398 
The Solar System [ch. xvi 
The matter which was first shed by the sun would form a ring of small 
mass rotating with the sun ; on account of the smallness of its mass, this would 
have no gravitational cohesion and, so far from increasing in density, would 
scatter under its own internal gaseous pressure. It seems impossible that such 
a ring could double its density before the disruptive influence of its rotation 
could come into play. Thus, on the whole, it seems necessary to admit Babinet’s 
contention that the solar system is not possessed of sufficient angular 
momentum to have broken up through excessive rotation*. The calculation 
needs some modification in view of the possibility discussed in Chapter x of 
the inner layers of the sun rotating faster than the outer, but the modification 
is found to make rotational break-up still more impossible. 
371 . The considerations which carry the most obvious condemnation of the 
theory of rotational break-up are of a somewhat different kind. If the sun 
once assumed the lenticular shape necessary for the shedding of matter by 
rotation, it is difficult to see how it could ever abandon it and become as 
nearly spherical as it now is ; it is also difficult to understand why the planets 
should be at such widely varying distances from the sun. 
These as well as other difficulties again confront the rotational theory 
when it attempts to explain the origin of the satellites of the planets. Many 
of these are so small that they can only have escaped scattering into space by 
liquefying or solidifying immediately after their birth. Thus their birth can 
not have been a long drawn out process such as the equatorial shedding of 
matter by a slowly shrinking mass ; the fact that these satellites survived the 
birth-process at all proves that they must have been born quickly. 
Tidal Theories. 
372 . As the genesis of the solar system cannot be explained in terms of 
a single mass rotating by itself in space, the only alternative which remains 
is to consider whether it can be explained as the result of the interaction of 
two or more masses. We can fix our attention on two, since encounters of 
three masses in space must be so rare as to be entirely unimportant. 
This brings us naturally and inevitably to tidal theories of the genesis 
of the solar system. The fundamental conception of these theories is, that at 
some time in the past a second mass approached so close to our sun as to break 
it up, through the action of intense tidal forces, into a number of detached 
masses. 
As between rotational and tidal theories, first appearances are wholly in 
favour of the latter. A rotating mass, and so also a system which has been 
formed without external interference out of a rotating mass, retains one 
* This argument was first given by myself in M.N. lxxvii. (1917), p. 186. Jeffreys has re 
investigated the question in M.N. lxxviii. (1918), p. 425, slightly modifying the argument, but 
confirming my original conclusions.