Figure 7.13. The rolling motion of one planetary sphere on another: In this illustration, the labeled axis emerging from the sphere at point B gives rise to motion along the arc B1 to B5. The comparison of this motion to that on the arc A1 to A5 (the celestial equator in this example) gives rise to the relative motion shown to the left. This is the hippopede. Drawn by E.F. Milone, after Pannekoek (1961/1989, Fig. 8, p. 109).

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