Comets by Sir Robert Ball

Let us suppose that there was
no other star in the universe
than our own sun, and let us
further, for the sake of making
the argument clearer, suppose
that the sun was deprived of its
system of attendant worlds. Next, let some
other object be introduced which we may
suppose to be extremely light, like a wisp of
vapour, and let it be situated at a distance
from the sun which we may regard as inde
finitely great. These two bodies, namely,
the sun and this wisp of vapour, are then
supposed to be abandoned to their mutual
attraction. Each of these objects will pull
the other, and the result of the attraction
between the two bodies will be to make them
approach each other. As, however, the
mass of the sun is so vast, while the mass
of the wisp is so small, we may fairly
assume that the greater part of this move
ment will be done by the wisp, while the sun
will remain comparatively at rest. The case
is indeed much of the same in this respect as in
the fall of a stone to the ground. The stone
goes down to meet the earth, but the earth
at the same time comes up to meet the stone.
As, however, the earth is more massive than
millions of millions of stones, the actual
movement performed by the earth is in this
case quite unappreciable. We may therefore
say, with truth enough for all practical
purposes, that it is the stone which does all
the moving, while the earth remains at rest.

In the same manner we may suppose the
sun to be at rest, while the wisp of vapour
is drawn towards it from the depths of space.
At first, no doubt the motion may be
extremely slow: for the attraction of the
sun decreases with its distance. Indeed,
the wisp of vapour might be so remote, that
it would require thousands of years to move
over an inch. But as the motion progresses
the body will gradually acquire speed, until
after the lapse of a time, so long that we shall
not attempt to express it in figures, the little
object will be found hurrying in towards the
sun with the speed of an express train; still
the pace will grow until the approaching
object will be moving as quickly as a rifle
bullet. The intervening distance is now
rapidly dimishing; but, as that distance

Vol. xxi.-50.

lessens, the intensity of the solar attraction
increases, and, consequently, the pace at
which the object is urged onwards becomes
greater and greater. From moving at the
rate of a mile in a second, the little object
would gradually attain a speed not less
than that of the earth in its orbit, namely,
about eighteen miles a second. Still
the body presses onwards, until a pace could
be reached of 100 or 200 miles a second.
Finally, when the vapour would be about to
make the terrific plunge into the glowing sun,
its speed would be upwards of 400 miles a
second. The vastness of this speed may
be realized from the fact that a body
animated by so great a velocity would
accomplish a complete circuit of the earth
in about a minute.

The case which I have supposed is, how
ever, not exactly that of a comet. The
movement would hardly take place in the
way just described, in which the sun and
the wisp of vapour were both orginally
at rest. Such a state of things could hardly
be possible in Nature. We may, no doubt,
suppose the sun to have been at rest, for it
is only the relative movements of the two
bodies which concern us. But we can hardly
imagine that the wisp of vapour could have
been so delicately placed as to have had
absolutely no motion whatever, except, in
deed, in the direct line towards the sun. If,
at the moment of starting, the object pos
essed a movement which would carry it in
the course of time out of the direct line
to the sun, then a totally different condition
of motion would be the result.

All the time that the sun was drawing this
wisp of vapour towards it, the transverse move
ment would be gradually moving the wisp
out of the direct line. Now, though the
speed of that movement may be very small,
yet in the lapse of those millions of
years that are required to draw the body into the
sun, this transverse movement will have
increased to such an extent that the object
will the miss the sun instead of hitting it. In
fact, after its stupendous voyage from the
indefinitely remote depths of space, during
which it has acquired its vast speed of scores
or hundreds of miles a second, the comet will
be found not plunging into the sun, but

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passing to one side of it. While the two
objects are in such close proximity their
mutual attraction is, of course, of tremendous
vehemence. In virtue of this attraction, the
rapidly moving comet is whirled round the
sun, and consequently begins to retreat again
towards the same side from which it has
come. In this majestic sweep the comet
describes a graceful curve. Coming in from
infinity it approaches the sun, wheels round
the sun, and then again retires to the depths
of space.

As the comet has swept in towards the
sun, in consequence of the attraction of that
body, it may seem difficult to understand
why it should then retreat outwards again,
notwithstanding the attraction which now
seeks to draw it back. This may, however,
be illustrated by a very simple contrivance.
Let a weight be hung from the ceiling by a
string. Let that weight be drawn to aside and
then released. It will, of course, swing down
to the lowest point, and then, having passed
through the lowest point, the weight will
begin to ascend. The attraction of the earth
pulls the body down, but as it descends it
acquires speed, and in virtue of this speed
it is enabled to pass the lowest point and to
ascend in opposition to gravity on the other
side. In the same way, the speed acquired
by the comet in its long voyage towards the
sun from the depths of space enables it to
sweep round the sun without being captured,
and then to pass away, perhaps, never more
to return. The nearer the comet is to the
solar surface the greater is the speed with
which it moves, and consequently the more
brief is its sojourn in the vicinity of the sun.
A comet has, in fact, been known to graze the
sun so closely that it passed within one-
seventh part of the sun's radius. In this
case a period of two hours sufficed for the
comet to completely turn round the sun
and commence its retreat into space.

The actual circumstances presented in
Nature are not quite so simple. We have
assumed that the sun and the comet were
the solitary objects in the universe. Of
course, this condition is not fulfilled. There
are the planets surrounding the sun, and
then there are the countless hosts of stars. Some
of these objects may attract the comet with
a vigour sufficient to sway it considerably
from the track which it would otherwise
follow. In consequence of these various
forces we are not justified in discussing the
problem actually presented in Nature as
being exactly the same as that in the case
hitherto supposed. But our illustrations will,

at all events, suffice to give a general idea of
what actually happens. The comets are
drawn in from the depths of space, they
approach the sun, they sweep round the sun,
and they then retreat again to the abyss from
which they have come. The laws of mathe
matics assure us that it is quite possible for
an object, after journeying from an immeasur
ably great distance for an immeasurably long
time, to enter our system, to wheel round the
sun and then again retreat to commence an
infinite voyage which should last for all
eternity. It is perfectly certain that this kind
of motion, which we know to be possible
does closely resemble that actually performed
by many of the comets. These bodies enter
our system, they come into the vicinity of the
earth, and, under these circumstances, they
are accessible to our observation. As they
retreat into space they gradually withdraw
from our view. Many of the comets which
come to visit us appear to be objects which
have never been within the ken of the earth
before, and which will never be within the
ken of the earth again.

There are, however, a few of these bodies
which describe orbits of a different kind.
They move round in elliptic or oval paths,
so that their visits to our vicinity and their
consequent visibility to the inhabitants of
the earth recur with more or less regularity.
Of such a nature is that most famous of all
comets, which bears the name of the illustrious
astronomer Halley. This splendid object
accomplishes a complete circuit around the
sun every seventy-five years. It will again
display its splendours for terrestrial admira
tion about the year 1910.

Our knowledge of comets has been greatly
extended in the last few years by the applica
tion of photographic methods to the investi
gation of the heavens. Indeed, we are
evidently now entering upon a new phase in
the history of these mysterious
objects. The advantages of photography for
such inquiries are obvious. In the first
place, the plates present to us pictures of
absolute accuracy. This is a matter of
special importance in this research, because
the appearance of comets changes so in
cessantly, that unless the portrait of the
comet obtained on any particular occasion
be absolutely faithful, it is impossible to
correct it on any subsequent occasion. Not
only from week to week does the comet alter
its appearance, but it changes even from day
to day. It is, therefore, of the utmost im
portance to obtain views of the body which
shall be of unquestioned accuracy so far as

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COMETS.

the aspect of the body is concerned at that
particular moment. There is also another
reason why photographic pictures of comets
are particularly instructive. It is a pecu
liarity of the sensitive plate that it is able to
perceive and record luminous expressions
quite too faint to produce any impression on
the eye.

When we examine the photograph of a
comet, we thus often find on it many details
which were quite unseen by the observer, no
matter how acute his vision may have been,
and no matter how powerful may be the
telescope which he as been employing. It
is, indeed, sometimes found that the tail of
the comet, as it is depicted on the plates, is
three times as extensive as the tail of the
same body as it is displayed through a
telescope.

An interesting comet, which has afforded
much occupation to the photographer, was
discovered on July 8th, 1893, by Mr. Alfred
Rordame, an astronomer residing in Salt
Lake City. Mr. W.J. Hussey obtained
some admirable photographs of this object

at the Lick Obser
vatory, and we are
also indebted to the
same astronomer
for a very interest
ing account of the
physical charac
teristics of this
body.

On looking at the
photograph of the
comet Rordame, on
the 12th of July,
and comparing it
with that shown on
the next page, taken
on the following
night, the observer
will be astonished
at the difference in
the structure of the
two tails. It would
seem as if some
violent dislocation
of the material of
the tail must have
taken place in the
interval which has
elapsed between the
times when the two
pictures were taken.
There is no doubt
that visual obser
vations would never

have established this point so clearly as the
photographs have done.

It will be noticed that the plates are marked
over by numbers of bright streaks: these
are the photographs of the stars which
happened to lie in the same field of view as
the comet. But it may well be asked how it
has come to pass that the stars are represented
by streaks instead of the round images which
we should expect from their sun-like character.
The explanation of this circumstance is not a
little curious and instructive. The comet is
in motion, and in moves so rapidly that in the
course of such a protracted exposure as that
on July 12th, which lasted for one hour and
twelve minutes, the comet changes its position
in the sky through a distance which is quite
apparent. If the camera had been directed
throughout the exposure to the same part of
the heavens, the comet, like the unquiet
sitter, would only have permitted us to obtain a
very blurred and indistinct portrait. To
obviate the effects of this motion it was
therefore necessary for the astronomer
who was engaged in taking the picture to

The Comet Rordame, July 12, 1893.

Photographed by W. J. Hussey, at the Lick Observatory

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THE STRAND MAGAZINE.

The Comet Rordame, July 13, 1893.

Photographed by W. J. Hussey, at the Lick Observatory.

shift the camera slowly during the course of
the exposure, and in that way to neutralize
the influence of the comet's motion. The
picture is thus made to represent the comet
as if that body had remained at rest during
the exposure. But the stars which were
strewn over the background remained quiet
all the time; as, however, the camera was
shifted, for the reason just mentioned, it
follows that each of the stars, instead of
being represented by a point, as it would
have been in an ordinary sidereal picture, is
manifested in this plate by a streak.

Such streaks, if
useless as stellar
pictures, are, never
theless, very in
structive. They
reveal to us the
nature and the
extent of the move
ment of the comet
during the period
which the exposure
has lasted. The
length of the streak
expresses the appa
rent distance
through which the
comet has moved,
while the direction
of the streak indi
cates the direction
in which the comet
is moving. At first
sight this latter
circumstance may
appear somewhat
puzzling. We are
accustomed to see
a shower of sparks
extending out be
hind a sky-rocket,
and this tail to the
rocket follows
generally the track
along which the
rocket has itself
advanced. But the
tail of a comet
bears a relation to
the comet very
different from that
which the stream
of sparks behind a
rocket bears to the
rocket itself. The
position of the
comet's tail is

governed by the remarkable law that it must
be turned away from the sun. In fact, it
would generally be found that a line drawn
through the comet from the tail to the head
would, when continued around the heavens,
point to the sun.

In the present case it is plain, from the
position of the star tracks, that the motion of
the comet happens to be almost perpendicular
to the direction of its tail. Considering the
very flimsy character of the tail of a comet,
it may seem rather surprising that this
structure should not be swept backwards so

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COMETS.

as to stream out along the comet's track.
Espcially is this the case when we think of
the enormous velocity at which the object is
moving. It must, however, be remembered
that no atmosphere exists in the open space
through which the comet wings its flight.
There would be no other medium to offer
any resistance to the flight of the tail, and
therefore there is no difficulty in explaining
how the object moves sideways through space
as the photographs show it actually does.

The photographs of Gale's comet shows
the rapid motion of that body in a very
instructive manner. The lengths of the star
tracks, of course, exhibit the distance through

Gale's Comet, May 8, 1894.

Photographed by E. E. Barnard, at the Lick Observatory.

which the comet has moved in the course of
a little more than two hours, which was the
duration of the exposure.

There is another very interesting circum
stance brought out by these photographs of
the stars, when we remember that they form
a background with respect to the comet. It
will be observed that many of the stellar
streaks are visible right through the tail. To
appreciate all that this implies we should note
that most of the stars here concerned are in
truth very faint objects. They would, under
any circumstances, be quite invisible to the
unaided eye. Yet they are nevertheless dis
tinctly seen, notwithstanding the interposition
of this stupendous volume of cometary matter.
Here, then, we see that stars, even though they
be faint stars, can nevertheless be discerned

through a thickness of hundreds of thousands
of miles, of the material which forms the
substance of the tail of the comet. This
enormously thick curtain is quite unavailing
for the purpose of hiding the stars. Yet the
light from those stars is so feeble that the
slightest film of haze on a summer sky would
suffice to extinguish them. This circum
stance shows, in a very striking manner,
how insignificant must be the quantity of
material which is contained in a comet.
We can admit this without, perhaps, going
quite so far as to agree with those who assert
that the tail of a notable comet may, never
theless, contain no greater quantity of solid

material than
could be packed
into a portman
teau.

The comet dis
covered by Swift
in 1892 is a very
interesting and
instructive object.
The picture shown
on the next page
was taken by Pro
fessor E. E. Bar
nard, at the Lick
Observatory, on
7th April, 1892.
This comet pos
esses a feature
which may be said
to be without a
parallel in the
photographs of
any other similar
object. It displays
the actual forma
tion of a second

comet as a part of the first. It can hardy be
doubted that this second comet is destined
to assume an independent existence. Astro
nomers had already known, in the case of
Biela's comet, that one of these objects
divided into two. Here the operation of
division may actually be witnessed.

The remarkable movements of all comets,
and the brilliant appearance which many of
these objects display, make every circum
stance with regard to them of much interest.
It will, therefore, not be a matter of suprise
to learn that the spectroscopic methods of
research, which have already taught us so
much with regard to the sun and the stars,
have been appplied to the examination of
comets. The results are very instructive, and
we here give some account of them.

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THE STRAND MAGAZINE.

Swift's Comet, April 7, 1892.

Photographed by E. E. Barnard, at the Lick Observatory.

We must first explain that the are two
totally different ways in which a body may be
rendered visible. In the first case, it may
shine by its own light; in the second case,
it may simply show the light reflected from
some other luminous body. The illumation
dispensed by a sun or a star is of the first
kind; that shed by the earth or any other
planet is of the second kind. The first
question which we have to ask with regard to
the light received from a comet may be thus
stated. Is this light due to some cause of
luminosity in the comet itself, or is it merely
sunlight reflected from the comet as from a
planet?

If we had been restricted to the use of
telescopes, however powerful, it would hardly
have been possible for us to have solved this
problem. The spectroscope has, however,
the power of disentangling the component
rays in a beam of light, and thus indicating
their character in such a way as enables us
to learn what the source of the light may
have been. We thus find that the light
emitted form a comet is, generally speaking,
of a two-fold character. Part of it is un

doubtedly reflected sunlight.
This is demonstrated by
observations with the spec
troscope, which show that
part of the radiation from a
comet exhibits a continuous
spectrum, marked by pre
cisely those lines and groups
of lines which are distinctly
characteristic of sunlight.
The evidence on this point
is quite convincing. We
should, indeed, have been
greatly surprised had it been
otherwise; for when the
comet adventures so near
the sun as it does in the
course of its wanderings it
must be brilliantly lighted
up by the great luminary,
and, of course, some por
tion of the splendour thus
produced is naturally re
flected to us.

But besides the bright
ness which comets possess
in virtue of the sunlight
which they receive, it is
quite certain that they are
also to be regarded as being
in a certain sense light
generators themselves. In
this respect the comet is at

once perceived to be a body of a totally
different character from a planet. The
splendour of Venus is due simply to the
sunlight which falls upon it. Nor does the
great Jupiter himself emit any rays beyond
those which he imperfectly reflects from the
sun. The comet is, however, of a very
different nature from the more robust planets.
Part of the light which the comet transmits
is unquestionably due to incandescense in
the body itself. If the sun were to be
suddenly deprived of light-giving power
while we were surveying the heavens con
taining the moon and a planet and a bright
comet, then the moon and the planet would
instantly disappear from view; but the
comet might still shine on. No doubt it
would lose some of its brightness, and
probably the tail would be to a great extent
shorn of its original proportions. There
would, however, be a certain amount of
cometary light independent of the sun still
forthcoming, so that extinction would not
necessarily overtake the comet as it
certainly would the moon and the planets.

The spectroscope not only tells us of the

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COMETS.

existence of light intrinsic to the comet, but
its evidence goes much further; it informs
us actually as to what the very elements must
be to whose presence in the comet the light
owes its origin. We here note the peculiar
advantage of the spectroscopic methods of
research. They detect special differences in
the rays of light, thus often enabling us to
trace each different type of light to its source.

The first notable achievement in the
determination of the peculiar character of
the radiation from a comet was made by
Dr. Huggins in 1868. He showed that
some of the rays of a comet which appeared
that year were indicative of the presence of
the element carbon in the body of the object.
In the case of this particular element the
available information carried us somewhat
further than is often the case. Not only was
the existence of the element demonstrated, but
the particular chemical combination in which
that element appeared was disclosed. By its
union with hydrogen, carbon gives rise to
an important series of compounds. The
substances thus produced are very familiar.
It need only be mentioned that the common
petroleum, which we use in our lamps, is a
combination of carbon and hydrogen! The
spectrum of a hydro-carbon, as one of these
compounds is termed, is of such a charac
teristic nature that it can be used as a
test to show whether the hydro-carbon
itself is present. Dr. Huggins compared the
spectrum of the comet now referred to with
the spectrum of these hydro-carbons. The
identity between the two spectra was noted,
and thus a splendid addition was a once
made to our knowledge. Subsequent research
has confirmed the important discovery that
hydro-carbons are characteristic components
of many comets.

For many years no further important
addition was made to our knowledge of the
elementary substances present in these
wandering bodies. The light they dispensed
appeared to be partly the reflected light from
the sun, and partly the light due to incan
descent hydro-carbons. But in 1882 a great
advance was made. A comet was discovered
that year in Albany, by Mr. Wells. At first,
this body showed the bright continuos
spectrum due to reflected sunlight, while the
indications of the presence of hydro-carbon

were mainly confined to the neighborhood
of the nucleus. After this interesting
object had adorned the heavens for a
couple of months Dr. Copeland, now the
distinguished Astronomer-Royal of Scotland,
discovered a bright yellow line in the spectrum
indicating the presence of sodium. This
observation was of particular importance,
inasmuch as it afforded at once direct
evidence of the presence in these celestial
wanderers of another element specially re
markable in its terrestrial relations. An
emphatic confirmation of Copeland's dis
covery was presently forthcoming. It is well
known that the bright yellow line indicative
of the presence of sodium is seen to be
double when examined under suitable cir
cumstances. As the comet appraoched the
sun the characteristic sodium light became
quite strong. The nucleus glared with the
distinct yellow hue belonging to this element;
and, indeed, by filtering away all other light
in a process with which a spectroscopist is
familiar, an outline of the head of the comet
was obtained which was produced by sodium
light alone.

To Dr. Copeland and Dr. Lohse at the
Earl of Crawford's observatory in Dunecht
we are also indebted for yet one more im
portant addition to our knowledge of the com
position of these bodies. In the autumn of
1882 we were visited by another comet, which
must rank as one of the most famous of these
objects which have appeared during this
century. On the 18th September, 1882, the
two astronomers I have named observed in
the spectrum of this comet, not only the
sodium line, but also six other lines.
The places of these were carefully measured,
and it was found afterwards that they were
undoubtedly the chief lines of the iron spec
trum. Here was, indeed, another notable dis
covery. The element iron, of such transcen
dent importance to us on the earth, is now
known to be a constituent of the cometary
wanderers throughout space.

Thus we have learned that the principal
elements in comets are among the common
substances on the earth. Here, again, we
find additional testimony to that fundamental
unity in the composition of heavenly bodies,
the perception of which is one of the most
notable results in modern science.

Originally published in The Strand Magazine, Volume 21 Month 1901