Harrisburg telegraph. [volume] (Harrisburg, Pa.) 1879-1948, 1919-01-16

q. ASScHe drtlcio L note you give
the dates Of the equinoxes as March
I'll and September go. This makes the
spring and summer seven days long
er than the autumn and winter:
VIII you please explain why this
happens? Is the distance from the
equator TO Caprieen that much great
er than to Cancer? Or Is the north
pole enough heavier than the south
pole to cause the latter to swing the
sreater distance? Were the distances
sqaai why sh9uld not the equnoxes
De exactly half way between the
equator and the solstlees and SO di-.
''ae 'ne year into 9Sus r. easfl aaancprK, psrss'
The best way to understand the
origin of the difference to which you
reier Is to consider the year as dl-
and a winter half, the summer half
ot the year for the northern hemis-
phefe, extending from the VernaI, or
spfing epulneH TO the Autumna1 equi-
hox, and the winter half from the
nox.
l.. During the summer half the sun
2 herth OF the equator, and during
the winter half It is south of the
eauasor, e Now, the dates of the equi.,
ne4es being respectively March 20
or q for the vernal and September
20 or 23 for the Autumnal (the ad-
Justments of the calendar make them
slightly yariable( It follows as you
Is a week longer than the winter
naif' How does this difference arise?
NOT, as you suggest from any diff
ference In distance Of the two tropi-
cal circles from the equator. or from
any wobling of the axis of the earth.
but from the eiliptical form of the
earth's orbit and from the varying
speed or the earth in Its motion
around the sun, the variation being
OF the orbit. -.
The length of the orbit is nearly
loo.Oo0,OO0 miles. and the sun, which
occupies the point around which the
earth revolves Is situated in one of
the foci of the. ellipse. about 1.6oO.O
miles from the centre measured
along the major axis. If. then. q per-
pendicular to, the major axis of the
eclipse be drawn through the sun 1t
VIII divide the orbit into two halves.
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.
since the plane Of the orbit is iden-
tical with that of the ,ecliptic, or the
apparent yearly. path of the sun
through the considerations. 1t follows
that the, line joining the equinoxes
divides the yearly path OF the e8rth
into two slightly unequal halves. and
that being so, the earth has q short
er distance to travel in describing the
half of its orbit which 10 nearest the
sun than In describing the other half.
difference of length between the VIM
ter and summer portions OF the year.
But the difference Is greatly increas-
ed by the fact that the speed of the
1nwaAIx ~ fs.NV ~1.a IT,
tance from the sun. so that when it is
in the part Of Its orbit which lies on
that side Of the center of the eclipse
where the sun is, 1t must travel fast
er than when it Is In the opposite part.
the sun It travels about 1877 miles
per second, and when 1t is farthest
from the sun about 1873 miles per
second. The motion of the earth be-
ing reflected In an apparent motion
about one minute of arc more than
a degree per day when the earth is
and about three minutes OF
arc less than o degree per day when
the earth Is farthest.
p1ete the explanation. 1t, so happens
that at the present time (and not
much change will occur for thous-
ands OF years.. the earth is nearest
the sun when winter prevails In the
from the sun when summer prevails
in the same hemisphere. The result
of this state of affairs. combined with
what has been said above. is that
our winters are about seven days
shorter. than our summers, counting
winter from the Autumnal to the
Vernal equinox and summer from the
Vernal to the Autumnal equinox.
The week of difference represents
what is gained by the relative short
ness of the half of the orbit nearest
the sun and the increased speed of
the earth in traversing that part of
the orbit.
In the southern hemisphere exact
ly the opposite state of affairs pre-
walls, the winter belonging to the
longer and the summer to the short
er half of the year, as divided by the
equinoxes. But, owing to a. combi-
nation of two causes. one the Pre-
cession of the equinoxes, which
makes the poles of the earth's axis
revolve around the poles of the &c
liptic, in q period of about $4,000
years, and the other a revolution of
the major axis of the earth's orbit.
which takes place in period OF 10s,-
900 years Gin a direction opposite to
the Precession), the present situation
will be reversed in about 10.5O0 years.
and then 1t will be the northern hem
isphere that will have winter longer
than summer, and the Icy winds OF
February may whistle q sharper tune
over our drifted hills than any that
historic generations have shrunk