THE PRIME RESULTANT 65 



definite line but also to and through a definite, unmarked, 

 mathematical point in that line. This line I call the 

 Prime Resultant, and this point, the Vertex, which latter, 

 in the present case, coincides with the dynamical (not the 

 celestial) pole of the ecliptic. The Vertex, then, may be 

 described as the hub or localized center of attraction for 

 our particular planetary system, just as the earth's 

 center is for all objects within, say, her atmospheric en- 

 velope. To make the matter perhaps still clearer, consult 

 again the example of the three bodies, A, B, and C, which 

 we assumed to be arranged in the form of an equilaterial 

 triangle. There the center of the triangle is the " ver- 

 tex " or converging point for all three bodies, and the dis- 

 tance from that point to either of the three severally is 

 the prime resultant for that one particular body. 



Let us return now to the position we originally oc- 

 cupied, directly over and far south of the sun, and looking 

 straight down on the broad plan of the solar system, and 

 let us suppose that we remain thus fixed in space so that 

 we may the better analyze the movements of the group. 

 It is customary with astronomers, repeating, in their way, 

 the awkward method of geographers, to imagine them- 

 selves surveying the ecliptic from the north. According 

 to their way of looking, the motions of the earth on its 

 axis, of the planets around the sun, and of the moon 

 around the earth, are all contra-clockwise (like a clock 

 dial reflected from a mirror), whereas viewed from the 

 south, as I advise, those motions all turn clockwise. It 

 is well to get clearly in mind right at the very outset the 

 fact that all of these motions run in the same direction. 

 This is the rule throughout the system (with a very few 

 minor exceptions among the satellites, which will be sub- 

 sequently considered). 



My object in viewing the ecliptic from the south in- 

 stead of from the north is, however, of much greater 

 significance than merely to aid the memory by substitut- 

 ing a right-handed motion in place of a left-handed one. 

 In fact, it is for the same reason that I recommend the 

 printing of terrestrial maps inverted, namely, to instill 

 and cultivate the sense that we are falling, and that north 



