Couldn't find element LayerAd

Error finding content

Kepler's 1st Law: Orbits are Elliptical - Windows to the Universe

Shop Windows to the Universe

Learn about planets outside our solar system through Exoplanets and Alien Solar Systems by Tahir Yaqoob, Ph.D., a book in our online store book collection.
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.

Shop Windows to the Universe Science Store!

The Summer 2010 issue of The Earth Scientist, available in our online store, includes articles on rivers and snow, classroom planetariums, satellites and oceanography, hands-on astronomy, and global warming.

Windows to the Universe Community

News

Opportunities

You might also be interested in:

Traveling Nitrogen Classroom Activity Kit

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

Kepler's 2nd Law: The Speeds of Planets

Kepler realized that the line connecting the planet and the Sun sweeps out equal area in equal time. Look at the diagram to the left. What Kepler found is that it takes the same amount of time for the...more

Elliptical Orbits

Do you think Earth moves around the Sun in a circle? That is almost true, but not quite. The shape of Earth's orbit isn't quite a perfect circle. It is more like a "stretched out" circle or an...more

New planetoid named Sedna discovered

Astronomers have found a new object in our Solar System. The new object is named Sedna. Astronomers are calling Sedna a "planetoid". A planetoid is smaller than a planet but bigger than asteroids or comets....more

Native American Astronomy

People have been living in North America for a long, long time. The first people to live there were the Native Americans. They didn't have clocks or calendars so they watched tides, the Sun, the Moon,...more

Archeoastronomy

Man has always observed the sky. By watching the Sun and Moon, early man could tell what season was coming next. They had to know this to be able to farm and hunt. Archeoastronomy started in the 1960's...more

The Stones of Carnac

The stones of Carnac, France, are very famous because there are a lot of them and because they are so old! The oldest stones found in Carnac are from about 4,500 B.C. That's older than the stones at Stonehenge!...more

The Cairns of Clava

You may have heard of the lake called Loch Ness, where people think they've seen the Loch Ness monster. Near Loch Ness there are three giant stone tombs you may not have heard of...they are called the...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF