Great Astronomers - Robert Stawell Ball (the little red hen ebook .TXT) 📗
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at a thousand miles, or at hundreds of thousands of miles. No
doubt the intensity of the attraction becomes weaker with every
increase in the altitude, but that action would still exist to
some extent, however lofty might be the elevation which had been
attained.
It then occurred to Newton, that though the moon is at a distance
of two hundred and forty thousand miles from the earth, yet the
attractive power of the earth must extend to the moon. He was
particularly led to think of the moon in this connection, not only
because the moon is so much closer to the earth than are any other
celestial bodies, but also because the moon is an appendage to the
earth, always revolving around it. The moon is certainly
attracted to the earth, and yet the moon does not fall down; how
is this to be accounted for? The explanation was to be found in
the character of the moon’s present motion. If the moon were left
for a moment at rest, there can be no doubt that the attraction of
the earth would begin to draw the lunar globe in towards our globe.
In the course of a few days our satellite would come down on the
earth with a most fearful crash. This catastrophe is averted by
the circumstance that the moon has a movement of revolution around
the earth. Newton was able to calculate from the known laws of
mechanics, which he had himself been mainly instrumental in
discovering, what the attractive power of the earth must be, so
that the moon shall move precisely as we find it to move. It then
appeared that the very power which makes an apple fall at the
earth’s surface is the power which guides the moon in its orbit.
[PLATE: SIR ISAAC NEWTON’S TELESCOPE.]
Once this step had been taken, the whole scheme of the universe
might almost be said to have become unrolled before the eye of the
philosopher. It was natural to suppose that just as the moon
was guided and controlled by the attraction of the earth, so the
earth itself, in the course of its great annual progress, should
be guided and controlled by the supreme attractive power of the
sun. If this were so with regard to the earth, then it would be
impossible to doubt that in the same way the movements of the
planets could be explained to be consequences of solar attraction.
It was at this point that the great laws of Kepler became
especially significant. Kepler had shown how each of the planets
revolves in an ellipse around the sun, which is situated on one of
the foci. This discovery had been arrived at from the
interpretation of observations. Kepler had himself assigned no
reason why the orbit of a planet should be an ellipse rather than
any other of the infinite number of closed curves which might be
traced around the sun. Kepler had also shown, and here again he
was merely deducing the results from observation, that when the
movements of two planets were compared together, the squares of
the periodic times in which each planet revolved were proportional
to the cubes of their mean distances from the sun. This also
Kepler merely knew to be true as a fact, he gave no demonstration of
the reason why nature should have adopted this particular relation
between the distance and the periodic time rather than any other.
Then, too, there was the law by which Kepler with unparalleled
ingenuity, explained the way in which the velocity of a planet
varies at the different points of its track, when he showed how
the line drawn from the sun to the planet described equal areas
around the sun in equal times. These were the materials with
which Newton set to work. He proposed to infer from these the
actual laws regulating the force by which the sun guides the
planets. Here it was that his sublime mathematical genius came
into play. Step by step Newton advanced until he had completely
accounted for all the phenomena.
In the first place, he showed that as the planet describes equal
areas in equal times about the sun, the attractive force which the
sun exerts upon it must necessarily be directed in a straight line
towards the sun itself. He also demonstrated the converse truth,
that whatever be the nature of the force which emanated from a
sun, yet so long as that force was directed through the sun’s
centre, any body which revolved around it must describe equal
areas in equal times, and this it must do, whatever be the actual
character of the law according to which the intensity of the force
varies at different parts of the planet’s journey. Thus the first
advance was taken in the exposition of the scheme of the universe.
The next step was to determine the law according to which the
force thus proved to reside in the sun varied with the distance of
the planet. Newton presently showed by a most superb effort of
mathematical reasoning, that if the orbit of a planet were an
ellipse and if the sun were at one of the foci of that ellipse,
the intensity of the attractive force must vary inversely as the
square of the planet’s distance. If the law had any other
expression than the inverse square of the distance, then the orbit
which the planet must follow would not be an ellipse; or if
an ellipse, it would, at all events, not have the sun in the
focus. Hence he was able to show from Kepler’s laws alone that
the force which guided the planets was an attractive power
emanating from the sun, and that the intensity of this attractive
power varied with the inverse square of the distance between the
two bodies.
These circumstances being known, it was then easy to show that the
last of Kepler’s three laws must necessarily follow. If a number
of planets were revolving around the sun, then supposing the
materials of all these bodies were equally affected by
gravitation, it can be demonstrated that the square of the
periodic time in which each planet completes its orbit is
proportional to the cube of the greatest diameter in that orbit.
[PLATE: SIR ISAAC NEWTON’S ASTROLABE.]
These superb discoveries were, however, but the starting point
from which Newton entered on a series of researches, which
disclosed many of the profoundest secrets in the scheme of
celestial mechanics. His natural insight showed that not only
large masses like the sun and the earth, and the moon, attract
each other, but that every particle in the universe must attract
every other particle with a force which varies inversely as the
square of the distance between them. If, for example, the two
particles were placed twice as far apart, then the intensity of
the force which sought to bring them together would be reduced to
one-fourth. If two particles, originally ten miles asunder,
attracted each other with a certain force, then, when the distance
was reduced to one mile, the intensity of the attraction between
the two particles would be increased one-hundred-fold. This
fertile principle extends throughout the whole of nature. In some
cases, however, the calculation of its effect upon the actual
problems of nature would be hardly possible, were it not for
another discovery which Newton’s genius enabled him to accomplish.
In the case of two globes like the earth and the moon, we must
remember that we are dealing not with particles, but with two
mighty masses of matter, each composed of innumerable myriads of
particles. Every particle in the earth does attract every
particle in the moon with a force which varies inversely as the
square of their distance. The calculation of such attractions is
rendered feasible by the following principle. Assuming that the
earth consists of materials symmetrically arranged in shells
of varying densities, we may then, in calculating its attraction,
regard the whole mass of the globe as concentrated at its centre.
Similarly we may regard the moon as concentrated at the centre of
its mass. In this way the earth and the moon can both be regarded
as particles in point of size, each particle having, however, the
entire mass of the corresponding globe. The attraction of one
particle for another is a much more simple matter to investigate
than the attraction of the myriad different points of the earth
upon the myriad different points of the moon.
Many great discoveries now crowded in upon Newton. He first of
all gave the explanation of the tides that ebb and flow around
our shores. Even in the earliest times the tides had been shown
to be related to the moon. It was noticed that the tides were
specially high during full moon or during new moon, and this
circumstance obviously pointed to the existence of some connection
between the moon and these movements of the water, though as to
what that connection was no one had any accurate conception until
Newton announced the law of gravitation. Newton then made
it plain that the rise and fall of the water was simply a
consequence of the attractive power which the moon exerted
upon the oceans lying upon our globe. He showed also that
to a certain extent the sun produces tides, and he was able
to explain how it was that when the sun and the moon both
conspire, the joint result was to produce especially high
tides, which we call “spring tides”; whereas if the solar
tide was low, while the lunar tide was high, then we had the
phenomenon of “neap” tides.
But perhaps the most signal of Newton’s applications of the
law of gravitation was connected with certain irregularities
in the movements of the moon. In its orbit round the earth
our satellite is, of course, mainly guided by the great
attraction of our globe. If there were no other body in the
universe, then the centre of the moon must necessarily perform an
ellipse, and the centre of the earth would lie in the focus of
that ellipse. Nature, however, does not allow the movements to
possess the simplicity which this arrangement would imply, for the
sun is present as a source of disturbance. The sun attracts the
moon, and the sun attracts the earth, but in different degrees,
and the consequence is that the moon’s movement with regard to the
earth is seriously affected by the influence of the sun. It is
not allowed to move exactly in an ellipse, nor is the earth
exactly in the focus. How great was Newton’s achievement in the
solution of this problem will be appreciated if we realise that he
not only had to determine from the law of gravitation the nature
of the disturbance of the moon, but he had actually to construct
the mathematical tools by which alone such calculations could be
effected.
The resources of Newton’s genius seemed, however, to prove
equal to almost any demand that could be made upon it. He saw
that each planet must disturb the other, and in that way he was
able to render a satisfactory account of certain phenomena which
had perplexed all preceding investigators. That mysterious
movement by which the pole of the earth sways about among the
stars had been long an unsolved enigma, but Newton showed that the
moon grasped with its attraction the protuberant mass at the
equatorial regions of the earth, and thus tilted the earth’s
axis in a way that accounted for the phenomenon which had been
known but had never been explained for two thousand years. All
these discoveries were brought together in that immortal work,
Newton’s Principia.”
Down to the year 1687, when the “Principia” was published, Newton
had lived the life of a recluse at Cambridge, being entirely
occupied with those transcendent researches
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