Kepler's 1st Law: Orbits are Elliptical



A diagram showing the elliptical orbits of some solar system objects.
Click on image for full size (15K GIF)
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.

A table of orbital data for the planets

Kepler's 1st Law: Orbits are Elliptical



A diagram showing the elliptical orbits of some solar system objects.
Click on image for full size (15K GIF)
With Tycho Brahe's observations, Kepler set out to determine if the paths of the planets 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. A circle has the same diameter whether you measure it across or up and down. But an ellipse has diameters of different lengths. The longest one is called the major axis, and the shortest 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.

Planets do move on ellipses, but they are nearly circular. 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.

A table of orbital data for the planets

Kepler's 1st Law: Orbits are Elliptical



A diagram showing the elliptical orbits of some solar system objects.
Click on image for full size (15K GIF)
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.

A table of orbital data for the planets


Last modified prior to September, 2000 by the Windows Team

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