continuous in one direction. It is what we call a periodic motion,
the pole swinging now to one side, and now to the other, of its
mean or average position. Thus this new discovery cannot be said
to unravel the mysterious puzzle of the geologists. Yet it is not
without the keenest interest, even from their point of view; for
the proof of any form of motion in a pole previously supposed to be
absolutely at rest may mean everything. No man can say what results
will be revealed by the further observations now being continued
with great diligence.
In the first place, it is important to explain that any such
motions as we have under consideration will show themselves to
ordinary observational processes principally in the form of changes
of terrestrial latitudes. Let us imagine a pair of straight lines
passing through the centre of the earth and terminating, one at the
observer's station on the earth's surface, and the other at that
point of the equator which is nearest the observer. Then, according
to the ordinary definition of latitude, the angle between these two
imaginary lines is called the latitude of the point of observation.
Now we know, of course, that the equator is everywhere just 90
degrees from the pole. Consequently, if the pole is subject to any
motion at all, the equator must also partake of the motion.
Thus the angle between our two imaginary lines will be affected
directly by polar movement, and the latitude obtained by
astronomical observation will be subject to quite similar changes.
To clear up the whole question, so far as this can be done by
the gathering of observational evidence, it is only necessary to
keep up a continual series of latitude determinations at several
observatories. These determinations should show small variations
similar in magnitude to the wabblings of the pole.
Let us now consider for a moment what is meant by the axis of the
earth. It has long been known that the planet has in general the
shape of a ball or sphere. That this is so can be seen at once
from the way ships at sea disappear at the horizon. As they go
farther and farther from us, we first lose sight of the hull, and
then slowly and gradually the spars and sails seem to sink down
into the ocean. This proves that the earth's surface is curved.
That it is more or less like a sphere is evident from the fact that
it always casts a round shadow in eclipses. Sometimes the earth
passes between the sun and eclipsed moon. Then we see the earth's
black shadow projected on the moon, which would otherwise be quite
bright. This shadow has been observed in a very large number of
such eclipses, and it has always been found to have a circular edge.
While, therefore, the earth is nearly a round ball, it must not be
supposed that it is exactly spherical in form. We may disregard
the small irregularities of its surface, for even the greatest
mountains are insignificant in height when compared with the
entire diameter of the earth itself. But even leaving these out of
account, the earth is not perfectly spherical. We can describe it
best as a flattened sphere. It is as though one were to press a
round rubber ball between two smooth boards. It would be flattened
at the top and bottom and bulged out in the middle. This is the
shape of the earth. It is flattened at the poles and bulges out
near the equator. The shortest straight line that can be drawn
through the earth's centre and terminated by the flattened parts of
its surface may be called the earth's axis of figure; and the two
points where this axis meets the surface are called the poles of
figure.
But the earth has another axis, called the axis of rotation. This
is the one about which the planet turns once in a day, giving
rise to the well-known phenomena called the rising and setting of
sun, moon, and stars. For these motions of the heavenly bodies
are really only apparent ones, caused by an actual motion of the
observer on the earth. The observer turns with the earth on its
axis, and is thus carried past the sun and stars.
This daily turning of the earth, then, takes place about the axis
of rotation. Now, it so happens that all kinds of astronomical
observations for the determination of latitude lead to values based
on the rotation axis of the earth, and not on its axis of figure.
We have seen how the earth's equator, from which we count our
latitudes, is everywhere 90 degrees distant from the pole. But this
pole is the pole of rotation, or the point at which the rotation
axis pierces the earth's surface. It is not the pole of figure.
It is clear that the latitude of any observatory will remain
constant only if the pole of figure and the rotation pole maintain
absolutely the same positions relatively one to the other. These
two poles are actually very near together; indeed, it was supposed
for a very long time that they were absolutely coincident, so that
there could not be any variations of latitude. But it now appears
that they are separated slightly.
Strange to say, one of them is revolving about the other in a
little curve. The pole of figure is travelling around the pole of
rotation. The distance between them varies a little, never becoming
greater than about fifty feet, and it takes about fourteen months
to complete a revolution. There are some slight irregularities in
the motion, but, in the main, it takes place in the manner here
stated. In consequence of this rotation of the one pole about the
other, the pole of figure is now on one side of the rotation pole
and now on the opposite side, but it never travels continuously
in one direction. Thus, as we have already seen, the sort of
continuous motion required to explain the observed geological
phenomena has not yet been found by astronomers.
Observations for the study of latitude variations have been made
very extensively within recent years both in Europe and the United
States. It has been found practically most advantageous to carry
out simultaneous series of observations at two observatories
situated in widely different parts of the earth, but having very
nearly the same latitude. It is then possible to employ the same
stars for observation in both places, whereas it would be necessary
to use different sets of stars if there were much difference in the
latitudes.
There is a special advantage in using the same stars in both
places. We can then determine the small difference in latitude
between the two participating observatories in a manner which
will make it quite free from any uncertainty in our knowledge of
the positions on the sky of the stars observed; for, strange
as it may seem, our star-catalogues do not contain absolutely
accurate numbers. Like all other data depending on fallible
human observation, they are affected with small errors. But if
we can determine simply the difference in latitude of the two
observatories, we can discover from its variation the path in
which the pole is moving. If, for instance, the observatories are
separated by one-quarter the circumference of the globe, the pole
will be moving directly toward one of them, when it is not changing
its distance from the other one at all.
This method was used for seven years with good effect at the
observatories of Columbia University in New York, and the Royal
Observatory at Naples, Italy. For obtaining its most complete
advantages it is, of course, better to establish several observing
stations on about the same parallel of latitude. This was done
in 1899 by the International Geodetic Association. Two stations
are in the United States, one in Japan, and one in Sicily. We
can, therefore, hope confidently that our knowledge as to the
puzzling problem of polar motion will soon receive very material
advancement.
SATURN'S RINGS
The death of James E. Keeler, Director of the Lick Observatory, in
California (p. 32), recalls to mind one of the most interesting
and significant of later advances in astronomical science.
Only seven years have elapsed since Keeler made the remarkable
spectroscopic observations which gave for the first time an ocular
demonstration of the true character of those mysterious luminous
rings surrounding the brilliant planet Saturn. His results have not
yet been made sufficiently accessible to the public at large, nor
have they been generally valued at their true worth. We consider
this work of Keeler's interesting, because the problem of the rings
has been a classic one for many generations; and we have been
particular, also, to call it significant, because it is pregnant
with the possibilities of newer methods of spectroscopic research,
applied in the older departments of observational astronomy.
The troubles of astronomers with the rings began with the invention
of the telescope itself. They date back to 1610, when Galileo
first turned his new instrument to the heavens (p. 49). It may
be imagined easily that the bright planet Saturn was among the
very first objects scrutinized by him. His "powerful" instrument
magnified only about thirty times, and was, doubtless, much
inferior to our pocket telescopes of to-day. But it showed, at
all events, that something was wrong with Saturn. Galileo put it,
"_Ultimam planet am tergeminam observavi_" ("I have observed the
furthest planet to be triple").
It is easy to understand now how Galileo's eyes deceived him. For
a round luminous ball like Saturn, surrounded by a thin flat ring
seen nearly edgewise, really looks as if it had two little attached
appendages. Strange, indeed, it is to-day to read a scientific
book so old that the planet Saturn could be called the "furthest"
planet. But it was the outermost known in Galileo's day, and
for nearly two centuries afterward. Not until 1781 did William
Herschel discover Uranus (p. 59); and Neptune was not disclosed by
the marvellous mathematical perception of Le Verrier until 1846 (p.
61).
Galileo's further observations of Saturn bothered him more and
more. The planet's behavior became much worse as time went on.
"Has Saturn devoured his children, according to the old legend?"
he inquired soon afterward; for the changed positions of earth
and planet in the course of their motions around the sun in their
respective orbits had become such that the ring was seen quite
edgewise, and was, therefore, perfectly invisible to Galileo's
"optic tube." The puzzle remained unsolved by Galileo; it was left
for another great man to find the true answer. Huygens, in 1656,
first announced that the ring _is_ a ring.
The manner in which this announcement was made is characteristic
of the time; to-day it seems almost ludicrous. Huygens published a
little pamphlet in 1656 called "_De Saturni Luna Observatio Nova_"
or, "A New Observation of Saturn's Moon." He gave the explanation
of what had been observed by himself and preceding astronomers in
the form of a puzzle, or "logogriph." Here is what he had to say of
the phenomenon in question:
"aaaaaaa ccccc d eeeee g h iiiiiii llll mm nnnnnnnnn oooo pp q rr s
ttttt uuuuu."
It was not until 1659, three years later, in a book entitled
"_Systema Saturnium_," that Huygens rearranged the above letters in
their proper order, giving the Latin sentence:
"_Annulo cingitur, tenui plano, nusquam cohaerente, ad eclipticam
inclinato._" Translated into English, this sentence informs us that
the planet "is girdled with a thin, flat ring, nowhere touching
Saturn, and inclined to the ecliptic"!
This was a perfectly correct and wonderfully sagacious explanation
of those complex and exasperatingly puzzling phenomena that had
been too difficult for no less a person than Galileo himself. It
was an explanation that _explained_. The reason for its preliminary
announcement in the above manner must have been the following:
Huygens was probably not quite sure of his ground in 1656,
while three years afterward he had become quite certain. By the
publication of the logogriph of 1656 he secured for himself the
credit of what he had done. If any other astronomer had published
the true explanation after 1656, Huygens could have proved his
claim to priority by rearranging the letters of his puzzle. On the
other hand, if further researches showed him that he was wrong,
he would never have made known the true meaning of his logogriph,
and would thus have escaped the ignominy of making an erroneous
explanation. Thus, the method of announcement was comparable in
ingenuity with the Huygenian explanation itself.
We are compelled to pass over briefly the entertaining history of
subsequent observations of the ring, in order to explain the new
work of Keeler and others. Cassini, about 1675, been able to show
that the ring was double; that there are really two independent
rings, with a distinct dark space between them. It was a case of
wheels within wheels. To our own eminent countryman, W. C. Bond,
of Cambridge, Mass., we owe the further discovery (Harvard College
Observatory, November, 1850) of the third ring. This is also
concentric with the other two, and interior to them, but difficult
to observe, because of its much smaller luminosity.
It is almost transparent, and the brilliant light of the planet's
central ball is capable of shining directly through it. For this
reason the inner ring is called the "gauze" or "crape" ring. If we
add to the above details the fact that our modern large telescopes
show slight irregularities in the surface of the rings, especially
when seen edgewise, we have a brief statement of all that the
telescope has been able to reveal to us since Galileo's time.
But of far greater interest than the mere fact of their existence
is the important cosmic question as to the constitution, structure,
and, above all, durability of the ring system. Astronomers often
use the term "stability" with regard to celestial systems like the
ring system of Saturn. By this they mean permanent durability. A
system is stable if its various parts can continue in their present
relationship to one another, without violating any of the known
laws of astronomy. Whenever we study any collection of celestial
objects, and endeavor to explain their motions and peculiarities,
we always seek some explanation not inconsistent with the continued
existence of the phenomena in question. For this there is, perhaps,
no sufficient philosophical basis. Probably much of the great
celestial procession is but a passing show, to be but for a moment
in the endless vista of cosmic time.
However this may be, we are bound to assume as a working theory
that Saturn has always had these rings, and will always have them;
and it is for us to find out how this is possible. The problem has
been attacked mathematically by various astronomers, including
Laplace; but no conclusive mathematical treatment was obtained
until 1857, when James Clerk Maxwell proved in a masterly manner
that the rings could be neither solid nor liquid. He showed,
indeed, that they would not last if they were continuous bodies
like the planets. A big solid wheel would inevitably be torn
asunder by any slight disturbance, and then precipitated upon
the planet's surface. Therefore, the rings must be composed of
an immense number of small detached particles, revolving around
Saturn in separate orbits, like so many tiny satellites.
This mathematical theory of the ring system being once established,
astronomers were more eager than ever to obtain a visual
confirmation of it. We had, indeed, a sort of analogy in the
assemblage of so-called "minor planets" (p. 64), which are known
to be revolving around our sun in orbits situated between Mars and
Jupiter. Some hundreds of these are known to exist, and probably
there are countless others too small for us to see. Such a swarm
of tiny particles of luminous matter would certainly give the
impression of a continuous solid body, if seen from a distance
comparable to that separating us from Saturn. But arguments founded
on analogy are of comparatively little value.
Astronomers need direct and conclusive telescopic evidence, and
this was lacking until Keeler made his remarkable spectroscopic
observation in 1895. The spectroscope is a peculiar instrument,
different in principle from any other used in astronomy; we study
distant objects with it by analyzing the light they send us, rather
than by examining and measuring the details of their visible
surfaces. The reader will recall that according to the modern
undulatory theory, light consists simply of a series of waves.
Now, the nature of waves is very far from being understood in the
popular mind. Most people, for instance, think that the waves of
ocean consist of great masses of water rolling along the surface.
This notion doubtless arises from the behavior of waves when
they break upon the shore, forming what we call surf. When a
wave meets with an immovable body like a sand beach, the wave is
broken, and the water really does roll upon the beach. But this
is an exceptional case. Farther away from the shore, where the
waves are unimpeded, they consist simply of particles of water
moving straight up and down. None of the water is carried by mere
wave-action away from the point over which it was situated at first.
Tides or other causes may move the water, but not simple
wave-motion alone. That this is so can be proved easily. If a chip
of wood be thrown overboard from a ship at sea it will be seen to
rise and fall a long time on the waves, but it will not move.
Similarly, wind-waves are often quite conspicuous on a field of
grain; but they are caused by the individual grain particles moving
up and down. The grain certainly cannot travel over the ground,
since each particle is fast to its own stalk.
But while the particles do not travel, the wave-disturbance
does. At times it is transmitted to a considerable distance from
the point where it was first set in motion. Thus, when a stone
is dropped into still water, the disturbance (though not the
water) travels in ever-widening circles, until at last it becomes
too feeble for us to perceive. Light is just such a travelling
wave-disturbance. Beginning, perhaps, in some distant star, it
travels through space, and finally the wave impinges on our eyes
like the ocean-wave breaking on a sand beach. Such a light-wave
affects the eye in some mysterious way. We call it "seeing."
The spectroscope (p. 21) enables us to measure and count the waves
reaching us each second from any source of light. No matter how far
away the origin of stellar light may be, the spectroscope examines
the character of that light, and tells us the number of waves set
up every second. It is this characteristic of the instrument that
has enabled us to make some of the most remarkable observations of
modern times. If the distant star is approaching us in space, more
light-waves per second will reach us than we should receive from
the same star at rest. Thus if we find from the spectroscope that
there are too many waves, we know that the star is coming nearer;
and if there are too few, we can conclude with equal certainty that
the star is receding.
Keeler was able to apply the spectroscope in this way to the planet
Saturn and to the ring system. The observations required dexterity
and observational manipulative skill in a superlative degree. These
Keeler had; and this work of his will always rank as a classic
observation. He found by examining the light-waves from opposite
sides of the planet that the luminous ball rotated; for one side
was approaching us and the other receding. This observation was,
of course, in accord with the known fact of Saturn's rotation
on his axis. With regard to the rings, Keeler showed in the same
way the existence of an axial rotation, which appears not to have
been satisfactorily proved before, strange as it may seem. But the
crucial point established by his spectroscope was that the interior
part of the rings rotates _faster_ than the exterior.
The velocity of rotation diminishes gradually from the inside to
the outside. This fact is absolutely inconsistent with the motion
of a solid ring; but it fits in admirably with the theory of a
ring comprised of a vast assemblage of small separate particles.
Thus, for the first time, astronomy comes into possession of an
observational determination of the nature of Saturn's rings, and
Galileo's puzzle is forever solved.
THE HELIOMETER
Astronomical discoveries are always received by the public with
keen interest. Every new fact read in the great open book of nature
is written eagerly into the books of men. For there exists a strong
curiosity to ascertain just how the greater world is built and
governed; and it must be admitted that astronomers have been able
to satisfy that curiosity with no small measure of success. But it
is seldom that we hear of the means by which the latest and most
refined astronomical observations are effected. Popular imagination
pictures the astronomer, as he doubtless once was, an aged
gentleman, usually having a long white beard, and spending entire
nights staring at the sky through a telescope.
But the facts to-day are very different. The working astronomer
is an active man in the prime of life, often a young man. He
wastes no time in star-gazing. His observations consist of
exact measurements made in a precise, systematic, and almost
business-like manner. A night's "watch" at the telescope is
seldom allowed to exceed about three hours, since it is found
that more continued exertions fatigue the eye and lead to less
accurate results. To this, of course, there have been many notable
exceptions, for endurance of sight, like any form of physical
strength, differs greatly in different individuals. Astronomical
research does not include "picking out" the constellations, and
learning the Arabic names of individual stars. These things are not
without interest; but they belong to astronomy's ancient history,
and are of little value except to afford amusement and instruction
to successive generations of amateurs.
Among the instruments for carefully planned measurements of
precision the heliometer probably takes first rank. It is at
once the most exquisitely accurate in its results, and the most
fatiguing to the observer, of all the varied apparatus employed by
the astronomer. The principle upon which its construction depends
is very peculiar, and applies to all telescopes, even ordinary
ones for terrestrial purposes. If part of a telescope lens be
covered up with the hand, it will still be possible to see through
the instrument. The glass lens at the end of the tube farthest from
the observer's eye helps to magnify distant objects and make them
seem nearer by gathering to a single point, or focus, a greater
amount of their light than could be brought together by the far
smaller lens in the unaided eye.
The telescope might very properly be likened to an enlarged eye,
which can see more than we can, simply because it is bigger. If
a telescope lens has a surface one hundred times as large as
that of the lens in our eye, it will gather and bring to a focus
one hundred times as much light from a distant object. Now, if
any part of this telescope be covered, the remaining part will,
nevertheless, gather and focus light just as though the whole lens
were in action; only, there will be less light collected at the
focus within the tube. The small lens at the telescope's eye-end is
simply a magnifier to help our eye examine the image of any distant
object formed at the focus by the large lens at the farther end of
the instrument. For of this simple character is the operation of
any telescope: the large glass lens at one end collects a distant
planet's light, and brings it to a focus near the other end of the
tube, where it forms a tiny picture of the planet, which, in turn,
is examined with the little magnifier at the eye-end.