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time in Aquarius, we may conclude that it was in the
winter; probably in the beginning of that year.

* Horrendae magnitudinis, Lublenietskl Historia omnium
Cometarum, p. 251, 2.
t Lubien. p. 270,



It appeared a third time in 1456. The sum of the
accounts then given' of it are,* that " a Comet of an
" unheard-of bigness was seen toward the east in the
" middle of Cancer, the whole month of June, with a
" tail of such a length as to reach almost two signs."
By this it seems, the tail was much longer than it has
been seen at any other time. But it is to be observ-
ed, that the Comet was then near its perihelion, where
the tail is always largest ; and the earth was in such a
position as to view the tail almost directly, and there-
fore under the largest angle that can be. However,
nothing is more likely than that the matter is exag-
gerated ; most of the old descriptions of Comets being
drawn up in very strong terms. Of which we have a
signal instance in this very quotation, wherein the
Comet is said to have been " of an unheard-of big-
ness.'* For these words refer to the body, and not to
the tail of the Comet ; as is plain in the original,t
'" Cometa inauditse magnitudinis apparuit, cum prae-
" longa Cauda." Now the bigness of its body could
scarcely deserve such an epithet ; as we just observ-
ed, under its first appearance. Whether the same
Comet may emit a longer tail in some revolutions
than others, is a matter which, I suppose, has never
been inquired into.

Hitherto the accounts are very defective ; and we
can learn nothing from them, of the Comet's course
in, the heavens.

* Lubien. p. 292.

^ flevel. Cometogr. Lib. viii, p. 461.


But the next time it was seen, which was in 1531, its
course was observed, though imperfectly, by Apian,
one of the most learned astronomers of those days,
but furnished with very indifferent instruments. He
it was who discovered,* that the tails of Comets are
turned from the Sun ; and made only such observa-
tions on this Comet, as served to establish that disco-
very It is said t to have been " seen from the 6th of
" August, till the 3d of September, in the N. W. at
" first in the morning before sun rise, and afterwards
" in the evening after sun set. It went through Leo,
" Virgo and Libra ; going sometimes 4*^ in a day,
" sometimes S'*, and sometimes 6^. It had north
" latitude, which was daily decreasing." It is also
said,| that it was a bearded " star, of a reddish or
" rather yellow colour.'* From Apian's observations
of its course. Dr. Halley computed its parabolic tra-
jectory ;— that its proper motion in it was retrograde ;
though its apparent motion was direct ; — and that it
was in perihelion, August 25.

Its fifth appearance was in 1 607, when its course
was observed by the famous Kepler with greater
care than it had been before. The substance of the
accounts, as we find them in Hevelius § and Lubie>-
NiETSKi,!! are, "that it was seen 41 days from the
" 16th of September ; at first in the evening, and af-
" tervvards all night ; — that it had a very small paral-
" lax, and so was no sublunary vapor, but elevated

* Hevel. Cometogr. Lib. viii. p. 461. f Lubien. p. 33,

I Hevel. Lib. xii. p. 846, § p. 871. || p. 407.


" to a great height in the aether ; — that it proceeded
" with a direct motion through Libra, Scorpio and
" Sagittarius ; at the end of September very swiftly,
« not less than 13*> in a day, but more slowly before
" and after ; and near the time of its disappearing,
" being almost stationary ; — that its face was spotted
<« and not round; — that it was larger than all the fixed
" stars, and almost as large as Jupiter himself ; — that
« its light was weak, pale and watery, like that of the
<• moon, when near the shadow of the earth ; — that
** toward the end of its appearance, its head was more
«* and more diminished; — -that its tail was about 7®
" long, and projected opposite to the sun, though with
" some deviation ;" and lastly, that, agreeable to the
terrific manner of describing Comets, which was not
yet laid aside, " it appeared like a fiery spear or flam-
" ing sword." From its apparent course, which Kep-
ler had particularly described for every night in
which he observed it. Dr. Halley determined its tra-
jectory ; fixing the time of the perihelion to Octo-
ber 16.

Its sixth appearance was in the year 1682, when
its motion was observed with all possible exactness
by the best astronomers in the world, and with the
best instruments ; telescopes having been then appli-
ed to the making astronomical observations. It was
seen from the 15th of August to the 9th of Septem-
ber ; at first in the morning, afterwards in the even-
ing. Its apparent motion was all the while direct,
through Leo, Virgo and Libra. In the beginning it


was accelerated, until it became almost 6^ in a day ;
after that, it was retarded, till towards the end it was
not more than 2° in a day.* It had north latitude,
which at first increased till it was above 26°, and then
decreased to less than 9^. Its head was brighter and
somewhat larger than the famous one of the year 1680,
but its tail not near so long-. By Mr. Flamsteed's
observations at the Royal Observatory in Greenivich^
the nucleus was hardly a tenth part so broad as the
atmosphere surrounding it ; which being measured by
a micrometer was 2' ; and therefore the nucleus was
only 11'' or 12" in diameter.f Hevelius found its
tail, when longest, to be about 15° or 16° ; but toward
the end it daily diminished ; and it deviated from the
precise point of opposition to the sun. By Dr. Hal-
ley's computation of its trajectory, it came to its pe-
rihelion September 4th.

Of its seventh appearance we ourselves are eye-
witnesses. It was first seen, m this part of the world,
on the 3d of April in the morning ; but might have
been seen much sooner, had not an uninterrupted suc-
cession of cloudy weather prevented it. On the 4th,
at 3|.h, I saw it, low in the east and not very bright,
with a tail about 3? or 4° long, directed obliquely up-
ward, toward the S. W. It was then in about 26° of
Aquarius, with 4° of north latitude. From hence it
\\xnt westerly^jor with a retrograde motion ; and south-
erly withal, approaching toward the ecliptic, which it

* Lo\vthorp*s Abridg-. Phil. Trans. Vol. I, p. 447. " "
t Newt. Princip. p. 481.


crossed on the 1 1th. At first it moved not above half
a degree in a day ; but with a motion greatly accele-
rated, by reason of its coming almost directly towards
the earth, it passed from the middle of the back of
Aquarius, over the tail of Capricorn, and ran so far
southward by the 22d, as not to rise above our hori-
zon. While it was invisible to us, it might be seen
in the southern countries, going over the tail of Pavo,
and south of Triangulum Australe, and through the
Crosiers, within 20° of the south pole of the equator.
After it had passed its nearest distance from the south
pole, the direction of its apparent motion must of ne-
cessity become northerly; by which means, it rose
again above our horizon on the 27th. The last time
I saw it before it left us, on its southern progress, was
dn the 19th in the morning; and the first time after
its return, on the 29th, in the evening ; when it ap-
peared bigger than before, though not so bright, but
rather of a dull and gloomy aspect ; and its tail was
longer and broader, seeming to be about 7^ long and
directed toward the eyst. These alterations were ow-
ing to its being nearer to the earth, and farther from
the sun.* Its apparent course was novv' N. W. toward
that part of Hydra, which is under Crater ; and it
crossed Hydra in about IS'* of Virgo. As it was now
going farther both from the earth and the sun, its mo-
tion became slower, and its bigness and brightness
were continually diminishing. The last time I saw

* See above, p. 25.


it, before the writing of this, was on the 22d of May
in the evening, when at 9 h^, I found it on the limb
of Sextans Uraniae, in about 8"^ of Virgo, and in 14<*
of south latitude ; with some uncertainty, arising from
the faint appearance it made ; which will increase
so as, in a little time, will make it difiicult to find the
Comet, especially when the moon-light nights come
on. Its diurnal motion is now scarcely one quarter
of a degree. Its apparent motion has all this while
been retrograde, as well as its real motion ; whereas
in its former returns, its apparent motion was direct.
This difference is occasioned by the earth's being
now on the north side of the Comet's orbit ; whereas
before, it happened to be always on the south side.
But the calculations made on the orbit of 1682 agree
as well with the last observations we have made of
the Comet, as they did with the first ; and this agree-
ment proves, both that it v/as the same Comet which
appeared the beginning of April in the morning, and
the end of April in the evening ; and that this is the
same Comet as that of 1682.

Such has been its course as seen from the earth. If
the reader be desirous to know the principal stages of
its present journey, they are as follow. In December
1720, it was in its aphelion, or greatest distance from
the sun ; — of above 28 hundred million miles. From
that time it approached the sun ; very slowly at first,
but with a motion continually accelerated. Two years
and a half ago, it entered the sphere of Saturn ; that
is, came as near to the sun as Saturn is ; and the be-
ginning of March 1758^ it entered the sphere of Ju-


piter. It was in its ascending node about the middle
of December last ; being then beyond the sphere of
Mars, which it entered on the last of that month ; as
it did the sphere of the earth on the first of February.
Possibly, in the beginning of January and for some
time after, it might have been seen in the western
hemisphere in the evening, at first not far from the
point of the vernal equinox, and then to the west of it ;
if one had known exactly where to look for it. At
that time it was as near the sun as it was on the 22d
of May, but not so near the earth ; and therefore not
bright enough to make people in general take notice
of it. On the 10th of February, it was got to that point
of its orbit where Kepler first saw it on the 16th of
September 1607; which is the most eastern point
that we have any account of its having been seen in ;
as that on the 22d of May inst. is the most western.
On the 20th of February it entered the sphere of Ve-
nus; but never went so low as that of Mercury. On
the last of February, it was in conjunction with the sun,
with but little more than 6<^ north latitude; and was
then hid in the sun's rays. But by its retrograde mo-
tion it soon got to the westward of the sun, and came
to its perihelion March 12 ; its distance from the
sun being then almost 47 million miles. Its geocen-
tric place was then in S'' of Pisces, with a little above
6** north latitude, as before. At that time, or soon
after, it might undoubtedly have been seen in the east ;
for it rose Ih. 10' before the sun. On the first of
April it went out of the sphere of Venus ; on the 1 1th,
passed its descending node ; and left the sphere


of the eartli, on the 20th. It came to its perigee or
nearest distance from the earth, on the 25th ; which
was about ten million miles. It was in opposition to
the sun, the 26th, having about 50° south latitude.
At this time its diurnal motion was about 17^^. It left
the sphere of Mars the 23d of May ; and will leave
that of Jupiter about the middle of next March, and
that of Saturn in September 1761 ; continuing its
course through the southern parts of the heavens till
the year 1834, when another visit may be expected
from it.

To compare its different periods, it will be best to
place in one view its several appearances, and the
times of its perihelion, so far as we know them.






Yeai's. Months, Periods.

1305, April,

13S0, about Jan. probably, 74 Years, and about 10 montlis.

1456, June, 76 and about 5 months.

1531, Aug'ust 25, 75 and two nnonths.

1607, October 16, 76 and 52 days.

1682, September 4, 74 ■ and 323 days.

1759, March 12.N.S.76 and 178 days.

Upon this view it is obvious, that the periods of the
Comet have been alternately greater and less ; though
the three first cannot be exactly determined. The
last seems to have been as long ; and the last but one,
as short as any ; the diiTerence between them being a
year and 220 days. This seems a great difference
when compared with the small irregularities which
the revolutions of the planets are subject to;* but

* See above, p. §3.

50 Appendix.

perhaps, ^vhen we know more of Comets, it may ap-
pear inconsiderable.

To find the mean revolution, we cannot come to any
greater exactness, than by supposing that in 1305 it
•was in its perihelion at the same time of the year as
it was this year; for both times it appeared in April.
Therefore in 454 years it has made six revolutions;
which gives 75|- years for a mean revolution. Whence,
supposing the earth's distance from the sun to be of
100 parts, we find, by Kepler's rule,* the longer axis
of the Comet's orbit to be 3578 of those parts; and
its perihelion-distance being 58, its a,phelion-dista.nce
is 3550. Thus the Comet's least distance from the sun
is to its greatest as 1 to above 60. In the perihelion,
the sun's diameter would appear to the Comet almost
double of what it does to us ; but in the aphelion, the
sun would not look bigger than Jupiter and Venus in
perigee do to us. And tlie sun's light and heat being
reciprocally as the squares of the distances, the light
and heat at the Comet in its perihelion is above 3600
times greater than it is at the aphelion. The Comet's
angular velocities are also in the same -proportion. In
its aphelion it moves but 18' of a degree in a year :
whereas in its perihelion it runs through an equal an-
gle in 2 h 24'. It continues on the north side of the
ecliptic, where its perihelion lies, but 1 14 days;.and
spends on the ^outh side all the rest of its long revo-

* Page 23.


As to the real bigness of the nucleus — Mr. Flam-
steed in 1682 found it but 11" or 12" in diameter,
as was remarked before, p. 45, which is very nigh the
bigness that Mercury appears of in his transits over
the sun at his ascending node : And the Comet's dis-
tance from the earth at that time could not differ much
from the planet's. Therefore the Comet may well be
supposed to be about the same bigness as the planet
Mercury, which is the least of all the planets, and but
■jyth part so big as the earth. If we could find on
what day Mr. Flamsteed made his observation, we
might determine this point with greater exactness.

The tail was of an enormous length, by the account
given of it in 145 6. But the later accounts are prob-
ably exactor ; of which the most distinct I can find is
thatof Hevelius before cited,* who in 1682 found it
about 15° or 16° long. According to this, its real
length was almost 12 million miles ; which is long
enough to reach from the descending node beyond
the earth's orbit. So that if the Comet had come to
that node about a month later, this year, or more ex-
actly on the 14th of May, the earth would have passed
through the end of its tail.

With regard to this Comet I shall only observe far-
ther, that the ascending node of its orbit lies pretty
near the orbit of Mars ; and the descending, near
that of Venus. After the ascending node, it may pos-
sibly come within about 12 million miles of the earth ;
and after the descending within 5 millions'; as it


would have done this year, had it come to its descend-
ing node bat nine days Ititer than it did. And these
are the nearest approaches which it can possibly
make to the earth.

We have not the like evidence of the return of any
other Comet, as we have of this. Dr. Halley was
inclined to believe that the Comet of autumn 1532
described the same orbit with that which was observ-
ed by Hevelius in the beginning of the year 1661 ;
and if so, its period was of 128^ years. * Though he
remarks that Apian's observations, which are the only
ones extant of the first, are not exact enough to de-
cide with certainty so nice an affuir. What makes it
probuble to me is, that on looking into the histories,
I find Comets appearing in those years, which a peri-
od of about 128 or 129 years, reckoned backward, falls
upon. LuiuENiETSKi spcaks of one that was seen in
the end of Marcii and beginning of April 1403 ; which
was 129-1 years before that of 1532, and might proba-
bly be the same. He speaks of another in July and
August, 1273, which was 129|. years before the last
mentioned; and of another in May, 1145, which was
1281- years before that; and of another, which ap-
peared for four months in 1017, or about 128 years
before the last mentioned. - But as I find none in the
period next before this, it is needless to look farther
back. The differences of these periods, being not
greater than what we found in our present Comet,
are no objection to all these Comets being the same ;

*Phil.Trans. No. 297.


though, as we have no account of their courses, one
cannot positively affirm that they were. If they were,
its mean period is 128^ years; and it will return in
the year 1789, or 170.

So far as an argument might be drawn from an
equality of periods and from similar phssnomina, * Dr,
Halley thought that the period of that wonderful
Cometof 1680 might be 575 years. Sir Isaac Newton,
from the curvature of its path, judged its period to be
above 500 years ;t and the learned Doctor, hy.ving
found mention in history of four appearances of fa-
mous Comets, with remarkably long tails, after in-
tervals of 575 years, was ready to conclude that these
were the same Comet. The appearances were, in
September after the death of Julius C^sar, 44 years
before the Christian 3era; in the year 531, in the
reign of the emperor Justinian; in February 1106,
in the reign of our K. Henry I ; and in December
1680. The Doctor added " similarity of phaenomi-
na ;'' being sensiole, without doubt, that the argimient
drawn from the equality of periods can, by itself, have
no great force. For so frequent have been the ap-
pearances of Comets, that a number of imaginary pe-
riods might easily be assumed, which, reckoned in
order, for three or four successions, might fall on
years wherein Come s were seen. It is pity the Doc-
tor could meet with no account of the courses of these
Comets in the heavens ; which might have decided
the question with certainty on one side or the other.

* Synops Astron. Comet. f As above, p. 25.



Since his death, a manuscript has oeen found in the
library of Pembroke-Hali in Cambridge^ which gives
some, though a very imperfect account of the course
of that Comet which appeared in 1 106. And the Au-
thor,* who mentions it, observes, that "there is such
" a wide difference between this manuscript account
'' and the Comet's places computed on the orbit of
" 1680, as must very much lessen, if it does not quite
" overbalance, the force of the arguments brought by
'' Dr. Halley to prove the identity of these two
« Comets."

This manuscript gives also a short account of a fa-
mous Comet which v/as seen in 1264. So far as its
trajectory could be determined from that slender de-
scription, there is found such a resemblance between
it and that of the Comet of 1 556, as gives some ground
for conjecture that they might possibly be tlie same.
If they were, its period is 292 years; and it may be
expected about the year 1848.

This is the substance of what we have collected
in regard to the revolutions of Comets. Our know-
ledge in this article is very imperfect ; but we are
now in the right way to have it enlarged. Every
Comet that appears will either increase the number
of Comets, or discover the period of some one that
was unknown. As the number of Comets increases
it may be best to distinguish them by names. Hi-
therto, each has been known only by the year of its
appearaixe ; as the Comet of 1680, or that of 1682,

♦ Mr. Dunthorne in Phil. Trans. Vol. xlvii. Art. xliii .


hue. But perhaps it may be more convenient to range
them in numerical order, as the astronomers do the
satellites of Jupiter and Saturn ; reckoning those first,
whose periods are shortest. In conformity to which
way of speaking, our present Comet is doubtless the
Jirsi Comet; and that of 1661 may probably be the
second. It is natural to expect that the periods of
others will be discovered in nearly this numerica*
order; for the shortest are likey to be discovered
soonest. But it will require the observations of
many hundreds, if not thousands of years, to complete
the Cometic system.





Coritainiiig an attempt to exphiiii the phsenomena of the tails or
COMETS, aud to account for their perpetual opposition to the
BITS', upon philosophical principles.

Po'.nting out some important ends for which these tails were pro-
bahly designed : Wherein it is shewn, that, in conseqiience of
tliese curious appendages, comets may be inhabited woslus,
and even comfortable habitations ; notwithstanding the vast ex-
centricities of their orbits.

The whole interspersed with observations and refiectionsou the

Sun and primary Planets.


la human works,, though labour'd on with pain,
A thousand movements scarce one purpose gain;
In God's, one single can its end produce ;
Yet serves to second too some other use.

Popt's Essay on Man,


T. B. Wait & Co....,.Printers.




Holllsian Professor of the Mathematics, and Natural Philosophy, and "Fellow of
the Corporation, of Harvard College : Member of the American Philosophical
Society of Philadelphia ; and Fellow of the Royal Society.


As an acknowledgement of the obligations which
result from an initiation, under his insvruction, in the
study of those sciences which more immediately be-
long to his province : In reliance upon his patronage
of a Treatise, which owes its publication to his caa-
dour and approbation : and, with all due deference to
his superior judgment; is gratefully inscribed, by

His obliged friend,

And most humble servant,



X HE design of the following Essay was, partly to
eradicate some absurd notions which have been hand-
ed down from the darkest periods of antiquity, and
which are still entertained by some, upon the ap-
pearance of a Comet ; and to remove the apprehen-
sions which may have been excited in the minds of
others, even by the writings of some great men among
the moderns : And partly to offer to those who indulge
themselves in more abstruse researches after the ope-
rations of natural causes, a few hints, the prosecution
of which may enlarge the field of philosophical spe-
culation, and open to all a new source for adoration
of the wisdom and beneficence of that Being, who has
made nothing in vain, and has disposed the various
parts of the universe by nveight and measure. It may
therefore be presumed, that, if the author is so hap-
py as to be understood by the generality of his readers
(which he has aimed at throughout the whole) the
gentlemen of science will pardon those minute dis-
cussions, which, they may judge unnecessary, and
that they will weigh with impartiality the arguments
which may be off'ered for their consideration, overlook
any immaterial inaccuracies, which may have arisen


through madvertency, and point out and refute with
candour any mistakes which he may have fallen into.
It is now w^ell known to astronomers that the mo-
tions of Comets are regulated by the universal law of
gravitation, and that they regard the sun as the com-
mon centre of all their motions, equally with the pla-
nets, although they are more numerous, and appa-
rently distinct in species from them. The planets,
since the discovery of the Satellites or Moons which
attend some of them, are generally, and with the
highest reason supposed to be inhabited worlds like
the earth. — Whereas it has been thought even by
some of the greatest modern astronomers, that the
extremes of heat and cold to which Comets are alter-

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