The Moon’s orbit around Earth is elliptical, though only slightly so. Its eccentricity sits at roughly 0.055, which means the orbit is close to circular but measurably stretched, with the Earth-Moon distance varying by about 50,000 km over each monthly cycle. That modest departure from a perfect circle has real consequences for tides, eclipses, and the apparent size of the Moon, and the full story of the orbit’s shape turns out to be considerably more interesting than a simple “yes.”
How Elliptical the Orbit Actually Is
An eccentricity of 0.055 means the Moon’s orbit is only slightly squished compared to a perfect circle. At its closest approach to Earth, called perigee, the Moon is about 363,000 km away. At its farthest point, apogee, that distance stretches to about 405,000 km. The difference works out to roughly 12 percent.
If you drew the orbit to scale on a sheet of paper, you’d have a hard time seeing that it isn’t circular. Many textbook diagrams wildly exaggerate the elliptical shape to make the concept visible, which leaves most people with the impression that the Moon’s orbit is far more elongated than it actually is. In reality, it’s one of the rounder orbits in the solar system.
Even so, the eccentricity is large enough to detect with surprisingly simple equipment. A paper in the American Journal of Physics described measuring it using nothing more than a piece of cardboard with a small hole slid along a yardstick, arriving at an approximate eccentricity of 0.039. The measurement was rough, but the point is striking: the Moon’s changing apparent size over a month is visible to careful observation without any telescope at all.1American Journal of Physics. Determining the eccentricity of the Moon’s orbit without a telescope
What You Actually See Because of the Ellipse
The 12 percent distance variation translates into roughly a 14 percent difference in apparent brightness and about a 7 percent difference in apparent diameter between perigee and apogee. When a full moon happens to coincide with perigee, it earns the popular label “supermoon.” A supermoon versus a full moon at apogee (sometimes called a “micromoon”) is a real difference, but a subtle one. Most people cannot tell by looking up at the sky on a given night; the comparison only becomes obvious in side-by-side photographs.
Where the ellipse becomes unmistakable is during eclipses. Because the Moon’s apparent size changes with distance, a solar eclipse that happens near perigee can produce a total eclipse, with the Moon completely covering the Sun’s disk. The same geometry near apogee produces an annular eclipse, where the Moon appears too small to cover the Sun and a ring of sunlight remains visible around the edge. The difference between a total and annular eclipse is a direct, observable consequence of that 12 percent distance swing.
How Astronomers Figured This Out
The Moon’s non-uniform motion across the sky caught the attention of ancient observers long before anyone had a word for “ellipse.” Hipparchos and Ptolemy identified two major anomalies in the Moon’s motion. One corresponded to a steady rotation of the line connecting perigee and apogee (the line of apses). The other involved fluctuations in both that line and the eccentricity itself, an irregularity later named “evection” by Bouillau. Tycho Brahe later discovered a further anomaly he called the “variation.”2Astronomy & Geophysics. Success and failure in Newton’s lunar theory
In the 17th century, Jeremiah Horrocks proposed modeling the Moon’s orbit as a rotating ellipse, a description that worked well for the predictions astronomers needed at the time.2Astronomy & Geophysics. Success and failure in Newton’s lunar theory Newton later tried to derive all these perturbations from his gravitational theory. He had mixed success. The lunar orbit was famously one of the hardest problems in classical mechanics, and Newton reportedly said it was the only problem that gave him headaches. The difficulty wasn’t that the orbit was elliptical; it was that the Sun, Earth, and Moon form a three-body system, and the Sun’s gravitational influence keeps warping the ellipse in complicated ways.
Measuring the Orbit With Lasers
The modern era of lunar distance measurement began when Apollo astronauts left retroreflector arrays on the Moon’s surface. Scientists on Earth fire short laser pulses at these reflectors and time how long the light takes to bounce back. The Lunar Laser Ranging Experiment at McDonald Observatory initially achieved an accuracy of about 1 nanosecond in round-trip travel time, corresponding to roughly 15 cm in one-way distance. Later improvements using shorter-pulse lasers pushed accuracy toward 2 to 3 cm.3PubMed. The Lunar Laser Ranging Experiment
That data was used to build a precise mathematical model of the Moon’s position, generated by directly computing the gravitational interactions of the Moon, Earth, and all the planets. Early fits matched the range to the three Apollo retroreflectors to an accuracy of about 5 meters, and the precision has improved enormously since.3PubMed. The Lunar Laser Ranging Experiment This level of tracking means scientists can monitor not just the average orbital shape but its month-to-month fluctuations, the slow precession of the orbit, and even the rate at which the Moon is receding from Earth (about 3.8 cm per year).
Why the Orbit Isn’t a Clean Ellipse
Describing the Moon’s orbit as “an ellipse” is the right starting point but a significant simplification. The Sun’s gravitational pull on the Moon is roughly 2.2 times stronger than Earth’s gravitational pull on the Moon.4GeoScienceWorld. Links of planetary energetics to moon size, orbit, and planet spin That number surprises most people. If the Sun pulls on the Moon more than twice as hard as Earth does, why doesn’t the Moon fly away? The answer is that both Earth and the Moon are falling around the Sun together. What matters for the Moon’s orbit around Earth is the difference in the Sun’s pull at different points in the orbit, and that difference is relatively small. But it’s not zero, and it continuously distorts the ellipse.
The solar influence produces several effects that make the real orbit more complex than a textbook ellipse:
- Apsidal precession: The perigee point doesn’t stay put. It rotates around Earth, completing a full circuit roughly every 8.85 years.
- Nodal precession: The plane of the orbit wobbles, completing a cycle about every 18.6 years.
- Eccentricity variation: The orbit’s eccentricity fluctuates over time, sometimes becoming more elongated and sometimes rounder.
The solar pull also causes orbital elongation and drives a roughly 1,000 km radial monthly excursion of the Earth-Moon barycenter within Earth’s mantle.4GeoScienceWorld. Links of planetary energetics to moon size, orbit, and planet spin So at any given moment the orbit is approximately elliptical, but that ellipse is always shifting its orientation, eccentricity, and tilt. Astronomers sometimes describe the trajectory as a “perturbed, precessing ellipse,” which is a more honest label than just “ellipse.”
What the Elliptical Orbit Means for Tides
The Moon’s varying distance from Earth directly affects tidal ranges. When the Moon is near perigee, its gravitational pull is stronger and tidal bulges are larger. When it’s near apogee, tidal forces weaken. Perigean spring tides, which occur when perigee coincides with a new or full moon, can be roughly 20 to 30 percent larger than average tides.
The interplay between the elliptical orbit and other lunar cycles creates longer-period patterns in extreme tides. The well-documented 4.4-year modulation in extreme high tides arises because the Moon’s declination effects and perigean effects come in and out of phase with each other.5Journal of Geophysical Research: Oceans. The Semiannual and 4.4-Year Modulations of Extreme High Tides When perigee lines up with the Moon passing directly over lower latitudes during a spring tide, the tidal bulge hits its maximum. These alignments drift in and out of sync over roughly 4.4 years. A semiannual pattern in extreme tides has also been identified as part of the same family of alignment effects.5Journal of Geophysical Research: Oceans. The Semiannual and 4.4-Year Modulations of Extreme High Tides
For coastal communities, this is more than an astronomical curiosity. The 4.4-year tidal cycle can push already-high tides over flooding thresholds, and as sea levels rise, each cycle’s peak pushes a little higher. Understanding when these extreme tide windows will occur helps cities and engineers plan for nuisance flooding events that are increasingly common.
How the Orbit Has Evolved Over Billions of Years
The Moon’s orbit hasn’t always had the shape and size it has today. Tidal friction, the process by which ocean tides on Earth slowly transfer rotational energy from Earth’s spin into the Moon’s orbit, has been pushing the Moon outward for billions of years. Early in the Earth-Moon system’s history, the Moon was dramatically closer.
Models of tidal dissipation suggest that 4.5 billion years ago, the Moon orbited at roughly 38 to 53 Earth radii, compared to about 60 Earth radii today. At that distance, a lunar month lasted somewhere between 330 and 550 hours, and Earth’s day was only 12 to 18 hours long.6Reviews of Geophysics. Secular effects of oceanic tidal dissipation on the Moon’s orbit and the Earth’s rotation The range in those estimates reflects uncertainty about how efficiently the ancient oceans dissipated tidal energy, which depends on the shape and depth of ocean basins that no longer exist.
The eccentricity of the early orbit was also likely different, though calculating exactly how different is tricky. Research on the secular changes of an elliptical satellite orbit under tidal friction shows that the eccentricity at earlier epochs depends heavily on assumptions about how Earth’s core evolved and how much tidal energy the Moon’s own interior absorbed.7Icarus. The earliest past of the Earth-Moon system What is clear is that both the orbital distance and shape have been evolving continuously since the Moon formed, and will continue to evolve. The Moon is still receding, Earth is still slowing down, and the orbital eccentricity is still subject to the competing influences of tidal friction and solar perturbation.
Independent geological evidence supports these models. Fossils of ancient coral preserve daily and monthly growth bands that record the number of days in a year and days in a month hundreds of millions of years ago. Tidal rhythmites in ancient sedimentary rocks tell a similar story. Both lines of evidence confirm that days were shorter and months contained more days in the geological past, consistent with a Moon that was closer and an Earth that spun faster.
The Barycenter Inside Earth
One underappreciated consequence of the Moon’s orbit involves the center of mass of the Earth-Moon system, the barycenter. Because Earth is about 81 times more massive than the Moon, this point sits inside Earth’s body, roughly 1,700 km below the surface. Both Earth and the Moon orbit this shared center of mass, which means Earth doesn’t sit still at the geometric center of the Moon’s orbit. Instead, Earth traces a small loop each month.
As the Moon moves along its elliptical path, getting closer and farther each month, the barycenter’s position inside Earth shifts. This produces a roughly 1,000 km radial monthly excursion within the mantle.4GeoScienceWorld. Links of planetary energetics to moon size, orbit, and planet spin The displacement is small relative to Earth’s radius, but it’s not negligible for precision work. Spacecraft navigation, satellite geodesy, and even some experiments in fundamental physics must account for this monthly wobble. The effect would be slightly different if the orbit were perfectly circular, since the barycenter would then trace a smoother path. The eccentricity adds a varying component to the motion, stretching and compressing the loop each month.
The Moon’s Path From the Sun’s Perspective
Everything discussed so far describes the Moon’s orbit as seen from Earth, which is the natural frame for questions about tides, eclipses, and lunar distance. But there’s a genuinely different way to look at the Moon’s trajectory: from the Sun’s point of view.
From the Sun, the Moon traces a path around the Sun that is always concave toward the Sun. Unlike the way we usually picture a moon looping around a planet in wide arcs, the Moon’s path in solar-centered coordinates is a gently wavy, roughly circular orbit around the Sun. The monthly trips around Earth show up as small scallops on that solar orbit. The Sun pulls on the Moon more than twice as hard as Earth does, which is precisely why the solar-centered path is so smooth. The Moon is, in a meaningful sense, a companion of Earth in solar orbit rather than a captive body whose trajectory is dominated by Earth’s gravity.
Whether the Moon’s orbit is “really” elliptical, then, depends on your frame of reference. From Earth, the answer is an unambiguous yes, and the elliptical shape matters for everything from tide prediction to eclipse planning. From the Sun, the orbit around Earth looks more like a small periodic deviation superimposed on a nearly circular solar orbit. Astronomers default to the Earth-centered ellipse because that is the useful frame for most practical purposes, but keeping the Sun-centered view in mind helps explain why solar perturbations are powerful enough to make the orbit so much messier than a clean Keplerian ellipse.