SOLAR SYSTEM AND RADAR — GREEN AND PETTENGILL 277 



SOUAR PARALL-AX in SECONDS OF ARC. 



Venus Radar Average of 



1958 1 I — 1889-1924 



::ir 



Figure 5. — Solar parallax is often used to specify the mean distance from the earth to 

 the sun. Lincoln Laboratory's value from Venus radar experiments in 1958 is here 

 compared with others, the probable errors being indicated by the shadings: 1931 Eros 

 0.001 second of arc; 1950 Eros, 0.0004; 1958 Venus, 0.0001; and 1889-1924 (seven de- 

 terminations), 0.001 second. 



compared in figure 5. II. Spencer Jones' 1931 result is from tri angula- 

 tion of Eros in that year; E. liabe's 1950 determination is from 

 perturbations of Eros, while the 1889-1924 figure is the average of 

 seven optical methods. The value of 8.800 seconds of arc is not an 

 observed but an adopted one, used in ephemerides. With these is 

 compared the 1958 radar evaluation. The distressing thing about 

 this compilation is the wide variance among the proposed numbers, 

 with even the regions of probable error failing to overlap. It is 

 hoped that additional radar observations will clear up the discordance. 



But there is more to the story of interplanetaiy radar distance 

 measurements than refining the value of the solar parallax. The 

 method should ultimately allow the determination of the orbits of 

 some planets to within a few miles. Wlien this accuracy is attained, 

 gravitational perturbations of higher order will have to be considered 

 in interpreting what is observed. It should also be possible to study 

 the relativistic motions of the perihelia of several other planets besides 

 Mercury. Mars is especially attractive since it comes fairly near to 

 us, has a rather eccentric orbit, and has an atmosphere whose retard- 

 ing effect on the radar signal is probably negligible. 



The effect of the intervening medium on the signal's speed of travel 

 is important. By far the largest effect is caused by the dielectric 

 constant differing slightly from unity, owing to free electrons in 

 interplanetary space and in the ionospheres of the earth and the target 

 planet. This retardation is greater at lower frequencies. For the 

 440-megacycle frequency used in the Venus experiment, it was calcu- 

 lated that a distance error of less than one part in a million would 

 result from the combined effects of our ionosphere and an average 

 of 1,000 electrons per cubic centimeter throughout the intervening 

 space. Had the measurement been made at 50 megacycles, the corre- 

 sponding discrepancy would have been 1 part in 60,000. 



We can put this difference to work in studying the electron content 



