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Dig into Montana Before History: 11K Years of Hunter-Gatherers in the Rockies and Plains by D. H. MacDonald, Ph.D. See our online store book collection.

# Orbit Shapes Interactive Animation

This animation explains about the shapes and sizes of orbits.

Orbits are ellipses. An ellipse is an oval, or a "stretched out" circle. An ellipse can be like a circle, or it can be long and skinny. Astronomers use a special word to describe the shape of an orbit. That word is "eccentricity". If an orbit is almost a circle, the eccentricity is small. If an orbit is a long, skinny ellipse, the eccentricity is bigger. A small eccentricity would be a number close to zero, like 0.1 or 0.2. A bigger eccentricity would be a number close to one, like 0.8 or 0.9. Eccentricity describes the shape of an orbit.

What about the size of an orbit? Think about a circle. The distance from the center of a circle to the edge is called the radius. The distance from the center of an ellipse to the edge is called the "semi-major axis". We measure the semi-major axis of an ellipse across the long direction of the ellipse.

Use the sliders in the animation (below) to change the shape and size of the orbit of "your planet". You can also see the orbit of Earth so you can compare your planet's orbit with Earth's orbit.

(Note: If you cannot see the animation below, or it is not working, you may need to download the latest Flash player.)

An astronomical unit (AU) is the length of the semi-major axis of Earth's orbit. AUs are used to measure distances in our Solar System.

Notice how a planet with an elliptical orbit moves closer to and further away from the Sun. The point of closest approach to the Sun is called perihelion. The furthest point is called aphelion.

An astronomer named Johannes Kepler figured out three important laws about the orbits of planets.

#### 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!

## Cool It! Game

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

## Eccentricity of an Orbit

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

## Dwarf Planets

In 2006 the International Astronomical Union (IAU) approved a new classification scheme for planets and smaller objects in our Solar System. Their scheme includes three classes of objects: "small solar...more

## Makemake: a Dwarf Planet

Makemake is a dwarf planet in our Solar System. Makemake was discovered in March 2005 by a team of astronomers led by Mike Brown. Makemake officially became a dwarf planet in July 2008. There were three...more

## The Magnetic Field

The force of magnetism causes material to point along the direction the magnetic force points. Here's another picture of how this works. This picture shows where the magnetic poles of the Earth are to...more

## Planetary Magnets

The Earth has a dipole magnetic field. This is when magnetic field lines point in a direction out of the South Pole and into the North Pole. Planets can also have quadrupoles (4 poles) and octupoles (8-poles)....more

## Magnetic Fields Near Planets

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

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.