34 The Outline of Science 



turn onec on its axis. So for fourteen days there is continuous 

 night, when the temperature must sink away down towards the 

 absolute eold of space. This will be followed without an instant 

 of twilight hy full daylight. For another fourteen days the 

 MIII'S rays will hear straight down, with no diffusion or absorp- 

 tion of their heat, or light, on the way. It does not follow, how- 

 that the temperature of the moon's surface must rise 

 enormously. Jt may not even rise to the temperature of melting 

 ire. Seeing there is no air there can be no check on radiation. 

 The heat that the moon gets will radiate away immediately. We 

 know that amongst the coldest places on the earth are the tops 

 of ven- high mountains, the points that have reared themselves 

 nearest to the sun but farthest out of the sheltering blanket of 

 the earth's atmosphere. The actual temperature of the moon's 

 surface hy day is a moot point. It may be below the freezing- 

 point or rfbove the boiling-point of water. 



The Mountains of the Moon 



The lack of air is considered by many astronomers to furnish 

 the explanation of the enormous number of "craters" which pit 

 the moon's surface. There are about a hundred thousand of 

 these strange rings, and it is now believed by many that they 

 are spots where very large meteorites, or even planetoids, 

 splashed into the moon when its surface was still soft. Other 

 astronomers think that they are the remains of gigantic bubbles 

 which were raised in the moon's "skin," when the globe was still 

 molten, hy volcanic gases from below. A few astronomers think 

 that they a IT. as is popularly supposed, the craters of extinct 

 volcanoes. Our craters, on the earth, are generally deep cups, 

 whereas these ring-formations on the moon are more like very 

 shallow and broad saucers. Clavius, the largest of them, is 123 

 miles across tin interior, yet its encircling rampart is not a mile 

 high. 



The mountains on the moon (Fig. 16) rise to a great height, 



