~~ Ses 
140. : | The Sky 
direct measurement, over one hundred miles in diameter. Be- | 
sides the giant craters there are numerous smaller ones of all ~ 
different sizes and in all different states of decay. On the © 
whole, the larger craters seem much older and present a more ~ 
tumbledown appearance than do the smaller ones, presum- — 
ably indicating that the smaller craters were formed at a later | 
date. Of course there is at present no erosion of the moon’s © 
surface due to water or winds because of the absence of — 
those elements, but there are very extreme changes in tem- — 
perature at least twice a month on the moon’s surface so that E 
surface rocks might easily be cracked and spalled off by this @ 
agency alone. Add to this the incessant hail of meteorites 
upon the moon, and the possibility of other agencies in the 
past, and it will be agreed that, whereas the moon’s features ~ 
are more permanent than geological formations of the earth, — 
they are not exactly eternal. 3 
tierce n pei Sy 
SEAS WITHOUT WATER 
Other features of the moon’s surface are the extensive 
“seas or maria—great expanses of dark, even material. Ge- 
ologists regard the moon’s maria as the remains of past out- 
flowing of molten rock from beneath the moon’s surface 
that have now cooled off and hardened into solid rock. These 
maria are responsible for those dark portions of the moon, | 
visible to the naked eye, whose irregular shapes have sug- 
gested such popular fancies as the “man in the moon’ or the 
“hunter and his dog.” 
LUNAR THEORY 
The precise prediction of the moon’s pathway through the 
sky has been a classic problem for the astronomers of many 
