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This animation shows the shapes of some elliptical orbits. These orbits have different eccentricities. If an ellipse has a high eccentricity, is it round like a circle or long like an oval?
Click on image for full size
Original animation by Windows to the Universe staff (Randy Russell).

# 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 out" circles or ovals. Mathematicians and astronomers call this oval shape an ellipse.

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.

The eccentricity of Earth's orbit is very small, so Earth's orbit is nearly circular. Earth's orbital eccentricity is less than 0.02. The orbit of Pluto is the most eccentric of any planet in our Solar System. Pluto's orbital eccentricity is almost 0.25. Many comets have extremely eccentric orbits. Halley's Comet, for instance, has an orbital eccentricity of almost 0.97!

The Sun is not at the center of an elliptical orbit. It is a little off to one side, at a point called a "focus" of the ellipse. Because of this offset 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. If an orbit has a large eccentricity, the difference between the perihelion distance and the aphelion distance will also be large. Earth is only 3% further from the Sun at aphelion than it is at perihelion. Pluto's aphelion distance from the Sun is 66% greater than its perihelion distance.

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## Traveling Nitrogen Classroom Activity Kit

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

## 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...more

## Pluto

Pluto is a frigid ball of ice and rock that orbits far from the Sun on the frozen fringes of our Solar System. Considered a planet, though a rather odd one, from its discovery in 1930 until 2006, it was...more

## 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

## The Magnetic Field

The force of magnetism causes material to point along the direction the magnetic force points. As shown in the diagram to the left, the force of magnetism is illustrated by lines, which represent the force....more

## Planetary Magnets

The Earth is a good example of a planetary dipole, where the lines of force point in a direction out of the South (magnetic) Pole and into the North (magnetic) Pole. Planets can also show evidence of quadrupoles...more

## Detecting Planetary Magnetism

A magnetometer is an instrument for measuring magnetic fields. Many spacecraft carry magnetometers to measure the magnetic fields around planets. When a spacecraft makes those measurements, what do the...more

## Tools for Math and Science

Some ideas are used throughout the sciences. They are "tools" that can help us solve puzzles in different fields of science. These "tools" include units of measurement, mathematical formulas, and graphs....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.