Evening star. [volume] (Washington, D.C.) 1854-1972, 1897-04-24
TIDAL FRICTION.
Its Effect on the Earth and ON the
Moon.

Dr. II. S. Williams in Harper's Magazine
Perhaps the most interesting accom-
plishments OF mathematical astronomy
from mundane standpoint, at any rate
are those that refer to the earth's own sat
elite. That seemingly staid body was long
ago discovered to have a propensity to
gain little on the Garth, appearing at
eolipses an inflnitesmal moment ahead
OF time. Astronomers were sorely puz-
zled by this act Of characterization:
but at last Laplace and Lagrange ex-
planned 1t as due to an oscillatory change
in the earth's orbit, thus fully generating
the moon, and seeming to demonstrate the
absolute stability and permanence of our
planetary system, which the moons misbe-
havior had appeared to threaten.
This highly satisfactory conclusion was
an orthodox belief of celestial mechanics
until 1858, when Prof Adams of Neptunian
Iame, with whom complex analyses were a
pastime, reviewed Laplace's calculation.
ana discovered an error, which, when cor
rected left about half the moons accel-
Grafton unaccounted for. This was a mo-
mentous discrepancy, which at first no one
could explain. But presently Prof Hem
hoitz' the great German physicist, sug-
sested that key might be found In tidal
friction, which, acting as a perpetual
brake on the earth's rotation, and affect
ing not merely the waters but the entire
substance OF our planet, must in the long
sweep OF time have changed its rate of ro-
tatlon Thus the seeming acceleration of
the moon might be accounted for as actual
retardation of the earth's rotation-a
lengthe1ing of the day, instead of a short
enlng of the month
Again the earth was shown IN he at fault
Out this time the moon could not be exon,
Grated, while the estimated stability of our
system, instead of being re-established,
was quite upset. For the tidal retardati
is not an oScillatory change which will
presently correct itself, like the orbital
Wooble, but perpetual change acting al
ways III one direction. Unless fully coun-
eelacteo py some opposing reaction, there
lore las II seems not to bel, the effect must
De cumulative, the ultimate consequences
disastrous. The exact character of these
consequences was first estimated bv prof
u. II. Darwin in 1879. He showed that
tidal friction in retarding the earth nnust
also push the moon out from the narent
planet on spiral orbit. Plainly, then. the
moon must formerly have been nearer the
eartn than at present. At some very re-
mote period 1t must have actually tonched
the earth; must, in other words. have heen
thrown on from the then plastic mass of
the earth, as polyp buds out from its ma
rent polyp. At that time the earth was
four hours WV "
NOW the day has been heightened IN
twehty-1our hours, and the moon has been
thrust out to distance of a quarter mill
lion miles; but the end is not vet The
same progress OF events must continue. till.
at some remote period in the future, the
Jay mas come to equal the month lunar
tldal 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 to-
ward her parent orb. Should we choose TO
take even greater liberties with the future
IL 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 toward coinci-
dence with the year; and that the moon
meantime must pause in it's outward flight.
ana come swinging back on a swinging
spiral, until finally, after the lapse of I'M
toid aeons, 1t ploughs and ricochets along
the surface of the earth, and plunges to
catastrophe destruction
But even though Imagination pause far
snort OF this direful elimination IL still Is
clear that modern calculations, based on
memorable tidal friction, suffice to reyolu-
tionlze the views formerly current as TO
the STABILITY of the planetary system. The
eighteenth century mathematician looked
upon TINS system as vast celestial ma
cnlne which had been in existence about
SIX thousand years, and which was des-
tIneo 10 run on forever. The ANALYST of
touay computes both the past and the fu-
lure OF TINS system in millions instead of
thousands or years, yet feels well assured
tnat the solar system offers no contradic-
lion 10 those laws of growth and decay
winch seem everywhere to represent IT,
Of