Although the outer confines of space have, as we have seen, been
compelled to bring their tribute of new knowledge by means of the
penetrating power possessed by modern telescopes, and the cameras and
spectroscopes attached to them, the study of the _near_ has by no means
been neglected, and for the reason that in astronomy especially we must
content ourselves in the case of the more distant bodies by surmising
what happens in them from the facts gathered in the region where alone
detailed observations are possible.
Thus what we can learn about the sun helps to explain what we discern
much more dimly in the case of stars; a study of the moon’s face we
are compelled to take as showing us the possibilities relating to the
surface condition of other satellites so far removed from us that they
only appear as points of light.
To begin, then, with the sun. Where a volume might be written, a few
words must suffice. I have already stated that at the beginning of the
nineteenth century the prevailing opinion was that it was a habitable
globe. It was limited to the fiery ball we see. At the end of the
century it is a body of the fiercest heat, and the ball we see is only
a central portion of a huge and terribly interesting mechanism, the
outer portions of which heave and throb every eleven years. Spots,
prominences, corona, everything feels this throbbing.
Although the discovery of spots on the sun was among Galileo’s first
achievements, it was reserved for the last half of the nineteenth
century to demonstrate their almost perfect periodicity.
Thanks to the labors of Schwabe, Wolf, Carrington, and De la Rue,
Stewart, and Loewy, we now know that every eleven years the spots wax
and wane; Tacchini and Ricco, during the last thirty years, have proved
that the prominences follow suit, and the fact that the corona also
obeys the same law was established during the American eclipse of 1878.
The study of solar physics consists in watching and recording the
thermal, chemical, and other changes which accompany this period. Some
of these effects can be best studied during those times when the ball
itself is covered by the moon in an eclipse. Then the outer portions of
the sun are revealed in all their beauty and majesty, and all the world
goes to see.
But it is the quiet daily work in the laboratory which has enabled us
to study the sun’s place in relation to the other stars, and so to
found a chemical classification of all the stars that shine.
From the sun we may pass to his system, and first consider the nearest
body to us—the moon.
While some astronomers have been discussing the movements and evolution
of our satellite, others have been engaged upon maps of its surface,
upon questions dealing with a lunar atmosphere, or a study of the
origin of the present conformations and of possible changes. The
science of selenology may be said to have been founded by Schröter
at the beginning of the century, but it required the application of
photography in later years to put it on a firm basis. Maps of the moon
have been prepared by Lohrmann, Beer and Mädler, and Schmidt, the
latter showing the positions of more than thirty thousand craters.
Very erroneous notions are held by some as to what we may hope to do
in the examination of the moon’s surface by a powerful telescope. A
power of a thousand enables us to see it as if we were looking at
York from London. It is recorded that Lassell once said that with his
largest reflector in a “fit” of the finest definition he thought he
might be able to detect whether a carpet as large as Lincoln’s Inn
Fields was round or square. Under these circumstances, then, we may
well understand that the question of changes on the surface has been
raised from time to time never to be absolutely settled one way or the
other. By many the existence of an atmosphere is denied, and this is a
condition which would negative changes, anything like the geological
changes brought about on the surface of the earth, but the idea is now
held by many that there is still an atmosphere, though of great tenuity.
The last few years of the century were rendered memorable from the
lunar point of view by the publication and minute study of a most
admirable series of photographs of the moon obtained by the great
equatorial Coudé of the Paris Observatory by Loewy and Puiseaux.
One of the chief points aimed at has been to determine the sequence
of the various events represented by the rills, craters, and walled
plains, the mountain ranges and seas. This work is still in progress,
the fourth part of the atlas being published in 1900; but enough has
already appeared to indicate that the results of the inquiry when
completed will be of the most important kind. The authors have already
come to the conclusion that the lunar and terrestrial sea-bottoms
much resemble each other, inasmuch as both have convex surfaces. The
lunar seas began by sinking of vast regions; the formidable volcanic
eruptions of which the moon has been the scene have taken place in
times equivalent to those labelled “recent” in geological parlance.
There is evidence that the axis of the moon has undergone great
displacements, and four great periods of change have been made out.
Finally they state that there is serious ground to believe that there
is an atmosphere of some sort remaining.
It may readily be understood that with each increase of optical power
new satellites of the various planets have been discovered. Soon after
the discovery of Neptune a satellite was noted by Lassell. In 1846
both he and the eagle-eyed observer Dawes independently discovered
another satellite (Hyperion) of Saturn. Lassell was rewarded in the
next year by the discovery of two more satellites of Uranus; but,
strangest observation of all, in 1877 Hall discovered at Washington
two satellites of Mars some six or seven miles only in diameter, one
of them revolving round the planet in seven and one-half hours at a
distance of less than four thousand miles. As the day on Mars is not
far different in duration from our own, this tiny satellite must rise
in the west and south three times a day!
Wonderful as this discovery was, it is certainly not less wonderful
when we consider it in connection with a passage in _Gulliver’s
Travels_, so true is it that truth is stranger than fiction. Swift,
in his satirical reference to the inhabitants of Laputa, writes:
“They have likewise discovered two lesser stars, or satellites, which
revolve round Mars, whereof the innermost is distant from the centre
of the primary planet exactly three of his diameters and the outermost
five; the former revolves in the space of ten hours; and the latter in
twenty-one and a half.”
The last discovery of this kind has been that of an inner satellite of
Jupiter by Barnard in 1892.
The planets from Mercury to Saturn were known to the ancients. I
have already referred to the discovery of Uranus by Herschel’s giant
telescope, not long before the nineteenth century was born, and of
Neptune, by analysis, towards the end of the first half of the century.
With regard to what modern observations have done in regard to their
physical appearance, the first place in general interest must be given
to Saturn and Mars.
Saturn has always been regarded as the most interesting of the
planetary family on account of its unique rings. Many subdivisions of
the rings, and a dusky ring, first seen by Dawes and Bond, have been
discovered during the last sixty years.
The meteoritic nature of the rings was suggested by Clerk Maxwell in
1857, and Keeler’s demonstration of the truth of this view by means
of the spectroscope, a few years ago, was brilliant in conception and
execution.
But during the last half of the century the interest centred in Mars
has been gradually increasing. The drawings made during the opposition
of 1862, when compared with those made by Beer and Mädler (1830–40),
made it perfectly clear that in this planet we had to deal with one
strangely like our own in many respects. There were obviously land and
water surfaces; the snow at the poles melted in the summer-time; clouds
were seen forming from time to time, and the changing tones of the
water surfaces suggested fine and rough weather.
Afterwards came the revelation of the hawk-eyed Schiaparelli, beginning
in the year 1877, and his wonderful map of the planet’s surface. The
land surfaces, instead of being unbroken, were cut up, as an English
farm is cut up by hedges; straight lines of different breadths and
tints crossed the land surfaces in all directions, and at times some of
them appeared double. Schiaparelli naturally concluded that they were
rivers—water channels—and being an Italian he used the appropriate
word _canali_. This, unfortunately, as it turned out, was translated
_canals_. Now canals are dug, _ergo_ there were diggers. From this the
demonstration, not of the habitability, but of the actual habitation,
of Mars was a small step, and the best way of signalling to newly found
kinsmen across some thirty millions of miles of space was discussed.
The world of science owes a debt of gratitude to Mr. Percival Lowell
for having taken out to the pure air and low latitude of Arizona an
eighteen-inch telescope for the sole purpose of accumulating facts
tending to throw light upon this newly raised question. This he did
in 1894. Schiaparelli has continued his magnificent observations
through each opposition when the planet is most favorably situated for
observation, and since 1896 Signor Cerulli, armed with a fifteen-inch
Cooke, in the fine climate of Italy, has joined in the inquiry, so
that facts are now being rapidly accumulated. It has been stated that
markings similar to the strange so-called “canals” on Mars are to be
seen on Mercury, Venus, and even on the satellites of Jupiter. Mr.
Percival Lowell does not hesitate to proclaim himself in favor of their
being due, _in Mars_, to an intelligent system of irrigation. Signor
Cerulli claims that _wherever seen_ they are mere optical effects.
We may be well content to leave to the twentieth century a general
agreement on this interesting subject.
Finally, in our survey of our own system, come comets and meteor
swarms. One of the most fruitful discoveries of the century, that
comets are meteor swarms, we owe to the genius of Schiaparelli, A. H.
Newton, and other workers on those tiny celestial messengers which give
rise to the phenomena of “falling” or “shooting” stars.
The magnificent displays of 1799, 1833, 1866, and, alas! that which
failed to come in 1899, we now know must be associated with Tempel’s
Comet. This is by no means the only case so far established; the
connection will in the future be closer still when the orbits of the
various swarms observed throughout the year shall be better known.
Comets which attract public attention by their brightness and grandeur
of form are rather rare, and, in fact, only twenty-five of such have
been seen since 1800. We have, however, with the great advance in
instrumental equipment, been able to discover many which are scarcely
visible to the naked eye, and this has swollen the number of comets
very considerably. In the seventeenth century we find that only
thirty-two were observed, while in the eighteenth this number was more
than doubled (seventy-two). In the nineteenth century more than three
hundred were placed on record, which is practically more than four
times the number seen in the eighteenth.
The last great comet visible any considerable time was that discovered
by Donati in 1858, and so carefully observed by Bond. It is unfortunate
that since the importance, in so many directions, of spectroscopic
observations of comets has been recognized they have been conspicuous
by their absence.