The Sun.
London, England, 1858-10-19 00:00:00

THE COMET.

Mr. J. It. Hind, in a letter to a contemporary,
says :—ln a communication of September 29, I
pointed out the near approach of the comet of Donati
to the planet Venus about the 18th of this month.
Calculations founded upon later elements of the
comet's orbit confirm the results there given, and
prove that Venus has had a rather narrow escape
from immersion in the denser part of the tail, if not
from actual collision with the nucleus.
The nearest approach of the two bodies will take
place early on the morning of Monday, next, the
18th inst., when their mutual distance will be less
than 9-100ths of the earth's distance from the sun;
consequently, on the present occasion no very near
approximation will occur, yet it is obvious that if
the comet had reached its least distance from the
sun a few days earlier than it has done the planet
might have passed through it, and I am very far
from thinking that close proximity to a comet of this
description would be unattended -with -danger,
though at a respectable distance its influence might
be salutary enough. The inhabitants of Venus will
witness a cometary spectacle far superior to that
which has recently attracted so much attention
here, inasmuch as the tail will doubtless appear
twice as long from that planet as from the earth,
and the nucleus proportionably more brilliant. It
is not unlikely that great uneasiness and alarm has
been prevalent on the " Virgin planet" for some
weeks past, on account of the uncertainty which
must have necessarily attached to the comet's exact
course, about the time of passage across the orbit of
Venus ; the wisest of her astronomers, unless they
are ahead of their terrestrial cousins in matters of
science, will have been sorely puzzled to decide
until very lately, whether he was hurrying on to a
transit through a comet or not. Such narrow
escapes may well lead to speculation on the crea-
tive object which belongs to these wanderers of the
solar system.
The following remarks bear upon a number of
queries which have appeared from time to time in
your columns, and, indeed, have formed every-day
questions during the last month. A few general
particulars are appended:—
The comet which is just now receding from view
in these latitudes was first discovered by Dr. G. B.
Donati, astronomer'at the Museum of Florence, on
the evening of the 2d of June, in right ascension
141 deg. 18 min., and north declination 23 deg. 47
min., corresponding to a position near the star / Leo-
nis. Previous to this date we had no knowledge of its
existence, and therefore it was not a predicted comet;
neither is it the one last observed in 1556. At the
date of discovery it was distant from the earth
228,000,000 miles, and was an excessively faint ob-
ject in the largest telescopes. This circumstance,
added to its very slow motion, rendered the calcu-
lation of its path in the heavens a matter of con-
siderable difficulty ; and it was not until the middle
of Aug. or later, that a trustworthy determination
of its future track among the stars could be obtained .
It is now fairly within our grasp, and astronomers
are not likely to be ignorant of its position to any
extent until it is again within the range of the
telescope in Europe more than 2,000 years hence.
The tail duringthe last fortnight, when the comet,
has been most conspicuous, appears to have main-
tained an average length of at least 40,000,000
miles, subtending an angle varying from 30 deg. to
40 deg. The dark line, or space down the centre,
frequently remarked in other great comets, has
been a striking characteristic in that of Donati.
The nucleus, though small, has been intensely bril-
liant in powerful instruments, and for some time
bore high magnifiers to much greater advantage
than is usual with these objects. In several respects
this comet has resembled the famous ones of 1744,
1680, and 1811, particularly as regards the signs of
violent agitation going on .in the vicinity of the
nucleus,' such as the appearance of luminous jets,
spiral offshoots, &c., which have rapidly emanated
from the planetary point and as quickly lost them-
selves in the general nebulosity of the head.
On the sth rust. the most casual observer would
have an opportunity of satisfying himself as to the
accuracy of the mathematical theory of the motions
of comets in the near approach of the nucleus of
Donati's to Arcturus, the principal star in the con-
stellation Bootes. The circumstances of the appulse
were very nearly as predicted in my last communi-
cation.

The curve described, by the comet in the heavens
has lately exhibited a tendency to deviate from the
parah pla, in which, for the sake of simplifying the
calculations, it is the custom of astronomers to
assume that these bodies move. Mr. Farley and I
remarked that it was impossible to represent accu-
rately three observations, extending from the begin-
ning of June to the middle of Sept., by a parabolic
curve, though the difference of the true path there-
from did not appear sufficiently great to require the
substitution of an ellipse for the mere
.purposes of
prediction, and accordingly the ephemeris published
in The Sun was founded on a parabolic orbit, which
of course supposes the period infinite. -More re-
cently several attempts have been made to deter-
mine the real nature of the-corhet's track in the sys
' -
tem and the last investigation by Mr. Loewy, of
the Observatory of Vienna, is no doubt entitled to
a considerable degree of confidence, agreeing as it
does well with four months' observations. I shall,
therefore, venture to place before your readers seve-
ral deductions from his calculations, premising that
the length of the major axis of the ellipse, and con-
, sequently the duration of the revolution, are liable
at present to more uncertainty than the other num-
bers, though we may perhaps depend upon them
within an eighth or a tenth of the whole. In a few
months' time more reliable quanties will doubtless
be in our possession.
The comet arrived at its least distance from the
sun a few minutes after 11 o'clock on the morning
of Sept._ 30 ; its longitude, as seen from the: Sun. at
this time, being 36 deg. 13 Min., and its distance
from him 55 million miles. It ascended from the
south to the north side of the 1864 days be-
fore the perihelion- passage, or on March 27; 1858,
distant from the sun 3.11, or situate among the
orbits of the minor planets, and will again traverse
the plane of the earth's path, moving south, on
Monday next, 18, 1.3 d days after perihelion, at a
distance of 0.71. The longer diameter of its orbit
is 184 times that of the earth's, or 35,100,000,000
miles, yet this enormous space is considerably
less than one-thousandth of the distance of the
nearest fixed star ! The smaller diameter of the el-
lipse is about 2,780,000,000. The orbit is inclined
to the ecliptic at an angle of 63 deg. 2 min., and
intersects it in longitude 165 deg: 19 min. and 345
deg. 19 min. From what has been here stated it
will be seen that the comet remains on the north
side of the earth's path only '205 days, so that
nearly the whole of its, trajectory is situate below
or rather to the south of that plane. The time of
revolution resulting from Mr. Loewy's calculations
is 2,495 years, which is about 500 years less than
that of the comet of 1811, during the period it was
visible from the earth. The hourly velocity- of the
comet in its orbit varies between 127,000 miles at
the perihelion and 480 miles at the aphelion.
The above numbers are probably a fair approxi-
mation to the true ones for this date ; but it does
not follow, because certain elements represent the
comet's path at any particular time, that they will
long continue to do so. This remark applies more
especially to the length of the revolution, and, as a
case in point, I may mention the result of Arge-
lander's calculation with respect. to the grand comet
of 1811. In the autumn of that year, when it was
most brilliant, the time of revolution corresponding
to the ellipse it was then describing was something
over 3,060 years, but in 1827 the comet was moving
in an ellipse of 2,890 years, showing an alteration
approaching two centuries due to planetary pertur-
bations, and for aught we know at present the
period of Donati's comet may be similarly altered.
The permanency of comets is strikingly exempli-
fied in the history of the celebrated comet of Hal-
ley, -which revolves round the sun in about 76 years,
and was last visible to us in the autumn of 1835.
With. the aid of the observations preserved in the
Chinese aullels there is strong reason for supposing