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Unformatted text preview: the planets that they look circular at first glance. For most of the planets one must measure the geometry carefully to determine that they are not circles, but ellipses of small eccentricity. Pluto and Mercury are exceptions: their orbits are sufficiently eccentric that they can be seen by inspection to not be circles. 3. The long axis of the ellipse is called the major axis , while the short axis is called the minor axis (adjacent figure). Half of the major axis is termed a semimajor axis . The length of a semimajor axis is often termed the size of the ellipse. It can be shown that the average separation of a planet from the Sun as it goes around its elliptical orbit is equal to the length of the semimajor axis. Thus, by the "radius" of a planet's orbit one usually means the length of the semimajor axis. For a more detailed investigation of the properties of ellipses, see this java applet...
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- Fall '10