Evening star. [volume] (Washington, D.C.) 1854-1972, 1910-07-26
WHAT AURORAS ARE
Sunlight Defracted in Air by
the Motion of the Earth.
WHAT ZODIACAL LIGHT IS
Not Cosmic, But Confined to Shell of
Air Around Globe.
BOTH LUMINOUS COUSINS
Dark Auroral Curtain Is Shadow
Arc Between the Observer and
Northern Lights.
WHAT ZODIACAL LIGHT IS
BOTH LUMINOUS
COUSINS
By Edwin Fairfax Naulty.

By Edwin Fairfax Naulty.
In the west after sunset, when the
sky is free from clouds, there appears
a great angle of soft light sometimes
sharply contrasted in color with the
rest Of the sky. always easy to trace,
once it is recognized, and usually green
ish golden after sunset, changing to a
blue white as the sun sinks lower and
lower. The base of this great angle of
light rests on the horizon and it's apex
extends upward to the zenith, some
times extending across the zenith to
the eastern hemisphere of the sky. The
same apparition may also be seen in
the morning sky preceding the rising
sun. The major axis of this angle of
light always lies along the line of the
Zodiacal constellation, being south of
the equator in the summer and north
of the equator in the winter. It's base,
which varies in width from twenty to
sixty times the diameter of the full
moon, rests in the twilight or dawn
light, but a trained eye under very
! favorable conditions can trace it even
through the dawn Or twilight. In ap-
pearance it is not unlike the Milky Way,
and it may be well compared to the
faint tall Of huge comet This is the
wonderful zodiacal light.
Many explanations of it's nature have
been offered. The most popular, quoted
by Young, Comstock, Newcomb and
fat ring, Or zone, extending outward from
the equator of the sun to beyond the
earth. The fact that the zodiacal light
has been seen to entirely cross the sky
fv.o
tate this theory. The rings of Saturn
furnished the hint for this explanation.
Nearly sixty years ago Chaplain Jones
OF the United States Navy, after two
years of observation, in ATTITUDES from
Al degrees north to % degrees south,
concluded that the zodiacal light was
ring of nebulous matter encircling the
earth and lying within the orbit of the
moon.
What the Zodiacal Light Is.
The zodiacal light is not a girdle of
nebulous matter encircling either the
sun or earth 1t is phenomena of
the earth's atmosphere and confined to
the great layer of air that clothes our
planet. 1t is the projection through
the earth's atmosphere of the equa-
torlal line or zone of greatest sunlight
Its major axis always agrees with the
equator of the sun. projected to the
earth and through it's atmosphere, in
Guiding the variation of the solar
equator during a solar revolution,
from the ecliptic Its change in APP
parent width is due to the obliquity
Of our point of view from it's plane. it's
curvature to the north, its apparent
flat ansularity and its curvature to the
south. at different seasons as seen from
north ATTITUDES, and the fact that
its appearance. south of the equator, is
just reversed at the same times, shows
that this appearance is due to our pro-
lection of vision of the light against
the curved outer shell of the earths
atmosphere.
Draw a circle half inch in diameter
around this draw another circle three
inches In diameter; around the second
draw third circle four inches in di-
ameter. The outer circle will show the
outer limits of the earths atmosphere,
the next circle will represent the sur
face of the earth, while the half-inch
inside circle will represent the disk of
greatest solar illumination of the
cles draw an equator and from the
north and south poles of the inner cir
Gle draw outward to the point of con-
tact of the equatorial line with the
outer circle four lines. You now have
a diamond-shaped outline inclosing
the inner circle. This drawing gives
you a chart of the illumination of the
earth's surface and atmosphere by the
sun at the vernal or autumnal equi-
nOX. If you assume your fiat plan to
be a sphere and your point of observa-
ton on it to be situated on the oppo-
site side and north of the equator line
the Zodiacal light will be represented
by that portion of the angle between
the two outer circles (the earth's sur
face and the circumsphere of the earth's
atmosphere). A movement of the dia-
mond north or south of the equator of
these two circles will show the sea-
the zodiacal light and a slight con-
sideration of optics will show why the
sides of the angle will appear to curve,
as they are seen from a curved line
against a curved line-from the sur
face of the earth against the circum-
sphere of the atmosphere.
A more correct way to plan this drawing
is to make the inner circle contact with
the second circle. on the equator. at its
right limb, and then draw angles to con-
projected equator al it's contact with the
outer circle. This will give a true chant
but it involves a longer and more techni-
cal explanation.
1t seems heartless to strip the sun and
earth of their mysterious Saturnine rings
of nebulous matter and to confine the
zodiacal light to the earths atmosphere,
but the confederation of many observa-
tions and reductions of observations COM
pels it, and it is further made neces-
sary by the fact that closely related to
the zodiacal light and caused by a diff
ferent manifestation of the same oondi-
tons are the aurora bOrealis and australis
those mysterious northern and southern
lights, with their glorious prismatic radi-
ance and their accompanying magnetic
phenomena.
Auroras Are Defracted Sunlight.
The auroras are caused by the defrac-
ton of the meridian line of solar illumina-
ton of the earth's atmosphere by the
actual motion of the earth on it's axis.
They are primarily neither electric nor
magnetic. Their appearance far north
or far south in either hemisphere, above
or below the geographical equator al
ways comedies with the position, at the
time of observation, of the poles of the
meridian are of solar illumination Of
the earth The auroras are strongest and
brightest near the first of April and the
first of October, when the poles of the
meridian arc of solar illumination and
the geographical poles coincide, and they
have a secondary period when the sun
light poles and the magnetic poles coin
eide. This last will be further dealt
with in the article on magnetism to fol-
low.
When the sun is at its greatest south
ing it's meridian line of light extends
north to the arctic circle, and south.
through the antarctic pole, to the antarc
H..