This interactive animation shows two important features of orbits - shape and size.

Orbits are ellipses. An ellipse can be like a circle, or it can be long and skinny. Mathematicians and astronomers use the term "eccentricity" to describe the shape of an orbit. An orbit shaped almost like a circle has a low eccentricity close to zero. A long, skinny orbit has a high eccentricity, close to one.

If you want to tell someone how big a circle is, you tell her or him the length of the radius (or the diameter) of the circle. The "semi-major axis" of an ellipse is like the radius of a circle. Imagine a long, skinny ellipse with a high eccentricity. Draw a line from one end of the ellipse to the other, through the middle, along the long direction of the ellipse. The distance from the center of the ellipse to one end of the line is the semi-major axis.

Use the sliders in the interactive animation (below) to change the shape and size of the orbit of "your planet". Earth's orbit is shown for comparison.