Wessington Springs herald. (Wessington Springs, Aurora County, Dakota [S.D.]) 1883-1891, 1883-04-14

Are the sails that move his social boat, /
Ana 4 S1 the ~ wind 19Ha Sl9 sgs loT dude Of T he!1l s aief n'sh'
THE SOLAR CORONA.
When a moderately magnifed image
of the sun, suitably %darkened, is thrown
upon a sheet Of white paper, the centre
Of the disc is seen to be brighter than the
edges. This fact, strange to relate, was
not early recognized. Calico distinctly
says that the image appears equally
bright in all its parts" Lambert also
held the same view, adding, that "there
is no person who does not admit this
fact" Bouguer, the inventor of the
heliometer, was the first to dispute it.
He fancied that the eye might be incapa-
BILE of detecting the real difference in the
luminosity of the SUMS surface, owing to
its insensible graduation from the centre
to the edges; and in order to put his idea
to the test of direct experiment, he iso-
lated the centre of the solar image, and
also a portion of equal extent near the
border, and compared them. The inter
mediate degrees of brightness being thus
got rid of, the relative dimness Of the
border region became at once apparent.
The conclusion arrived at by Bouguer
was that "the brightness Of the central
portion of the sun is to the brightness Of
q portion situate at three-fourths of the
radius, measuring from the centre, as
forty-eight to thirty five" With slight
modification, this conclusion has been
adopted by Laplace, Sir John Herschel,
Airy, and other modern astronomers.
But if ocular evidence other than that
obtainable by photometric methods be
wanted, it i is to be found in the results of
photography. Photographic pictures Of
the sun Enthusiastically show a shading off
towards the border of the solar disc.
Now, this circumstance is not without
its important significance. Of course,
the center of the suns disc is slightly
nearer to us than the edges; but any diff
ference Of brightness due to this cause
must be totally inappreciable. The fact
Of the luminous intensity diminishing as
it does can only be due to the absorption
transparent envelope. The reason Of this
is obvious enough. The rays of the
moon, when overhead, have to pass
through a smaller extent Of our atmos-
phere to reach the eye of IN observer
when she is near the horizon; and con-
versely, the eye Of the observer has to
penetrate a thinner layer Of air in the
former case than in the latter. So with
the sun. A solar beam emanating from
the center of his apparent disc corresponds
to the case Of a moon that is right over
head; and a beam from his extreme edge
to moon on the horizan; and if we sup
pose the envelope which surrounds him
to be absorptive, the beam in the latter
instance runs a far greater chance of be-
ing absorbed than in the former, having
80 much farther to travel through the
absorbing medium. AS observation
shows that more rays are
absorbed near the borders Of the
disc than in the center, producing
a corresponding diminution of light, it
is reasonably inferred that an invisible
solar envelope, or atmosphere, does exist,
and that its nature, whatever it be, is such
as to make it an absorbent Of light.
AS everybody knows, a solar eclipse is
produced by the dark body Of the moon
coming between us and the sun, and so
intercepting his beams. When a total
eclipse takes place, the moon entirely
covers the suns disc; hence, on such occa
sions we should expect the orb to be blot.
ted out of the sky for the time being.
But as a matter Of fact he is not so. The
black moon is seen super-imposed upon
what we ordinarily regard as the sun
but all around the somber disc, a bright
glory of light is visible, extending far out
from the edges and throwing into clea1
relief the body Of u eclipsing satellite.
What is this light? 1t may be one Of
two things. 1t may be a lunar atmos.
phere brilliantly Illuminated by the sun
behind; or it may be a solar atmosphere.
of inferior brightness to the sun himself
which only becomes visible when the
superior light is withdrawn or concealed
That it is not the first, we have album
dant proof. Leaving out of consideration
the many evidences that the moon ha.
not an atmosphere Of appreciate heigh,
or density, we have the circumstance tina
the glory in question, when the lunar anc
solar discs do not exactly coincide, form.
a pretty uniform fringe around the latter
disc, quite irrespective Of the position O.
the former. The inference is unquestion
able. The fringe of light belong to the
sun not to the moon. It is, in fact, the
corona, or c%oronal atmosphere, whose ex
istence we have already been led to infer
from the appearance of the suns surface
Respecting the nature of this solar ap
pendage, we know but little. From it
shading effect upon the solar disc, We
gather that it has the power of absorbing
light; but anything more than that We
are not warranted in assuming from the
phenomenon. Then its luminosity is SC
faint-compared with that of the sun
that it sonly visible during the totality Of
an eclipse; and eclipses are so rare, and
and of such exceedingly short duration
when they do occur, that knowledge oh.
tainable only while they last must neces.
sarily be slow of growth. On this
ground, the approaching eclipse of May
6, 1883. is being looked forward TO with
unusual interest. Whilst on ordinary
1a,o~n, H. Ir~, &c &k.-