THE SOLAR SYSTEM — LOVELL 289 



of uniform strength after scattering from the Imiar surface. In fact, 

 very marked irregularities in the strength of the returned echoes were 

 found. The individual pulses, separated in time by a second or so, 

 varied in strength and there was also a long-period variation in the 

 average strength of the returned signals with periods of 15 or 30 

 minutes. It appears that these short-period and long-period effects 

 are quite different phenomena. The long-period variation is the re- 

 sult of an influence on the radio waves of the earth's magnetic 

 field as they traverse the space between the earth and the moon. 

 Most of this influence occurs in the ionized regions of the earth at a 

 height of about 200 to 400 kilometers, and the variation is caused by 

 the rotation of the plane of polarization of the radio waves — the 

 Faraday effect occurring in the earth's ionosphere. The exploitation 

 of this effect in a systematic manner has provided a method of 

 measuring the total number of electrons between the earth and the 

 moon. 



The short-period fading which takes place in periods of seconds 

 has a different origin. This fading is an effect of the libration of the 

 moon. Because of the irregularities of the slight ellipticity of the 

 motion of the moon around the earth, it never presents exactly the 

 same face but gives the effect of a slight oscillation laiown as libration. 

 It seems that the nature of the lunar surface is such that even for 

 radio wavelengths it does not reflect as a smooth body but has a num- 

 ber of plateaus which reflect the radio waves back to earth. The re- 

 flecting qualities of adjacent parts of the limar surface differ so much 

 that we get these very large variations in amplitude. An investiga- 

 tion of the statistics of this phenomenon leads to a surprising con- 

 clusion. In the case of the reflection of light, the moon behaves 

 like a ball of chalk which appears almost uniformly bright in a beam 

 of light. On the other hand, when radio waves are directed toward 

 it the moon scatters similarly to a polished ball-bearing in a beam of 

 light — the central region of the ball appearing much brighter than 

 the remainder of the surface. Wlien radio waves are reflected from 

 the moon it seems that they are not returned uniformly from the whole 

 forward hemisphere of the moon but predominantly from a small part 

 of the forward hemisphere — a hemispherical cap only about a fifth of 

 the radius of the lunar surface. This is a striking illustration of the 

 overall smoothness of the moon as far as wavelengths of the order of 

 a meter or so are concerned. 



This discovery had an interesting practical result. The suitability 

 of the moon had often been considered in relation to the problem 

 of bouncing radio messages from one side of the earth to the other, 

 using radio wavelengths so short that the earth's ionosphere was 

 penetrated and, therefore, there could be no interference from sun- 

 spots. It had been decided that this was impossible because the moon, 



