The Washington times., 1920-01-17


the autumn and winter,
Will you please explain why
this hAppensr Is the distance
from the equator TO Capricorn
that much greater than to Can
cert Or Is the north pole
enough heavier than the south
pole to cause the latter to
swing the greater distanceY
We're the distances equal why
should not the equinoxes be
exactly half way between the
equator and the solstices and so
divide tho year into fair equal
partsr
A N. BABcocK.
THE best way to understand
the origin of the difference
TO which you refer is to con-
slider the year as divided into two
parts summer half and a win
ter half. the summer half Of the
year for the northern hemisphere,
extending from the Vernal, or
spring. equinox to the autumnal
equinox, and the winter half from
the autumnal equinox to the Ver-
nal equinox.
The Summer Half.
During the summer half the sun
Is north Of the equator, and during
the winter half 1t is south Of the
equator. Now, the dates Of the
equinoxes being. respectively,
March 20 or 21 for the Vernal and
September 20 or % for the au-
tumnal (the adjustments OF the
calendar make them slightly vari-
able). 1t follows, as you have no
ticed, that the summer half is a
week longer than the winter half.
How does this difference arise?
Not, as you suggest, from any diff
ference in distance Of the two trop-
ical circles from the equator, or
from any wobbling of the axis of
the earth, but from the elliptical
form Of the earth's orbit and from
the varying speed of the earth In
its motion around the sun, this
variation being a consequence Of
the elliptical from of the orbit.
The length Of the orbit is nearly
1S6,O00,000 miles, and the sun.
which occupies the point around
which the earth revolves Is situ-
ated In one of the foci Of the
ellipse, about 100000 miles from
the center measured along the
major axis. II, then, a perpendic-
ular to the major axis of the
ellipse be drawn through the sun
it will divide the orbit into two
halves, one slightly larger than
the other. But because the orbit
is inclined to the plane Of the
equator the points where 1t
crosses that plane must lie on
the perpendicular just described.
Now, these points are the eQuin-
oxes, since the plane Of the orbit
is identical with that of the
ecliptic, or the apparent yearly
path of the sun through the con-
stellations. 1t follows that the
line joining the equinoxes divides
the yearly path of the earth into
two slightly unequal halves. and,
that being so, the earth has
shorter distance to travel in
describing the half of its orbit
which is nearest the sun than in
describing the other half
Speed of Travel.
This alone would make a slight
difference of length between the
winter and summer portions OF
the year. But the difference Is
greatly increased by the fact that
the speed of the earth varies in
versely with its distance from the
sun, so that when it is in the part
of it's orbit which lies on tnat side
of the center of the elHpse where
the sun is, it must travel faster
than when It is in the opposite
part.
In fact, when the earth is near
est the sun it travels about 1877
miles per second, and when it is
farthest from the sun about 1873
miles per second. The motion Of
the earth being reflected in an
apparent motion Of the sun. the
latter appears to travel about one
minute of arc more than de-
gree per day when the earth is
nearest, and about three minutes
Of arc less than a degree per day
when the earth is farthest.
One thing more is needed to
complete the explanation. 1t so
happens that, at the present time
land not much change will occur
for thousands Of years), the earth
is nearest the sun when winter
prevails in the northern hemis-
phere, and farthest from the sun
when summer prevails in the same
hemisphere. The result Of this
tate of affairs, combined with
what has been said above, is that
our winters are about seven days
shorter than our summers, count
ing winter from the autumnal to
the vernal equinox and summer