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THE POPULAR SCIENCE MONTHLY. 



serving its displacement in the sky as seen from remote points on the 

 earth. The relative distances of the planets are easily found in sev- 

 eral different ways, 1 and are known with very great accuracy the 

 possible error hardly reaching the ten-thousandth in even the most 

 unfavorable cases. In other words, we are able to draw for any mo- 

 ment an exceedingly accurate map of the solar system the only ques- 

 tion being as to the scale. Of course, the determination of any line 

 in the map will fix this scale ; and for this purpose one line is as good 

 as another, so that the measurement of the distance from the earth to 

 the planet Mars, for instance, will settle all the dimensions of the 

 system. 



Fig. 2. 



The figure illustrates the method of observation. Suppose two 

 observers, situated one near the north pole of the earth, the other 

 near the south. Looking at the planet, the northern observer will see 

 it at JV (in the upper figure), while the other will see it at /S, farther 

 north in the sky. If the northern observer sees it as at A (in the 

 lower part of the figure), the southern will at the same time see it as at 

 B ; and, by measuring carefully at each station the apparent distance 

 of the planet from several of the little stars (a, 5, c) which appear in 

 the field of view, the amount of the displacement can be accurately as- 

 certained. The figure is drawn to scale. The circle E being taken 

 to represent the size of the earth as seen from Mars when nearest us, 

 the black disk represents the apparent size of the planet on the same 

 scale, and the distance between the points JVand S, in either figure A 

 or , represents, on the same scale also, the displacement which would 



1 One method of determining the relative distances of a planet and the sun from each 

 other and from the earth is the following, known since the days of Hipparchus : First, 

 observe the date when the planet comes to its opposition i. e., when sun, earth, and 

 planet, are in line, as in the figure, where the planet and earth are represented by M a,nd 

 R Next, after a known number of days, say one hundred, when the planet has advanced 



