COMETS
Great comets are almost always unexpected visitors. There is only one
great comet that we know has been seen more than once, and expect with
reasonable certainty to see again. This is Halley's comet, which has
been returning to a near approach to the sun at somewhat irregular
intervals of seventy-five to seventy-eight years during the last
centuries: indeed, it is possible that it was this comet that was
coincident with the invasion of England by William the Conqueror.
There are other small comets that are also regular inhabitants of the
solar system; but, as with Halley's comet, so with these, two
circumstances are to be borne in mind. First, that each successive
revolution round the sun involves an increasing degradation of their
brightness, since there is a manifest waste of their material at each
near approach to the sun; until at length the comet is seen no more, not
because it has left the warm precincts of the sun for the outer
darkness, but because it has spent its substance. Halley's comet was not
as brilliant or as impressive in 1835 as it was in 1759: in 1910 it may
have become degraded to an appearance of quite the second order.
Next, we have no knowledge, no evidence, that any of these comets have
always been members of the solar family. Some of them, indeed, we know
were adopted into it by the influence of one or other of the greater
planets: Uranus we know is responsible for the introduction of one,
Jupiter of a considerable number. The vast majority of comets, great or
small, seem to blunder into the solar system anyhow, anywhere, from any
direction: they come within the attractive influence of the sun; obey
his laws whilst within that influence; make one close approach to him,
passing rapidly across our sky; and then depart in an orbit which will
never bring them to his neighbourhood again. Some chance of direction,
some compelling influence of a planet that it may have approached, so
modified the path of Halley's comet when it first entered the solar
system, that it has remained a member ever since, and may so remain
until it has ceased to be a comet at all.
It follows, therefore, that, as to the number of great comets that may
be seen in any age, we can scarcely even apply the laws of probability.
During the last couple of thousand years, since chronicles have been
abundant, we know that many great comets have been seen. We may suppose,
therefore, that during the preceding age, that in which the Scriptures
were written, there were also many great comets seen, but we do not
know. And most emphatically we are not able to say, from our knowledge
of comets themselves and of their motions, that in the days of this or
that writer a comet was flaming in the sky.
If a comet had been observed in those ages we might not recognize the
description of it. Thus in the fourth year of the 101st Olympiad, the
Greeks were startled by a celestial portent. They did not draw fine
distinctions, and posterity might have remained ignorant that the
terrifying object was possibly a comet, had not Aristotle, who saw it as
a boy at Stagira, left a rather more scientifically worded description
of it. It flared up from the sunset sky with a narrow definite tail
running "like a road through the constellations." In recent times the
great comet of 1843 may be mentioned as having exactly such an
appearance.
So we cannot expect to find in the Scriptures definite and precise
descriptions that we can recognize as those of comets. At the most we
may find some expressions, some descriptions, that to us may seem
appropriate to the forms and appearances of these objects, and we may
therefore infer that the appearance of a comet may have suggested these
descriptions or expressions.
The head of a great comet is brilliant, sometimes starlike. But its tail
often takes on the most impressive appearance. Donati's comet, in 1858,
assumed the most varied shapes--sometimes its tail was broad, with one
bright and curving edge, the other fainter and finer, the whole making
up a stupendous semi-circular blade-like object. Later, the tail was
shaped like a scimitar, and later again, it assumed a duplex form.
Though the bulk of comets is huge, they contain extraordinarily little
substance. Their heads must contain some solid matter, but it is
probably in the form of a loose aggregation of stones enveloped in
vaporous material. There is some reason to suppose that comets are apt
to shed some of these stones as they travel along their paths, for the
orbits of the meteors that cause some of our greatest "star showers" are
coincident with the paths of comets that have been observed.
But it is not only by shedding its loose stones that a comet diminishes
its bulk; it loses also through its tail. As the comet gets close to the
sun its head becomes heated, and throws off concentric envelopes, much
of which consists of matter in an extremely fine state of division. Now
it has been shown that the radiations of the sun have the power of
repelling matter, whilst the sun itself attracts by its gravitational
force. But there is a difference in the action of the two forces. The
light-pressure varies with the surface of the particle upon which it is
exercised; the gravitational attraction varies with the mass or volume.
If we consider the behaviour of very small particles, it follows that
the attraction due to gravitation (depending on the volume of the
particle) will diminish more rapidly than the repulsion due to
light-pressure (depending on the surface of the particle), as we
decrease continually the size of the particle, since its volume
diminishes more rapidly than its surface. A limit therefore will be
reached below which the repulsion will become greater than the
attraction. Thus for particles less than the 1/25000 part of an inch in
diameter the repulsion of the sun is greater than its attraction.
Particles in the outer envelope of the comet below this size will be
driven away in a continuous stream, and will form that thin, luminous
fog which we see as the comet's tail.
We cannot tell whether such objects as these were present to the mind of
Joel when he spoke of "blood and fire and pillars of smoke"; possibly
these metaphors are better explained by a sand- or thunder-storm,
especially when we consider that the Hebrew expression for the "pillars
of smoke" indicates a resemblance to a palm-tree, as in the spreading
out of the head of a sand- or thunder-cloud in the sky. The suggestion
has been made,--following the closing lines of _Paradise Lost_ (for
Milton is responsible for many of our interpretations of Scripture)
"High in front advanced,
The brandished sword of God before them blazed,
Fierce as a comet,"
--that a comet was indeed the "flaming sword which turned every way, to
keep the way of the tree of life." There is less improbability in the
suggestion made by several writers that, when the pestilence wasted
Jerusalem, and David offered up the sacrifice of intercession in the
threshing floor of Ornan the Jebusite, the king may have seen, in the
scimitar-like tail of a comet such as Donati's, God's "minister,"--"a
flame of fire,"--"the angel of the Lord stand between the earth and the
heaven, having a drawn sword in his hand stretched out over Jerusalem."
The late R. A. Proctor describes the wanderings of a comet thus:--
"A comet is seen in the far distant depths of space as a
faint and scarcely discernible speck. It draws nearer and
nearer with continually increasing velocity, growing
continually larger and brighter. Faster and faster it rushes
on until it makes its nearest approach to our sun, and then,
sweeping round him, it begins its long return voyage into
infinite space. As it recedes it grows fainter and fainter,
until at length it passes beyond the range of the most
powerful telescopes made by man, and is seen no more. It has
been seen for the first and last time by the generation of men
to whom it has displayed its glories. It has been seen for the
first and last time by the race of man itself."[108:1]
"These are . . . wandering stars, to whom is reserved the
blackness of darkness for ever."
FOOTNOTES:
[108:1] R. A. Proctor, _The Expanse of Heaven_, p. 134.
[Illustration: FALL OF AN AEROLITE.
"There fell a great star from heaven, burning as it were a lamp." (_see_
p. 116).]