The Bossier banner. [volume] (Bellevue, Bossier Parish, La.) 1859-1952, 1883-02-22
Air and Seas on Venus.

are full of interest to those who wish to
learn something of the real condition of
other planets than our own. We have
been apt, perhaps, to regard Venus in
transit more as an adjunct to our
telescope measuring instruments (mi-
crometers, heliometers, and SO forth,
forgetting how much interest attaches to
the passage of a world like our own lin
all probability between us and the
source of our light and heat. But even
the telescope scrutiny of planet so stu
ated is likely to reveal much that is in
structive, If carefully studied; while with
so wonderful an instrument of research
as the spectroscope we may hope to learn
such things about Venus or Mercury in
transit as men century ago would have
deemed hopelessly beyond our means of
investigation. Fortunately, these in
quiries have not wholly been overlooked
during the late transit, though the OBJ
servations for determining the SUMS
distance have occupied so large share
of attention, at least among official as
tronomers.
The most important of these physical
inquiries are those relating to the atmos-
phere of Venus; and of these the most
promising are those directed to her APP
pearanee during the few minutes when
nearly bnt not the whole of her disk is
on the face of the sun. If we place
ourselves in Imagination at that point
of Venus surface which, as seen
from the earth at such !. time,
lies farthest from the solar disk, it
will be manifest on q little consideration
that from that point no part of the sun
can be visible unless Venus has an at
mosphere. But if Venus has an atmos-
/ phere ike our own, then from the point
we are considering large part of the
SUMS disk must be visible, being raised
into view by the retroactive power of that
atmosphere, precisely as our sun when
i in a geometrieal sense he has set THAT
is, when straight line from the eye to
him encounters the convexitv of the
earthl is brought wholly into view by at
nospheric refraetion. Only part (about
four-fifths) of the sun of Venus would
be brought into view in this way, suppos-
ing her air of the same retroactive power
as ours, simply because the sun, as seen
from Venus looks larger than as seen
from the earth, while the retroactive effect
(onthis supposition) would be no greater.
But, still, a large portion of the solar disk
would be visible were, there no clouds
over that part of Venus. Now, an OBJ
server on earth, directing his sight to
that point of Venus, is looking along pre-
cisely the same visual line toward the
sun as our supposed observer on the
| planet, only from greater distance.
The terrestrial observer, then, would see
the sun in the direction of his visual line
toward that part of Venus, as certainly
as the observer on Venus would, and,
for the same reason, he would see the
sun through the retroactive action of the
| planets atmosphere. 1t follows that,
supposing Venus to have an atmosphere
like that of our earth, and still more if,
as other observations have tended to
show, Venus has an even denser atmos-
phere, Ight--real but refracted sunlight
would come to the terrestrial observer
around that part of the convexity of
Venus disk. And it is clear, further,
that the whole of that part of Venus cir
cumferenee which lies outside the sun
would, on this supposition, be surrounded
by an are of light; whereas, if Venus
has no atmosphere, or an atmosphere
very much rarer thantile earth's no such
arc Of light would be seen.
Now, during the transit of 1874. not
only was such an arc of light visible
around the part of Venus which was
outside the SUMS disk, before the first
internal contact and after the second,
but this arc was photographed. The sig
nilcance OF this will be seen when we re-
member that photographs of thesun must
be taken with very short exposures, other
wise the image is destroyed by excess of
light. Jansen has obtained good pict-
| ures of the sun three feet in diameter.
with an exposure of less than the two
| thousandth part of a second, and although
in 1874 the exposures were nothing like
SO short as this. they lasted much less
than second. If, during this short ex-
posure, the arc of light round Venus left
its Photographic record, we may be sure
it was very much brighter light than that
of Venus herself as we see her when she
is amorning or evening star. 1t was, in
fact, as certainly sunlight as that which
| we receive from the setting sun when the
air is at its .clearest. Thus we have in
that Photographic record proof positive
! that Venus has an atmosphere with a re-
fractive power not falling much short, if
it do not equal or even exceed, that of
our own air. -
During the recent transit this obserya-
ton was repeated, with results not pre-
cisely the same, yet perfectly accordant
with those obtained in 1874, and even to
the thoughtful mind more specially inter-
esting. The arc of light was not perfect,
but broken near the middle, so that only
two fine horns of light could be seen,
extending from either eusp of the solar
disk chat is, from the two corners, so to
speak. where the outlines of the sun and
Venus intersected). These horns of light
were unequal in length. We learn from
the inequality of these horns that the at
mosphere of Venus is Of variable trans
parency, like that of our, earth. Over
that part of Venus where the longer horn
was seen, the atmosphere was Cat the
time) clearer than where the shorter
arc was seen. But, remembering that
in 1874 the arc of light was complete.
we may go further, recognizing that at
different times, as well as in different
places, the atmosphere of Venus varies