"Now between the circle and the ellipse there is no other intermediary except a different ellipse. Therefore the path of the planet is an Ellipse ..."  Johannes Kepler ( 1571  1630 )
Source: "The New Astronomy": Astronomia nova ( Heidelberg, 1609 ) Chapter 58, 284  85, KGW 3 366, from School of Mathematics and Statistics,
University of St Andrews, Scotland
Johannes Kepler ( 1571 – 1630 )
German mathematician, astronomer and astrologer
portrait circa 1610  artist unknown
Kepler's 1st Law ( Planetary Law of Ellipses: Sun  centered model ):
All planetary orbits are ellipses with the Sun at one of the two foci.
An ellipse is defined as the locus of points, the sum of whose distance from two fixed points ( the foci ) is constant. That is, an ellipse is a special curve where the sum of the distances from every point on the curve to two other points is a fixed constant.
The ellipse equation is therefore
An ellipse is drawn by using two tacks into a piece of cardboard with a taut string and by moving a pencil held just inside the string.
Using this picture you can draw an ellipse as follows:
The closer together which these points are, the more closely that the ellipse resembles the shape of a circle. In fact, a circle is the special case of an ellipse in which the two foci are at the same location.
Where the two tacks ( foci ) are come closer, the ellipse will approach a circle. In fact, every circle is a special case ellipse where the two foci are identical.
Hence,
e = eccentricity of a circle = ea/a = c/a = 0. See below for this definition.
0 < eccentricity of an ellipse < 1
eccentricity of a straight line = ∞, infinity [ ∞ = lemniscate, latin for ribbon ]
• Sun = red circle, one of two foci; stationary yellow circle is an imaginary 2nd foci
• Planet = moving yellow circle
• Blue arrow = initial condition
• Red arrow = moving planet and is proportional to planet's velocity
• The Perigee: Closer to the sun, the faster the planet passes in its transit orbit
• The Apogee: Further from the sun, the slower the planet passes in its transit orbit
[ note: the last two observations hold because of Kepler's 2nd Law of Equal Areas where a planet sweeps out equal areas during equal intervals of time. ]
Sun at one of the two foci
Major Axis = Rp + Ra = 2a
Minor Axis = 2b
a = Semimajor axis of ellipse
Rp = perihelion radius
Ra = aphelion radius
Rav = a = 1/2(Ra + Rp) = average orbital radius
c =ea = 1/2(Ra  Rp) = interfocal radius
e = eccentricity of ellipse = ea/a = c/a.
source: http://dev.physicslab.org/Document.aspx?doctype=3&filename=OscillatoryMotion_KeplersLaws.xml
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