A diagram showing the elliptical orbits of some solar system objects.
Click on image for full size
Kepler's 1st Law: Orbits are Elliptical
With Tycho Brahe's observations in hand, Kepler set out to determine
if the paths of the planets against the background stars could be
described with a curve. By trial and error, he discovered that an
ellipse with the Sun at one focus could accurately describe the orbit
of a planet about the Sun.
Ellipses are described mainly by the length of their two axes. The
longest one is called the major axis, and the short one is the minor
axis. The ratio of these two lengths determines the
eccentricity (e) of the ellipse; it's a measure of
how elliptical it is. Circles have e=0, and very stretched-out
ellipses have an eccentricity nearly equal to 1.
It's important to note that planets, while they do move on ellipses,
have nearly circular orbits. Comets are a good example of objects in
our solar system that may have very elliptical orbits. Compare the
eccentricities of the objects in the diagram.
Once Kepler figured out that planets move around the Sun on
ellipses, he then discovered another interesting fact about
the speeds of planets as they go around
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