A diagram showing the elliptical orbits of some solar system objects.
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# 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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