yet apparently above the horizon. |