ital wobble, but a perpetual change, acting always in one direction.
Unless fully counteracted by some opposing reaction, therefore (as
it seems not to be), the effect must be cumulative, the ultimate
consequences disastrous. The exact character of these consequences was
first estimated by Professor G. H. Darwin in 1879. He showed that tidal
friction, in retarding the earth, must also push the moon out from the
parent planet on a spiral orbit. Plainly, then, the moon must formerly
have been nearer the earth than at present. At some very remote period
it must have actually touched the earth; must, in other words, have been
thrown off from the then plastic mass of the earth, as a polyp buds out
from its parent polyp. At that time the earth was spinning about in a
day of from two to four hours.

Now the day has been lengthened to twenty-four hours, and the moon has
been thrust out to a distance of a quarter-million miles; but the end is
not yet. The same progress of events must continue, till, at some remote
period in the future, the day has come to equal the month, lunar tidal
action has ceased, and one face of the earth looks out always at the
moon with that same fixed stare which even now the moon has been brought
to assume towards her parent orb. Should we choose to take even greater
liberties with the future, it may be made to appear (though some
astronomers dissent from this prediction) that, as solar tidal action
still continues, the day must finally exceed the month, and lengthen out
little by little towards coincidence with the year; and that the moon
meantime must pause in its outward flight, and come swinging back on a
descending spiral, until finally, after the lapse of untold aeons, it
ploughs and ricochets along the surface of the earth, and plunges to
catastrophic destruction.

But even though imagination pause far short of this direful culmination,
it still is clear that modern calculations, based on inexorable tidal
friction, suffice to revolutionize the views formerly current as to the
stability of the planetary system. The eighteenth-century mathematician
looked upon this system as a vast celestial machine which had been in
existence about six thousand years, and which was destined to run on
forever. The analyst of to-day computes both the past and the future of
this system in millions instead of thousands of years, yet feels well
assured that the solar system offers no contradiction to those laws of
gr