What term quantifies the degree to which a planet's orbit deviates from being a perfect circle?
Answer
Eccentricity ($e$)
Eccentricity, denoted by the variable $e$, is the specific measurement used to describe the 'squashedness' or deviation of an orbital path from perfect circularity. A value of $e=0$ signifies a mathematically perfect circle because the two foci coincide, meaning the distance to the central body remains constant. If $0 < e < 1$, the path is elliptical, as is the case for all gravitationally bound planets. If $e$ were to equal or exceed 1, the path would be an open trajectory, such as a parabola or hyperbola, meaning the object would escape the gravitational influence of the star and not return.
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