502 THE POPULAR SCIENCE MONTHLY 



and Seeliger's results to fortunate chance, and it is scarcely possible 

 to doubt that in the zodiacal-light materials lie the causes of the dis- 

 crepancies referred to. I have little hesitation in venturing the 

 opinion that Seeliger's investigation marks an epoch in the application 

 of Newton's law of gravitation to the motions within the solar system. 

 At one stroke he appears to have removed a group of discrepancies 

 which served as bases for many inquiries as to the preciseness and 

 sufficiency of the Great Law. With all respect to Seeliger's genius and 

 labor, however, scientific caution will value confirmation of his results 

 by other investigators. 



Seeliger's assumptions as to the distribution and mass of the zodiacal 

 material are of interest, especially when we recall that the zodiacal light 

 within some 20 degrees of the sun is unobservable, on account of the 

 glare, and that the brightness of the light is a poor index to the mass : 

 a given quantity of matter, finely divided, would reflect sunlight more 

 strongly than the same quantity existing in larger particles. For the 

 mathematical development of the subject he assumed that the material 

 is distributed throughout a space represented by a much-flattened 

 ellipsoid of revolution whose center is at the sun's center, whose axis 

 of revolution coincides more or less closely with the sun's axis, whose 

 polar surfaces extend 20 or 30 degrees north and south of the sun 

 (as viewed from the earth), whose equatorial regions extend consider- 

 ably beyond the earth's orbit, and in which the density-distribution 

 of materials decreases as a function both of the linear distance out from 

 the sun and of the angular distance out from the equatorial plane 

 of symmetry. According to these assumptions, surfaces of equal 

 densities are concentric ellipsoidal surfaces, and the number of such 

 ellipsoids can be increased or decreased according as the computer 

 may desire to represent more or less closely any assumed law of 

 density-variation within the one great spheroid. Practically, Seeliger 

 found that the disturbing effects on the planets are almost independent 

 of the law of distribution of the material, as related to distance from 

 the sun, as far out as two thirds of the distance to Mercury. He 

 made use of only two ellipsoids : One with equatorial radius 0.24 unit" 

 and polar radius 0.024, of uniform density; and the other with corre- 

 sponding radii 1.20 and 0.24, of uniform but much smaller density. 

 The total mean densities determined for his volumes, on the basis of 

 unity as the mean density of the sun, are, respectively, 2.18 X 10"^^ 

 and 3.1 X 10"^^; and the resulting combined mass of the two ellipsoids 

 is 3.1 X lO"'^ of the sun's mass, which is roughly twice the mass of 

 Mercury. The corresponding density of mass-distribution is sur- 

 prisingly low. In the inner and denser ellipsoid, the matter, if as 

 dense as water, would occupy 1 part in 30,000,000,000 of the space; 

 if as dense as the earth, only 1 part in 160,000,000,000. 

 ^The distance from the sun to the earth being 1.00. 



