When we see a comet approaching the sun with its tail following in the
orbit of the nucleus, we have no great difficulty in believing the
common theory that a comet consists of nucleus attracted toward the sun,
while the tail is repelled; and that we see the whole of it. But as it
approaches the sun, difficulties arise that make us doubt whether the
theory be true.
Let us suppose a comet with a tail 50,000,000 miles in length, and that
it will require two days to pass round the sun. Now the tail, being
always in a line drawn through the center of the sun and center of the
nucleus, will, when it reaches the long axis of the elliptical orbit,
stand perpendicularly to the orbit of the nucleus. That is, the
extremity of the tail farthest from the sun, in addition to its onward
motion, has acquired a lateral motion that has lifted it 50,000,000
miles in the first day of its perihelion. The velocity of the extremity
has been vastly accelerated over that of the nucleus, and it has
moreover a sheer lift above the orbit of the nucleus. Now this lift is
in opposition to gravity; neither is it in consequence of any previous
momentum, for its velocity is accelerated and its previous momentum
would be a hindrance; nor is the lift in consequence of any repelling
force from the sun, for such force would be diminished in proportion to
the square of the distance, and the far end would be acted on less than
the nucleus end of the tail, whereas the velocity of the former is
increased a hundred fold over that of the latter. A polar force in the
comet would merely draw the comet into the sun. We therefore find no
force adequate for such a lift, but on the contrary all the forces are
opposed to it.
But if the first day of the perihelion overwhelms us with difficulty,
the second day will prove disastrous to the common theory. For the
extremity of the tail farthest from the sun will be required to pass
with lateral motion from its perpendicular 100,000,000 miles, so that it
may be in advance of the nucleus and again rest on its orbit. This orbit
is an impassable line, and therefore instantly arrests the prodigious
lateral velocity of the tail. That impassable orbital line is to it
as solid and inflexible as a wall of adamant. The motion so instantly
arrested would be disastrous to any tail, whether composed of gas,
meteorites, or electricity, whatever that may be.
Having shown that the common theory of comets is filled with insuperable
difficulties, I will again call attention to a theory proposed about
eighteen months ago in the SCIENTIFIC AMERICAN.
According to this theory, a comet consists of a nucleus and an
atmosphere, for the most part invisible, surrounding it on all sides to
an extent at least equal to the length of the tail. The rays of the sun
in passing through or near the nucleus are so modified as to become
visible in their further progress through the cometic atmosphere, while
all the rest remain invisible. What we call the tail is merely a radius
of the cometic atmosphere made visible, and as the comet moves through
space, only different portions of the atmosphere come in sight, in
obedience to the ordinary laws of light. There is no difficulty in
accounting for the rise and fall of the tail at perihelion, nor for the
tail preceding the nucleus afterward.
The spherical theory accounts easily for the different forms of tail
seen in different comets. The sword shaped tails, at variance with the
common theory, can be accounted for by supposing a slight difference in
density or material in the cometic atmosphere, which will deflect the
light as seen. The comet of 1823, which cannot be explained on the
common theory, is very easily explained on the spherical. That comet
showed two tails, apparently of equal length, which moved opposite to
each other, and perpendicularly to the orbit of the nucleus, and showing
no signs of repulsive force from the sun. On the spherical theory it is
only necessary to suppose such an arrangement of the nucleus as would
reflect the rays of the sun laterally; a slight modification of the
nucleus would give not only two but any number of tails pointing in
different directions.
It may be objected to the spherical theory that a tail 50,000,000 miles
long would call for a sphere 100,000,000 miles in diameter, and that
would be too vast for our solar system. But it is claimed for this
sphere that it consists of the same material as the so-called tail, and
that it has the same capability of moving among planets without manifest
disturbance to either.
The sphere at the perihelion would envelop the sun, and as a noticeable
reduction is sometimes found in its so-called tail, the cometic
atmosphere may impart to the sun at that time whatever is necessary to
its use.
That there is something in common between the sun's corona and cometary
matter was shown by the last solar eclipse observed in South Pacific
Ocean, where the spectrum of sun's corona was found to be the same as
that of a comet's tail. Are we to attribute in any degree the different
appearances of the sun's corona to the presence or absence of a comet
at its perihelion? At the eclipse of the sun seen in Upper Egypt two or
three years ago, a comet was seen close to the sun, but I have seen no
account of the appearance of the corona at that time.