Myths and Marvels of Astronomy
horizon is irregular. It is here that the text-book pictures mislead;
for they show the depression as far too great to be overlooked, setting
the observer sometimes about two thousand miles above the sea-level. The
eye, then, judges the horizon to be where it usually is--on the same
level as the observer; but looking downwards, the eye perceives, and at
once appreciates if it does not even exaggerate, the great depth at
which the earth lies below the balloon. The appearance, then, as judged
by the eye, is that of a mighty basin whose edge rises up all round to
the level of the balloon, while its bottom lies two or three miles or
more below the balloon.
The zetetic faithful reason about this matter as though the impressions
of the senses were trustworthy under all conditions, familiar or
otherwise; whereas, in point of fact, we know that the senses often
deceive, even under familiar conditions, and almost always deceive under
conditions, which are not familiar. A person, for example, accustomed to
the mist and haze of our British air, is told by the sense of sight,
when he is travelling where a clearer atmosphere prevails, that a
mountain forty miles from him is a hill a few miles away. On the other
hand, an Italian travelling through the Highlands is impressed with the
belief that all the features of the scenery are much larger (because he
supposes them much more remote) than they really are. A hundred such
instances of deception might easily be cited. The conditions under which
the aeronaut observes the earth are certainly less familiar than those
under which the Briton views the Alps and Apennines, or the Italian
views Ben Lomond or Ben Lawers. It would be rash, therefore, even if no
other evidence were available, to reject the faith that the earth is a
globe because, as seen from a balloon, it looks like a basin. Indeed, to
be strictly logical, the followers of Parallax ought on this account to
adopt the faith that the earth is not flat, but basin-shaped, which
hitherto they have not been ready to do.
We have seen that Parallax describes a certain experiment on the Bedford
Level, which, if made as he states, would have shown certainly that
something was wrong in the accepted system--for a six-mile straight-edge
along water would be as severe a blow to the belief in a round earth, as
a straight line on the sea-surface from Queenstown to New York. Another
curious experiment adorns his little book, which, if it could be
repeated successfully before a dozen trustworthy witnesses, would rather
astonish men of science. Having, he says, by certain
reasoning--altogether erroneous, but that is a detail--convinced himself
that, on the accepted theory, a bullet fired vertically upwards ought to
fall far to the west of the place whence it was fired, he carefully
fixed an air-gun in a vertical position, and fired forty bullets
vertically upwards. All these fell close to the gun--which is not
surprising, though it must have made such an experiment rather
dangerous; but two fell back into the barrel itself--which certainly was
very surprising indeed. One might fairly challenge the most experienced
gunner in the world to achieve one such vertical shot in a thousand
trials; two in forty bordered on the miraculous.
The earth-flatteners I have been speaking of claim, as one of their
objects, the defence of Scripture. But some of the earth-flatteners of
the last generation (or a little farther back) took quite another view
of the matter. For instance, Sir Richard Phillips, a more vehement
earth-flattener than Parallax, was so little interested in defending
the Scriptures, that in 1793 he was sentenced to a year's imprisonment
for selling a book regarded as atheistic. In 1836 he attempted the
conversion of Professor De Morgan, opening the correspondence with the
remark that he had 'an inveterate abhorrence of all the pretended wisdom
of philosophy derived from the monks and doctors of the Middle Ages, and
not less those of higher name who merely sought to make the monkish
philosophy more plausible, or so to disguise it as to mystify the mob of
small thinkers.' He seems himself to have succeeded in mystifying many
of those whom he intended to convert. Admiral Smyth gives the following
account of an interview he had with Phillips: 'This pseudo-mathematical
knight once called upon me at Bedford, without any previous
acquaintance, to discuss "those errors of Newton, which he almost
blushed to name," and which were inserted in the "Principia" to "puzzle
the vulgar." He sneered with sovereign contempt at the "Trinity of
Gravitating Force, Projectile Force, and Void Space," and proved that
all change of place is accounted for by motion.' [Startling hypothesis!]
'He then exemplified the conditions by placing some pieces of paper on a
table, and slapping his hand down close to them, thus making them fly
off, which he termed applying the momentum. All motion, he said, is in
the direction of the forces; and atoms seek the centre by "terrestrial
centripetation"--a property which causes universal pressure; but in what
these attributes of pushing and pulling differ from gravitation and
attraction was not expounded. Many of his "truths" were as mystified as
the conundrums of Rabelais; so nothing was made of the motion.'
A favourite subject of paradoxical ideas has been the moon's motion of
rotation. Strangely enough, De Morgan, who knew more about past
paradoxists than any man of his time, seems not to have heard of the
dispute between Keill and Bentley over this matter in 1690. He says,
'there was a dispute on the subject, in 1748, between James Ferguson and
an anonymous opponent; and I think there have been others;' but the
older and more interesting dispute he does not mention. Bentley, who was
no mathematician, pointed out in a lecture certain reasons for believing
that the moon does not turn on her axis, or has no axis on which she
turns. Keill, then only nineteen years old, pointed out that the
arguments used by Bentley proved that the moon does rotate instead of
showing that she does not. (Twenty years later Keill was appointed
Savilian Professor of Astronomy at Oxford. He was the first holder of
that office to teach the Newtonian astronomy.)
In recent times, as most of my readers know, the paradox that the moon
does not rotate has been revived more than once. In 1855 it was
sustained by Mr. Jellinger Symons, one of whose staunchest supporters,
Mr. H. Perigal, had commenced the attack a few years earlier. Of course,
the gist of the argument against the moon's rotation lies in the fact
that the moon always keeps the same face turned towards the earth, or
very nearly so. If she did so exactly, and if her distance from the
earth were constantly the same, then her motion would be exactly the
same as though she were rigidly connected with the earth, and turned
round an axis at the earth. The case may be thus illustrated: Through
the middle of a large orange thrust one short rod vertically, and
another long rod horizontally; thrust the further end of the latter
through a small apple, and now turn the whole affair round the short
vertical rod as an axis. Then the apple will move with respect to the
orange as the moon would move with respect to the earth on the
suppositions just made. No one in this case would say that the apple was
turning round on its axis, since its motion would be one of rotation
round the upright axis through the orange. Therefore, say the opponents
of the moon's rotation, no one should say that the moon turns round on
her axis.
Of course, the answer would be obvious even if the moon's motions were
as supposed. The moon is not connected with the earth as the apple is
with the orange in the illustrative case. If the apple, without rigid
connection with the orange, were carried round the orange so as to move
precisely as if it were so connected, it would unquestionably have to
rotate on its axis, as any one will find who may try the experiment.
Thus for the straight rod thrust through the apple substitute a straight
horizontal bar carrying a small basin of water in which the apple
floats. Sway the bar steadily and slowly round, and it will be found (if
a mark is placed on the apple) that the apple no longer keeps the same
face towards the centre of motion; but that, to cause it to do so, a
slow motion of rotation must be communicated to the apple in the same
direction and at the same rate (neglecting the effects of the friction
of the water against the sides of the basin) as the bar is rotating. In
my 'Treatise on the Moon' I have described and pictured a simple
apparatus by which this experiment may easily be made.
But, of course, such experiments are not essential to the argument by
which the paradox is overthrown. This argument simply is, that the moon
as she travels on her orbit round the sun--the real centre of her
motion--turns every part of her equator in succession towards him once
in a lunar month. At the time of new moon the sun illuminates the face
of the moon turned from us; at the time of full moon he illuminates the
face which has been gradually brought round to him as the moon has
passed through her first two quarters. As she passes onwards to new
moon again, the face we see is gradually turned from him until he
shines full upon the other face. And so on during successive lunations.
This could not happen unless the moon rotated. Again, if we lived on the
moon we should find the heaven of the fixed stars turning round from
east to west once in rather more than twenty-seven days; and unless we
supposed, as we should probably do for a long time, that our small world
was the centre of the universe, and that the stars turned round it, we
should be compelled to admit that it was turning on its own axis from
west to east once in the time just named. There would be no escape. The
mere fact that all the time the stars thus seemed to be turning round
the moon, the earth would not so seem to move, but would lie always in
the same direction, would in no sort help to remove the difficulty.
Lunarian paradoxists would probably argue that she was in some way
rigidly connected with the moon; but even they would never think of
arguing that their world did not turn on its axis, _unless_ they
maintained that it was the centre of the universe. This, I think, they
would very probably do; but as yet terrestrial paradoxists have not, I
believe, maintained this hypothesis. I once asked Mr. Perigal whether
that was the true theory of the universe--the moon central, the earth,
sun, and heavens carried round her. He admitted that his objections to
accepted views were by no means limited to the moon's rotation; and, if
I remember rightly, he said that the idea I had thrown out in jest was
nearer the truth than I thought, or used words to that effect. But as
yet the theory has not been definitely enunciated that the moon is the
boss of the universe.
Comets, as already mentioned, have been the subjects of paradoxes
innumerable; but as yet comets have been so little understood, even by
astronomers, that paradoxes respecting them cannot be so readily dealt
with as those relating to well-established facts. Among thoroughly
paradoxical ideas respecting comets, however, may be mentioned one whose
author is a mathematician of well-deserved repute--Professor Tait's
'Sea-Bird Theory' of Comets' Tails. According to this theory, the rapid
formation of long tails and the rapid changes of their position may be
explained on the same principle that we explain the rapid change of
appearance of a flight of sea-birds, when, from having been in a
position where the eye looks athwart it, the flight assumes a position
where the eye looks at it edgewise. In the former position it is