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.
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