What is the Moon's orbital eccentricity?

The Moon's orbital eccentricity is 0.0549. This dimensionless number describes how much the lunar orbit departs from a perfect circle: 0 would be circular, while larger values indicate a more elongated ellipse.

Because the orbit is elliptical, the Moon's distance from Earth changes. It averages roughly 363,300 kilometres at perigee and 405,500 kilometres at apogee. The difference affects the Moon's orbital speed: it moves faster near perigee and slower near apogee, as required by Kepler's laws.

The value 0.0549 is a mean or standard orbital value, not an unchanging constant. Solar and terrestrial gravitational perturbations make the Moon's eccentricity vary over time, approximately between 0.026 and 0.077 in commonly cited descriptions. Those changes also influence the apparent size of the Moon and the conditions for eclipses.

A frequent mix-up is 0.0167, which is Earth's orbital eccentricity around the Sun. The value 0.206 belongs to Mercury, while 0.093 is associated with Mars. The Moon's orbit is therefore noticeably less circular than Earth's, though still far from a highly elongated ellipse.