The Moon is Earth’s sole natural satellite. Most people picture the Moon orbiting Earth in a simple circle, but the reality is far more complex and dynamic. The gravitational relationship between Earth and Moon results in a path that is slightly oval, constantly shifting, and influenced by the distant Sun. Understanding the true shape of the Moon’s orbit requires appreciating this intricate celestial geometry.
The Primary Ellipse
The Moon’s path is geometrically defined as an ellipse, not a perfect circle. This elliptical shape means the distance between the Earth and the Moon constantly changes throughout the lunar cycle. The deviation from a perfect circle is quantified by eccentricity, which for the Moon’s orbit averages about 0.0549.
The closest point to Earth is known as perigee, averaging approximately 363,300 kilometers. Conversely, the farthest point is apogee, where the Moon recedes to about 405,500 kilometers. This difference of over 42,000 kilometers means the Moon’s apparent size in the sky varies by roughly 12% between these two extremes. The elliptical geometry is the fundamental description of the Moon’s motion relative to Earth.
Gravitational Forces Shaping the Orbit
While Earth’s gravity is the primary force binding the Moon, the Sun’s gravitational influence constantly perturbs the elliptical path. The Sun’s pull on the Moon is more than twice as strong as Earth’s, but Earth’s closer proximity makes its force the dominant one for holding the Moon in orbit. This competition between forces prevents the Moon’s orbit from being a fixed, stable ellipse.
The Moon’s speed varies, traveling fastest at perigee and slowest at apogee. This change in speed is essential for balancing the Moon’s momentum against Earth’s gravitational pull. The Sun’s influence causes the entire elliptical orbit to rotate, a phenomenon called apsidal precession. This rotation means the perigee and apogee points slowly shift eastward, completing one full revolution in about 8.85 years.
The Moon’s True Path Through Space
When viewed from the solar system, the Moon’s path is not a series of tight loops around Earth. Instead, the Earth and Moon orbit a common center of mass called the barycenter, located approximately 4,670 kilometers from Earth’s center, beneath the planet’s surface. As this Earth-Moon system orbits the Sun, the Moon’s true path is a vast, wave-like trajectory.
The Moon’s orbital path around the Sun is always convex; it never curves inward or makes a loop around the Earth. The Sun’s overwhelming gravitational influence ensures the curvature of the Moon’s path is always directed toward the Sun. The Moon’s orbital motion is more like a gentle, weaving pattern that slightly deviates from Earth’s yearly path. This highlights that the Moon fundamentally follows the Sun, with Earth introducing a localized perturbation.
How Orbital Variation Affects Observation
The Moon’s elliptical orbit affects our sky, notably through the concepts of the Supermoon and Micromoon. A Supermoon occurs when a full moon coincides with the Moon being near perigee, making it appear up to 14% larger and 30% brighter than the farthest full moon. Conversely, a Micromoon is the popular term for a full moon that occurs when the Moon is near apogee.
The varying speed of the Moon also influences the geometry of solar eclipses. When an eclipse occurs with the Moon near apogee, its slower speed and smaller apparent size mean it cannot fully block the Sun, resulting in an annular eclipse. If the eclipse happens when the Moon is near perigee, its faster speed reduces the maximum possible duration of totality, as its shadow crosses Earth’s surface more quickly.