Page:Popular Science Monthly Volume 51.djvu/377

Rh But after the autumnal equinox and during the winter season all this is changed. Not only do the rings cause daily eclipses of the sun, but they give no illumination at night, for their dark side is then toward the observer, and they can be only "negatively visible," so to speak—that is, their position in the sky is shown merely by the absence of stars in that portion.

As to their appearance from various positions on the planet, it might be said that the whole system is visible above the horizon as far as latitude 41°—that of New York and Constantinople in our northern hemisphere, and Tasmania and New Zealand in the southern. At this latitude the inner edge of the dark ring will be upon the south point of the horizon, and the arch will extend about a third of the way toward the zenith. When latitude 51° is reached, that of Dresden and Winnipeg, Manitoba, the dark ring will have sunk below the horizon, but the whole width of the bright rings will be above it; and, finally, at latitude 66° 30′, that of our Arctic and Antarctic Circles, the entire system will have disappeared.

Of the illumination given by the moons in the absence of the rings we must say a little, since one often sees some statement to the effect that so many moons must compensate in some measure for the diminution of sunlight. But as the moons are illuminated by this very sunlight, their brilliancy is reduced in the same ratio, and in Saturn's case their total light in no wise makes up for this loss.

Reckoning from the best estimates of their sizes, we find that the total area on the sky covered by the moons when full is about two and a half times the area of our own moon, but their illumination, could they all be full at once, would be only the fortieth part of what we are accustomed to at the full. Then, again, as all of them except Japetus, the outer one, lie in the plane of the equator, it is evident that at the equinoxes, when this plane passes through the sun, they will all suffer total eclipse at the full, and will continue thus until the increasing inclination of the axis toward the sun brings their orbits one by one outside the shadow at this point. Thus we see that this numerous retinue does not amount to so much, after all, in the matter of illumination.

One other feature, and one which would doubtless be noticed first of all were any of us suddenly transferred to another planet, would be our change in weight due to the change in surface gravity. If we take the dimensions of Saturn as revealed by the telescope to represent its true size, we should find much less difference than one would expect, considering the tremendous size of the planet. The combination of three factors—the much greater distance of the surface from the center of the planet,