EVOLUTION OF THE SOLAR SYSTEM. 
529 
From these considerations we may conclude that the effect of the separation of a 
satellite is to destroy planetary rotation, but to preserve angular momentum within 
the planetary subsystem. 
Hence we ought to find that those planets which have large satellites have a slow 
rotation, but have a relatively large amount of angular momentum within their 
systems. 
A proper method of comparison between the several planets is difficult of attain¬ 
ment, but these ideas seem to agree with the fact that the earth, which is large 
compared with Mars, rotates in the same time, but that the whole angular momentum 
of earth and moon is large."' 
* A method of comparing the various members of the solar system has occurred to me, but it is not 
founded on rigorous argument. 
It seems probable that the small density of the larger planets is due to their not being so far advanced 
in their evolution as the smaller ones, and it is likely that they are continuing to contract and will some 
day be as dense as the earth. 
The proposed method of comparison is to estimate how fast each of the planets must rotate if, with 
their actual rotational momenta, they were as condensed as the earth, and had the same law of internal 
density. 
The period of this rotation may be called the “ effective period.” 
With the data used above, taking the earth’s mean density as unity, the mean density of Mars is '675, 
that of Jupiter '235, that of Saturn T25 or '111 or '074, according to the data used. 
To condense these planets we must reduce their radii in the proportion of the cube-roots of these 
numbers. 
Their actual moments of inertia must be reduced by multiplying by the -frd power of these numbers, 
and as we suppose the law of internal density to be the same as in the earth, the moments of inertia of 
Jupiter and Saturn must be also increased in the proportion '33438 to '26138. 
Then the “ effective period ” will be the actual period reduced by the same factors as have been given 
for reducing the moments of inertia. 
In this way I find that the Martian day is to be divided by 1'3; the Jovian day by 2; and the Saturnian 
day by 3'14 to 4'44 according to the data adopted. The earth’s day of course remains unchanged. 
The following table gives the results. 
Table IV. 
Planet. 
Actual period of 
Effective period of 
rotation. 
rotation. 
Earth .... 
23 1 * 56 m 
23 h 56 m 
Mars. 
24 h 37 m 
19 h 
Jupiter .... 
9 h 55 m 
5 h 
Saturn .... 
1CF 29 m 
3 h 20 m to 2 h 20 m 
This seems to me to illustrate the arguments used above. For there should in general be a diminution 
of effective period as we recede from the sun. 
It will be noted that the earth, although ten times larger than Mars, has a longer effective period. The 
larger masses should proceed in their evolution slower than the smaller ones, and therefore the greater 
proximity of the earth to the sun does not seem sufficient to account for this, more especially as it is 
