BY SIR ROBERT BALL, LOWNDEAN PROFESSOR OF ASTRONOMY AT THE UNIVERSITY
OF CAMBRIDGE, ENGLAND.
One of the most remarkable chapters in the astronomy of the past
century was commenced on the very first night with which that century
began. It was, indeed, on the 1st of January, 1801, that the discovery
of a new planet was announced. The five great orbs--Jupiter, Saturn,
Mercury, Mars, and Venus--had been known from the earliest times of
which we have records, and the planet Uranus had been discovered nearly
twenty years before the previous century closed. The solar system
was thus thought to consist of these six planets and, of course, the
earth. On the memorable night to which I have referred, Piazzi, the
astronomer, made a remarkable advance. He discovered yet another
planet--the seventh, or eighth, if the earth be included. The new body
was a small object in comparison with those which were previously
known. It was invisible to the unaided eye, and seemed no more than a
starlike point even when viewed through a telescope. It revolved around
the sun in the wide region between the orbits of Mars and Jupiter.
This discovery was speedily followed by others of the same kind,
and, as the century has advanced to its close, the numbers of these
planets--asteroids, as they are generally called--has been gradually
increasing, so much so that now, of these little bodies known to
astronomers, the number amounts to about four hundred and fifty.
But just as the beginning of the century was heralded by the discovery
of the first of these asteroids, so the close of the century will be
signalized in the history of astronomy by the detection among these
little objects of one which has entirely cast into the shade all other
discoveries of the same nature. On the night of the 13th of August,
1898, a German astronomer, Herr Witt, exposed a photographic plate to
the heavens in his telescope in the Observatory of Urania, at Berlin.
On that plate a picture of the heavens was obtained, and in that
picture a new planet was revealed. At first the discovery of one more
asteroid does not imply very much. Hundreds of such planets might be
found, and indeed have been found, and yet no particular comment has
been called forth. But this planet found by Witt is a unique object; it
is more interesting than the whole of the four hundred and thirty-two
other minor planets which have preceded it--not, indeed, on account
of its size, for Witt’s planet is a wholly insignificant object from
this point of view. The special interest which this new planet has
for us dwellers on the earth lies in the fact that it seems to be the
nearest to the earth of all the other worlds in space--the moon, of
course, excepted. This is the reason why the attention of all who are
interested in the science of astronomy has been concentrated on Witt’s
discovery. It is certainly the most interesting telescopic revelation
which has been made for many years.
It may illustrate a characteristic feature in the progress of modern
astronomy if I describe how Witt succeeded in obtaining this picture.
He had selected one of the most rapid plates that the skilled
manufacturer can supply to the photographer. He put this plate into
his telescope, and he directed it to the heavens. If that plate had
been used in broad daylight for the more ordinary purpose of obtaining
a photographic portrait, an exposure of half a second would have been
quite long enough. But the very faint stars can not work their charm on
the plate with equal rapidity; a second is not long enough, nor is ten
seconds, nor even ten minutes. If we desire to secure an imprint of the
faintest stars we must expose the plate for an hour, and sometimes for
even much longer than an hour. Of course, an exposure of such duration
would utterly ruin the picture if a gleam of any other light obtained
access. But in the darkness of night the plate is secure from this
danger. Each star is thus given time enough to impress its little image
at leisure.
The photographer has often occasion to deplore the poorness of his
light. It is, of course, in the endeavor to counteract the poorness
of the light that so long an exposure is frequently given. But it
will not be any longer supposed that, from the astronomer’s point of
view, a tedious exposure must necessarily be a disadvantage. Let it
be henceforth recollected that it was the very requirement of a long
exposure which led to the present important discovery. If the stars had
been bright enough to be photographed by an exposure not longer than
a few seconds or even than a few minutes, then this new and wonderful
planet Eros would not have been revealed.
Many points of light which were undoubtedly stars, and merely stars,
were shown on this picture taken by the German astronomer at Urania.
Among these points of light was, however, one object which, though in
appearance hardly distinguishable from a faint star, was in truth a
body of a very different character. No telescope, however powerful,
would show by mere inspection any appreciable difference between the
dot of light indicating a star and the dot of light indicating the
asteroid Eros. The fundamental difference between the star and Eros
was, however, revealed by the long exposure. The stars in such a
picture are, of course, at rest. They have occupied for years and for
centuries the places where we now find them. If they are moving at
all, their movements are so slow that they need not now be considered.
But this starlike point, or, as we may at once call it, this asteroid,
Eros, is moving. Not that its movements seem very rapid from the
distance at which alone we are compelled to view it. No casual glance
would indicate that Eros was flying along. The ordinary observer would
see no change in its place in a second--no change in its place even in
a minute. But when the exposure has lasted for an hour this asteroid,
in the course of the hour, has moved quite appreciably. Hence arose a
great difference between the representation which the photograph has
given of the stars, properly so called, and of the asteroid. Each star
is depicted as a sharp, well-defined point. This little body which
is not a star, this unsteady sitter in the picture, could not be so
represented; it merely appeared as a streak. The completed photograph
accordingly shows a large number of well-marked dots for the stars, and
among them one faint line for the asteroid.
Such a feature on a picture, though very unusual, does sometimes
present itself. To detect such a streak on a photograph of the stars
is a moment of transcendent joy to the astronomer. It is often for
him the exciting occasion on which a discovery is made. This little
moving point is in actual fact as different from a star as a pebble
is different from a brilliant electric light. The resemblance of the
asteroid to a star is merely casual; the resemblance would wholly
disappear if we were able to make a closer inspection. The star is a
brilliant blazing orb like a sun, but so far away that its luster is
diminished to that of a point; the planet is comparatively near us;
it is a dark body like our earth, and is like our earth also in this
further respect that all the light it enjoys has been derived from the
sun.
Though there is this immense difference between a star and a planet,
yet the observer must not expect to notice any such difference by
merely taking a peep through the telescope. It was only the long
exposure in the photograph that revealed the little body.
Such is the manner in which an asteroid is generally discovered in
these latter days. A discovery like this comes as the well-earned
reward of the skill and patience of the astronomical photographer.
There are, indeed, a large number of known asteroids; our catalogues
contained four hundred and thirty-two of them up to the time when Witt
exposed his now famous plate. Had the asteroid Witt then found been
merely as other asteroids, it would never have received the prominent
position that has now to be assigned to it in any account of the
astronomy of the century. That object found by Witt on this night which
is to be henceforth memorable in astronomy is of a wholly exceptional
kind. Had Eros been merely an ordinary asteroid, Witt might no doubt
have received the credit to which his labors and success would have
entitled him. Another asteroid would have been added to the long list
of such objects already known, but the newspapers would never have
troubled their readers about the matter, and the only persons who
would have been affected would have been the astronomers, and perhaps
even among them no particular sympathy would have been felt in certain
quarters. Those particular astronomers to whom has been intrusted the
special work of looking after the asteroids and of calculating the
tables of their movements might even have received with no very great
enthusiasm the announcement of this further addition to the burden on
their heavily laden shoulders.
I have said that Eros is quite a small globe; it may be well for us
fully to realize how small that asteroid actually is. If the moon were
to be crushed into two million equal fragments, each of those parts
would be as big as Eros. If the whole of Eros were to be covered with
houses, the city thus formed would not be so large as greater London.
So far as mere size is concerned, Eros is quite unimportant. We can
further illustrate this if we compare Eros with some of the other
planets. The well-known evening star, Venus, the goddess of love, is a
hundred million times as big as that tiny orb we now call Eros, the god
of love. After all this it may seem strange to have to maintain what
is, however, undoubtedly the fact, that the discovery of Eros is one of
the most remarkable discoveries of this century.
Until Eros was discovered, our nearest neighbors among the planets
were considered to be Venus on one side and Mars on the other. The
other great planets are much more distant, while, of course, the stars
properly so called are millions of times as far.
Great, then, was the astonishment of the astronomers when, by the
discovery of Eros, Mars and Venus were suddenly dethroned from their
position of being the earth’s nearest neighbors among the planetary
host. This little Eros will, under favorable circumstances, approach
the earth to within about one third the distance of Mars when nearest,
or about one half the distance of Venus when nearest. We thus
concentrate on Eros all the interest which arises from the fact that,
the moon of course excepted, Eros is the nearest globe to the earth in
the wide expanse of heaven. To the astronomer this statement is of the
utmost significance; when Eros comes so close it will be possible to
determine its distance with a precision hitherto unattainable in such
measurements. Once the distance of Eros is known, the distance of the
sun and of all the other planets can be determined. The importance of
the new discovery arises, then, from the fact that by the help of Eros
all our measurements in the celestial spaces will gain that for which
every astronomer strives--namely, increased accuracy.
Seeing that the existence of intelligence is a characteristic feature
of this earth, we feel naturally very much interested in the question
as to whether there can be intelligent beings dwelling on other
worlds around us. It is only regrettable that our means of solving
this problem are so inadequate. Indeed, until quite lately it would
have been almost futile to discuss this question at all. All that
could then have been said on the subject amounted to little more
than the statement that it would be intolerable presumption for man
to suppose that he alone, of all beings in the universe, was endowed
with intelligence, and that his insignificant little earth, alone
amid the myriad globes of space, enjoyed the distinction of being the
abode of life. Recent discovery has, however, given a new aspect to
this question. At the end of this century certain observations have
been made disclosing features in the neighboring planet, Mars, which
have riveted the attention of the world. On this question, above most
others, extreme caution is necessary. It is especially the duty of the
man of science to weigh carefully the evidence offered to him on a
subject so important. He will test that evidence by every means in his
power, and if he finds the evidence establishes certain conclusions,
then he is bound to accept such conclusions irrespective of all other
circumstances.
Mr. Percival Lowell has an observatory in an eminently favorable
position at Flagstaff, in Arizona. He has a superb telescope,
and enjoys a perfect climate for astronomical work. Aided by
skillful assistants, he has observed Mars under the most favorable
circumstances with great care for some years. I must be permitted to
say that, having carefully studied what Mr. Lowell has set forth, and
having tested his facts and figures in every way in my power, most
astronomers have come to the conclusion that, however astonishing his
observations may seem to be, we can not refuse to accept them.
No one has ever seen inhabitants on Mars, but Mr. Percival Lowell
and one or two other equally favored observers have seen features on
that planet which, so far as our experience goes, can be explained in
no other way than by supposing that they were made by an intelligent
designer for an intelligent purpose. Mr. Lowell has discovered that
there are certain operations in progress on the surface of Mars which,
if we met with on this earth, we should certainly conclude, without the
slightest hesitation, were the result of operations conducted under
what we consider rational guidance.
A river, as Nature has made it, wends its way to and fro; it never
takes the shortest route from one point to another; the width of the
river is incessantly changing; sometimes it expands into a lake,
sometimes it divides so as to inclose an island. If we could discern
through our telescopes a winding line such as I have described on Mars
it might perhaps represent a river.
But suppose, instead of a winding line, there was a perfectly straight
line, or rather a great circle on the globe drawn as straight as a
surveyor could lay it out--if we beheld an object like that on Mars I
think we should certainly infer that it was not a river made in the
ordinary course of natural operations; no natural river ever runs in
that regular fashion. If such a straight line were indeed a river, then
it must have been designedly straightened by human agency or by some
other intelligent agency for some particular purpose. In its larger
features Nature does not work by straight lines. A long and perfectly
straight object, if found on our earth, might be a canal or it might be
a road; it might be a railway or a terrace of some kind; but assuredly
no one would expect it to be a natural object.
We have the testimony of Schiaparelli, now strengthened by that of Mr.
Lowell and his assistants, that there are many straight lines of this
kind on Mars. They appear to be just as straight as a railway would
have to be if laid across the flat and boundless prairie, where the
engineer encountered no obstacle whatever to make him swerve from the
direct path. These lines on Mars run for hundreds of miles, sometimes,
indeed, I should say for thousands of miles. They are far wider than
any terrestrial river, except perhaps the Amazon for a short part of
its course. The lines on Mars are about forty miles wide. Indeed, the
planet is so distant that if these lines were much narrower than
forty miles they would be invisible. Each of them is marvelous in its
uniformity throughout its entire length.
The existence of these straight lines on the planet contains perhaps
the first suggestion of the presence of some intelligent beings on
Mars. The mere occurrence of a number of perfectly straight, uniform
lines on such a globe would in itself be a sufficiently remarkable
circumstance. But there are other features exhibited by these objects
which also suggest the astonishing surmise that they have been
constructed by some intelligent beings for some intelligent purposes.
Sometimes two of these lines will start from a certain junction,
sometimes there will be a third or a fourth from the same junction; in
one case there are as many as seven radiating from the same point. Such
an arrangement of these straight lines is certainly unlike anything
that we find in Nature. We are led to seek for some other explanation
of the phenomenon, and here is the explanation which Mr. Lowell offers:
It has recently been found that there are no oceans of water on the
planet Mars. In earlier days it used no doubt to be believed that the
dark marks easily seen in the telescope could represent nothing but
oceans, but I think we must now give up the notion that these are
watery expanses. Indeed, there is not much water on that globe anywhere
in comparison with the abundance of water on our earth. It is the
scarcity of water which seems to give a clew to some of the mysteries
discovered on Mars by Schiaparelli and Lowell.
As our earth moves round the sun we have, of course, the changing
seasons of the year. In a somewhat similar manner Mars revolves around
the sun, and accordingly this planet has also its due succession of
seasons. There is a summer on Mars, and there is a winter; during the
winter on that globe the poles of the planet are much colder than at
other seasons, and the water there accumulates in the form of ice or
snow to make those ice-caps that telescopic observers have so long
noticed. In this respect Mars, of course, is like our earth. The
ice-cap at each pole of our globe is so vast that even the hottest
summer does not suffice to melt the accumulation; much of the ice and
snow there remains to form the eternal snow which every arctic explorer
so well knows. It would seem, however, that the contrast between winter
and summer on Mars must be much more deeply marked than the contrast
between winter and summer on our earth. During the summer of Mars ice
and snow vanish altogether from the poles of that planet.
Mr. Lowell supposes that water is so scarce on Mars that the
inhabitants have found it necessary to economize to the utmost whatever
stock there may be of this most necessary element. The observations at
Flagstaff tend to show that the dark lines on Mars mark the course of
the canals by which the water melted in summer in the arctic regions
is conducted over the globe to the tracts where the water is wanted.
Not that the line as we see it represents actually the water itself;
the straight line so characteristic of Mars’s globe seems rather to
correspond to the zones of vegetation which are brought into culture
by means of water that flows along a canal in its center. In much the
same way would the course of the Nile be exhibited to an inhabitant
on Mars who was directing a telescope toward this earth: the river
itself would not be visible, but the cultivated tracts which owe their
fertility to the irrigation from the river would be broad enough to be
distinguishable. The appearance of these irrigated zones would vary, of
course, with the seasons; and we observe, as might have been expected,
changes in the lines on Mars corresponding to the changes in the
seasons of the planet.
A noteworthy development of astronomy in the last century has been the
erection of mighty telescopes for the study of the heavens. It must
here suffice to mention, as the latest and most remarkable of these,
the famous instrument at the Yerkes Observatory, which belongs to the
University of Chicago. Just as the century is drawing to its close, the
Yerkes telescope has begun to enter on its sublime task of exhibiting
the heavens under greater advantages than have ever been previously
afforded to any astronomers since the world began.
The University of Chicago having been recently founded, it was desired
to associate with the university an astronomical observatory which
should be worthy of the astonishing place that this wonderful city has
assumed in the world’s history. Mr. Yerkes, an American millionaire,
generously undertook to provide the cost of this observatory. Two noble
disks of glass, forty inches in diameter, were produced at the furnaces
of Messrs. Mantois, in Paris; these disks were worked by Mr. Alvan
Clark, of Boston, into the famous object glass which, weighing nearly
half a ton, has now been mounted in what we may describe as a temple or
a palace such as had never been dreamed of before in the whole annals
of astronomy.
Perhaps if we could now place the science of the nineteenth century in
its proper perspective the most remarkable discovery which it contains
would be that of the planet Neptune. Indeed, the whole annals of
science present no incident of a more dramatic character.
It will be remembered that at the latter part of the eighteenth century
William Herschel had immortalized himself by the discovery of a great
planet, to which was presently assigned the name of Uranus. After the
movements of Uranus had been carefully studied, it was found that
on many previous occasions Uranus had been unwittingly observed by
astronomers, who regarded it as a star. When these observations were
all brought together, and when the track which Uranus followed through
the heavens was thus opened to investigation, it was found that the
movements of the planet presented considerable anomalies. The planet
did not move precisely as it would have moved had it been subjected
solely to the supreme attractive power of the sun. Astronomers are,
of course, accustomed to irregularities of this description in the
movements of the planets. These irregularities have as their origin
the attractions of the various other members of the solar system.
It is possible to submit these attractions to calculation and thus
to estimate their amount. The effect, for instance, of Saturn in
disturbing Jupiter can be allowed for, and the nature of Jupiter’s
motion as thus modified can be precisely estimated. In like manner,
the influence of the earth on Venus can be determined, and so for the
other planets; and thus, generally speaking, it was found that when
the proper allowances had been made for the action of known causes
of disturbance, then the calculated movement of each planet could be
reconciled with observation.
The circumstances of Uranus were, however, in this respect wholly
exceptional. Due allowance was first made for the attraction of Uranus
by Saturn, and for the attraction of Uranus by Jupiter, as well as by
the other planets. It was thus found that the irregularities of Uranus
could be to some extent explained, but that it was not possible in this
manner to account for those irregularities completely. It was therefore
evident that some influence must be at work affecting the movement
of Uranus, in addition to those arising from any planet of which
astronomers hitherto had cognizance. The only available supposition
would be that some other planet, at present unrecognized, must be in
our system, and that the attraction of this unknown body must give rise
to those irregularities of Uranus which remained still outstanding.
A great problem was thus proposed for mathematicians. It was nothing
less than to affect the determination of the orbit and the position
of this unknown planet, the sole guide to the solution of the problem
being afforded by the discrepancies between the places of Uranus
as actually observed and the places which were indicated by the
calculations, when every allowance had been made for known causes.
The problem was indeed a difficult one, but, fortunately, two
mathematicians proved to be equal to the task of solving it--Adams,
in England, and Le Verrier, in France. Each of these astronomers, in
independence of the other, succeeded in determining the place of the
planet in the sky. The dramatic incident of this discovery was afforded
when the mathematicians had done their work. When the place of the
planet had been ascertained, then the telescopic search was undertaken
to verify if it were indeed the case that a planet hitherto unknown
did actually lurk in the spot to which the calculations pointed. Every
one who has ever read a book on astronomy is well acquainted with the
wonderful manner in which this verification was made. Just where the
mathematicians indicated, there was the great planet discovered! To
this object the name of “Neptune” has been assigned, and its discovery
may be said to mark an epoch in the history of gravitation. It provided
a most striking illustration of the truth of those great laws which
Newton had discovered.
The latter half of the century will be also remarkable in the history
of science from the fact that within that period mankind has been
enabled to make some acquaintance with the chemistry of the celestial
bodies. It was in 1859 that Kirchhoff and Bunsen first expounded to the
world the true meaning of the dark lines in the solar spectrum. In this
they were following out a line of reasoning that had been previously
suggested by Prof. Sir G. Stokes, of Cambridge, England. Those who
are at all conversant with that wonderful branch of knowledge known
as spectrum analysis are aware how these discoveries have rendered it
possible for us to determine in many cases the actual material elements
found in the most distant bodies.
One of the striking results to which this investigation has led is the
demonstration of the substantial unity of the materials from which the
earth and the various heavenly bodies have been constructed. Those
elements which enter most abundantly into the composition of the earth
are also the elements which appear to enter most abundantly into the
composition of the sun and of the stars. The iron and the hydrogen, the
sodium and the many other materials of which our globe is so largely
formed, are also the selfsame materials which, in widely different
proportions and in very different associations, go to form the heavenly
bodies. This conclusion is as interesting as it was unexpected. It
might naturally have been thought that, seeing the sun is separated
from us by nearly a hundred million miles, and seeing that the stars
are separated from us by millions of millions of miles, all these
celestial bodies must be constructed in quite a different manner and
of substances quite distinct from the substances which we know on
this earth. But this is not the case. Indeed, at the present moment
it seems doubtful if there be any element which spectrum analysis
has hitherto disclosed in the celestial bodies which is not also a
recognized terrestrial body. The well-known case of helium gives a
striking illustration. In the year 1868 Sir Norman Lockyer detected
the presence of rays in the solar spectrum which were unknown at that
time in terrestrial chemistry. These rays appeared to emanate from
some substance which, though present in the sun, did not then appear
to belong to the earth. This element was accordingly named “helium,”
to indicate its solar origin. Twenty-five years later Professor
Ramsay discovered a substance on the earth which had been hitherto
unrecognized, and which, on examination, yielded in the spectrum
precisely those same rays which had been found in the so-called
helium from the sun. In consequence of this discovery this element
is now recognized as a terrestrial body. It is indeed a remarkable
illustration of the extraordinary character of modern methods of
research that a substance should have first been discovered at a
distance of nearly one hundred million miles, that same substance being
all the time, though no doubt in very small quantities, a constituent
of our earth as well as of the sun.
Much has been done within the past century in many other branches of
astronomy. I must especially mention the important subject of meteoric
showers. For the development of our knowledge of this attractive part
of astronomy we are largely indebted to the labors of the late Prof.
H. Newton, of Yale. By his investigations, in conjunction with those
of the late Professor Adams, it was demonstrated that the shower of
shooting stars which usually appears in the middle of November is
derived from a shoal of small bodies which revolve around the sun in an
elliptic track, and accomplish that circuit in about thirty-three years
and a quarter. The earth crosses the track of these meteors in the
middle of November. If it should happen that the great shoal is passing
through the junction at the time the earth also arrives there, then the
earth rushes through the shoal of little bodies. These plunge into our
atmosphere, they are ignited by the friction, and a great shower is
observed. It is thus that we account for the recurrence of specially
superb displays at intervals of about thirty-three years.
But one more great astronomical discovery of this century must be
mentioned, and here again, as in so many other instances, we are
indebted to American astronomers. It was in 1877 that Prof. Asaph Hall
discovered that the planet Mars was attended by two satellites. This
was indeed a great achievement, and excited the liveliest interest and
attention. Since the days when telescopes were first invented all
the astronomers have been looking at Mars, and yet they never noticed
(their telescopes were not good enough) those interesting satellites
which the acute observation of Professor Hall detected with the help
of the great telescope of the Naval Observatory at Washington. This
discovery was followed by another of a still more delicate nature,
when that consummate observer, Professor Barnard, using the great Lick
telescope, detected the fifth satellite of Jupiter. This is indeed a
most difficult object to observe, requiring, as it does, the highest
optical power, the most perfect atmospheric conditions, and the most
skillful of astronomical observers. We may take this observation to
represent the high-water mark of telescopic astronomy in the nineteenth
century. This being so, it may fitly conclude this brief account of
some of the most remarkable astronomical discoveries which that century
has produced.