The Moon is Earth’s only natural satellite and the fifth largest moon in the solar system. Its characteristics shape ocean tides, stabilize axial tilt, and enable centuries of scientific study. This profile summarizes verified measurements and models of the Moon’s size, orbit, surface, composition, and observable phenomena, emphasizing data used by planetary science and astronomy over time.
Size and Mass
The Moon’s dimensions are consistently reported across telescopic, orbital, and landed measurements. Diameter, equatorial and polar flattening, and mass define its scale and gravitational influence.
Equatorial and Polar Diameters
Because the Moon is an oblate spheroid, equatorial and polar diameters differ slightly. Laser altimetry and spacecraft tracking refine these values with millimeter-level precision where possible.
Mass and Mean Density
Mass is derived from tracking spacecraft orbits and gravitational interactions. Mean density indicates how mass is distributed internally, informing models of formation and differentiation.
| Attribute | Verified Detail | Source Type |
|---|---|---|
| Mean Diameter | 3,474 kilometers | LRO and historical telescopic |
| Equatorial Diameter | 3,476 km | LRO altimetry |
| Polar Diameter | 3,472 km | LRO altimetry |
| Mass | 7.342×10^22 kg | Orbiter tracking (GRAIL) |
| Mean Density | 3.344 g/cm^3 | GRAIL and seismic data |
| Surface Gravity | 1.62 m/s² | Orbiter and lander data |
| Escape Velocity | 2.38 km/s | Modeled from mass and radius |
Orbit and Motion
The Moon’s orbit governs tidal patterns, eclipse cycles, and observational planning. Key parameters include semi-major axis, eccentricity, inclination, and period, all tracked by radar, laser ranging, and spacecraft.
Semi-Major Axis and Eccentricity
The average Earth–Moon distance is about 384,400 km, but the orbit is not a perfect circle. Eccentricity causes the distance to vary, affecting apparent size and tidal strength.
Inclination and Orbital Period
Inclination relative to Earth’s orbit and nodal regression influence eclipse seasons. Sidereal and synodic months describe different time frames for orbital and phase cycles.
| Metric | Estimate or Range | Context |
|---|---|---|
| Semi-major Axis | 384,400 km | Average Earth–Moon distance |
| Eccentricity | 0.0549 | Perigee ~363,300 km; Apogee ~405,500 km |
| Orbital Inclination | 5.145° | Relative to Earth’s orbit (ecliptic) |
| Sidereal Month | 27.32 days | Orbit relative to fixed stars |
| Synodic Month | 29.53 days | Cycle of lunar phases |
| Orbital Speed (avg) | 1.022 km/s | Derived from period and distance |
Surface Features and Topography
The lunar surface records billions of years of impact history and volcanic activity. Highlands, maria, craters, and regolith define its character. Remote sensing and sample return constrain albedo, roughness, and age distributions.
Lunar Highlands vs. Maria
Highlands are light, heavily cratered, and composed of anorthosite. Maria are darker, younger basaltic plains filling ancient basins. Their contrasting brightness is visible from Earth without magnification.
Regolith and Micrometeorites
Regolith is a layer of fragmented rock and dust generated by constant impacts. It ranges from powdery fines at the surface to consolidated breccia, influencing landing safety and equipment design.
- Maria cover about 16% of the near side despite being younger
- Regolith can be 4 to 5 meters deep in mare regions
- The lunar soil contains metallic iron grains formed by micrometeorite impacts
Composition and Internal Structure
Seismic experiments, gravity models, and samples indicate a crust, mantle, and small metallic core. Understanding differentiation helps date the Moon and compare it with Earth and other bodies.
Crust, Mantle, and Core
The crust is asymmetric, thicker on the far side. The mantle contains minerals like olivine and pyroxene. Evidence suggests a partially molten outer core, but its size remains uncertain.
Notable Minerals and Elements
Plagioclase feldspar is abundant in highlands. Ilmenite and pyroxene dominate maria. Trace elements such as titanium and thorium vary regionally, visible in spectral maps.
| Parameter | Estimate or Range | Context |
|---|---|---|
| Crust Thickness (near side) | 30–50 km | Thinner under maria |
| Crust Thickness (far side) | 60–80 km | Consistent with gravity and seismic data |
| Core Radius (approx.) | ~330 km | Partially molten, Fe–S rich |
| Lunar Mantle Depth | ~1,000 km | Between crust and core |
| Albedo (Bond) | 0.12 | Reflects 12% of sunlight |
Observable Phenomena and Interactions
The Moon influences tides, eclipses, and nocturnal illumination. Its phases and libration affect visibility from Earth, while synchronous rotation keeps one hemisphere Earth-facing. Accurate models support navigation, photography, and education.
Lunar Phases and Libration
Phases cycle monthly as the Sun–Moon–Earth angle changes. Libration in longitude and latitude allows observers on Earth to see slightly more than 50% of the surface over time.
Tidal Effects and Eclipses
Gravitational pull creates ocean tides and solid Earth tides. Eclipses occur when the Sun, Earth, and Moon align within nodes, with the Moon taking on a reddish hue during total lunar eclipses.
Exploration and Measurement Techniques
Robotic missions and human landings have refined size, gravity, and composition data. Orbital altimetry, laser ranging, and sample analysis continuously improve model accuracy for science and future exploration.
Measurement Methods
Radar ranging, lunar laser ranging arrays, and spacecraft Doppler tracking provide precise distance, rotation, and deformation measurements. These constrain tidal dissipation and interior structure.