A diagram showing the elliptical orbits of some solar system objects.
Click on image for full size
Kepler's 1st Law: Orbits are Elliptical
After many experiments, Kepler discovered that the planets move on
ellipses around the Sun. An ellipse is kind of a stretched out
circle. A real circle has the same width, or diameter, whether you
measure it across or up and down. But an ellipse has diameters of
different lengths. How long the longest diameter is compared to the
shortest one determines the
eccentricity (e) of the
ellipse; it's a measure of how stretched out the ellipse is.
Circles have e=0 because their diameters are all the same. If an
ellipse has one very short diameter, and one very long one, then it is
a very stretched-out ellipse, and has an eccentricity nearly equal
to 1.
Planets do move on ellipses, but they are nearly circular (e
very close to 0). Comets are a good example of objects in our solar
system that may have very elliptical orbits. Compare the
eccentricities and orbits 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
the Sun.
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