Note that if you follow the Starry Night instructions on the previous page to observe the orbits of Earth and Mars from above, you can also see the shapes of these orbits and how circular they appear.
The elliptical orbits diagram at "Windows to the Universe" includes an image with a direct comparison of the eccentricities of several planets, an asteroid, and a comet. For an animation showing orbits with varying eccentricities, see the eccentricity diagram at "Windows to the Universe." Note that the orbit with an eccentricity of 0.2, which appears nearly circular, is similar to Mercury's, which has the largest eccentricity of any planet in the Solar System. In reality the orbits of most planets in our Solar System are very close to circular, with eccentricities of near 0 (e.g., the eccentricity of Earth's orbit is 0.0167). Studies have shown that astronomy textbooks introduce a misconception by showing the planets' orbits as highly eccentric in an effort to be sure to drive home the point that they are ellipses and not circles. So you can think of a circle as an ellipse of eccentricity 0. In the limiting case where the foci are on top of each other (an eccentricity of 0), the figure is actually a circle. The larger the distance between the foci, the larger the eccentricity of the ellipse. In the image above, the green dots are the foci (equivalent to the tacks in the photo above). The line that is perpendicular to the major axis at its center is called the minor axis, and it is the shortest distance between two points on the ellipse. The line that passes from one end to the other and includes both foci is called the major axis, and this is the longest distance between two points on the ellipse.
However, in an ellipse, lines that you draw through the center vary in length. We know that in a circle, all lines that pass through the center (diameters) are exactly equal in length.
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