373 



A Short History of Astronomy 



[CH. XIII. 



which goes farthest from it. All the orbits are described 

 in the standard, or west to east, direction. The most 

 interesting characteristic in the distribution of the minor 

 planets, first noted in 1866 by Daniel Kirkwood{ 1815-1895), 

 is the existence of comparatively clear spaces in the regions 

 where the disturbing action of Jupiter would by Lagrange's 



FIG. 89. Paths of minor planets. 



principle (chapter XL, 243) be most effective : for instance, 

 at a distance from the sun about five-eighths that of Jupiter, 

 a planet would by Kepler's law revolve exactly twice as fast 

 as Jupiter ; and accordingly there is a gap among the minor 

 planets at about this distance. 



Estimates of the sizes and masses of the minor planets 

 are still very uncertain. The first direct measurements 



