COMETS
The reader has, no doubt, been struck by the marked uniformity which
exists among those members of the solar system with which we have dealt
up to the present. The sun, the planets, and their satellites are all
what we call solid bodies. The planets move around the sun, and the
satellites around the planets, in orbits which, though strictly
speaking, ellipses, are yet not in any instance of a very oval form. Two
results naturally follow from these considerations. Firstly, the bodies
in question hide the light coming to us from those further off, when
they pass in front of them. Secondly, the planets never get so far from
the sun that we lose sight of them altogether.
With the objects known as Comets it is, however, quite the contrary.
These objects do not conform to our notions of solidity. They are so
transparent that they can pass across the smallest star without dimming
its light in the slightest degree. Again, they are only visible to us
during a portion of their orbits. A comet may be briefly described as an
illuminated filmy-looking object, made up usually of three portions--a
head, a nucleus, or brighter central portion within this head, and a
tail. The heads of comets vary greatly in size; some, indeed, appear
quite small, like stars, while others look even as large as the moon.
Occasionally the nucleus is wanting, and sometimes the tail also.
[Illustration: FIG. 18.--Showing how the Tail of a Comet is directed
away from the Sun.]
These mysterious visitors to our skies come up into view out of the
immensities beyond, move towards the sun at a rapidly increasing speed,
and, having gone around it, dash away again into the depths of space. As
a comet approaches the sun, its body appears to grow smaller and
smaller, while, at the same time, it gradually throws out behind it an
appendage like a tail. As the comet moves round the central orb this
tail is always directed _away_ from the sun; and when it departs again
into space the tail goes in advance. As the comet's distance from the
sun increases, the tail gradually shrinks away and the head once more
grows in size (see Fig. 18). In consequence of these changes, and of the
fact that we lose sight of comets comparatively quickly, one is much
inclined to wonder what further changes may take place after the bodies
have passed beyond our ken.
The orbits of comets are, as we have seen, very elliptic. In some
instances this ellipticity is so great as to take the bodies out into
space to nearly six times the distance of Neptune from the sun. For a
long time, indeed, it was considered that comets were of two kinds,
namely, those which actually _belonged_ to the solar system, and those
which were merely _visitors_ to it for the first and only time--rushing
in from the depths of space, rapidly circuiting the sun, and finally
dashing away into space again, never to return. On the contrary,
nowadays, astronomers are generally inclined to regard comets as
permanent members of the solar system.
The difficulty, however, of deciding absolutely whether the orbits of
comets are really always _closed_ curves, that is to say, curves which
must sooner or later bring the bodies back again towards the sun, is,
indeed, very great. Comets, in the first place, are always so diffuse,
that it is impossible to determine their exact position, or, rather, the
exact position of that important point within them, known as the centre
of gravity. Secondly, that stretch of its orbit along which we can
follow a comet, is such a very small portion of the whole path, that the
slightest errors of observation which we make will result in
considerably altering our estimate of the actual shape of the orbit.
Comets have been described as so transparent that they can pass across
the sky without dimming the lustre of the smallest stars, which the
thinnest fog or mist would do. This is, indeed, true of every portion
of a comet except the nucleus, which is, as its name implies, the
densest part. And yet, in contrast to this ghostlike character, is the
strange fact that when comets are of a certain brightness they may
actually be seen in full daylight.
As might be gathered from their extreme tenuity, comets are so
exceedingly small in mass that they do not appear to exert any
gravitational attraction upon the other bodies of our system. It is,
indeed, a known fact that in the year 1886 a comet passed right amidst
the satellites of Jupiter without disturbing them in the slightest
degree. The attraction of the planet, on the other hand, so altered the
comet's orbit, as to cause it to revolve around the sun in a period of
seven years, instead of twenty-seven, as had previously been the case.
Also, in 1779, the comet known as Lexell's passed quite close to
Jupiter, and its orbit was so changed by that planet's attraction that
it has never been seen since. The density of comets must, as a rule, be
very much less than the one-thousandth part of that of the air at the
surface of our globe; for, if the density of the comet were even so
small as this, its mass would _not_ be inappreciable.
If comets are really undoubted members of the solar system, the
circumstances in which they were evolved must have been different from
those which produced the planets and satellites. The axial rotations of
both the latter, and also their revolutions, take place in one certain
direction;[24] their orbits, too, are ellipses which do not differ much
from circles, and which, furthermore, are situated fairly in the one
plane. Comets, on the other hand, do not necessarily travel round the
sun in the same fixed direction as the planets. Their orbits, besides,
are exceedingly elliptic; and, far from keeping to one plane, or even
near it, they approach the sun from all directions.
Broadly speaking, comets may be divided into two distinct classes, or
"families." In the first class, the same orbit appears to be shared in
common by a series of comets which travel along it, one following the
other. The comets which appeared in the years 1668, 1843, 1880, 1882,
and 1887 are instances of a number of different bodies pursuing the same
path around the sun. The members of a comet family of this kind are
observed to have similar characteristics. The idea is that such comets
are merely portions of one much larger cometary body, which became
broken up by the gravitational action of other bodies in the system, or
through violent encounter with the sun's surroundings.
The second class is composed of comets which are supposed to have been
seized by the gravitative action of certain planets, and thus forced to
revolve in short ellipses around the sun, well within the limits of the
solar system. These comets are, in consequence, spoken of as "captures."
They move around the sun in the same direction as the planets do.
Jupiter has a fairly large comet family of this kind attached to him. As
a result of his overpowering gravitation, it is imagined that during the
ages he must have attracted a large number of these bodies on his own
account, and, perhaps, have robbed other planets of their captures. His
family at present numbers about thirty. Of the other planets, so far as
we know, Saturn possesses a comet family of two, Uranus three, and
Neptune six. There are, indeed, a few comets which appear as if under
the influence of some force situated outside the known bounds of the
solar system, a circumstance which goes to strengthen the idea that
other planets may revolve beyond the orbit of Neptune. The terrestrial
planets, on the other hand, cannot have comet families; because the
enormous gravitative action of the sun in their vicinity entirely
overpowers the attractive force which they exert upon those comets which
pass close to them. Besides this, a comet, when in the inner regions of
the solar system, moves with such rapidity, that the gravitational pull
of the planets there situated is not powerful enough to deflect it to
any extent. It must not be presumed, however, that a comet once captured
should always remain a prisoner. Further disturbing causes might
unsettle its newly acquired orbit, and send it out again into the
celestial spaces.
With regard to the matter of which comets are composed, the spectroscope
shows the presence in them of hydrocarbon compounds (a notable
characteristic of these bodies), and at times, also, of sodium and iron.
Some of the light which we get from comets is, however, merely reflected
sunlight.
The fact that the tails of comets are always directed away from the sun,
has given rise to the idea that this is caused by some repelling action
emanating from the sun itself, which is continually driving off the
smallest particles. Two leading theories have been formulated to account
for the tails themselves upon the above assumption. One of these, first
suggested by Olbers in 1812, and now associated with the name of the
Russian astronomer, the late Professor Brédikhine, who carefully worked
it out, presumes an electrical action emanating from the sun; the other,
that of Arrhenius, supposes a pressure exerted by the solar light in its
radiation outwards into space. It is possible, indeed, that repelling
forces of both these kinds may be at work together. Minute particles are
probably being continually produced by friction and collisions among the
more solid parts in the heads of comets. Supposing that such particles
are driven off altogether, one may therefore assume that the so-called
captured comets are disintegrating at a comparatively rapid rate. Kepler
long ago maintained that "comets die," and this actually appears to be
the case. The ordinary periodic ones, such, for instance, as Encke's
Comet, are very faint, and becoming fainter at each return. Certain of
these comets have, indeed, failed altogether to reappear. It is notable
that the members of Jupiter's comet family are not very conspicuous
objects. They have small tails, and even in some cases have none at all.
The family, too, does not contain many members, and yet one cannot but
suppose that Jupiter, on account of his great mass, has had many
opportunities for making captures adown the ages.
Of the two theories to which allusion has above been made, that of
Brédikhine has been worked out so carefully, and with such a show of
plausibility, that it here calls for a detailed description. It appears
besides to explain the phenomena of comets' tails so much more
satisfactorily than that of Arrhenius, that astronomers are inclined to
accept it the more readily of the two. According to Brédikhine's theory
the electrical repulsive force, which he assumes for the purposes of his
argument, will drive the minutest particles of the comet in a direction
away from the sun much more readily than the gravitative action of that
body will pull them towards it. This may be compared to the ease with
which fine dust may be blown upwards, although the earth's gravitation
is acting upon it all the time.
The researches of Brédikhine, which began seriously with his
investigation of Coggia's Comet of 1874, led him to classify the tails
of comets in _three types_. Presuming that the repulsive force emanating
from the sun did not vary, he came to the conclusion that the different
forms assumed by cometary tails must be ascribed to the special action
of this force upon the various elements which happen to be present in
the comet. The tails which he classes as of the first type, are those
which are long and straight and point directly away from the sun.
Examples of such tails are found in the comets of 1811, 1843, and 1861.
Tails of this kind, he thinks, are in all probability formed of
_hydrogen_. His second type comprises those which are pointed away from
the sun, but at the same time are considerably curved, as was seen in
the comets of Donati and Coggia. These tails are formed of _hydrocarbon
gas_. The third type of tail is short, brush-like, and strongly bent,
and is formed of the _vapour of iron_, mixed with that of sodium and
other elements. It should, however, be noted that comets have
occasionally been seen which possess several tails of these various
types.
We will now touch upon a few of the best known comets of modern times.
The comet of 1680 was the first whose orbit was calculated according to
the laws of gravitation. This was accomplished by Newton, and he found
that the comet in question completed its journey round the sun in a
period of about 600 years.
In 1682 there appeared a great comet, which has become famous under the
name of Halley's Comet, in consequence of the profound investigations
made into its motion by the great astronomer, Edmund Halley. He fixed
its period of revolution around the sun at about seventy-five years, and
predicted that it would reappear in the early part of 1759. He did not,
however, live to see this fulfilled, but the comet duly returned--_the
first body of the kind to verify such a prediction_--and was detected on
Christmas Day, 1758, by George Palitzch, an amateur observer living near
Dresden. Halley also investigated the past history of the comet, and
traced it back to the year 1456. The orbit of Halley's comet passes out
slightly beyond the orbit of Neptune. At its last visit in 1835, this
comet passed comparatively close to us, namely, within five million
miles of the earth. According to the calculations of Messrs P.H. Cowell
and A.C.D. Crommelin of Greenwich Observatory, its next return will be
in the spring of 1910; the nearest approach to the earth taking place
about May 12.
On the 26th of March, 1811, a great comet appeared, which remained
visible for nearly a year and a half. It was a magnificent object; the
tail being about 100 millions of miles in length, and the head about
127,000 miles in diameter. A detailed study which he gave to this comet
prompted Olbers to put forward that theory of electrical repulsion
which, as we have seen, has since been so carefully worked out by
Brédikhine. Olbers had noticed that the particles expelled from the head
appeared to travel to the end of the tail in about eleven minutes, thus
showing a velocity per second very similar to that of light.
The discovery in 1819 of the comet known as Encke's, because its orbit
was determined by an astronomer of that name, drew attention for the
first time to Jupiter's comet family, and, indeed, to short-period
comets in general. This comet revolves around the sun in the shortest
known period of any of these bodies, namely, 3-1/3 years. Encke
predicted that it would return in 1822. This duly occurred, the comet
passing at its nearest to the sun within three hours of the time
indicated; being thus the second instance of the fulfilment of a
prediction of the kind. A certain degree of irregularity which Encke's
Comet displays in the dates of its returns to the sun, has been supposed
to indicate that it passes in the course of its orbit through some
retarding medium, but no definite conclusions have so far been arrived
at in this matter.
A comet, which appeared in 1826, goes by the name of Biela's Comet,
because of its discovery by an Austrian military officer, Wilhelm von
Biela. This comet was found to have a period of between six and seven
years. Certain calculations made by Olbers showed that, at its return in
1832, it would pass _through the earth's orbit_. The announcement of
this gave rise to a panic; for people did not wait to inquire whether
the earth would be anywhere near that part of its orbit when the comet
passed. The panic, however, subsided when the French astronomer, Arago,
showed that at the moment in question the earth would be some 50
millions of miles away from the point indicated!
[Illustration: PLATE XVII. DONATI'S COMET
From a drawing made on October 9th, 1858, by G.P. Bond, of Harvard
College Observatory, U.S.A. A good illustration of Brédikhine's theory:
note the straight tails of his _first_ type, and the curved tail of his
_second_.
(Page 257)]
In 1846, shortly after one of its returns, Biela's Comet divided into
two portions. At its next appearance (1852) these portions had separated
to a distance of about 1-1/2 millions of miles from each other. This
comet, or rather its constituents, have never since been seen.
Perhaps the most remarkable comet of recent times was that of 1858,
known as Donati's, it having been discovered at Florence by the Italian
astronomer, G.B. Donati. This comet, a magnificent object, was visible
for more than three months with the naked eye. Its tail was then 54
millions of miles in length. It was found to revolve around the sun in a
period of over 2000 years, and to go out in its journey to about 5-1/2
times the distance of Neptune. Its motion is retrograde, that is to say,
in the contrary direction to the usual movement in the solar system. A
number of beautiful drawings of Donati's Comet were made by the American
astronomer, G.P. Bond. One of the best of these is reproduced on Plate
XVII., p. 256.
In 1861 there appeared a great comet. On the 30th of June of that year
the earth and moon actually passed through its tail; but no effects were
noticed, other than a peculiar luminosity in the sky.
In the year 1881 there appeared another large comet, known as Tebbutt's
Comet, from the name of its discoverer. This was the _first comet of
which a satisfactory photograph was obtained_. The photograph in
question was taken by the late M. Janssen.
The comet of 1882 was of vast size and brilliance. It approached so
close to the sun that it passed through some 100,000 miles of the solar
corona. Though its orbit was not found to have been altered by this
experience, its nucleus displayed signs of breaking up. Some very fine
photographs of this comet were obtained at the Cape of Good Hope by Mr.
(now Sir David) Gill.
The comet of 1889 was followed with the telescope nearly up to the orbit
of Saturn, which seems to be the greatest distance at which a comet has
ever been seen.
The _first discovery of a comet by photographic means_[25] was made by
Professor Barnard in 1892; and, since then, photography has been
employed with marked success in the detection of small periodic comets.
The best comet seen in the Northern hemisphere since that of 1882,
appears to have been Daniel's Comet of 1907 (see Plate XVIII., p. 258).
This comet was discovered on June 9, 1907, by Mr. Z. Daniel, at
Princeton Observatory, New Jersey, U.S.A. It became visible to the naked
eye about mid-July of that year, and reached its greatest brilliancy
about the end of August. It did not, however, attract much popular
attention, as its position in the sky allowed it to be seen only just
before dawn.
[24] With the exception, of course, of such an anomaly as the retrograde
motion of the ninth satellite of Saturn.
[25] If we except the case of the comet which was photographed near the
solar corona in the eclipse of 1882.
[Illustration: PLATE XVIII. DANIEL'S COMET OF 1907
From a photograph taken, on August 11th, 1907, by Dr. Max Wolf, at the
Astrophysical Observatory, Heidelberg. The instrument used was a 28-inch
reflecting telescope, and the time of exposure was fifteen minutes. As
the telescope was guided to follow the moving comet, the stars have
imprinted themselves upon the photographic plate as short trails. This
is clearly the opposite to what is depicted on Plate XIII.
(Page 258)]