What is the fundamental property that quantifies an orbit's deviation from circularity?
Answer
Eccentricity
The primary mathematical measure used to characterize and discuss the shape of an orbit, specifically how much it deviates from a perfect circle, is called eccentricity, often denoted by the symbol $e$. This unitless parameter is the defining characteristic used in physics and mathematics to categorize orbits as circular ($e=0$), elliptical ($0 < e < 1$), parabolic ($e=1$), or hyperbolic ($e>1$). Although the term 'ellipticity' exists and may sometimes be used synonymously or defined based on axis ratios, 'eccentricity' remains the universally preferred and unambiguous descriptor for quantifying this specific degree of non-circularity in orbital mechanics.
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