What is the eccentricity ($e$) value defining a perfectly circular orbit path?

Answer

The eccentricity ($e$) is the unitless parameter used to quantify the deviation of an orbit from a perfect circle. By definition, a perfect circle represents the absolute minimum deviation possible. This minimum deviation corresponds mathematically to an eccentricity value of exactly zero ($e = 0$). When $e$ is zero, the distance from the orbiting body to the central focus remains constant throughout the entire orbit, which is the geometric definition of a circle. Any value greater than zero introduces elongation or deviation from this perfect circularity. For instance, values between 0 and 1 define ellipses, and $e=1$ defines a parabola, signifying that $e=0$ is the unique marker for the circular case.

What is the eccentricity ($e$) value defining a perfectly circular orbit path?

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