Why the Moon's orbit is not a simple circle
If the Moon orbited a perfectly spherical Earth with no other forces, its path would be a fixed ellipse. But the Moon's orbit is noticeably elliptical, meaning its distance from Earth changes over the course of a month. Historical and modern analyses show that this changing distance is not just a matter of the ellipse's shape: the Sun's gravity perturbs the Earth-Moon system, adding further variations to the Moon's distance and to its position along its orbit [1]. The result is that the Moon's angular size in our sky changes by at least a few arc minutes, a variation that ancient observers could have detected with careful naked-eye measurements [1].
The ellipticity and the solar perturbation together produce what astronomers call anomalies—departures from a simple uniform motion. These anomalies in the Moon's longitude and distance are correlated with the Moon's phase, meaning the Moon's apparent position and distance shift in a pattern tied to whether it is new, full, or in between [1]. So even the basic shape of the orbit is not fixed; it is continually nudged by the Sun.
The Sun and tides keep changing the orbit
The Sun's gravitational pull on the Moon is not negligible. Over long periods, this third-body perturbation alters the Moon's orbit in ways that are difficult to capture with simple equations. A recent analytical solution for the lunar orbit explicitly treats the third-body perturbation from the Sun and also accounts for tidal effects, which slowly push the Moon away from Earth [5]. By fitting their solution to precise long-term ephemerides, the authors recovered the receding speed of the Moon due to tides, showing that tidal interactions are an essential part of the orbit's long-term evolution [5].
Tidal effects arise because Earth's rotation drags its tidal bulge slightly ahead of the Moon, transferring angular momentum to the Moon and causing it to spiral outward. This is a slow but persistent change, and it means the Moon's orbit is not static over geological time. The combination of the Sun's periodic tugs and the slow tidal recession makes the lunar orbit a moving target, requiring increasingly sophisticated models to predict its path accurately [5].
What this means for spacecraft and prediction
The complexity of the Moon's orbit is not just an academic curiosity; it directly affects spacecraft operations. When Korea's Danuri orbiter arrived at the Moon, the flight dynamics team encountered both expected and unexpected issues during lunar orbit insertion. They had to make real-time updates to their design reference mission, and in the end they achieved the final mission orbit using only three orbit insertion burns instead of the planned five plus a trim maneuver [2]. This operational flexibility was necessary because the actual dynamical environment around the Moon did not perfectly match the pre-flight plan.
Part of that environment includes the Moon's own gravity field, which is lumpy and non-spherical. The Moon's J2 perturbation—a measure of how its gravity field deviates from a perfect sphere—causes orbits around the Moon to change over time. Engineers designing frozen orbits, which minimize the variation of orbital eccentricity, must account for these perturbations to keep a satellite stable with minimal maintenance [4]. The same J2 effect, along with the Moon's orbital eccentricity, contributes to the complex dynamical environment that spacecraft must navigate [3].
For precise prediction of spacecraft trajectories in the Earth-Moon region, higher-fidelity models are needed. A full ephemeris model that includes the gravitational fields of perturbing bodies and the eccentricity of the Moon's orbit alters trajectories compared to simpler models, and researchers have shown that attitude (orientation) can be predicted in such a model by coupling translational and rotational motion [3]. This coupling adds another layer of complexity: a spacecraft's orientation affects its motion, and vice versa, in a way that is sensitive to the Moon's orbital eccentricity and external perturbations [3].
Sources used in this answer
A Mixture of Ancient and Modern Understanding Concerning the Distance and Motion of the Moon
The Moon's distance varies due to the ellipticity of its orbit and solar perturbations, and these variations are correlated with the Moon's phase.
Lunar Orbit acquisition of the Korea Pathfinder lunar orbiter: Design reference vs actual flight results
Korea's Danuri orbiter required real-time adjustments to its lunar orbit acquisition plan and achieved its final orbit with only three insertion burns instead of the planned five plus a trim maneuver.
Orbit and Attitude Coupling in the Full Higher-Fidelity Ephemeris Model within the context of the Geometric Mechanics Framework
The eccentricity of the Moon's orbit and gravitational perturbations from other bodies create a complex dynamical environment in which spacecraft attitude can be predicted by coupling translational and rotational motion in a full ephemeris model.
Design of Elliptical Lunar Frozen Orbit Considering Lunar J2 Perturbations
Frozen orbits around the Moon can exist even at low eccentricity when lunar J2 perturbations and the eccentricity of the perturbed body are considered.
The Main Problem of Lunar Orbit Revisited
An analytical solution of the lunar orbit that accounts for third-body perturbations and tidal effects can recover the Moon's receding speed from Earth.