What does eccentricity measure in terms of orbits?

Eccentricity specifically measures how elliptical an orbit is compared to a perfect circle. An eccentricity of 0 indicates a circular orbit, while values closer to 1 indicate increasingly elongated or elliptical shapes. The concept of eccentricity quantifies the deviation of an orbit from being circular; therefore, it reflects the degree of ‘flattening’ in the orbital path. This characteristic is crucial in understanding the dynamics of celestial bodies, as it affects their distance from the central body in varying positions throughout their orbit. The other options do not accurately describe eccentricity, as they refer to different properties such as speed or orientation of the orbit rather than its shape.