The Worthington advance. [volume] (Worthington, Minn.) 1874-1908, 1898-06-23
THE ACTION OF TIDES.
The Time Coming When the Day
and the Month VIII Be
SynchrOnOus.

dCI1OA dS dAIcCl1HS SLc11dI SlIuCluic,
George Howard Darwin, son of Charles
Darwin, and professor of astronomy at
Cambridge (England) university, con-
tributes to the Atlantic a suggest
i've and instructive paper on the past
history and future relations of the earth
and moon, as typical of the general evo-
luton of satellites.
Beginning with the tidal phenomena
(caused by the attraction of the moon),
and the continual variation in the
earth's shape produced by them, he
traces the counter action exerted upon
the moon by them in return, and espe-
cially by the results of tidal friction;
the resultant effects upon the rotation
and revolution Of both bodies, and he
indicates their future history down to
the point, many ages distant, when
the earth and the moon will revolve to.
gether in unison, and when our day and
our month will be of equal length, and
that length will be IS of ourpresent days.
IF familiarity does not always breed
contempt, yet at least it generally
breeds Indifference" says Prof. Darwin.
''This is the case with most of us in re-
gard to the rise and fall of the tide by
the seashore, and so the problem as tc
whether the tide will serve conven-
iently to allow the children to dig in
the sand or search for seaweed looms
larger than that presented by the gil
gantic forces which now produce only
these somewhat insignificant pulsations
of the sea. Yet the tides should call
forth in us deeper INTEREST," might
almost say an emotion-for they are
the feeble residue of influences which
have probably exercised predominant
control over the history of the earth
and the moon since an indeterminate
but remote epoch in the past, and will
continue that control into the distant
future. According to the law of uni
versal gravitation, the moon attracts
matter which stands near to her
more strongly than that which is
more remote. It follows that the
attraction on the ocean, at the
side of the earth which is nearest
to the moon, must be greater than that
exercised on the solid earth itself.
Hence there is tendency of the sea to
depart from it's natural spherical shape,
and to bulge outward toward the moon.
So far the matter is simple; but it is per-
picking to many that the moon should
app; rently repel the water lying on the
further side of the earth. This action,
however, is not due to any real repul-
sion from. the moon, but results from
the fact that on the further side the
moon must attract the solid earth more
strongly than it does the water. On the
nearer side the moon pulls the water
away from the earth, and on the further
side she pulls the earth away from the
water, thus producing an apparent re-
pulsion of the water to an extent equal
to the attraction on the other side. In
this way there arises tendency for the
ocean to bulge equally toward and away
from the moon, and to assume an egg
like shape.
"A day is the name for the time in
which the earth rotates once, and a
month for the time in which the moon
revolves once. Then since tidal fric-
ton retards the earth's rotation and
the moons revolution, we may state
that the day and the month are now
frightening at different rates which are
calculable, although the absolute rates
In time are unknown. It will suffice for
general comprehension of the prob
lem to know that the present rate of
increase of the day is much more rapid
than that of the month, and that this
will hold good in the future. Thus
the number of rotations of the earth in
the interval comprised in one revolu-
ton Of the moon diminishes; or, in other
words, the numberof days in the month
diminishes, although the length of each
day increases so rapidly that the month
itself is longer than atpresent. For ex-
ample, when the day shall be equal in
length to two of our actual days, the
month may be as long as thirty-seven of
our days, and the ear tn will spin round
only about eighteen times in the month.
49, ~. .E+s
"Thes gradual change in the day and
the month now taking place proceeds
continuously until the duration Of ro-
tationSof the earth is prolonged to IS of
our present days. At the same time,
the month, or the time of a revolution
of theSmoon round the earth, will also.
occupy s. Of our days. Since the month
here means the period Of the return Of
the moon to the same place amongst the
stars, and since the day is to be esti-l
mated in the same way, the moon must
then always face the same part Of the
earth s surface, and the two bodies must
move as though they were fastened to-
gether by bar. The outcome Of the