164 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1921. 



cheating that the sun was in a gaseous state with high central con- 

 densation when the planets were born. The jets of matter thrown 

 out would also be gaseous, but would rapidly cool in the process of 

 ejection, and might soon liquefy or even solidify. It can be shown 

 that the planets which would be formed out of the middle portion 

 of such a jet ought to be much more massive than those formed near 

 the ends, and this may possibly provide an explanation of the com- 

 paratively great masses of Jupiter and Saturn. We imagine that 

 the planets at first described orbits under the combined gravitational 

 action of the sun and the passing star by which the cataclysm was 

 caused, but as this star receded they were left revolving, as at pres- 

 ent, around the sun. During their earlier motion they may them- 

 selves have been broken up by the tidal action of one or both of 

 the big masses present, and such a process may explain the origin 

 of the satellites of the planets. 



Such, in its main outlines, is the tidal theory of the genesis of 

 the solar system. So far as can be seen, a vast amount of further 

 mathematical research is needed before it can be either definitely 

 accepted or finally condemned. For myself, I find it more acceptable 

 than the rotational theory, or any other hypothesis so far offered, 

 of the origin of the solar system. Time does not permit of a dis- 

 cussion of its difficulties, but I may perhaps conclude by stating what 

 seem to me to be its main advantages over the rotational theory. 



(1) It escapes the well-known criticism of the rotational theory 

 that the present angular momentum of the solar system is too small 

 to be compatible with a previous rotational break-up, and I do not 

 know of any similar quantitative criticism which can be brought 

 against the tidal theory. 



(2) The solar system is arranged with reference to two planes — 

 the invariable plane of the system, which contains the orbits of the 

 outer planets, and a second plane inclined at about 6° to the former 

 plane, which contains the sun's equator and the orbit of Mercury. 

 A system which had broken up by rotation alone ought to be ar- 

 ranged symmetrically about one plane — the original invariable plane 

 of the system. On the tidal theory the two planes of the solar 

 system are readily explained as being the plane in which the tide- 

 raising star moved past the sun, and the original plane of the sun's 

 rotation. 



(3) Theoretical investigations suggest that there is only one pos- 

 sible end for a rotating system, namely, a binary or multiple star of 

 the type familiar to astronomers, and it is quite certain our system 

 is not of this type. Similar investigations on tidal action suggest 

 that the final end of a system broken up by a tidal cataclysm ought 

 to show many of the features of our present solar system. 



