RADIO ASTRONOMY — HAWKINS 291 



believe that meteor fragments are shed by a comet as the icy nucleus 

 of the comet evaporates in the heat from the sun. 



Farther out from the earth we come to the moon, and radio echoes 

 have been obtained from the moon by many experimenters. At a dis- 

 tance of 200,000 miles, radar astronomers have to wait for a period of 

 about 2 seconds before the echo returns. The echo is subjected to 

 many effects on its journey to and from the moon and from the way it 

 has changed we can learn many interesting things about the atmos- 

 phere of the earth and the surface of the moon. The radio wave form- 

 ing the echo is formed, of course, from oscillatory electric and mag- 

 netic fields which are at right angles to each other. When the electric 

 field is parallel to the receiving dipole a maximum signal is produced. 

 In this way the direction of the field can be determined. It is found 

 that the field is rotated many times as the echo pulse travels to the 

 moon and back. Most of the rotation occurs in the ionosphere of the 

 earth, as it is proportional to the electron density of the transmitting 

 medium and the strength of the magnetic field of the earth. This ro- 

 tation gives us information about the ionosphere at great heights above 

 the earth's surface. 



As the radio pulse is reflected from the surface of the moon the 

 mountain ranges and craters cause interference so that the echo power 

 fluctuates. This effect is not unlike the glitter that is seen when light 

 falls on a rough, shiny object. There are other things that cause the 

 signal to fluctuate more rapidly than the interference from a rough 

 surface, but the origin of these rapid variations is at present unknown. 



Radar astronomy will probably never become as spectacular as 

 radio astronomy. With pulse techniques we certainly are making our 

 first venture out into space, and the radio pulse can certainly visit and 

 explore the moon even if mankind at present is limited to the earth. 

 But we will require tremendously powerful transmitters if we are to 

 bounce an echo off our neighboring planets such as Venus and Mars. 

 To reach the nearest star is impossible : even if we did have sufficient 

 transmitter power we would have to wait eight whole years for the 

 echo to return. The output of the natural transmitters of the cosmos 

 is far greater than any we can make on the earth. Cygnus A, for 

 example, on the edge of the visible universe, puts out a power which 

 is more than a billion times greater than our man-made signals. Such 

 considerations help us to realize our insignificant position as earth- 

 bound mortals, and impress upon us the grandeur of the natural 

 universe. 



