astronomers have acquired of the motions of comets, and the power they
have gained of predicting their return. Indeed, every thing appertaining
to this class of bodies is so wonderful, as to seem rather a tale of
romance than a simple recital of facts. Comets are truly the
knights-errant of astronomy. Appearing suddenly in the nocturnal sky,
and often dragging after them a train of terrific aspect, they were, in
the earlier ages of the world, and indeed until a recent period,
considered as peculiarly ominous of the wrath of Heaven, and as
harbingers of wars and famines, of the dethronement of monarchs, and the
dissolution of empires.
Science has, it is true, disarmed them of their terrors, and
demonstrated that they are under the guidance of the same Hand, that
directs in their courses the other members of the solar system; but she
has, at the same time, arrayed them in a garb of majesty peculiarly her
own.
Although the ancients paid little attention to the ordinary phenomena of
Nature, hardly deeming them worthy of a reason, yet, when a comet blazed
forth, fear and astonishment conspired to make it an object of the most
attentive observation. Hence the aspects of remarkable comets, that have
appeared at various times, have been handed down to us, often with
circumstantial minuteness, by the historians of different ages. The
comet which appeared in the year 130, before the Christian era, at the
birth of Mithridates, is said to have had a disk equal in magnitude to
that of the sun. Ten years before this, one was seen, which, according
to Justin, occupied a fourth part of the sky, that is, extended over
forty-five degrees, and surpassed the sun in splendor. In the year 400,
one was seen which resembled a sword in shape, and extended from the
zenith to the horizon.
Such are some of the accounts of comets of past ages; but it is probable
we must allow much for the exaggerations naturally accompanying the
descriptions of objects in themselves so truly wonderful.
A comet, when perfectly formed, consists of three parts, the nucleus,
the envelope, and the tail. The nucleus, or body of the comet, is
generally distinguished by its forming a bright point in the centre of
the head, conveying the idea of a solid, or at least of a very dense,
portion of matter. Though it is usually exceedingly small, when compared
with the other parts of the comet, and is sometimes wanting altogether,
yet it occasionally subtends an angle capable of being measured by the
telescope. The envelope (sometimes called the _coma_, from a Latin word
signifying hair, in allusion to its hairy appearance) is a dense
nebulous covering, which frequently renders the edge of the nucleus so
indistinct, that it is extremely difficult to ascertain its diameter
with any degree of precision. Many comets have no nucleus, but present
only a nebulous mass, exceedingly attenuated on the confines, but
gradually increasing in density towards the centre. Indeed, there is a
regular gradation of comets, from such as are composed merely of a
gaseous or vapory medium, to those which have a well-defined nucleus. In
some instances on record, astronomers have detected with their
telescopes small stars through the densest part of a comet. The tail is
regarded as an expansion or prolongation of the coma; and presenting, as
it sometimes does, a train of appalling magnitude, and of a pale,
portentous light, it confers on this class of bodies their peculiar
celebrity. These several parts are exhibited in Fig. 67, which
[Illustration Figures 67, 68. COMETS OF 1680 AND 1811.] represents the
appearance of the comet of 1680. Fig. 68 also exhibits that of the comet
of 1811.
The _number_ of comets belonging to the solar system, is probably very
great. Many no doubt escape observation, by being above the horizon in
the day-time. Seneca mentions, that during a total eclipse of the sun,
which happened sixty years before the Christian era, a large and
splendid comet suddenly made its appearance, being very near the sun.
The leading particulars of at least one hundred and thirty have been
computed, and arranged in a table, for future comparison. Of these,
_six_ are particularly remarkable; namely, the comets of 1680, 1770, and
1811; and those which bear the names of Halley, Biela, and Encke. The
comet of 1680 was remarkable, not only for its astonishing size and
splendor, and its near approach to the sun, but is celebrated for having
submitted itself to the observations of Sir Isaac Newton, and for having
enjoyed the signal honor of being the first comet whose elements were
determined on the sure basis of mathematics. The comet of 1770 is
memorable for the changes its orbit has undergone by the action of
Jupiter, as I shall explain to you more particularly hereafter. The
comet of 1811 was the most remarkable in its appearance of all that have
been seen in the present century. It had scarcely any perceptible
nucleus, but its train was very long and broad, as is represented in
Fig. 68. Halley's comet (the same which reappeared in 1835) is
distinguished as that whose return was first successfully predicted, and
whose orbit is best determined; and Biela's and Encke's comets are well
known for their short periods of revolution, which subject them
frequently to the view of astronomers.
In _magnitude and brightness_, comets exhibit great diversity. History
informs us of comets so bright, as to be distinctly visible in the
day-time, even at noon, and in the brightest sunshine. Such was the
comet seen at Rome a little before the assassination of Julius Cæsar.
The comet of 1680 covered an arc of the heavens of ninety-seven
degrees, and its length was estimated at one hundred and twenty-three
millions of miles. That of 1811 had a nucleus of only four hundred and
twenty-eight miles in diameter, but a tail one hundred and thirty-two
millions of miles long. Had it been coiled around the earth like a
serpent, it would have reached round more than five thousand times.
Other comets are exceedingly small, the nucleus being in one case
estimated at only twenty-five miles; and some, which are destitute of
any perceptible nucleus, appear to the largest telescopes, even when
nearest to us, only as a small speck of fog, or as a tuft of down. The
majority of comets can be seen only by the aid of the telescope. Indeed,
the same comet has very different aspects, at its different returns.
Halley's comet, in 1305, was described by the historians of that age as
the comet of terrific magnitude; (_cometa horrendæ magnitudinis_;) in
1456 its tail reached from the horizon to the zenith, and inspired such
terror, that, by a decree of the Pope of Rome, public prayers were
offered up at noonday in all the Catholic churches, to deprecate the
wrath of heaven; while in 1682 its tail was only thirty degrees in
length; and in 1759 it was visible only to the telescope until after it
had passed its perihelion. At its recent return, in 1835, the greatest
length of the tail was about twelve degrees. These changes in the
appearance of the same comet are partly owing to the different positions
of the earth with respect to them, being sometimes much nearer to them
when they cross its track than at others; also, one spectator, so
situated as to see the comet at a higher angle of elevation, or in a
purer sky, than another, will see the train longer than it appears to
another less favorably situated; but the extent of the changes are such
as indicate also a real change in magnitude and brightness.
The _periods_ of comets in their revolutions around the sun are equally
various. Encke's comet, which has the shortest known period, completes
its revolution in three and one third years; or, more accurately, in
twelve hundred and eight days; while that of 1811 is estimated to have
a period of thirty-three hundred and eighty three years.
The _distances_ to which different comets recede from the sun are
equally various. While Encke's comet performs its entire revolution
within the orbit of Jupiter, Halley's comet recedes from the sun to
twice the distance of Uranus; or nearly thirty-six hundred millions of
miles. Some comets, indeed, are thought to go a much greater distance
from the sun than this, while some are supposed to pass into curves
which do not, like the ellipse, return into themselves; and in this case
they never come back to the sun. (See Fig. 34, page 153.)
Comets shine _by reflecting the light of the sun_. In one or two
instances, they have been thought to exhibit distinct _phases_, like the
moon, although the nebulous matter with which the nucleus is surrounded
would commonly prevent such phases from being distinctly visible, even
when they would otherwise be apparent. Moreover, certain qualities of
_polarized_ light,--an affection by which a ray of light seems to have
different properties on different sides,--enable opticians to decide
whether the light of a given body is direct or reflected; and M. Arago,
of Paris, by experiments of this kind on the light of the comet of 1819,
ascertained it to be reflected light.
The tail of a comet usually increases very much as it approaches the
sun; and it frequently does not reach its maximum until after the
perihelion passage. In receding from the sun, the tail again contracts,
and nearly or quite disappears before the body of the comet is entirely
out of sight. The tail is frequently divided into two portions, the
central parts, in the direction of the axis, being less bright than the
marginal parts. In 1744 a comet appeared which had six tails spread out
like a fan.
The tails of comets extend in a direct line from the sun, although more
or less curved, like a long quill or feather, being convex on the side
next to the direction in which they are moving,--a figure which may
result from the less velocity of the portion most remote from the sun.
Expansions of the envelope have also been at times observed on the side
next the sun; but these seldom attain any considerable length.
The _quantity of matter_ in comets is exceedingly small. Their tails
consist of matter of such tenuity, that the smallest stars are visible
through them. They can only be regarded as masses of thin vapor,
susceptible of being penetrated through their whole substance by the
sunbeams, and reflecting them alike from their interior parts and from
their surfaces. It appears perhaps incredible, that so thin a substance
should be visible by reflected light, and some astronomers have held
that the matter of comets is self-luminous; but it requires but very
little light to render an object visible in the night, and a light vapor
may be visible when illuminated throughout an immense stratum, which
could not be seen if spread over the face of the sky like a thin cloud.
"The highest clouds that float in our atmosphere," says Sir John
Herschel, "must be looked upon as dense and massive bodies, compared
with the filmy and all but spiritual texture of a comet."
The small quantity of matter in comets is proved by the fact, that they
have at times passed very near to some of the planets, without
disturbing their motions in any appreciable degree. Thus the comet of
1770, in its way to the sun, got entangled among the satellites of
Jupiter, and remained near them four months; yet it did not perceptibly
change their motions. The same comet, also, came very near the earth; so
that, had its quantity of matter been equal to that of the earth, it
would, by its attraction, have caused the earth to revolve in an orbit
so much larger than at present, as to have increased the length of the
year two hours and forty-seven minutes. Yet it produced no sensible
effect on the length of the year, and therefore its mass, as is shown by
La Place, could not have exceeded 1/5000 of that of the earth, and
might have been less than this to any extent. It may indeed be asked,
what proof we have that comets have any matter, and are not mere
reflections of light. The answer is, that, although they are not able by
their own force of attraction to disturb the motions of the planets, yet
they are themselves exceedingly disturbed by the action of the planets,
and in exact conformity with the laws of universal gravitation. A
delicate compass may be greatly agitated by the vicinity of a mass of
iron, while the iron is not sensibly affected by the attraction of the
needle.
By approaching very near to a large planet, a comet may have its orbit
entirely changed. This fact is strikingly exemplified in the history of
the comet of 1770. At its appearance in 1770, its orbit was found to be
an ellipse, requiring for a complete revolution only five and a half
years; and the wonder was, that it had not been seen before, since it
was a very large and bright comet. Astronomers suspected that its path
had been changed, and that it had been recently compelled to move in
this short ellipse, by the disturbing force of Jupiter and his
satellites. The French Institute, therefore, offered a high prize for
the most complete investigation of the elements of this comet, taking
into account any circumstances which could possibly have produced an
alteration in its course. By tracing back the movements of this comet,
for some years previous to 1770, it was found that, at the beginning of
1767, it had entered considerably within the sphere of Jupiter's
attraction. Calculating the amount of this attraction from the known
proximity of the two bodies, it was found what must have been its orbit
previous to the time when it became subject to the disturbing action of
Jupiter. It was therefore evident why, as long as it continued to
circulate in an orbit so far from the centre of the system, it was never
visible from the earth. In January, 1767, Jupiter and the comet happened
to be very near to one another, and as both were moving in the same
direction, and nearly in the same plane, they remained in the
neighborhood of each other for several months, the planet being between
the comet and the sun. The consequence was, that the comet's orbit was
changed into a smaller ellipse, in which its revolution was accomplished
in five and a half years. But as it approached the sun, in 1779, it
happened again to fall in with Jupiter. It was in the month of June that
the attraction of the planet began to have a sensible effect; and it was
not until the month of October following, that they were finally
separated.
At the time of their nearest approach, in August, Jupiter was distant
from the comet only 1/491 of its distance from the sun, and exerted an
attraction upon it two hundred and twenty-five times greater than that
of the sun. By reason of this powerful attraction, Jupiter being further
from the sun than the comet, the latter was drawn out into a new orbit,
which even at its perihelion came no nearer to the sun than the planet
Ceres. In this third orbit, the comet requires about twenty years to
accomplish its revolution; and being at so great a distance from the
earth, it is invisible, and will for ever remain so unless, in the
course of ages, it may undergo new perturbations, and move again in some
smaller orbit, as before.
With the foregoing leading facts respecting comets in view, I will now
explain to you a few things equally remarkable respecting their
_motions_.
The paths of the planets around the sun being nearly circular, we are
able to see a planet in every part of its orbit. But the case is very
different with comets. For the greater part of their course, they are
wholly out of sight, and come into view only while just in the
neighborhood of the sun. This you will readily see must be the case, by
inspecting the frontispiece, which represents the orbit of Biela's
comet, in 1832. Sometimes, the orbit is so eccentric, that the place of
the focus occupied by the sun appears almost at the extremity of the
orbit. This was the case with the orbit of the comet of 1680. Indeed,
this comet, at its perihelion, came in fact nearer to the sun than the
sixth part of the sun's diameter, being only one hundred and forty-six
thousand miles from the surface of the sun, which, you will remark, is
only a little more than half the distance of the moon from the earth;
while, at its aphelion, it was estimated to be thirteen thousand
millions of miles from the sun,--more than eleven thousand millions of
miles beyond the planet Uranus. Its _velocity_, when nearest the sun,
exceeded a million of miles an hour. To describe such an orbit as was
assigned to it by Sir Isaac Newton, would require five hundred and
seventy-five years. During all this period, it was entirely out of view
to the inhabitants of the earth, except the few months, while it was
running down to the sun from such a distance as the orbit of Jupiter and
back. The velocity of bodies moving in such eccentric orbits differs
widely in different parts of their orbits. In the remotest parts it is
so slow, that years would be required to pass over a space equal to that
which it would run over in a single day, when near the sun.
The appearances of the same comet at different periods of its return are
so various, that we can never pronounce a given comet to be the same
with one that has appeared before, from any peculiarities in its
physical aspect, as from its color, magnitude, or shape; since, in all
these respects, it is very different at different returns; but it is
judged to be the same if its _path_ through the heavens, as traced among
the stars, is the same.
The comet whose history is the most interesting, and which both of us
have been privileged to see, is Halley's. Just before its latest visit,
in 1835, its return was anticipated with so much expectation, not only
by astronomers, but by all classes of the community, that a great and
laudable eagerness universally prevailed, to learn the particulars of
its history. The best summary of these, which I met with, was given in
the Edinburgh Review for April, 1835. I might content myself with barely
referring you to that well-written article; but, as you may not have the
work at hand, and would, moreover, probably not desire to read the
whole article, I will abridge it for your perusal, interspersing some
remarks of my own. I have desired to give you, in the course of these
Letters, some specimen of the labors of astronomers, and shall probably
never be able to find a better one.
It is believed that the first recorded appearance of Halley's comet was
that which was supposed to signalize the birth of Mithridates, one
hundred and thirty years before the birth of Christ. It is said to have
appeared for twenty-four days; its light is said to have surpassed that
of the sun; its magnitude to have extended over a fourth part of the
firmament; and it is stated to have occupied, consequently, about four
hours in rising and setting. In the year 323, a comet appeared in the
sign Virgo. Another, according to the historians of the Lower Empire,
appeared in the year 399, seventy-six years after the last, at an
interval corresponding to that of Halley's comet. The interval between
the birth of Mithridates and the year 323 was four hundred and
fifty-three years, which would be equivalent to six periods of
seventy-five and a half years. Thus it would seem, that in the interim
there were five returns of this comet unobserved, or at least
unrecorded. The appearance in the year 399 was attended with
extraordinary circumstances. It was described in the old writers as a
"comet of monstrous size and appalling aspect, its tail seeming to reach
down to the ground." The next recorded appearance of a comet agreeing
with the ascertained period marks the taking of Rome, in the year
550,--an interval of one hundred and fifty-one years, or two periods of
seventy-five and a half years having elapsed. One unrecorded return
must, therefore, have taken place in the interim. The next appearance of
a comet, coinciding with the assigned period, is three hundred and
eighty years afterwards; namely, in the year 930,--five revolutions
having been completed in the interval. The next appearance is recorded
in the year 1005, after an interval of a single period of seventy-five
years. Three revolutions would now seem to have passed unrecorded, when
the comet again makes its appearance in 1230. In this, as well as in
former appearances, it is proper to state, that the sole test of
identity of these cornets with that of Halley is the coincidence of the
times, as near as historical records enable us to ascertain, with the
epochs at which the comet of Halley might be expected to appear. That
such evidence, however, is very imperfect, must be evident, if the
frequency of cometary appearances be considered, and if it be
remembered, that hitherto we find no recorded observations, which could
enable us to trace, even with the rudest degree of approximation, the
paths of those comets, the times of whose appearances raise a
presumption of their identity with that of Halley. We now, however,
descend to times in which more satisfactory evidence may be expected.
In the year 1305, a year in which the return of Halley's comet might
have been expected, there is recorded a comet of remarkable character:
"A comet of terrific dimensions made its appearance about the time of
the feast of the Passover, which was followed by a Great Plague." Had
the terrific appearance of this body alone been recorded, this
description might have passed without the charge of great exaggeration;
but when we find the Great Plague connected with it as a consequence, it
is impossible not to conclude, that the comet was seen by its historians
through the magnifying medium of the calamity which followed it. Another
appearance is recorded in the year 1380, unaccompanied by any other
circumstance than its mere date. This, however, is in strict accordance
with the ascertained period of Halley's comet.
We now arrive at the first appearance at which observations were taken,
possessing sufficient accuracy to enable subsequent investigators to
determine the path of the comet; and this is accordingly the first comet
the identity of which with the comet of Halley can be said to be
conclusively established. In the year 1456, a comet is stated to have
appeared "of unheard of magnitude;" it was accompanied by a tail of
extraordinary length, which extended over sixty degrees, (a third part
of the heavens,) and continued to be seen during the whole month of
June. The influence which was attributed to this appearance renders it
probable, that in the record there is more or less of exaggeration. It
was considered as the celestial indication of the rapid success of
Mohammed the Second, who had taken Constantinople, and struck terror
into the whole Christian world. Pope Calixtus the Second levelled the
thunders of the Church against the enemies of his faith, terrestrial and
celestial; and in the same Bull excommunicated the Turks and the comet;
and, in order that the memory of this manifestation of his power should
be for ever preserved, he ordained that the bells of all the churches
should be rung at mid-day,--a custom which is preserved in those
countries to our times.
The extraordinary length and brilliancy which was ascribed to the tail,
upon this occasion, have led astronomers to investigate the
circumstances under which its brightness and magnitude would be the
greatest possible; and upon tracing back the motion of the comet to the
year 1456, it has been found that it was then actually in the position,
with respect to the earth and sun, most favorable to magnitude and
splendor. So far, therefore, the result of astronomical calculation
corroborates the records of history.
The next return took place in 1531. Pierre Appian, who first ascertained
the fact that the tails of comets are usually turned from the sun,
examined this comet with a view to verify his statement, and to
ascertain the true direction of its tail. He made, accordingly, numerous
observations upon its position, which, although rude, compared with the
present standard of accuracy, were still sufficiently exact to enable
Halley to identify this comet with that observed by himself.