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MR. G. H. DARWIN ON THE 
§ 9. General discussion and summary. 
According to the nebular hypothesis the planets and the satellites are portions 
detached from contracting nebulous masses. In the following discussion I shall accept 
that hypothesis in its main outline, and shall examine what modifications are neces¬ 
sitated by the influence of tidal friction. 
In § 7 it is shown that the reaction of the tides raised in the sun by the planets must 
have had a very small influence in changing the dimensions of the planetary orbits round 
the sun, compared with the influence of the tides raised in the planets by the sun. 
From a consideration of numerical data with regard to the solar system and the 
planetary subsystems, it appears improbable that the planetary orbits have been much 
enlarged by tidal friction, since the origin of the several planets. But it is possible 
that part of the eccentricities of the planetary orbits is due to this cause. 
We must therefore examine the several planetary subsystems for the effects of tidal 
friction. 
From arguments similar to those advanced with regard to the solar system as a 
whole, it appears unlikely that the satellites of Mars, Jupiter, and Saturn originated 
very much nearer the present surfaces of the planets than we now observe them. But 
the data being insufficient, we cannot feel sure that the alteration in the dimensions 
of the orbits of these satellites has not been considerable. It remains, however, nearly 
certain that they cannot have first originated almost in contact with the present 
surfaces of the planets, in the same way as, in previous papers, has been shown to be 
probable with regard to the moon and earth. 
The numerical data in Table II., § 7, exhibit so striking a difference between the 
terrestrial system and those of the other planets, that, even apart from the considera¬ 
tions adduced in this and previous papers, we should have grounds for believing that 
the modes of evolution have been considerably different. 
This series of investigations shows that the difference lies in the genesis of the moon 
close to the present surface of the planet, and we shall see below that solar tidal 
shown above that the efficiency of solar tidal friction is of abont the same magnitude for the two planets. 
It is explicable however by the considerations in the text, for it was there shown that a large satellite 
was destructive of planetary rotation. 
If we estimate how fast the earth must rotate in order that the whole internal momentum of moon and 
earth should exist in the form of rotational momentum, then we find an effective period for the earth of 
4 h 12 m . This ag’ain illustrates what was stated above, viz.: that a large satellite is preservative of the 
internal momentum of the planet’s system. 
The orbital momentum of the satellites of the other planets is so small, that an effective period for the 
other planets, analogous to the 4 h 12 m of the earth, would scarcely differ sensibly from the periods given 
in the table. 
If Jupiter and Saturn will rdtimately be as condensed as the earth, then it must be admitted as possible 
or even probable that Saturn (and perhaps Jupiter) will at some future time shed another satellite; for 
the efficiency of solar tidal friction at the distance of Saturn is small, and a. period of two or three hour’s 
gives a very rapid rotation. 
