Johannes Kepler
was the first, by his boldness, and in opposition to the ancient philosophers,
to give the order which banished perfectly circular orbits from the sky.
He proved most clearly from Tycho Brahe's observations that confirmation
was provided in the case of the orbit of Mars, and he then noticed that
the ellipticity of the orbit ought of necessity to occur in the case of
Mercury; but in the case of the Sun he was unable to demonstrate the elliptical
shape of its orbit, although he ascribed such to all the planets on the
basis of extremely ingenious, but false, physical arguments. This
view was so agreeable to all the learned that it was readily accepted,
especially as the most learned and famous astronomer Boulliau had largely
perfected it, though he had deduced it from different principles, and had
formed the said elliptical figures from different elements. For this
reason, it will perhaps be useful to give a brief account of what Boulliau
proposed so we may later show in what manner this doctrine of ellipticity
is confirmed (Figure 1).
 Imagine
a scalene cone with its apex at A and having a circular base of diameter
BC. Let its axis be AI; and let the triangle through the axis perpendicular
to the circular base be ABC, so that angle AIC is acute, its complementary
angle obtuse. Draw the straight line EK to subtend an angle at the
apex so that EK is divided into two equal parts at point X by straight
line VT, which is equal to EK, is parallel to the base BC, and cuts the
axis at point Z. It follows that the triangle MXZ will be isosceles,
having MX equal to ZX. Therefore triangle AEK will not be sub-contrary
to triangle ABC. Through straight line EK raise a plane surface
perpendicularly to the plane of triangle ABC, which surface will develop
the ellipse ERK in the section of the cone. The transverse axis
of this ellipse will be EK, its conjugate axis will be ON, its center
will be X, and one of the foci or poles will be the point M on the axis
of the cone. The segment XH being equal to XM, the other focus of
the ellipse will be H.
This
being presupposed, Boulliau then assumes that the Sun is at point H, and
that the planet moves with uniform motion about the axis AMI of the cone
in circles which are always equidistant from the circle of the base BC
of the cone, which circles may be called equant circles. The point
M, or rather, indeed, the entire axis, will be called the center of uniform
motion. Seeing that it is the nature of uniform circular motion
to sweep out equal angles at the center in equal times, and given that
these angles at the center belong to similar circumferences, which are,
nevertheless, proportional to their radii, it thus follows that when the
planet passes through point E belonging to the circle whose semi-diameter
is SE, then its motion will be slowest, because this circle is the smallest
of those described by the celestial body in its proper period, so too,
when the celestial body arrives at point Y and describes the circumference
of circle FG on the cone (which circle passes through the focus M) its
motion will be rapid because this circle is larger.
Later,
when it traverses the peripheries of other circles, the greatest of which
passes through K, its motion will be fastest because it describes the
periphery of the largest circle PK. At position K the planet is
nearest to the pole H. Consequently, from aphelion E to perihelion
K, the celestial body will traverse the peripheries of innumerable circles
[parallel to the base] which increase successively [in diameter], and
for this reason, the uniform motion which sweeps out equal angles about
axis AI in equal times is [also simultaneously] represented on the elliptical
periphery ERK by increases which will have the same relationship to each
other as the radii of the said circles above the minimum [SE = HK].
Although,
by deducing the physical equation as well as the optical equation from
this hypothesis, Boulliau, as pointed out by Seth Ward, omitted certain
things concerning the mean motion; nevertheless, it cannot be denied that
this first invention of his is admirable, ingenious, and praiseworthy.
It is no doubt true that Cosmologists raise two objections; in the first
place, these cones for each planet are fictitious, consequently, it is
not clear how it happens that the planet moves on a certain conical surface
which has no existence in the world; and second, it seems contrary to
fact that these motions are performed about a certain point [M] and a
certain line of uniform motion passing through M, for this point is indivisible
and assumed by the imagination as being in the aether, and has absolutely
no substance or [physical] faculty. Consequently, there is absolutely
no reason why the planet should revolve about the said point and imaginary
line in accordance with a perfectly constant rule and, on the contrary,
move in a quite irregular manner with respect to the very large globe
of the Sun itself, placed at point H, as if the main object of the star
were not to turn round the Sun itself but to turn around the said imaginary,
fantastic point, which is without perfection or faculty. Indeed, this
is such a strong objection that it seems very difficult to answer.
With
regard to the first point, I believe I can not only deal with it satisfactorily
but, perhaps, also give enlightenment on some things concerning the secrets
of nature. In the first place, we shall imagine the planet to move
under two motions, the one circular, the other, on the contrary, linear,
and we shall show from these two motions, taken as elements, that an elliptical
motion can result. Let us assume the Sun is at H and the planet,
to begin with, is at aphelion E but has two motions, the first is orbital
about the Sun, the second is linear in the direction from A towards P.
Let us also assume the said motions are commensurable with each other
in such a manner that when the planet describes a semi-circle starting
from E, it must go from E to P in the same time with a linear motion;
but that during the following semi-circle the planet returns from P to
E. We must assume also that the plane ED of the circular motion
is always inclined with respect to line EP of linear motion, whence it
follows that the planet, by its motion in a straight line, traverses the
circumferences of innumerable circles which are always equidistant from
each other, and if, during this time, the circular motion were uniform,
especially if equal angles were swept out in equal times with respect
to the center, then the planet would describe an elliptical orbit as we
have said above. We see therefore, even though no real cone be supposed
to exist in the world, how it is nevertheless possible for elliptical
motion to take place in exactly the same way as if we assumed a solid
cone of that kind.
It
can be shown that the hypothesis of the aforesaid two motions is possible,
in the first place, by the example of all those planets that have orbits
similar to circles, but never follow the exact periphery of circles.
Furthermore, the curves described by them incline more uniformly to the
plane of the planet's orbit than is postulated by the inclination on which
its latitude depends.
Seeing that
the plane of the solar whirlpool is definitely inclined to the plane of
the ecliptic, then, if it were true, as Kepler thought, that the planets
are in some way caught up and carried round the Sun by the solar rays as
they revolve, the planets should necessarily be carried round along circle
LQ and others parallel to it. In fact, if we imagine the Sun to remain
at the point H and to describe circles parallel to LQ by its own whirlpool,
in such a way that the axis of this whirlpool is raised perpendicularly
to the plane LQ of the whirlpool, the solar rays would move in the plane
of the said circle LQ and others parallel to it; moreover, the planets,
being carried around by these rays, would advance on this plane and, since
in the meantime the planet would execute its proper motion from E to P
and from P to E, it would be obliged to travel first in circle PK, then
in circle LQ, and so on, without remaining for any definite time in any
one of them (because motion on EP is supposed continuous). Consequently,
the planets must be carried round along the peripheries of innumerable
circles of different sizes, which provide a measure of the irregular velocity
with which the planet moves round the Sun; and we see that such a motion,
far from being impossible, is on the whole rational and probable, provided
it does not encounter other objections.
Furthermore,
it must be noted that while the solar body always revolves in the same
place, nevertheless, the equidistant circles described by its rays as
the Sun itself revolves on its axis are always in the same plane with
respect to the situation and extent of the world. Therefore, in
order that the elliptical paths described by the different planets may
be saved, we need only: 1) place the aphelia at various distances
from the Sun in various places in the starry sky and Zodiac in such
a way that the line of aphelion drawn from the Sun to the planet is
more or less inclined with respect to the plane of the Sun's whirlpool;
and 2) ascribe to each of them a motion along the straight line inclined
to the plane of the solar whirlpool, and 3) having the exact value necessary
to form an ellipse suitable to the planet in question, with its periods
and all other circumstances which have been observed in its motion.
One
final difficulty remains, namely, if it is possible for the planet to
move around the focus of the ellipse, or the point of equality, while
the Sun is placed at the other focus or pole. Most certainly,
it seems difficult and incomprehensible that the planet, whether carried
around by its own innate power or by some external power, should be
moved around this point of equality which possesses no power or entity.
This view is indeed refuted by the reality of nonuniform speed of planetary
motion, which speed, according to this hypothesis, should increase in
exactly the same proportion as the distances drawn from the axis of
the cone through the focus increase; or else to the same extent as the
semi-diameters of the equant circles increase. This clearly cannot
occur either in the case where the motion of the planet on the circumferences
of the equant circles is effected by a proper force existing in the
planet itself; or in the case where it results from an external faculty
which propels the planet. For in the former case, the speed of
the planet should always be uniform and constant, whereas in the latter
case, the velocity of the planet should decrease in proportion to the
increase in the semi-diameters and circumferences of the equant circles,
as will be shown later. For these reasons we have been obliged
to abandon the above hypothesis, and if possible, to try to find another
more probable one, or to show that the same elliptical path of planetary
motion can be retained but only on the basis of firmer hypotheses more
conformable to physical arguments.
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