The Washington times. [volume] (Washington [D.C.]) 1902-1939, 1921-09-26
OUR PUZZLING EARTH
Y MANY LOTIONS AND MOYEMENTS HIS ire

; happens to be directed q few degrees
aside from the bright star Vega in
the northern hemisphere. It Is not
TO be concluded that Vega has any
special connection with this motion.
The motion IL about one-third slow.
er than that Of the revolution of
the earth around the sun, and
amounts to twelve miles per second.
Another conspicuous motion Of the
earth as Q whole Is its daily rota-
ton on its axis. But while this
alters its aspect with regard to the
heavenly bodies, causing different
sides OF the globe to be successfully
turned toward them. 1t does not at
feet the position in space. Conceiv-
ably, the earth might stand still In
space and yet rotate on its axis like
q spinning ball.
The three motions Just mentioned
may be called the three primary mo-
tons Of the earth. But these are
all subjects to variations and modifi-
cations which might be treated as
more or less independent, thus ex-
tending the list Of movements to a
dozen or more. For instance, the
varying attractions of the other
planets upon the earth cause Il not
only to wander q little aside from
the smooth ellipse that 1t would
otherwise describe around the sun,
but also slightly quicken it's pace
at one time and retard 1t at another.
This might be called q slight stag.
ger OF the earth, due to gravitation-
al pushing and pulling.
Another similar irregularity of
motion Is impaired to the earth by
Its little travelling companion, the
moon. Then, the general shape Of
the earth's elliptical orbit Is con-
tinually. though very slowly, chang.
ing, the eccentricity becoming alter
nately greater and less; and this
variation Is one Of the causes that
have been alphabetically assigned
to account, or help to account, for
the great ice ages in the earth's
geologic history.
The earths orbit, considered as
whole and treated as II It were
real thing instead Of a mere in
tangible path In space, In also sub
1ect to peculiar motion which in
volVes, we may say, a correspond
ing motion OF the earth, and which
is called the revolution of the an
sides'-the apsides being opposite
points of the orbit of which one Ia
the nearest and the other the far
thest from the sun. This revolu-
ton takes about 100000 years. -
Next we come to motion
which affects the position of the
axis OF rotation with regard to
the plane Of revolution. This Ia
called the change Of the obli.
guilty Of the ecliptic. The obliauity
is greatest, when the axis leans
most from a perpendicular to the
plane of the orbit. This motion IN
very slow and irregular, requiring
thousands of years to produce nota.
BILE effects.
At present the obliquity Is 23 de-
grees ~ minutes 81 seconds of arc,
and Is decreasing about half a sec.
ond per year. About $4,000 years
from now the obliquity will be only
about 22h degrees, after which Il
will begin to increase again. The
earth's action also has a motion of
revolution, like that OF the leaning
peg of a top which gradually turns
round. This motion Is called the
procession OF the equinoxes, and
nearly 10000 years are required for
one revolution OF the pole of the
earth's axis around the pole Of the
ecliptic,