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

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA