astronomers have at last acquired over these erratic bodies, than to
read in the Edinburgh Review for April, 1835, the paragraph containing
the final results of all the labors and anticipations of astronomers,
matured as they were, in readiness for the approaching visitant, and
then to compare the prediction with the event, as we saw it fulfilled a
few months afterwards. The paragraph was as follows: "On the whole, it
may be considered as tolerably certain, that the comet will become
visible in every part of Europe about the latter end of August, or
beginning of September, next. It will most probably be distinguishable
by the naked eye, like a star of the first magnitude, but with a duller
light than that of a planet, and surrounded with a pale nebulosity,
which will slightly impair its splendor. On the night of the seventh of
October, the comet will approach the well-known constellation of the
Great Bear; and between that and the eleventh, it will pass directly
through the seven conspicuous stars of that constellation, (the Dipper.)
Towards the end of November, the comet will plunge among the rays of the
sun, and disappear, and will not issue from them, on the other side,
until the end of December."
Let us now see how far the actual appearances corresponded to these
predictions. The comet was first discovered from the observatory at
Rome, on the morning of the fifth of August; by Professor Struve, at
Dorpat, on the twentieth; in England and France, on the twenty-third;
and at Yale College, by Professor Loomis and myself, on the
thirty-first. On the morning of that day, between two and three o'clock,
in obedience to the directions which the great minds that had marked out
its path among the stars had prescribed, we directed Clarke's telescope
(a noble instrument, belonging to Yale College) towards the
northeastern quarter of the heavens, and lo! there was the wanderer so
long foretold,--a dim speck of fog on the confines of creation. It came
on slowly, from night to night, increasing constantly in magnitude and
brightness, but did not become distinctly visible to the naked eye until
the twenty-second of September. For a month, therefore, astronomers
enjoyed this interesting spectacle before it exhibited itself to the
world at large. From this time it moved rapidly along the northern sky,
until, about the tenth of October, it traversed the constellation of the
Great Bear, passing a little above, instead of "through" the seven
conspicuous stars constituting the Dipper. At this time it had a
lengthened train, and became, as you doubtless remember, an object of
universal interest. Early in November, the comet ran down to the sun,
and was lost in his beams; but on the morning of December thirty-first,
I again obtained, through Clarke's telescope, a distinct view of it on
the other side of the sun, a moment before the morning dawn.
This return of Halley's comet was an astronomical event of transcendent
importance. It was the chronicler of ages, and carried us, by a few
steps, up to the origin of time. If a gallant ship, which has sailed
round the globe, and commanded successively the admiration of many great
cities, diverse in language and customs, is invested with a peculiar
interest, what interest must attach to one that has made the circuit of
the solar system, and fixed the gaze of successive worlds! So intimate,
moreover, is the bond which binds together all truths in one
indissoluble chain, that the establishment of one great truth often
confirms a multitude of others, equally important. Thus the return of
Halley's comet, in exact conformity with the predictions of astronomers,
established the truth of all those principles by which those predictions
were made. It afforded most triumphant proof of the doctrine of
universal gravitation, and of course of the received laws of physical
astronomy; it inspired new confidence in the power and accuracy of that
instrument (the calculus) by means of which its elements had been
investigated; and it proved that the different planets, which exerted
upon it severally a disturbing force proportioned to their quantity of
matter, had been correctly weighed, as in a balance.
I must now leave this wonderful body to pursue its sublime march far
beyond the confines of Uranus, (a distance it has long since reached,)
and take a hasty notice of two other comets, whose periodic returns have
also been ascertained; namely, those of Biela and Encke.
Biela's comet has a period of six years and three quarters. It has its
perihelion near the orbit of the earth, and its aphelion a little beyond
that of Jupiter. Its orbit, therefore, is far less eccentric than that
of Halley's comet; (see Frontispiece;) it neither approaches so near the
sun, nor departs so far from it, as most other known comets: some,
indeed, never come nearer to the sun than the orbit of Jupiter, while
they recede to an incomprehensible distance beyond the remotest planet.
We might even imagine that they would get beyond the limits of the sun's
attraction; nor is this impossible, although, according to La Place, the
solar attraction is sensible throughout a sphere whose radius is a
hundred millions of times greater than the distance of the earth from
the sun, or nearly ten thousand billions of miles.
Some months before the expected return of Biela's comet, in 1832, it was
announced by astronomers, who had calculated its path, that it would
cross the plane of the earth's orbit very near to the earth's path, so
that, should the earth happen at the time to be at that point of her
revolution, a collision might take place. This announcement excited so
much alarm among the ignorant classes in France, that it was deemed
expedient by the French academy, that one of their number should prepare
and publish an article on the subject, with the express view of
allaying popular apprehension. This task was executed by M. Arago. He
admitted that the earth would in fact pass so near the point where the
comet crossed the plane of its orbit, that, should they chance to meet
there, the earth would be enveloped in the nebulous atmosphere of the
comet. He, however, showed that the earth would not be near that point
at the same time with the comet, but fifty millions of miles from it.
The comet came at the appointed time, but was so exceedingly faint and
small, that it was visible only to the largest telescopes. In one
respect, its diminutive size and feeble light enhanced the interest with
which it was contemplated; for it was a sublime spectacle to see a body,
which, as projected on the celestial vault, even when magnified a
thousand times, seemed but a dim speck of fog, still pursuing its way,
in obedience to the laws of universal gravitation, with the same
regularity as Jupiter and Saturn. We are apt to imagine that a body,
consisting of such light materials that it can be compared only to the
thinnest fog, would be dissipated and lost in the boundless regions of
space; but so far is this from the truth, that, when subjected to the
action of the same forces of projection and solar attraction, it will
move through the void regions of space, and will describe its own orbit
about the sun with the same unerring certainty, as the densest bodies of
the system.
Encke's comet, by its frequent returns, (once in three and a third
years,) affords peculiar facilities for ascertaining the laws of its
revolution; and it has kept the appointments made for it with great
exactness. On its return in 1839, it exhibited to the telescope a
globular mass of nebulous matter, resembling fog, and moved towards its
perihelion with great rapidity. It makes its entire excursions within
the orbit of Jupiter.
But what has made Encke's comet particularly famous, is its having first
revealed to us the existence of a _resisting medium_ in the planetary
spaces. It has long been a question, whether the earth and planets
revolve in a perfect void, or whether a fluid of extreme rarity may not
be diffused through space. A perfect vacuum was deemed most probable,
because no such effects on the motions of the planets could be detected
as indicated that they encountered a resisting medium. But a feather, or
a lock of cotton, propelled with great velocity, might render obvious
the resistance of a medium which would not be perceptible in the motions
of a cannon ball. Accordingly, Encke's comet is thought to have plainly
suffered a retardation from encountering a resisting medium in the
planetary regions. The effect of this resistance, from the first
discovery of the comet to the present time, has been to diminish the
time of its revolution about two days. Such a resistance, by destroying
a part of the projectile force, would cause the comet to approach nearer
to the sun, and thus to have its periodic time shortened. The ultimate
effect of this cause will be to bring the comet nearer to the sun, at
every revolution, until it finally falls into that luminary, although
many thousand years will be required to produce this catastrophe. It is
conceivable, indeed, that the effects of such a resistance may be
counteracted by the attraction of one or more of the planets, near which
it may pass in its successive returns to the sun. Still, it is not
probable that this cause will exactly counterbalance the other; so that,
if there is such an elastic medium diffused through the planetary
regions, it must follow that, in the lapse of ages, every comet will
fall into the sun. Newton conjectured that this would be the case,
although he did not found his opinion upon the existence of such a
resisting medium as is now detected. To such an opinion he adhered to
the end of life. At the age of eighty-three, in a conversation with his
nephew, he expressed himself thus: "I cannot say when the comet of 1680
will fall into the sun; possibly after five or six revolutions; but
whenever that time shall arrive, the heat of the sun will be raised by
it to such a point, that our globe will be burned, and all the animals
upon it will perish."
Of the _physical nature_ of comets little is understood. The greater
part of them are evidently mere masses of vapor, since they permit very
small stars to be seen through them. In September, 1832, Sir John
Herschel, when observing Biela's comet, saw that body pass directly
between his eye and a small cluster of minute telescopic stars of the
sixteenth or seventeenth magnitude. This little constellation occupied a
space in the heavens, the breadth of which was not the twentieth part of
that of the moon; yet the whole of the cluster was distinctly visible
through the comet. "A more striking proof," says Sir John Herschel,
"could not have been afforded, of the extreme transparency of the matter
of which this comet consists. The most trifling fog would have entirely
effaced this group of stars, yet they continued visible through a
thickness of the comet which, calculating on its distance and apparent
diameter, must have exceeded fifty thousand miles, at least towards its
central parts." From this and similar observations, it is inferred, that
the nebulous matter of comets is vastly more rare than that of the air
we breathe, and hence, that, were more or less of it to be mingled with
the earth's atmosphere, it would not be perceived, although it might
possibly render the air unwholesome for respiration. M. Arago, however,
is of the opinion, that some comets, at least, have a solid nucleus. It
is difficult, on any other supposition, to account for the strong light
which some of them have exhibited,--a light sufficiently intense to
render them visible in the day-time, during the presence of the sun. The
intense heat to which comets are subject, in approaching so near the sun
as some of them do, is alleged as a sufficient reason for the great
expansion of the thin vapory atmospheres which form their tails; and the
inconceivable cold to which they are subject, in receding to such a
distance from the sun, is supposed to account for the condensation of
the same matter until it returns to its original dimensions. Thus the
great comet of 1680, at its perihelion, approached within one hundred
and forty-six thousand miles of the surface of the sun, a distance of
only one sixth part of the sun's diameter. The heat which it must have
received was estimated to be equal to twenty-eight thousand times that
which the earth receives in the same time, and two thousand times hotter
than red-hot iron. This temperature would be sufficient to volatilize
the most obdurate substances, and to expand the vapor to vast
dimensions; and the opposite effects of the extreme cold to which it
would be subject in the regions remote from the sun would be adequate to
condense it into its former volume. This explanation, however, does not
account for the direction of the tail, extending, as it usually does,
only in a line opposite to the sun. Some writers, therefore, suppose
that the nebulous matter of the comet, after being expanded to such a
volume that the particles are no longer attracted to the nucleus, unless
by the slightest conceivable force, are carried off in a direction from
the sun, by the impulse of the solar rays themselves. But to assign such
a power to the sun's rays, while they have never been proved to have any
momentum, is unphilosophical; and we are compelled to place the
phenomena of comets' tails among the points of astronomy yet to be
explained.
Since comets which approach very near the sun, like the comet of 1680,
cross the orbits of all the planets, the possibility that one of them
may strike the earth has frequently been suggested. Still it may quiet
our apprehensions on this subject, to reflect on the vast amplitude of
the planetary spaces, in which these bodies are not crowded together, as
we see them erroneously represented in orreries and diagrams, but are
sparsely scattered at immense distances from each other. They are like
insects flying, singly, in the expanse of heaven. If a comet's tail lay
with its axis in the plane of the ecliptic when it was near the sun, we
can imagine that the tail might sweep over the earth; but the tail may
be situated at any angle with the ecliptic, as well as in the same plane
with it, and the chances that it will not be in the same plane are
almost infinite. It is also extremely improbable that a comet will cross
the plane of the ecliptic precisely at the earth's path in that plane,
since it may as probably cross it at any other point nearer or more
remote from the sun. A French writer of some eminence (Du Sejour) has
discussed this subject with ability, and arrived at the following
conclusions: That of all the comets whose paths had been ascertained,
none _could pass_ nearer to the earth than about twice the moon's
distance; and that none ever _did pass_ nearer to the earth than nine
times the moon's distance. The comet of 1770, already mentioned, which
became entangled among the satellites of Jupiter, came within this
limit. Some have taken alarm at the idea that a comet, by approaching
very near to the earth, might raise so high a _tide_, as to endanger the
safety of maritime countries especially: but this writer shows, that the
comet could not possibly remain more than two hours so near the earth as
a fourth part of the moon's distance; and it could not remain even so
long, unless it passed the earth under very peculiar circumstances. For
example, if its orbit were nearly perpendicular to that of the earth, it
could not remain more than half an hour in such a position. Under such
circumstances, the production of a tide would be impossible. Eleven
hours, at least, would be necessary to enable a comet to produce an
effect on the waters of the earth, from which the injurious effects so
much dreaded would follow. The final conclusion at which he arrives is,
that although, in strict geometrical rigor, it is not physically
impossible that a comet should encounter the earth, yet the probability
of such an event is absolutely nothing.
M. Arago, also, has investigated the probability of such a collision on
the mathematical doctrine of chances, and remarks as follows: "Suppose,
now, a comet, of which we know nothing but that, at its perihelion, it
will be nearer the sun than we are, and that its diameter is equal to
one fourth that of the earth; the doctrine of chances shows that, out of
two hundred and eighty-one millions of cases, there is but one against
us; but one, in which the two bodies could meet."
La Place has assigned the consequences that would result from a direct
collision between the earth and a comet. "It is easy," says he, "to
represent the effects of the shock produced by the earth's encountering
a comet. The axis and the motion of rotation changed; the waters
abandoning their former position to precipitate themselves towards the
new equator; a great part of men and animals whelmed in a universal
deluge, or destroyed by the violent shock imparted to the terrestrial
globe; entire species annihilated; all the monuments of human industry
overthrown;--such are the disasters which the shock of a comet would
necessarily produce." La Place, nevertheless, expresses a decided
opinion that the orbits of the planets have never yet been disturbed by
the influence of comets. Comets, moreover, have been, and are still to
some degree, supposed to exercise much influence in the affairs of this
world, affecting the weather, the crops, the public health, and a great
variety of atmospheric commotions. Even Halley, finding that his comet
must have been near the earth at the time of the Deluge, suggested the
possibility that the comet caused that event,--an idea which was taken
up by Whiston, and formed into a regular theory. In Gregory's Astronomy,
an able work, published at Oxford in 1702, the author remarks, that
among all nations and in all ages, it has been observed, that the
appearance of a comet has always been followed by great calamities; and
he adds, "it does not become philosophers lightly to set down these
things as fables." Among the various things ascribed to comets by a late
English writer, are hot and cold seasons, tempests, hurricanes, violent
hail-storms, great falls of snow, heavy rains, inundations, droughts,
famines, thick fogs, flies, grasshoppers, plague, dysentery, contagious
diseases among animals, sickness among cats, volcanic eruptions, and
meteors, or shooting stars. These notions are too ridiculous to require
a distinct refutation; and I will only add, that we have no evidence
that comets have hitherto ever exercised the least influence upon the
affairs of this world; and we still remain in darkness, with respect to
their physical nature, and the purposes for which they were created.
LETTER XXVII.
METEORIC SHOWERS.
"Oft shalt thou see, ere brooding storms arise,
Star after star glide headlong down the skies,
And, where they shot, long trails of lingering light
Sweep far behind, and gild the shades of night."--_Virgil._
FEW subjects of astronomy have excited a more general interest, for
several years past, than those extraordinary exhibitions of shooting
stars, which have acquired the name of meteoric showers. My reason for
introducing the subject to your notice, in this place, is, that these
small bodies are, as I believe, derived from nebulous or cometary
bodies, which belong to the solar system, and which, therefore, ought to
be considered, before we take our leave of this department of creation,
and naturally come next in order to comets.
The attention of astronomers was particularly directed to this subject
by the extraordinary shower of meteors which occurred on the morning of
the thirteenth of November, 1833. I had the good fortune to witness
these grand celestial fire-works, and felt a strong desire that a
phenomenon, which, as it afterwards appeared, was confined chiefly to
North America, should here command that diligent inquiry into its
causes, which so sublime a spectacle might justly claim.
As I think you were not so happy as to witness this magnificent display,
I will endeavor to give you some faint idea of it, as it appeared to me
a little before daybreak. Imagine a constant succession of fire-balls,
resembling sky-rockets, radiating in all directions from a point in the
heavens a few degrees southeast of the zenith, and following the arch of
the sky towards the horizon. They commenced their progress at different
distances from the radiating point; but their directions were uniformly
such, that the lines they described, if produced upwards, would all have
met in the same part of the heavens. Around this point, or imaginary
radiant, was a circular space of several degrees, within which no
meteors were observed. The balls, as they travelled down the vault,
usually left after them a vivid streak of light; and, just before they
disappeared, exploded, or suddenly resolved themselves into smoke. No
report of any kind was observed, although we listened attentively.
Beside the foregoing distinct concretions, or individual bodies, the
atmosphere exhibited _phosphoric lines_, following in the train of
minute points, that shot off in the greatest abundance in a
northwesterly direction. These did not so fully copy the figure of the
sky, but moved in paths more nearly rectilinear, and appeared to be much
nearer the spectator than the fire-balls. The light of their trains was
also of a paler hue, not unlike that produced by writing with a stick of
phosphorus on the walls of a dark room. The number of these luminous
trains increased and diminished alternately, now and then crossing the
field of view, like snow drifted before the wind, although, in fact,
their course was towards the wind.
From these two varieties, we were presented with meteors of various
sizes and degrees of splendor: some were mere points, while others were
larger and brighter than Jupiter or Venus; and one, seen by a credible
witness, at an earlier hour, was judged to be nearly as large as the
moon. The flashes of light, although less intense than lightning, were
so bright, as to awaken people in their beds. One ball that shot off in
the northwest direction, and exploded a little northward of the star
Capella, left, just behind the place of explosion, a phosphorescent
train of peculiar beauty. This train was at first nearly straight, but
it shortly began to contract in length, to dilate in breadth, and to
assume the figure of a serpent drawing itself up, until it appeared like
a small luminous cloud of vapor. This cloud was borne eastward, (by the
wind, as was supposed, which was blowing gently in that direction,)
opposite to the direction in which the meteor itself had moved,
remaining in sight several minutes. The point from which the meteors
seemed to radiate kept a fixed position among the stars, being
constantly near a star in Leo, called Gamma Leonis.
Such is a brief description of this grand and beautiful display, as I
saw it at New Haven. The newspapers shortly brought us intelligence of
similar appearances in all parts of the United States, and many minute
descriptions were published by various observers; from which it
appeared, that the exhibition had been marked by very nearly the same
characteristics wherever it had been seen. Probably no celestial
phenomenon has ever occurred in this country, since its first
settlement, which was viewed with so much admiration and delight by one
class of spectators, or with so much astonishment and fear by another
class. It strikingly evinced the progress of knowledge and civilization,
that the latter class was comparatively so small, although it afforded
some few examples of the dismay with which, in barbarous ages of the
world, such spectacles as this were wont to be regarded. One or two
instances were reported, of persons who died with terror; many others
thought the last great day had come; and the untutored black population
of the South gave expression to their fears in cries and shrieks.