when brightest. Nevertheless, as if to make up for its seeming
insignificance by the interest attaching to it in a physical point
of view, it exhibited, at its appearance in 1846, a phenomenon
which struck every astronomer with amazement, as a thing without
previous example in the history of our system. It was actually seen
to separate itself into two distinct comets, which, after thus
parting company, continued to journey along amicably through an arc
of upward of 70° of their apparent orbit, keeping all the while
within the same field of view of the telescope pointed toward them.
The first indication of something unusual being about to take place
might be, perhaps, referred to the 19th of December, 1845, when the
comet appeared to Mr. Hind pear-shaped, the nebulosity being unduly
elongated in a direction inclining northward. But on the 13th of
January, at Washington, in America, and on the 15th and subsequently
in every part of Europe, it was distinctly seen to have become
double; a very small and faint cometic body, having a nucleus of its
own, being observed appended to it, at a distance of about 2′ (in
arc) from its centre, and in a direction forming an angle of about
328° with the meridian, running northward from the principal or
original comet. From this time the separation of the two comets went
on progressively, though slowly. On the 30th of January the apparent
distance of the nucleus had increased to 3′, on the 7th of February
to 4′, and on the 13th to 5′, and so on, until on the 5th of March
the two comets were separated by an interval of 9′ 19″, the apparent
direction of the line of junction all the while varying but little
with respect to the parallel.
During this separation, very remarkable changes were observed to
be going on, both in the original comet and its companion. Both
had nuclei, both had short tails, parallel in direction and nearly
perpendicular to the line of junction; but whereas at its first
observation, on January 13th, the new comet was extremely small and
faint in comparison with the old, the difference both in point of
light and apparent magnitude diminished. On the 10th of February
they were nearly equal, although the day before the moonlight had
effaced the new one, leaving the other bright enough to be well
observed. On the 14th and 16th, however, the new comet had gained a
decided superiority of light over the old, presenting at the same
time a sharp and star-like nucleus, compared by Lieutenant Maury to a
diamond spark. But this state of things was not to continue. Already,
on the 18th, the old comet had regained its superiority, being nearly
twice as bright as its companion, and offering an unusually bright
and star-like nucleus. From this period the new companion began to
fade away, but continued visible up to the 15th of March. On the
24th the comet appeared again single, and on the 22d of April both
had disappeared.
While this singular interchange of light was going forward,
indications of some sort of communication between the comets were
exhibited. The new or companion comet, besides its tail, extending
in a direction parallel to that of the other, threw out a faint
arc light which extended as a kind of bridge from the one to the
other; and after the restoration of the original comet to its former
pre-eminence, it, on its part, threw forth additional rays, so as
to present the appearance of a comet with three faint tails forming
angles of about 120° with each other, one of which extended toward
its companion.
On the 22d of August, 1844, Signor de Vico, director of the
observatory of the Collegio Romano, discovered a comet, the motions
of which, a very few observations sufficed to show, deviated
remarkably from a parabolic orbit. It passed its perihelion on the
2d of September, and continued to be observed until the 7th of
December. Elliptic elements of this comet, agreeing remarkably well
with each other, were accordingly calculated by several astronomers,
from which it appears that the period of revolution is about 1,990
days, or 5½ (5.4357) years, which (supposing its orbit undisturbed in
the interim) would bring it back to the perihelion on or about the
13th of January, 1850, on which occasion, however, by reason of its
unfavorable situation with respect to the sun and earth, it could not
be observed.
This comet, when brightest, was visible to the naked eye, and had
a small tail. It is especially interesting to astronomers from the
circumstance of its having been rendered exceedingly probable by
the researches of M. Leverrier, that it is identical with one which
appeared in 1678, with some of its elements considerably changed
by perturbation. This comet is further remarkable from having been
concluded, by Messrs. Laugier and Mauvais, to be identical with the
comet of 1585 observed by Tycho Brahe, and possibly also with those
of 1743, 1766, and 1819.
By far the most remarkable comet, however, which has been seen during
the present century, is that which appeared in the spring of 1843,
and whose tail became visible in the twilight of the 17th of March
in England as a great beam of nebulous light, extending from a point
above the western horizon, through the stars of Eridanus and Lepus,
under the belt of Orion. This situation was low and unfavorable; and
it was not till the 19th that the head was seen, and then only as
a faint and ill-defined nebula, very rapidly fading on subsequent
nights. In more southern latitudes, however, not only the tail was
seen, as a magnificent train of light extending 50° or 60° in length;
but the head and nucleus appeared with extraordinary splendor,
exciting in every country where it was seen the greatest astonishment
and admiration. Indeed, all descriptions agree in representing it as
a stupendous spectacle, such as in superstitious ages would not fail
to have carried terror into every bosom. In tropical latitudes in the
Northern Hemisphere, the tail appeared on the 3d of March, and in
Van Diemen’s Land so early as the 1st, the comet having passed its
perihelion on the 27th of February.
There is abundant evidence of the comet in question having been seen
in full daylight, and in the sun’s immediate vicinity. It was so
seen on the 28th of February, the day after its perihelion passage,
by every person on board the H.E.I.C.S. “Owen Glenndower,” then off
the Cape, as a short dagger-like object close to the sun a little
before sunset. On the same day at 3^h 6^m P. M., and consequently in
full sunshine, the distance of the nucleus from the sun was actually
measured with a sextant by Mr. Clarke of Portland, United States, the
distance centre from centre being then only 3° 50′ 43″.
[Illustration: Fig. 28.—Orbits of the Nine Comets Captured by Jupiter
_Scale: 5 millimetres = 1 radius of the Earth’s orbit_]
It is by no means merely as a subject of antiquarian interest, or
on account of the brilliant spectacle which comets occasionally
afford, that astronomers attach a high degree of importance to all
that regards them. Apart even from the singularity and mystery which
appertains to their physical constitution, they have become, through
the medium of exact calculation, unexpected instruments of inquiry
into points connected with the planetary system itself, of no small
importance. We have seen that the movements of the comet Encke, thus
minutely and perseveringly traced by the eminent astronomer whose
name is used to distinguish it, have afforded ground for believing in
the presence of a resisting medium filling the whole of our system.
Similar inquiries, prosecuted in the cases of other periodical
comets, will extend, confirm, or modify our conclusions on this
head. The perturbations, too, which comets experience in passing
near any of the planets, may afford, and have afforded, information
as to the magnitude of the disturbing masses, which could not well
be otherwise obtained. Thus the approach of this comet to the planet
Mercury in 1838 afforded an estimation of the mass of that planet
the more precious, by reason of the great uncertainty under which
all previous determinations of that element labored. Its approach
to the same planet in the year 1848 was still nearer. On the 22d
of November their mutual distance was only fifteen times the moon’s
distance from the earth.
It is, however, in a physical point of view that these bodies offer
the greatest stimulus to our curiosity. There is, beyond question,
some profound secret and mystery of nature concerned in the
phenomenon of their tails. Perhaps it is not too much to hope that
future observation, borrowing every aid from rational speculation,
grounded on the progress of physical science generally (especially
those branches of it which relate to the ethereal or imponderable
elements), may ere long enable us to penetrate this mystery, and to
declare whether it is really _matter_ in the ordinary acceptation of
the term which is projected from their heads with such extravagant
velocity, and if not impelled, at least _directed_, in its course by
a reference to the sun, as its point of avoidance. In no respect is
the question as to the materiality of the tail more forcibly pressed
on us for consideration than in that of the enormous sweep which it
makes round the sun in perihelio, in the manner of a straight and
rigid rod, in defiance of the law of gravitation, nay, even of the
received laws of motion, extending (as we have seen in the comets
of 1680 and 1843) from near the sun’s surface to the earth’s orbit,
yet whirled round unbroken: in the latter case through an angle of
180° in little more than two hours. It seems utterly incredible that
in such a case it is one and the same material object which is thus
brandished. If there could be conceived such a thing as a _negative
shadow_, a momentary impression made upon the luminiferous ether
behind the comet, this would represent in some degree the conception
such a phenomenon irresistibly calls up. But this is not all. Even
such an extraordinary excitement of the ether, conceive it as we
will, will afford no account of the projection of lateral streamers;
of the effusion of light from the nucleus of a comet toward the sun;
and its subsequent _re_jection; of the irregular and capricious mode
in which that effusion has been seen to take place; none of the clear
indications of alternate evaporation and condensation going on in
the immense regions of space occupied by the tail and coma—none, in
short, of innumerable other facts which link themselves with almost
equally irresistible cogency to our ordinary notions of matter and
force.
LIFE IN OTHER WORLDS.—J. E. GORE
The question is often asked, Are the stars inhabited? To this we
can confidently answer, No. The stars themselves are certainly not
habitable by any forms of life with which we are familiar. That the
stars are luminous incandescent bodies, similar to the sun, seems
almost self-evident. That they shine by their own inherent light, and
not by light reflected from another body, like the planets of the
Solar System, is a fact which scarcely needs demonstration. There are
no bright objects near them from which they could derive their light,
and they are too far from the sun to obtain any illumination from
that source. But if any proofs were necessary, we have the evidence
of the spectroscope, which shows unmistakably that their light
emanates from incandescent bodies. Many of the stars show spectra
very similar to that of the sun. The light of others, although
differing somewhat in quality when analyzed by the prism, indicates
clearly that they are at a very high temperature—in many cases,
indeed, suggesting that they are actually hotter than the sun. It may
be objected, however, that in the case of binary or revolving double
stars, the smaller component may possibly shine by light reflected
from the brighter star. Indeed, this has been suggested in the case
of Sirius and its faint companion. But, if the companion of Sirius
shone merely by reflected light from its primary, it would be much
fainter than it is, and, indeed, would be utterly invisible in our
largest telescopes. Further, in some double stars, spectroscopic
observations suggest that the component stars have different spectra.
This is, of course, conclusive evidence against the hypothesis of
borrowed light; for were the smaller star to shine by reflected
light from the larger, the spectra of both would be identical, as
in the case of the sun and moon. We may therefore conclude that all
the visible stars are suns, and totally unfit for the habitation of
living creatures.
But may not the stars have planets revolving round them, forming
solar systems similar to our own? As they are evidently suns
shining by inherent light, may they not form centres of planetary
systems? In the case of those stars having spectra differing from
the solar spectrum, we can not speak with any confidence; but for
those which show spectra similar to that of our sun, and having,
therefore, probably a similar chemical constitution, the existence
of planets revolving round them seems from analogy very probable.
I refer to _single_ stars, that is stars which have no telescopic
close companion; for the double stars may, perhaps, form systems
differently constituted. In any case these binary systems would not
be strictly comparable with ours, for the sun is certainly a single
star.
Whether systems of planets really revolve round the stars referred
to, is a question which, unfortunately, can not be decided by
observation. If a planet equal in size to the “giant planet,”
Jupiter, were revolving round the nearest star—Alpha Centauri—at the
same distance from that star that Jupiter is from the sun, it would
be utterly invisible in our largest telescopes. The invisibility of
planets circling round the stars is therefore no proof whatever of
their non-existence. Each star of the solar type may possibly be
attended by a retinue of planets which may, perhaps, remain forever
invisible in the largest telescopes which man can construct. We can,
therefore, draw our conclusions only from analogy. If other suns
exist resembling our own sun in chemical constitution, which we know
to be a fact, is it not reasonable to suppose that they also form
centres of planetary systems similar to the Solar System?
“Consult with reason, reason will reply,
Each lucid point which glows in yonder sky,
Informs a system in the boundless space,
And fills with glory its appointed place;
With beams unborrowed brighten other skies,
And worlds to the unknown with heat and light supplies.”
The suns, which we call stars, were clearly not created for our
benefit. They are of very little practical use to the earth’s
inhabitants. They give us very little light; an additional small
satellite—one considerably smaller than the moon—would have been much
more useful in this respect than the millions of suns revealed by
the telescope. They must, therefore, have been formed for some other
purpose.
On Laplace’s Nebular Hypothesis, the condensation of an original
nebulous mass endowed with a motion of rotation would result not only
in the formation of a sun, similar to ours, but also in a system of
planets revolving round the central body. If, indeed, the primitive
nebula had no rotation or motions of any kind, the result would be
a sun without planets or satellites; but the motions with which all
the stars seem to be animated lead us to suppose that this would be a
case of very rare occurrence. We may therefore conclude, with a high
degree of probability, that the stars—at least those with spectra of
the solar type—form centres of planetary systems somewhat similar to
our own.
This being surmised, let us consider the conditions necessary for
the existence of life on these planets. There are various conditions
which must be complied with before we can imagine life, as we know
it, to be possible on any planet. Perhaps the most important of
these is the question of temperature. We know that in the universe
a great range of temperature exists, from the cold of interstellar
space—estimated at about 460° below the freezing-point of water—to
the intense heat which rages in the solar photosphere. In this long
thermal scale life is, at least on the earth, restricted within
rather narrow limits. Below a certain low temperature life can not
exist. The point is, however, far above the temperature of space. On
the other hand, above a certain high temperature—a low one, however,
compared with the intense heat of the solar surface—life is also
impossible, at least for highly organized beings like man and the
larger animals. For minute microscopic organisms the scale may,
perhaps, be somewhat extended; but even in its widest limits, the
range of temperature within which life is possible is, so far as we
know, certainly a narrow one.
For the support of life and vegetation, light is also necessary,
for without it no flowers would bloom, nor corn grow and ripen to
maturity. To obtain this supply of light and heat it is necessary
that a life-bearing planet should revolve at a suitable distance
from, and in a nearly circular orbit round, a central sun. These
conditions, it is hardly necessary to say, are fulfilled in the case
of the earth. Were we much nearer to the sun than we are, we should
suffer from excessive heat, and were we much further away, we should
probably perish from the cold. For this reason the existence of life
on the other planets of the Solar System seems very doubtful. Mercury
is probably too hot, and the other planets are certainly too cold,
so far as heat from the sun is concerned, unless, indeed, their
internal heat is sufficient to raise the temperature of their surface
to a point sufficient for the maintenance of life. Indeed, there is
good reason to suppose that in the planets Jupiter, Saturn, Uranus,
and Neptune, this internal heat is still so great that life would be
quite impossible on their surface. Venus, inside the earth’s orbit,
and Mars, outside, are the two planets which seem to approach nearest
to the required conditions. We know that both these planets possess
atmospheres somewhat similar to ours, and, in Mars at least, land and
water most probably exist on its surface. Venus is, of course, much
hotter than the earth, and Mars much colder, but possibly the polar
regions of Venus and the equatorial regions of Mars may form suitable
abodes for some forms, at least, of animal and vegetable life.
Let us proceed, however, to consider some other conditions necessary
for the existence of life on a planet. A suitable temperature is,
of course, indispensable, but this is not all. There are other
conditions which must be complied with. The planet must have a
rotation on its axis, so that every portion shall in turn receive its
due share of light and heat. Each point on its surface must have its
day and night, the day for work and the night for rest. The axis of
rotation must not lie in the plane of the planet’s orbit, but must
have a suitable inclination, so that each hemisphere may enjoy its
seasons, summer and winter, “seed-time and harvest,” in due course.
Further the velocity of rotation on its axis must not be too rapid.
If the earth rotated in a period of one and a quarter hours, bodies
at the equator would have no weight, and life would be impossible in
those regions.
The planet must also possess a mass sufficient to retain bodies
on its surface by the force of gravity. In the case of very small
bodies, such as the moons of Mars and some of the minor planets
between Mars and Jupiter, objects thrown into the air would pass away
into space never to return.
The planet should also have a mean density greater than that of
water, otherwise the seas would possess no stability, and destructive
waves would quickly destroy all life on its surface. All these
conditions are fulfilled in the case of Mars as well as on the earth.
In the planet Saturn, however, the density is less than that of
water, and in Uranus and Neptune only slightly greater.
The planet must also possess a suitable atmosphere. This is an
all-important condition for the support of animal life—at least
for the existence of man and the higher orders of animals. This
atmosphere must consist—so far as we know—of oxygen and nitrogen
gases mechanically mixed in proper proportions, and with a small
quantity of carbonic acid gas. Were the oxygen in smaller quantity
than it exists in the earth’s atmosphere, life could not be
supported. On the other hand, were it much in excess of its present
amount, a fever would be produced in the blood which would very soon
put an end to animal life. The presence of other gases in excessive
quantities would also render the air unfit for breathing. We see,
therefore, that a comparatively slight change in the composition of
a planet’s atmosphere would—so far as our experience goes—render the
planet uninhabitable by any of the higher forms of life with which we
are familiar.
For the support of life on a planet, water is also absolutely
necessary. Without this useful fluid the world would soon become
a desert, and life and vegetation would speedily vanish from its
surface.
Geological conditions must also be considered. It is clearly
necessary for the welfare of human beings at least that the surface
soil and rocks should contain coal, iron, lime, and other minerals,
substances almost indispensable for the ordinary wants of civilized
existence.
[Illustration: Nine Views of the Hour-Glass Sea on Mars
1, Nov. 26, 1864; 2, June 29, 1873; 3, Oct. 28, 1879; 4, June 2,
1888; 5, June 20, 1890; 6, Aug. 6, 1892; 7, Oct., 1894; 8, Dec. 3,
1896; 9, Dec. 7, 1896]
That all or any of the conditions considered would be complied with
in the case of a planet revolving round a star it is, of course,
impossible to say. But when we find stars showing by their spectra
that they contain chemical elements identical with those which exist
in the sun and the earth, analogy would lead us to suppose that very
possibly a planet resembling our earth may revolve round each of
these distant suns. I say _a_ planet, for evidently there would be
only _one_ distance from the central luminary—a distance depending
on its size—at which the temperature necessary for the support of
life would exist, as in the case of the earth, over the whole of
the planet’s surface. For other planets of the stellar system, life
would be, if it existed at all, most probably confined to restricted
regions of the planet’s surface. There would, therefore, be in each
system one planet, and only one, _especially_ suitable for the
support of animal life as we know it. This is with reference to light
and heat. If the other conditions were not complied with, then life
would probably not exist even on this one planet. In the case of a
star larger than the sun, the planet should be placed at a greater
distance than the earth is from the sun, but in this case the length
of the year and the seasons would be longer than ours.
The star which more nearly resembles the sun in the character of
the light which it emits is the bright star Capella. Arcturus has a
somewhat similar spectrum. But these are probably suns of enormous
size, if any reliance can be placed on the measures of their distance
from the earth. Other bright stars with spectra of the solar type are
Pollux, Aldebaran, Beta Andromedæ, Alpha Arietis, Alpha Cassiopeiæ,
Alpha Cygni, and Alpha Ursæ Majoris. Another star is Eta Herculis.
The magnitude of this star as measured with the photometer is about
3½. A parallax found by Bélopolsky and Wagner places it at a distance
of 515,660 times the sun’s distance from the earth. If the sun were
placed at this distance, I find that it would be reduced to a star of
the third magnitude. This result would imply that Eta Herculis is a
slightly smaller sun than ours; and a planet placed a little nearer
to the star than the earth is to the sun might, perhaps, fulfil the
conditions of a life-bearing world.
The number of stars visible in our largest telescopes is usually
estimated at 100,000,000. Of these we may perhaps assume that
10,000,000 have a spectrum of the solar type, and therefore closely
resemble our sun in their chemical constitution. If we suppose that
only one in ten of these is similar in size to the sun, and has a
habitable planet revolving round it, we have a total of 1,000,000
worlds in the visible universe fitted for the support of animal life.