290 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 7 



varies between the Equator and the Poles. The Equator rotates 

 once in 9 hours 50 minutes 26 seconds, and the corresponding figure 

 at the Pole is 9 hours 55 minutes 24 seconds. By timing the variation 

 of the signals the latitude of the source can be obtained. This is, of 

 course, not a very exact determination, and the method is further 

 complicated by the presence of more than one transmitting area. 

 Despite these difficulties the main noise area has already been located. 

 It is close to the famous red spot which has been observed in Jupi- 

 ter's atmosphere since 1664. Surprisingly little is known about the 

 spot from the optical observations. One hypothesis suggests that 

 it is an island of solid ammonia or methane floating in the dense at- 

 mosphere, while at the other extreme it is considered to be the product 

 of an active volcano. Perhaps the radio observations will help us to 

 determine the true nature of this disturbance. 



Radio observations have given indications that Jupiter may be 

 surrounded by an ionosphere. The red-spot region does not produce 

 signals at every position as it rotates. There appears to be an attenua- 

 tion of the noise as the spot approaches the east or west limb and this 

 has been explained by reflection effects in the ionosphere. The double 

 and triple pulses forming the rumble are also explained in terms of 

 the ionosphere. A signal from some disturbance in the atmosphere 

 is received by direct transmission to produce the first pulse, while the 

 second pulse is the echo produced by the surface of Jupiter. The 

 third component is reflected from the ionosphere back to the surface 

 before reaching the receiver on the earth. 



RADAR ASTRONOMY 



We are not limited to passive reception of signals. Great advances 

 were made during the Second World War in the detection of aircraft 

 by means of radio echoes. In the same way a high-power transmitter 

 can be made to send out a series of pulses which will be reflected off 

 celestial objects. 



Meteors are the nearest bodies of interest in astronomy, for although 

 they spend many years circulating between the planets, they spend the 

 last second of their life in the atmosphere of the earth about 60 miles 

 up. The meteor particle collides with the atmosphere at such a high 

 velocity that it completely evaporates, producing heat, light, and 

 ionization. By studying the echoes from the column of ionization it 

 is possible to measure the velocity of the meteor with fair precision. 

 With three or more radar stations one can determine the direction of 

 motion of the meteor. Velocity and direction together define its orbit, 

 or life history, and we can then trace back its path among the planets. 

 Radar observations have shown that meteors are members of the solar 

 system and do not come from the space between the stars. We now 



