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2S ON COMETS.

Hrvelius, tftat some* have cast shadows behind
them. Their shining only by the sun's light sup-
plies us with a reason why they are most frequently
seen near the sun, or in that hemisphere of the hea-
vens, in the middle of which the sun is placed. For
in their descent to the planetary regions, they are not
sufficiently illuminated by the sun to discover them-
selves, till they come within the sphere of Jupiter.f
Now the greater part of this sphere is on that side of
the earth, which looks toward the sun, and therefore
mort? Comets may be expected on that side than on
the other.

You have now seen in what respects the Comets
luay be said to be a kind of planets. There is a con-
formity between them in these particulars : both of
them are compact, permanent, opake bodies ; both
shine, not by native light, as meteors do, but by re-
flecting the sun's light ; and both revolve in conic
sections round the sun placed in one focus of their
orbits, which, in all probability, are of the same kind,
elliptic. But in other circumstances there is a dif-
ference. Their orbits, though elliptic, are vastly
more eccentric or oblong than those of the planets ;
the former being almost purabolic ; the latter, almost
circular. There is a difference, as well in the situa-
tion as in the figure of their orbits. The planes of
all the planetaiy orbits are almost coincident with the
ecliptic ; that which differs most, making with it an

* As that of 1665, whose shadow made a kind of fissure in
the tail, Hevelii Cometogr, Lib. xii. p. 898. f p. 8.



ON COMETS. ' 27

angle of but 7*. But the orbits of Comets are in-
clined to the ecliptic in very different angles, some of
them being almost perpendicular to it. The dispo-
sition of their orbits is indeed as various as possible
in every respect ; as any one will see, that looks into
Dr. Halley's table of them. Again : the real mo-
tion of all the planets round the sun is constantly di-
rect; whereas the real motion of many Comets*
is retrograde, or contrary to that of the planets, as
was intimated before. f This diversity in the real po-
sition of their planes, and in the rea/direction of their
motions, occasions that great difference in their afi'
parent courses, which we took notice of, under the
head of their general phsenomena.:): Such a variety
in the direction of these motions is, by the way, a full
proof, that there are neither solid orbs in the heavens,
as some of the ancients imagined ; nor such whirl-
pools of fluid matter, as many of the moderns have
introduced, in order to account for the planetary re-
volutions.

But Comets seem to differ from the planets in no-
thing more, than in that particular constitution of
their bodies, or atmospheres, or both, which disposes
them to have tails ;«— an appendage, not in the least
degree belonging to any of the planets. You have
already seen the insuffxciency of some hypotheses
which had been proposed to account for this appear-
ance; and that the cause of it must be some ex-

* About one half of the known Comets are retrograde.
t p. 7. % p. 3.



28 ON COMETS.

tremely rare vapor reflecting the sun's light. That
the tail consists of some matter derived from the nu-
cleus, may be argued from hence, that Comets have
the greatest and brightest tails, quickly after they have
passed by the sun ; and those have the longest tails,
which go nearest to him. By this it appears, that
the heat, which the Comet acquires from the sun,
tends to increase the tail. Thus, the Comet of 1680,
when it was first discovered in its descent to the sun,
had no tail ; after some days, a small one arose, which
increased in the approach to the sun ; but soon after
the Comet had passed its perihelion, which was not a
sixth part of the sun's diameter distant from the sun,
where it contracted a prodigious degree of heat,* it
emitted a very splendid tail of a surprising magni-
tude, extending in length above 70°; which, consi-
dering its distance from the earth, fell little short of
a hundred million miles. But this afterwards gradu-
ally diminished, until it totally disappeared. From
hence it may be collected, that the tail is nothing but
an extremely thin vapor, excited by heat from the
nucleus of the Comet. Which is farther confirmed by
a rule our great philosopher has given,! to determine
at what time the vapor in the end of the tail began
to ascend from the nucleus ; by which rule he found,
that almost all the vapor, which composed the tail of
this Comet after it had passed its perihelion, arose at,
and soon after the time of the perihelion, when it sus-
tained the greatest heat.

* See above p. 8. f Newt. Prlncip. p. 513.



ox COMETS. 29

The tail of a Comet, being a vapor excited by the
heat of the sun, must consist of matter of a texture
different from what is in the planets. For some Co-
mets have never come so near to the sun, as the earth
always is; many have not been nearer^ to him than
Venus or Murcury, and so have never been exposed
to a greater heat than these planets continually are ;
yet have all these Comets sent forth considerable tails.
Which shews that the matter of these tails was of
such a peculiar texture as disposed it to be easily ra-
refied, without a great degree of heat. Since the at-
mospheres of Comets are vastly larger in proportion
to their bodies, than those of -the planets are, so far
as we can judge of the other planets, by analogy,
from our own ; and since they abound much more
with vapors, as is evident from their thick and tur-
bid appearance; they seem to be a proper fund, to
supply matter in sufficient plenty for the formation of
the tails. For though some of these tails are vastly
long, yet they are so thin, that an inconsiderable
quantity of matter, duly rarefied, may suffice to pro-
duce them. We find, that a very little fuel will make
smoke enough to fill a very large space.

The ascent of the tails in a direction opposite to the
sun, our illustri6us author thinks owing to the rare-
faction of the matter of which they consist. For as in
our atmosphere, where every thing gravitates to the
earth, the smoke of any body mounts upward from
the earth ; and that either perpendicularly, if the body
be at rest ; or obliquely, if that be moved sideways :
*3



30 ON COMETS.

SO in the heavens, where all things gravitate to the
sun, smoke and vapors ought to ascend from the sun ;
either ri^ht upward, if the fuming body were at rest;
or obliquely, when that, by being in motion, leaves
the places from which the superior parts of the va-
por had ascended. '* The air in a chimney, says Sir
Isaac,* being rarefied by heat, becomes specifically
lighter than the neighbouring air, and therefore ne-
cessarily rises in it, and carries up the smoke with it ;
and thus the vapor in the tail of a Comet being
warmed, will warm the setherial aura or solar atmos-
phere, wherein it is involved ; which by this warmth
being rarefied, and its specific gravity towards the
sun diminished, it will ascend, and carry with it the
matter which niakes the tail ;" and this matter, by its
rarefaction, will continually expand on all sides, and
make the upper end of the tail the broadest and thin-
nest. The setherial aura must be supposed ex-
tremely rare and next to a vacuum, at the distance at
which some Comets pass from the sun ; yet, be it
ever so rare, if that part of it, which is involved
within the atmosphere and tail of a Comet, be made
rarer by heat than the ambient parts are, it must rise
in them, by the known laws of specific gravity.

From what has been said it appears, that the vapor,
which makes the tail, has a compound motion ; — a mo-
tion of ascent right upward from the sun, and a pro-
gressive motion, which it had in common with the
nucleus. All the parts of it must therefore, by their

* Princip. p. Sli.



ON COMETS. 31

gravity toward the sun, revolve round him in conic
sections, for the same reason as the nucleus itself does ;
and, though detached from that, will accompany it in
its progress, till by slow degrees they are dissipated.
Here you may see the reason, why the tail is often a
little incurvated, and sometimes bifurcated; and why
it is not turned exactly to the point opposite to the
sun. To keep the tail straight, and exactly turned
from the sun, all the parts of it should describe grea-
ter arcs in the same time, and therefore move with
greater velocities than the nucleus ; — greater, in the
same proportion as they are more distant from the
sun. But the progressive motion, which they receive
from the nucleus, can never give them a greater ve-
locity than that has. Therefore the upper parts will
be left a little behind, or on that side front which the
Comet is moving ; and the tail will be bent, with its
convexity on that side toward which the Comet is
moving. Farther: As the orbit of a Comet is very
curve near the perihelion, those vapours, which make
a tail before the perihelion, rise in a different direc-
tion from those after the perihelion. The consequence
of this may be a forked tail ; being, as it were, two
tails, joined together near the nucleus, and divaricat-
ing toward their upper ends.

The tail, being always nearly opposite to the sun,
will follow the head of the Comet, while that is de-
scending to its perihelion ; and will precede it, in its
ascent. And if the Comet be near a straight line pass-
ing through the sun and the earth, and so be near its



32 ON COMETS.

conjunction or opposition with the sun; the upper
end of the tail being broadest, will appear to surround
the head of the Comet on all sides, like hair.

Thus have we endeavoured to explain the general
phsenomena of Comets, enumerated in the beginning
of the foregoing discourse. Concerning their uses
and effects, two or three things are now to be said.

It is not to be doubted, that the allwise Author of
nature designed so remarkable a sort of bodies for im-
portant purposes, both natural and moral, in His crea-
tion. The moral purposes seem not very difficult to
be found. Such grand and unusual appearances tend
to rouse mankind, who are apt io fall asleefi, while all
things continue as they naere ; to awaken their at-
tention, and to direct it to the supreme Governor of
the universe ; whom they would be in danger of to-
tally forgetting, were nature always to glide along
with an uniform tenor. These exotic stars serve to
raise in our minds most sublime conceptions of God,
and particularly display his exquisite skill. The mo-
tions of many Comets being contrary to those of the
planets, shew that neither of them proceed from ne-
cessity or fate, but from choice and design. The same
thing is to be seen in the figure and situation of their
orbits ; which indeed have not the appearance of re-
gularity, as those of the planets, and yet are the result
of admirable contrivance By means of their great
eccentricity, they run so swiftly through the planetary
regions, as to have but very little time to disturb their
own motions, or those of the planets. And this end



ON COMETS. 33

is still more effectually answered in those Comets
wiiose motion is retrograde, or contrary to that of the
planets. In this case, the relative velocity wherewith
the Comet and planet run by each other is the sum;
but when Comets move the same way as the planets,
it is the difference^ of their real velocities.* By this
great eccentricity likewise, as well as by the very dif-
ferent situation of their planes, they are at vast dis-
tances from each other in their aphelia ; where their
motions are so slow, and their gravitation to the sun
so weak, that their mutual gravitation might produce
irregularities, and perhaps throw the system into con-
fusion; which this precaution has guarded against..

But without enlarging on these things, I must has-
ten to speak a little of the natural purposes, to which
Comets may be subservient. To assign ail these in
particular, may perhaps never be in the power of mor-
tals ; to be sure, it is not to be expected at present,
while our knowledge of the coraetic system is but in
its dawn. It does not seem very likely, that they

* The velocity of a Comet is to that of a planet, at the same
distance from the sun, nearly as 7 to 5. Therefore the relative
velocity of retrograde Comets is to that of direct ones as 7-|-5
to 7 — 5, that is, as 12 to 2. So that a retrograde Comet in its
transit by a planet will continue w^ithin a given distance from
it, but one sixth part of the time that it would, if it were di-
rect. This would be strictly so, in case a Comet and planet
should pass by each other in parallel directions. But the grea-
ter the deviation from parallelism is, so much the less diffe-
rence wiil be misde, in the time of their continuing' near one
another, by a Comet's being retrograde or direct.



34 ON COMETS.

should be intended, as the planets, to afford an habi-
tation for animals. The very great excentricity of
their orbits exposes them to such mighty changes of
light and darkness, of heat and cold, as seem to im-
ply that they are designed for purposes very different
from the planets. If the Comet of 1680 revolves in
575 years, as some have supposed (and its period can-
not be much shorter than this ;* though it may be
much longer) its greatest distance from the sun must
be above 23,000 times greater than its least distance;
and the light and heat of the sun being every where
reciprocally as the squares of the distances, the Co-
met's greatest light and heat is above five hundred
million times greater than its least. Extremes like
these must, one would think, destroy all vegetables
and animals, in the least degree resembling those
we are acquainted with. And yet, such is the in-
exhaustible variety of the works of God, that it is
more than possible, that there may be vegetables and
animals, to which such extremes may be as necessary
as those, to which our globe is subject, are to ours.
However, it seems most probable, that the Comets
are designed to be some way or other serviceable to
the planetary worlds. Those who see with unphilo-
sophic eyes are, I suppose, generally alarmed at the
sight of a Comet ; and indeed the bulk of mankind
seem disposed, by a sort of natural superstition, to
turn every unusual phsenomenon into a presage of

* See above, p. 25.



ON COMETS. 35

some dire event. Of the truth of this observation, so
far as it relates to Comets, the histories of them af-
ford an incontestable proof. But these objects of ter-
ror may be productive of great advantages. It is the
judgment of Sir Isaac Newton,* theit the tails of
Comets maybe intended to recruit the perpetual v/aste
of fluids in the planets by vegetation and putrefaction.
The tails sometimes reach to an immense distance;!
and the rarefied vapor, dilating itself toward their up-
per end, must gradually be diffused throughout the
heavens, and by its gravity be drawn into the atmos-
pheres of the planets, and mingle with them. He
farther suspects, that the most subtle and spirituous
part of our air, which is most requisite to maintain,
life, may come principally from the Comets. Ano-
ther end of them may be, to. supply the perpetual con-
sumption of the sun. I The Comet of 1680 descend-
ed so near to the sun,t that it probably entered, and
that with an excessive velocity, into the denser part
of his atmosphere, where meeting with a sensible re-
sistance, it might be a little retarded ; and if so, in its
next revolution it will go nearer the sun, and be more
retarded ; and being thus retarded in every revolution,
it must at length fall upon the sun, and may serve for
new fuel to feed his flame. It may be more retarded by
the attraction of other Comets in its aphelion, where its
motion is very slow ; and by that means may the sooner,
fall into the sun. Such may probably be some of the

* Princip. p. 515, f See above, p. 28.

I Princip. p. 525,



36 ON COMETS.

uses of Comets ; and as our knwvledge of their niyn-
ber, their periods, and the figure and situation of their
orbits advances, more may be discovered.

On the other side, it oue^ht not to be concealed, that
they seem fitted to be the ministers of divine justice
as well as goodness, and capable of producing very
great and destructive changes in the planetary worlds j
some of them having their nodes situated not far from
the orbits of the planets. Were a Comet to pass very
near a planet, it would increase the tides in the ocean
(if the other planets have oceans as well as our earth)
and might swell ihem so as to drown the lower coun-
tries, if not the mountains ; and should the planet at
the same time pass through the tail of the Comet, the
vapours of that, supposing them to be aqueous, might,
by turning into rain, greatly increase the inundation.
By such a near approach of a Comet to the earth, has
the ingenious and learned Mr. Whiston endeavoured
to account for the universal deluge spoken of in the
holy scriptures. And if he has not satisfactorily ac-
counted for every particular upon this hypothesis, at
least he has produced much more evidence of various
kinds than could have been expected in a matter of
such early antiquity, to prove that, in fact, a Comet
came very near and passed by the earth, on the day
the deluge began ; and if so, it could not but have a
very great influence in bringing on that most astonish-
ing catastrophe. Indeed, according to the laws of na-
ture, particularly those of gravity, it is not possible
but that the near approach of a Comet to a planctj



ON (JOMETS. 37

either in its descent to the sun or ascent from him,
should draw after it a train of dangerous, if not fatal
consequences. Several of these might easily be men-
tioned; and many others there may be, of which w<5
have not at present the least suspicion.

But instead of entering here into a detail, which
would probably answer no valuable end, I choose ra-
ther to turn your thoughts to that consummate Wis-
dom, which presides over this vast machine of nature,
and has so regulated the several movements in it, as
to obviate the damage that might arise from this quar-
ter. None but an eye able to pierce into the remotest
futurity, and to foresee, throughout all ages, all the
situations which this numerous class of bodies would
have towards the planets, in consequence of the laws
of their respective motions, could have given so just
an arrangement to their several orbits, and assigned
them their places at first on their orbits with such
perfect accuracy, that their motions have ever since
continued without interfering, and no disasters of this
sort have taken place; unless we except the case of
the deluge. For though so many Comets have tra-
versed this planetary system, and some of their orbits
run near to those of the planets ; yet the planets have
never been in the way,* but always at a distance from

* The Comet of 1618 came to its descending node, the 19th
o^ September. Dr. Halley observes, that if it had come to
it about the middle o^ March [I think he should have said
May] it would have passed within one tvyrentieth of the sun's
distance from the earth. And so would the Comet of 1686,
4



'38 ©N COMETS.

the nearest point, when the Comets have passed by
it. The foresight of that great Being, which has
hitherto prevented such disorders, will continue to
prevent them, so long as He sees it fit the present
frame of nature should subsist. Longer than that, it
is not fit that it should subsist.

It may not be unseasonable to remark, for a conclu-
sion, that as, on the one hand, it argues a temerity
unworthy a philosophic mind, to explode every ap-
prehension of danger from Comets, as if it were im-
possible that any damage could ever be occasioned
by any of them; because some idle and superstitious
fancies have in times of ignorance prevailed concern-
ing them : So on the other, to be thrown into a panic,
whenever a Comet appears, on account of the ill ef-
fects which some few of these bodies might possibly
produce, if they were not under a proper direction,
betrays a weakness equally unbecoming a reasonable
being. The wisest course is, to aim at such a recti-
tude of intention and firmness of resolution, that, as
Horace says.

Si fractus illabatur orbis,
Impavidum ferient ruinse.

Read, 18th of April, 1759.

had it arrived at its ascending node a little sooner. But no Co-
met has threatened the earth with a nearer approach than that
of 1680; which, had it come down to tlie sun a month later,
would have passed as near the earth as the moon is.



APPENDIX.

CONCERNING THE REVOLUTIONS OF THE PRESENT
COMET5 AND OF SOME OTHERS,

X HIS year, 1759, has done great honour to astrono-
my. It has added a new confirmation to Sir Isaac
Newton's theory of Comets, which was indeed very
well established before ; and it has verified the first
prediction that ever was made of the appearance of a
Comet ; — the first, I mean, that ever was made upon
astronomical principles : For astrological or enthu-
siastical predictions of these or any other unusual
phsenomena deserve no regard, but are to be treated
as the dreams of visionaries. And yet, trifling and
contemptible as they are, such have been made both
in former and later times.* This first astronomical
prediction was made by Dr. Halley, in pursuance of
the theory now mentioned; and the grounds, upon
which he went in making it, have been laid open in
the foregoing papers, p. 23. They were in brief:
That the Comets, which appeared in the years 1531,
1607 and 1682 described trajectories in the heavens,
the observed part of which was sensibly parabolic ;
and upon computation, their trajectories were found

* Halley's Synops, Astron. Comet.



4© APPENDIX.

to be nearly the same. The inference from "which,
according to the principles of this theory, is, that
these were one and the same Comet, revolving in a
very long elliptic orbit, in about 75 or 76 years*
From hence the Dr. predicted, that " it would return
" in the year 1758;" as he first published in 1705.*
Having afterward looked over the catalogues of more
ancient Comets, and discovered that three others had
preceded the aforesaid three, in like intervals of time,
viz. in the years 1305, 1380 and 1456, he was more
confirmed in his opinion that they were the same
Comet. But having remarked the inequality of its
periods, that some were under 75 years, and others
above 76, and considered the effect which the action
x)f Jupiter upon the Comet, by the force of gravity,
might probably have had, about the time of its last
appearance in 1682, he concluded, that " it would not
" return till after the longer period of above 76 years,
« about the end of the year 1758, or the beginning of
" the next."t With what sagacity and accuracy of
judgment this conclusion was drawn, the event has
sufficiently declared. At the same time he expressed
his confidence, that " if it should return again at the
•* time he had foretold, impartial posterity would not
" refuse to acknowledge that this was first discovered
*' by an Englishman.''* It is to be hoped, the present
age will not forfeit the character of impartiality ; but
that all the world will now unite in doing justice to
such distinguishing merit.

* Pliil. Tnms. No. 097. j Synops. Astron. Comet.



APPENDIX.



41



As this Comet is on some accounts more remarka-
ble than the rest, I persuade myself a short history oi
it will not be unacceptable to the curious. I shall
therefore throw together the principal things that I
have met with concerning it.

Its first appearance, that may be depended on, is
that in 1305. As to which, all we find is, that it was
seen about Easter, that is, in April, as it was this pre-
sent year; and that it was,* according to the language
of those days, "of a horrible bigness:'* Though it
is likely it did not appear bigger at that time, than it
did on the 29 th of April last. For it could hardly
ever have been seen much nearer to the earth, than
it was this last time. It is to little purpose to look
for any appearances of it prior to this. For though I
find in Hevelius's and Lubienietski's Histories of
Comets mention of the appearance of Comets about
some of those times v/hen, it is probabley this ap-
peared, as within a year or two before and after ; yet
there is no way to determine which of these was the
same as the present Comet.

Its next appearance was in 1380; but the time of
the year is not mentioned. All the account isjf that
" a burning Comet appeared in Aquarius for three.
" months together." From its appearing for so long


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Online LibraryJohn WinthropTwo lectures on comets → online text (page 4 of 14)