THE PLANET SATURN, 365 



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 " nega- 

 tively 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 GG 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 illumi- 

 nation, 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 illumi- 

 nation. 



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 dif- 

 ference 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. 



