South Bend news-times. (South Bend, Ind.) 1913-1938, 1917-03-07
Strange Zodiacal
Light due to Swarm
Of Small Particles
By Garrett p. serriss.

\ correspondent asks. W nat Is
the cause Of the ZodiacaI light. and
when can II best be SEEM"
II is believed that the cause OF
the Zodiacal light is the reflection
OF sunlight from vast numbers Of
small particles. circulating around
the sun in practically circular or
bits. the swarm being condensed in
the neighborhood OF the sun, and
thinning out with increase Of dis
lance.
It's farthest limits are not known
because the light gradually fades TO
invisibility, without a sharp border,
but il is evident that the rejecting
particles, though greatly reduced in
number. extend somewhat beyond
the earth's orbit, since the light Is
in favorable circumstances family
visible clear across the sky.
A puzzling fact is that there seems
70 be a condensation at a point in
the sky exactly opposite TO the sun,
where a curious oval illumination is
visible 10 practiced eyes. This has
been named the Gesenschein, or
counterglow A very interesting sug-
gestion concerning the cause of the
Gegenschein is thus stated by Prof.
+. r. Moulton:
AT a point $4,000 miles from the
earth, which is beyond the apex Of
its shadow, there is a region where.
ia consequence of the combined
forces of the earth and sun, wander
ing particles tend TO circulate in "A
sort Of dynamic whirlpool. 1t has
heen suggested that the circulating
particles which produce the Zodi-
acal light are caught in this whirl
and are virtually condensed enough
to produce the observed phenome-
non of the Gegenschein."
But while anybody can see the
ZOdiacal light. when the circum-
stances are favorable, no ordinary
eye Is likely TO catch sight OF the
Gesenschein. / was once on Mt.
Wilson in company with Prof. l..
l.. Barnard. on one of those nights
when from all quarters the heaS-
ems speak to men," and he endeav-
Ored 10 make me see the Gegen-
schein, pointing out the stars which
staked its limits, but my eyes were
not trained TO that work and l could
only feel the presence of the mys-
terry. knowing that it must be there.
since Barnard saw IL.
The spring months. from Febru
airy to April. are the best time for
seeing the Zodiacal light in the
evening sky, while the autumn
months are the most favorable TO
it's visibility in the morning sky. II
appears after the end of twilight in
the evening. and before the begin
ming Of twilight in the morning. TIE
reason why the months named are
the best for its observation is that
then the ecliptic, or apparent an
nual track of the sun through the
sky, meets the horizon at a very
high angle, and consequently the
axis of the light rises nearly per-
pendicular, thus carrying il out of
the obscuring effects of the dense.
misty air nearer the horizon. on
an autumn morning in 1896, from
the summit of Mt Etna, which, be-
ing two miles high and far from any
other mountains, offers one of the
most unobstructed sky views in the
world, 1 saw the ZOdiacal light
CLOWNS with a splendor that aston
ished and fascinated me. / could
hardly believe that ii was the same
phenomenon had sometimes
glimpsed through the dimming at
mosphere of New York.
Yet often the Zodiacal light is ex-
ceedingly beautiful in our ATTITUDES
and longitudes. even when viewed
from near sea-level. It's light is
usually described as pearly, and it
has, perhaps. at times. a slight tinge
of color 1t is Of a lenticular shape.
or, perhaps better, saw edge-shape,
narrowing TO vanishing point
which may reach nearly. or quite,
TO the zenith IL is brightest in the
line of the axis. fading out at the
sides. It's width at the bottom va
ries, at different times, from 80 10
40 degrees.
Whether the substance whose re-
Election produces the Zodiacal light
is in a dusty or a gaseous state re-
mains an unsettled question. al
though observations or the spectrum
of the light favor the view that the
rejecting particles are solid. 1t has
even been supposed that the con-
Centration OF these particles near
the sun is sufficient TO account (by
the resistance which they offer to
any body moving through them) for
certain Irregularities in the motion
of the planet Mercury, and in the
motions of comets.
The Swedish savant, Svante A.
Arrhenius. has suggested that the
Gegenschein is not a part of the
Zodiacal light but is kind of
cometary tail of the earth. COM
posed. like the tails of comets, Of
corpuscles emitted from the electri-
fed earth and driven out into space
in a long streamer by the pressure
of the waves of sunlight.