The Birth of a Satellite
BY GEORGE HOWARD DARWIN, F.R.S., LL.D., D.Sc.
Plumian Professor of Astronomy, Cambridge, England
THE celebrated nebular hypothesis of Kant and Laplace has been expounded very often, and in this article it will suffice to remind the reader that in that theory the solar system is supposed to originate from a lens-shaped nebula of highly rarefied gas rotating slowly about an axis perpendicular to the present orbits of the planets. As the gas cooled, the central portion condensed and its temperature rose. The speed of rotation increased in consequence of the contraction, according to a well-known law of mechanics; the edges of the lenticular mass of gas then ceased to be continuous with the more central portion, and a ring of matter was detached. Further cooling led to further contraction and to increased rotation, until a second ring was shed, and so on successively. The rings then ruptured and aggregated themselves into planets, whilst the central nucleus formed the sun.
Since the time of Laplace celestial photography has furnished conclusive evidence of the general truth of the nebular hypothesis, but we have also learnt, principally by means of the spectroscope, much as to the mechanical characteristics of systems of a wholly different kind, namely, double stars, which are found in general to consist of two bodies of not very unequal masses revolving about one another in close proximity. While the nebular hypothesis may be accepted as affording a fairly satisfactory explanation in many cases, yet in others it seems altogether inappropriate. We are thus led to conjecture that there may be more ways than one in which a celestial body may start on its own individual career, and this view is confirmed by certain details of the solar system itself.
The planets Mars, Jupiter, Saturn, Uranus, and Neptune have their satellites, and it would not be unnatural to classify our own moon as simply one more in a series of objects with precisely similar histories. Of course the earth is similar in that it is a planet attended by a satellite, but text-books of astronomy scarcely give sufficient emphasis to the fact that the earth and moon really do differ widely from other planets and satellites. The earth is, in fact, only 80 times as heavy as the moon, whereas Saturn is 4600 times as heavy as its satellite Titan, itself by far the largest satellite in the solar system. It seems, then, that there may be reason to suspect that the mode of genesis of a satellite relatively so large as the moon may have differed materially from that of all other satellites. Such a suspicion is confirmed by the investigation of the part which tidal friction has probably played in the evolution of our planet. The present article would be expanded to undue proportion if I were to attempt to touch further on this point, but the argument would have seemed weaker than it is if it had been entirely passed over.*
Accepting, then, the substantial correctness of the nebular hypothesis, I throw out the conjecture that there is a second type of birth in which the subordinate body is born all in one piece. It is easy to imagine a continuous gradation between these two extremes, for we may imagine a lopsided ring, and if the absence of balance were extreme, it might be more exact to regard it from the first as being a single satellite. When, then, I say that the birth of the moon was probably unlike that of all the other bodies of our system, I mean that we shall be near the truth in classifying the origin of the planets and of the other satellites as belonging to the Laplacian
