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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1957 



are varying in a random manner about a certain mean level. These 

 ripples can be greatly reduced by integrating the signals over long 

 periods of time. 



One of the most impressive ways of displaying the noise from the 

 cosmos is to use a loudspeaker system. The sun and local galaxy can 

 be heard as a gentle hiss; the galactic noise remains steady but the 

 storms on the sun swell and fade many times during the course of an 

 hour. Jupiter is the performer that really dominates the air. When 

 heard over a high-fidelity system, its roars and rumbles almost con- 

 vince one that the Romans were right in their ideas about the gods. 

 For quantitative work, however, it is essential to obtain a permanent 

 record in a form amenable to analysis. If the signal is fed to a 



* RADIO 



STAR 



RECEIVER 



SIGNAL 



TIME 



Figure 2. — A radio interferometer and the signal it produces when a radio star passes 

 through the antenna pattern. 



milliammeter with a pen attached to the arm, a mark will be made 

 which is proportional to the intensity of the signal. If the mark 

 is made on a roll of paper driven at a constant speed then a precise 

 intensity-time graph is produced. Radio stars can be observed by 

 sweeping the telescope slowly across the sky, for when the star is in 

 the center of the beam the pen gives a maximum deflection. One of 

 the most convenient scanning arrangements is to clamp the telescope 

 and utilize the rotation of the earth. This has been the preferred 

 method with an interferometer because the baseline is long and the 

 instrument is mechanically unwieldy. The sensitive beams are there- 

 fore allowed to drift across a star as the earth rotates and the pen 

 record varies rhythmically as shown in figure 2. A star of small 

 diameter produces well-defined maxima and minima, but a large 



