The Moon, our planet’s largest celestial neighbor, is formally classified in astronomy as a natural satellite of Earth. This designation defines its relationship to the primary body it orbits.
The Standard Classification of Natural Satellites
A natural satellite, often called a moon, is an astronomical body that orbits a planet, dwarf planet, or minor planet. The Moon meets this criterion by being gravitationally bound to Earth, completing an orbit approximately every 27.3 days. The International Astronomical Union (IAU) governs the naming and classification of celestial objects, and the Moon’s status as a satellite is universally accepted. The IAU distinguishes our satellite by capitalizing its name, referring to it simply as “the Moon,” while other satellites are generally referred to with a lowercase “m.”
The Moon is a large, solid body that has achieved hydrostatic equilibrium, meaning its gravity has pulled it into a nearly spherical shape. This physical characteristic is shared with planets and dwarf planets. However, the Moon’s orbit around Earth prevents it from being classified as a planet itself. Its motion is dominated by the gravitational pull of its primary planet, placing it within the standard definition of a natural satellite.
Unique Characteristics of the Earth-Moon System
The Earth-Moon system is structurally distinct from nearly all other planet-satellite pairings due to the Moon’s unusually large size relative to Earth. The Moon’s diameter is about 3,474 kilometers, roughly one-quarter of Earth’s diameter. Most other satellites are tiny fractions of their parent planet’s size, making the Earth-Moon ratio unique.
This disproportionate size means the Moon exerts a significant gravitational influence on Earth, causing both bodies to orbit a common center of mass known as the barycenter. The barycenter’s location is determined by the mass and separation of the two objects. Since Earth is approximately 81 times more massive than the Moon, the barycenter is located on average about 4,670 kilometers from Earth’s center.
This common center of mass lies approximately three-quarters of the way to Earth’s surface, meaning the barycenter is technically inside the Earth. Because Earth constantly wobbles around this point, some scientists occasionally refer to the pair as a “double planet” or “binary system.” This descriptive term is not an official IAU classification but highlights the Moon’s profound effect on Earth’s movement and its unique status.
Origin Story: The Giant Impact Hypothesis
The Moon’s unusual size and composition are best explained by the leading scientific theory of its formation, the Giant Impact Hypothesis. This model proposes that about 4.5 billion years ago, a Mars-sized protoplanet, sometimes named Theia, collided with the proto-Earth. The catastrophic impact vaporized and ejected vast amounts of material from both bodies’ outer layers into orbit.
This hot, swirling debris disc, composed largely of silicates, eventually cooled and coalesced due to gravity, forming the Moon. The theory explains the Moon’s low density and small iron core, as the material came mostly from the less-dense mantles and crusts of the two colliding bodies. Supporting evidence includes the Moon’s depletion of volatile elements, such as water, which would have boiled away during the impact’s extreme heat.
Analysis of Apollo lunar samples shows that the Moon and Earth share similar stable isotope ratios for elements like oxygen. This isotopic fingerprint suggests a common origin for their material, supporting the idea that the Moon was formed from a thorough mixing of material from the proto-Earth and the impacting body. The Giant Impact Hypothesis provides context for why the Earth-Moon system is gravitationally and structurally distinct.