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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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Cool It! Game

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

Kepler's Second Law: The Speeds of Planets

Kepler's second law he again discovered by trial and error. After some experimentation, Kepler realized that the line connecting the planet and the Sun sweeps out equal area in equal time. Look at the...more

Elliptical Orbits

When one object is in orbit around another object, the orbit is usually an elliptical orbit. For example, all of the planets in our Solar System move around the Sun in elliptical orbits. An ellipse is...more

New planetoid named Sedna discovered

Astronomers have announced the discovery of a large new planetoid named Sedna. Mike Brown of the California Institute of Technology, Chad Trujillo of the Gemini Observatory in Hawaii, and David Rabinowitz...more

Native American Astronomy

By 30,000 B.C,. Asian hunter-gatherers had crossed the Bering Strait into North America. These people were the first to inhabit this new land and so they are known as the Native Americans of North America....more

Archeoastronomy

"The movements of the heavenly bodies are an admirable thing, well known and manifest to all peoples. There are no people, no matter how barbaric and primitive, that do not raise up their eyes, take note,...more

The Stones of Carnac

The stones of Carnac, France, are probably the most famous stones markings outside of those found at Stonehenge in England. Where Stonehenge is composed of standing stones, the Carnac area has many different...more

The Cairns of Clava

Not too far from Loch Ness, where the green highlands of Scotland rise and fall there lies three giant cairns of stones. They are called the Balnuaran of Clava. The Balnuaran of Clava, giant tombs encased...more

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.