"Moon" - what is it, definition of the term
A natural satellite orbiting a planet, formed from debris left after the planet’s early accretion, typically composed of silicate rock and metal, exhibiting a near‑circular, prograde trajectory; it reflects sunlight, creating observable phases, and exerts measurable gravitational influence on its primary, affecting tides and orbital dynamics.
Detailed information
The Earth's natural satellite is a rocky body approximately 3,474 km in diameter, composed primarily of silicate minerals and a small iron‑rich core. Surface features include vast basaltic plains, towering impact craters, and narrow mountain ranges formed by ancient tectonic activity.
Its orbital period around the planet is roughly 27.3 days, while the synodic cycle—the interval between successive full appearances—lasts about 29.5 days. The elliptical path causes variations in apparent size and illumination, influencing tidal forces that shape oceanic bulges and affect geological stress.
The exterior is covered by a fine layer of pulverized rock and glass, known as regolith. This material ranges from coarse fragments several centimeters across to dust particles finer than a micron. Regolith depth varies, reaching up to 20 m in some highland regions and thinning to a few meters in mare basins.
Gravitational interaction generates measurable effects on the planet’s rotation, extending the length of a day by approximately 2.3 milliseconds per century. The same interaction drives the gradual recession of the satellite, increasing its average distance by about 3.8 cm each year.
Scientific exploration has yielded detailed maps, compositional analyses, and seismic data. Key findings include:
- Presence of water‑bearing minerals in permanently shadowed craters.
- Detection of trace gases such as helium‑3 and argon.
- Evidence of ancient volcanic activity forming basaltic plains.
- Seismic recordings revealing a partially molten interior.
These observations support models of formation through a giant impact event, followed by accretion of debris and subsequent cooling. The body continues to serve as a reference point for navigation, timekeeping, and calibration of astronomical instruments.