SOLAR SYSTEM AND RADAR — GREEN AND PETTENGILL 273 



TURNING AXIS 



ECHO 



TO EARTH C 



ECHO 



in 



RECEDING 



Decreased 

 Frequency 





Normal 

 Frequency 



APPROACHING 

 SIDE 



MOON 



MOON 



APPROACHING 



Increased 

 Frequency 



Figure 3. — Mapping the moon by radar. In diagram I, by selecting thatpart of the echo 

 within a limited interval in range, we observe only the shaded area, a ring on the moon 

 as seen from the earth In IL C is the center of the disk, and AB is the axis around which 

 the moon appears to turn, because of changing libration. Here AB has been arbitrarily 

 drawn perpendicular to the line of sight. Half of the turning disk is approaching, half 

 receding. By further selecting a limited frequency interval, we observe only the narrow 

 strip of disk between the vertical lines. In III, looking down on the moon, we see one 

 of two small areas thus isolated by the combined selection of range and frequency. Com- 

 pare III with the observational record in figure 4. 



