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In mathematics, eccentricity is a parameter associated with every conic section. It can be thought of as a measure of how much the conic section deviates from being circular. In particular:

To calculate the eccentricity of any ellipse, measure the semi-major axis and call it a. Measure the semi-minor axis and call that measurement b. Now:

In astronomy, the eccentricity of an orbit can be calculated using this formula. For example, the eccentricity of the Earth's orbit is 0.0167.

In popular usage, eccentricity refers to unusual or odd behavior on the part of a person, as opposed to being normal. Eccentric behavior is often considered whimsical or quirky, although it can also be strange and disturbing. American millionaire Howard Hughes, for example was considered to be very eccentric, and stored his urine in glass jars and never cut his hair or nails. Other people may have eccentric taste in clothes, or have eccentric hobbies or collections.

Many of history's most brilliant minds have displayed many unusual behaviors and habits, and eccentricity is sometimes thought to be a sign of genius. However, many eccentrics are crankss, rather than geniuses.

Extravagance is a kind of eccentricity, related to abundance and wastefulness.

List of notable eccentrics