Moon

Moon
Gregory H. Revera · CC BY-SA 3.0 · source
Name	Moon
Caption	The Moon as seen from Earth during a full phase.
Designations	Luna, Selene, Cynthia
Adjectives	Lunar, selenic
Epoch	J2000
Inclination	5.145° to the ecliptic
Satellites	None

Moon. Earth's only permanent natural satellite, it is the fifth-largest moon in the Solar System and the largest relative to its host planet. Its gravitational influence is the main driver of Earth's tides and has played a crucial role in stabilizing the planet's axial tilt. The prevailing scientific theory, the giant-impact hypothesis, suggests it formed from the debris of a collision between Earth and a Mars-sized body named Theia.

Characteristics

The Moon is a differentiated body with a crust, mantle, and core. Its surface is dominated by ancient, heavily cratered highlands and darker, younger plains called maria, which are vast basins filled with solidified basalt from volcanic eruptions. The crust is on average about 50 kilometers thick and is composed primarily of rocks like anorthosite. The lack of a significant atmosphere and global magnetic field means its surface is exposed directly to the solar wind and impacted by micrometeorites, creating a layer of fine dust called regolith. Measurements from missions like the Gravity Recovery and Interior Laboratory have provided detailed maps of its internal structure and gravitational field.

Orbit and rotation

The Moon orbits Earth at an average distance of about 384,400 kilometers, completing a revolution in approximately 27.3 days, a period known as a sidereal month. Its rotation period is synchronized with its orbital period, a state called tidal locking, which results in the same hemisphere, the near side of the Moon, always facing Earth. This synchronous rotation is a consequence of gravitational interactions over billions of years. The Moon's orbital plane is inclined relative to Earth's equator, and its orbit is slightly elliptical, leading to variations in apparent size and phenomena like the supermoon and micro moon.

Phases and appearance

The changing illumination of the Moon as seen from Earth creates the cyclical lunar phases, which progress from new moon to full moon and back over a synodic month of about 29.5 days. During a full moon, the entire near side is illuminated by the Sun. Occasionally, the orbital alignment allows Earth to cast its shadow on the Moon, creating a lunar eclipse. The Moon's apparent color can vary due to atmospheric conditions on Earth, and its surface features, such as the Mare Imbrium and the Tycho crater, are visible with the naked eye or through instruments like those used by Galileo Galilei.

Exploration

Human exploration of the Moon began in earnest during the Cold War rivalry between the United States and the Soviet Union. The Soviet Union's Luna program achieved the first unmanned impact, photograph of the far side, and soft landing. The United States' Apollo program, culminating in Apollo 11 with astronauts Neil Armstrong and Buzz Aldrin, landed the first humans in 1969. Subsequent missions, like Apollo 17, conducted extensive scientific work. In the 21st century, numerous nations and agencies, including NASA, the European Space Agency, China National Space Administration, and Indian Space Research Organisation, have launched orbiters, landers, and rovers, such as the Chang'e 4 mission to the far side and NASA's Artemis program aiming for a sustainable human presence.

Cultural significance

The Moon has profound cultural and symbolic importance across human civilizations. In Greek mythology, it was personified as the goddess Selene, while in Roman mythology, it was Luna. It features prominently in religious calendars, such as the Islamic calendar, and festivals like the Mid-Autumn Festival in China. It has inspired countless works of art, literature, and music, from the poetry of William Wordsworth to the film by Georges Méliès, *A Trip to the Moon*. Its symbolism is diverse, often representing cycles, femininity, and the unconscious, as explored in the works of Carl Jung. The historic Apollo 8 photograph, *Earthrise*, taken from lunar orbit, became a powerful icon for the environmental movement.

Category:Moons of Earth Category:Astronomical objects