The Moon’s orbit around the Earth is definitively elliptical, not a perfect circle. This slightly stretched path is a fundamental outcome of the laws of physics governing celestial bodies. The resulting non-uniform distance between the Earth and the Moon drives many observable phenomena, from variations in the Moon’s apparent size to the strength of ocean tides.
Defining the Moon’s Elliptical Path
The Moon’s orbit is a defined ellipse, a closed curve where the Earth sits at one of the two focal points. The distance between the two bodies constantly changes during the Moon’s approximately 27.5-day orbital period. The degree to which the orbit deviates from a perfect circle is measured by its eccentricity, which for the Moon averages about 0.0549.
The closest point in the orbit to Earth is called perigee, with a mean distance of approximately 363,300 kilometers. Conversely, the farthest point is known as apogee, where the mean distance stretches out to about 405,507 kilometers. This difference of over 42,000 kilometers, or roughly 11.6%, is substantial and is the direct result of the orbit’s elliptical geometry.
These two extreme points, perigee and apogee, are collectively referred to as the apsides of the orbit. The line connecting them forms the major axis of the ellipse. The changing distance and resulting variations in the Moon’s speed are predictable consequences of this orbital shape.
The Forces That Shape the Orbit
The elliptical path is a direct consequence of the gravitational interaction between the Earth and the Moon, adhering to the principles of Newtonian mechanics. This relationship is governed by the inverse square law of gravity, where the force of attraction weakens rapidly as the distance between the two bodies increases. This varying gravitational pull ensures the Moon follows a curved, non-circular trajectory.
The Moon’s movement within this ellipse is described by Kepler’s Second Law of Planetary Motion, often called the law of equal areas. This law states that a line connecting the Earth and the Moon sweeps out equal areas over equal intervals of time. To achieve this, the Moon must accelerate and move faster when it is nearer to Earth at perigee, where the gravitational pull is strongest.
Conversely, the Moon slows down considerably as it approaches apogee, the farthest point in its orbit, where the Earth’s gravitational influence is at its weakest. This constant adjustment of speed ensures angular momentum is conserved throughout the orbit, establishing the elliptical path as the natural state of a two-body system under gravity.
Orbital Perturbations and Precession
While the Earth-Moon system establishes the basic elliptical shape, the orbit is not a simple, static ellipse due to the influence of other celestial bodies. The gravitational pull of the Sun constantly interferes with the Moon’s path, causing orbital perturbations. The Sun’s gravitational force on the Moon is more than twice as strong as the Earth’s pull, although the Earth’s proximity maintains the Moon in its orbit.
One of the most significant effects of these perturbations is apsidal precession, which is the slow rotation of the Moon’s elliptical path itself. The entire ellipse, including the positions of perigee and apogee, gradually rotates in the same direction as the Moon’s orbit. This rotation means that the orientation of the major axis of the ellipse is constantly changing in space.
This apsidal precession cycle takes approximately 8.85 Earth years to complete one full 360-degree rotation. The Sun’s tidal forces also cause the Moon’s eccentricity to fluctuate, making the ellipse more or less stretched over time. These complex, ever-changing parameters mean that the Moon’s orbit is best described as an elliptical path that is continually being tugged and reshaped by the surrounding solar system.
How the Ellipse Affects Our View of the Moon
The varying distance of the Moon due to its elliptical orbit has observable consequences for people on Earth, most notably affecting the Moon’s apparent size. When the Moon is at perigee, its closest point, it appears larger and brighter in the sky than when it is at apogee. This difference in apparent size can be as much as 14%, and the resulting difference in brightness is about 30%.
When a Full Moon occurs precisely when the Moon is near perigee, it is popularly referred to as a “Supermoon”. Conversely, a Full Moon near apogee is sometimes called a “Micromoon”. This phenomenon is a direct visual demonstration of the orbital geometry and the physical distance change.
The elliptical orbit also has a measurable impact on Earth’s ocean tides. Tidal forces are generated by the Moon’s gravity, and since gravity is distance-dependent, these forces are significantly stronger when the Moon is at perigee. This closer alignment produces above-average tidal ranges, meaning higher high tides and lower low tides. The maximum tidal ranges, sometimes called “King Tides,” occur when a perigee Moon aligns with the Sun and Earth during a New or Full Moon phase.