However, in reality, perfect circles don't exist in nature. All orbits are slightly elliptical, meaning they are slightly oval-shaped. The closer an orbit is to being circular, the lower its eccentricity is. Eccentricity is a measure of how much an orbit deviates from a perfect circle. An eccentricity of 0 represents a perfect circle, while an eccentricity of 1 represents a completely flattened ellipse.

So, while no orbit is truly circular, some orbits are much closer to circular than others. For example, the orbit of the Earth around the Sun has an eccentricity of only 0.0167, meaning it's almost perfectly circular.