Image 1 Caption: The parametric equation of an ellipse (with center at origin) dynamically illustrated in the animation.
Features:
x(t)=a*cos(E) ,where E is the angle measured counterclockwise from the positive x-axis to the blue line and in astro jargon is called the eccentric anomaly. From the ellipse equation (with "b" being the semi-minor axis), we find:
(x/a)**2+(y/b)**2 = 1 . (y/b)**2=1-(x/a)**2 = 1-cos(E)**2 = sin(E)**2 (y/b) = ± sin(E) where the valid case is clearly +sin(E) y = b*sin(E)
The ordered pair [x(E)=a*cos(E),y(E)=b*sin(E)] constitute the parametric equation of an ellipse. The radius R to the blue dot is
R=sqrt[(a*cos(E))**2+(b*sin(E))**2] .Only in the case of a circle where a=b is R=a and the parametric equation reduces to [x(E)=a*cos(E),y(t)=a*sin(E)].
Note Apollonius did NOT use formulae in a modern sense. He worked purely in words and diagrams. His understanding of ellipses was based on his study of conic sections which are illustrated in Image 2 and described in the next list item.
Something always to remember aobut
ancient Greek astronomers:
they could NOT measure distances to
astro-bodies, except that
eventually they got a fairly good distance to the
Moon
of ∼ 60
Earth equatorial radii (with R_eq_⊕ = 6378.1370 km).
This meant that could NOT know directly where anything was in
3-dimensional outer space, except for
the Moon.
For more information on their distance problem, see
Ancient Astronomy file:
distance_problem.html.
The approach they took to deal with non-uniform motions in
Heavens
was decompose them into
uniform circular motions
that compounded reproduced to some
accuracy/precision
the motions of the
planets.
In Aristotelian cosmology, the
uniform circular motions
were of compounded
celestial spheres that
carried the planets around.
In Ptolemaic system, the
uniform circular motions
were those on
the deferent and epicycle
of epicycle models.