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Ready, Set, SCIENCE!, by the National Research Council, focuses on K-8 science classsrooms. Check out the other publications in our online store, as well as classroom materials.

This animation shows elliptical orbits with different eccentricities. It also shows how the Sun is at the focus of an ellipse, and some of the math behind elliptical orbits.
Click on image for full size
Original animation by Windows to the Universe staff (Randy Russell).

# Elliptical Orbits

You may think that most objects in space that orbit something else move in circles, but that isn't the case. Although some objects follow circular orbits, most orbits are shaped more like "stretched out" circles or ovals. Mathematicians and astronomers call this oval shape an ellipse. All of the planets in our Solar System, many satellites, and most moons move along elliptical orbits.

An ellipse can be very long and thin, or it can be quite round - almost like a circle. Scientists use a special term, "eccentricity", to describe how round or how "stretched out" an ellipse is. If the eccentricity of an ellipse is close to one (like 0.8 or 0.9), the ellipse is long and skinny. If the eccentricity is close to zero, the ellipse is more like a circle.

Earth moves around the Sun in an elliptical orbit. Earth's orbit is almost a perfect circle; its eccentricity is only 0.0167! Pluto has the least circular orbit of any of the planets in our Solar System. Pluto's orbit has an eccentricity of 0.2488.

The Sun isn't quite at the center of a planet's elliptical orbit. An ellipse has a point a little bit away from the center called the "focus". The Sun is at the focus of the ellipse. Because the Sun is at the focus, not the center, of the ellipse, the planet moves closer to and further away from the Sun every orbit. The close point in each orbit is called perihelion. The far away point is called aphelion.

Johannes Kepler, a German astronomer who lived in the early 17th century, discovered some important laws about orbits. Kepler's First Law of Planetary Motion states that planets move in elliptical orbits. His Second Law explains how planets move faster when they are close to the Sun (near perihelion) than when they are far away (near aphelion).

#### Shop Windows to the Universe Science Store!

Our online store includes issues of NESTA's quarterly journal, The Earth Scientist, full of classroom activities on different topics in Earth and space science, ranging from seismology, rocks and minerals, oceanography, and Earth system science to astronomy!

## Traveling Nitrogen Classroom Activity Kit

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## Eccentricity of an Orbit

You may think that most objects in space that orbit something else move in circles, but that isn't the case. Although some objects follow circular orbits, most orbits are shaped more like "stretched...more

## Pluto

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## Kepler's 2nd Law: The Speeds of Planets

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## The Solar System

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## Mechanics

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## Eris - a dwarf planet

Eris is a dwarf planet in our Solar System. Eris was one of the first three objects classified as a dwarf planet, along with Pluto and Ceres. Eris was first spotted in January 2005. Eris is a large sphere...more

## Orionid Meteor Shower

The Orionid meteor shower happens every year in October. Meteor showers are times when you can see many meteors or "shooting stars" in one night. There are several meteor showers each year. Most meteor...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.