The Moon: Earth's Companion and Gateway to the Cosmos

Updated: January 11, 2026 • Created: January 20, 2025

Science/Planetary Science & Space planetary-science

The Moon is Earth's only natural satellite and the largest moon relative to its planet in the solar system. Its presence has shaped Earth in profound ways: stabilizing Earth's axial tilt, creating tides, and possibly playing a role in the origin of life. The Moon's synchronous rotation means we only ever see one face from Earth, but the far side, first imaged by the Soviet Luna 3 mission in 1959, revealed a dramatically different landscape. The Moon's surface tells a story of violent impacts, ancient volcanism, and billions of years of geological history preserved in the absence of weather or plate tectonics. The Apollo missions (1969-1972) remain humanity's greatest achievement in space exploration, landing 12 astronauts on the Moon and returning 382 kg of lunar samples that continue to be studied today. These samples revealed that the Moon formed from a giant impact early in Earth's history, when a Mars-sized object collided with the young Earth, ejecting material that coalesced into the Moon. Today, the Moon is the target of a new era of exploration, with multiple nations planning missions to establish permanent bases and use the Moon as a stepping stone to Mars and beyond. This article explores the Moon's formation, geology, exploration history, and its role in Earth's past and humanity's future.

In Simple Terms

The Moon is Earth's constant companion in space—it's always there, lighting up the night sky and creating the tides in our oceans. What's really cool is that the Moon always shows us the same face (we never see the "dark side" from Earth), but when spacecraft finally got around to the other side, they found it looks completely different—much more cratered and rough. The Moon is like a giant history book—because it has no atmosphere or weather, all the craters and features from billions of years ago are still there, perfectly preserved. Scientists think the Moon formed when something the size of Mars crashed into the young Earth billions of years ago, blasting material into space that eventually came together to form the Moon. The Moon does amazing things for Earth—it stabilizes our planet's tilt (which keeps our seasons stable), creates the tides (which have shaped life in the oceans), and might have even helped life get started. The Apollo missions in the 1960s and 1970s were the first and only time humans have visited another world, and the rocks they brought back are still teaching us new things today. Now, countries around the world are planning to go back to the Moon, this time to stay, and use it as a stepping stone to explore Mars and beyond!

Abstract

The Moon orbits Earth at an average distance of 384,400 km, with a radius of 1,737 km and a mass of 7.34 × 10²² kg. The Moon's synchronous rotation means it rotates once for every orbit, keeping the same face toward Earth. The lunar surface is divided into two main terrains: the bright, heavily cratered highlands (covering 83% of the surface) and the dark, smooth maria (covering 17%), which are ancient lava flows. The Moon has no atmosphere, no magnetic field, and experiences extreme temperature variations from 127°C during the day to -173°C at night. The Moon's geology is dominated by impact craters, with the largest being the South Pole-Aitken Basin, 2,500 km in diameter. The Moon formed approximately 4.5 billion years ago from debris ejected when a Mars-sized object collided with the young Earth, a theory supported by the similarity of oxygen isotopes between Earth and Moon and the Moon's low iron content. The Apollo missions provided the first and only human visits to another world, with 12 astronauts walking on the Moon between 1969 and 1972. Recent missions have discovered water ice in permanently shadowed craters near the poles, making the Moon a potential resource for future exploration. The Moon continues to be a target for exploration, with NASA's Artemis program planning to return humans to the Moon and establish a sustainable presence. This article reviews the Moon's formation, geology, exploration history, and future prospects.

../../images/moon-apollo The Moon as seen from Earth, showing its heavily cratered surface. Credit: NASA (Public Domain)

Introduction

The Moon has been humanity's constant companion for billions of years, influencing our culture, science, and understanding of the cosmos. It's the only celestial body humans have visited, and the Apollo missions remain one of humanity's greatest achievements. But the Moon is more than a destination—it's a time capsule preserving the early history of the solar system, a laboratory for understanding planetary formation, and a stepping stone to the rest of the solar system.

The Moon's influence on Earth is profound. Its gravity creates tides that have shaped coastlines and may have played a role in the origin of life. The Moon stabilizes Earth's axial tilt, preventing chaotic climate variations. Without the Moon, Earth's tilt could vary dramatically, making the planet less habitable.

Understanding the Moon is essential for understanding Earth. The Moon's surface, unchanged for billions of years except by impacts, preserves a record of the early solar system that has been erased on Earth by plate tectonics, weather, and life. The Apollo samples continue to reveal new insights about the Moon's formation, Earth's early history, and the processes that shaped the inner solar system.

Physical Characteristics

Basic Properties

The Moon is the fifth-largest moon in the solar system:

Radius: 1,737 km (0.27 Earth radii)
Mass: 7.34 × 10²² kg (0.0123 Earth masses)
Density: 3.34 g/cm³ (less dense than Earth, indicating less iron)
Surface gravity: 1.62 m/s² (0.165 times Earth's gravity)
Escape velocity: 2.38 km/s

The Moon's low density compared to Earth supports the giant impact theory of formation, as the Moon would have formed from Earth's mantle material, which is less dense than the core.

Orbit and Rotation

The Moon's orbit and rotation are synchronized:

Semi-major axis: 384,400 km (varies from 356,400 to 406,700 km)
Orbital period: 27.3 days (sidereal month)
Synodic period: 29.5 days (time between full moons)
Rotation period: 27.3 days (synchronous with orbit)
Orbital inclination: 5.1 degrees to Earth's equator

The synchronous rotation means the Moon always shows the same face to Earth, though libration (wobbling) allows us to see about 59% of the surface over time.

Tidal Locking

The Moon is tidally locked to Earth:

Cause: Earth's gravity created a tidal bulge on the Moon
Effect: The bulge was pulled ahead of the Moon's rotation, slowing it until rotation matched orbital period
Result: Same face always toward Earth

Tidal locking is common for moons and occurs when the orbital period matches the rotation period.

Formation: The Giant Impact Theory

The Collision

The Moon likely formed from a giant impact:

Timing: ~4.5 billion years ago, shortly after Earth formed
Impactor: Mars-sized object (Theia) collided with young Earth
Angle: Oblique impact, not head-on
Result: Material from both Earth and Theia ejected into orbit

The impact would have been catastrophic:

Earth's surface melted
Atmosphere vaporized
Debris formed a ring around Earth
Ring material coalesced into the Moon

Evidence

Multiple lines of evidence support the giant impact theory:

Oxygen isotopes: Moon and Earth have identical oxygen isotope ratios, suggesting they formed from the same material
Low iron: Moon has less iron than Earth, consistent with forming from mantle material
Angular momentum: Earth-Moon system has high angular momentum, consistent with an impact
Lunar samples: Apollo samples show the Moon was once molten (magma ocean)
Computer simulations: Models show a giant impact can create a Moon-sized object

Alternative Theories

Other theories have been proposed but are less supported:

Co-accretion: Moon formed alongside Earth (doesn't explain low iron)
Capture: Moon formed elsewhere and was captured (doesn't explain similar composition)
Fission: Moon split from Earth (doesn't explain angular momentum)

The giant impact theory remains the most widely accepted.

Surface Geology

Two Terranes

The Moon's surface is divided into two distinct regions:

Highlands (83% of surface):

Bright, heavily cratered terrain
Older (4.0-4.5 billion years)
Composed of anorthosite (calcium-rich feldspar)
Formed from the lunar magma ocean

Maria (17% of surface):

Dark, smooth plains
Younger (3.1-3.9 billion years)
Composed of basalt (volcanic rock)
Formed from lava flows filling impact basins

The maria appear dark because the basalt is less reflective than the highland rocks.

Impact Craters

Impact craters dominate the lunar surface:

Number: Millions, ranging from micrometers to hundreds of kilometers
Formation: Meteoroids, asteroids, and comets striking the surface
Preservation: Craters remain unchanged for billions of years (no weather or tectonics)

Largest craters:

South Pole-Aitken Basin: 2,500 km diameter, 13 km deep (largest impact basin in solar system)
Imbrium Basin: 1,160 km diameter
Orientale Basin: 930 km diameter (best-preserved multi-ring basin)

Volcanism

The Moon was once volcanically active:

Lunar maria: Formed from flood basalts 3.1-3.9 billion years ago
Volcanic features: Domes, cones, and rilles (collapsed lava tubes)
Recent activity: Some evidence of volcanism as recent as 100 million years ago

The volcanism was driven by:

Impact heating: Large impacts created basins
Radioactive decay: Heat from radioactive elements
Tidal heating: Early in lunar history when closer to Earth

Regolith

The lunar surface is covered in regolith:

Composition: Broken rock and dust from billions of years of impacts
Thickness: Averages 4-5 meters in maria, 10-15 meters in highlands
Properties: Fine-grained, sharp (no weathering to round edges)
Hazards: Can damage equipment and pose health risks

The Far Side

The far side of the Moon is dramatically different:

More cratered: Fewer maria (only 1% vs. 31% on near side)
Thicker crust: ~50 km vs. ~30 km on near side
Different composition: More anorthosite, less basalt

Why the difference?

Crustal asymmetry: Far side has thicker crust, making it harder for lava to reach surface
Impact distribution: Near side may have received more impacts that created basins
Thermal evolution: Different cooling history on two sides

The far side is also ideal for radio astronomy, as it's shielded from Earth's radio emissions.

Water on the Moon

Discovery

Water ice was discovered in permanently shadowed craters:

Location: Near the poles, in craters that never receive sunlight
Temperature: Below -170°C
Source: Comets, asteroids, solar wind
Amount: Estimated billions of tons

Significance

Lunar water is crucial for future exploration:

Life support: Can be converted to oxygen and hydrogen
Rocket fuel: Hydrogen and oxygen can power rockets
Radiation shielding: Water can protect against radiation
Agriculture: Could support future lunar bases

Multiple missions are planned to map and extract lunar water.

Exploration History

Early Observations

Ancient times: Moon observed with naked eye
1610: Galileo first observes Moon with telescope
1651: Riccioli creates first detailed lunar map
1800s: Telescopic observations reveal surface features

Space Age

Luna program (Soviet Union, 1959-1976):

Luna 3: First images of far side (1959)
Luna 9: First soft landing (1966)
Luna 16: First automated sample return (1970)

Ranger program (NASA, 1961-1965):

Impact probes that photographed surface before crashing

Surveyor program (NASA, 1966-1968):

Soft landings to test landing technology
Analyzed soil composition

Lunar Orbiter program (NASA, 1966-1967):

Mapped 99% of lunar surface
Identified Apollo landing sites

Apollo Program (1969-1972)

The Apollo program remains humanity's greatest achievement in space:

6 landings: 12 astronauts walked on the Moon
Samples: 382 kg of lunar rocks and soil
Experiments: Seismometers, laser reflectors, solar wind collectors
Legacy: Continues to inspire and inform

Apollo 11 (July 20, 1969):

First human landing
Neil Armstrong and Buzz Aldrin
Collected 21.5 kg of samples

Apollo missions: Each added to our understanding of the Moon's geology, formation, and history.

Recent Missions

Clementine (1994):

First evidence of water ice at poles

Lunar Prospector (1998-1999):

Confirmed water ice
Mapped composition

Lunar Reconnaissance Orbiter (2009-present):

High-resolution mapping
Identified landing sites
Studying water distribution

Chang'e program (China, 2007-present):

Orbiters, landers, and rovers
First landing on far side (2019)
Sample return (2020)

Artemis program (NASA, planned):

Return humans to Moon
Establish sustainable presence
Use Moon as stepping stone to Mars

The Moon's Influence on Earth

Tides

The Moon's gravity creates tides:

Mechanism: Gravitational pull varies across Earth
Effect: Ocean bulges on side toward and away from Moon
Frequency: Two high tides and two low tides per day
Importance: Shaped coastlines, may have influenced evolution

Axial Tilt Stabilization

The Moon stabilizes Earth's axial tilt:

Without Moon: Tilt could vary chaotically (0-85 degrees)
With Moon: Tilt varies only 2-3 degrees
Effect: Prevents extreme climate variations
Importance: May be crucial for long-term habitability

Cultural Impact

The Moon has influenced human culture:

Calendars: Many cultures based on lunar cycles
Mythology: Moon features in myths worldwide
Science: First target for space exploration
Inspiration: Continues to inspire exploration

Future Exploration

Artemis Program

NASA's Artemis program plans to:

Return humans to Moon by 2025
Establish sustainable presence
Build Gateway space station in lunar orbit
Use Moon as testbed for Mars missions

International Efforts

Multiple nations are planning lunar missions:

China: Continuing Chang'e program, planning base
Russia: Planning Luna missions
India: Chandrayaan program
Private companies: SpaceX, Blue Origin planning missions

Scientific Goals

Future missions will study:

Lunar interior: Structure and composition
Water resources: Distribution and extraction
Geology: Formation and evolution
Astrobiology: Potential for life
Astronomy: Far side observatories

Conclusion

The Moon is far more than a bright object in the night sky—it's a world with a rich geological history, a key to understanding Earth's formation, and humanity's gateway to the solar system. The Apollo missions showed that humans can explore other worlds, and the samples they returned continue to reveal new insights about the Moon, Earth, and the early solar system. As we prepare to return to the Moon with the Artemis program, we're entering a new era of lunar exploration that will establish a permanent human presence and use the Moon as a stepping stone to Mars and beyond. The Moon, our constant companion, will continue to inspire and challenge us as we explore the cosmos.

For related topics:

Earth - The Moon's parent planet
Mars - The next destination for human exploration
Mercury - A planet similar in appearance to the Moon
Phobos and Deimos - Mars's moons
Planetary Science & Space - Overview of planetary science topics

^[NASA Solar System Exploration - Moon] NASA. (2024). Moon: In Depth. NASA Solar System Exploration. https://solarsystem.nasa.gov/moons/earths-moon/in-depth/

^[Apollo Program] NASA. (2024). Apollo Program. NASA. https://www.nasa.gov/mission_pages/apollo/index.html

^[Giant Impact Theory] Canup, R. M. (2012). Forming a Moon with an Earth-like composition via a giant impact. Science, 338(6110), 1052-1055.

^[Lunar Water] Colaprete, A., et al. (2010). Detection of water in the LCROSS ejecta plume. Science, 330(6003), 463-468.

^[Lunar Reconnaissance Orbiter] NASA. (2024). Lunar Reconnaissance Orbiter. NASA Goddard Space Flight Center. https://lunar.gsfc.nasa.gov/

^[Artemis Program] NASA. (2024). Artemis Program. NASA. https://www.nasa.gov/specials/artemis/

^[Lunar Geology] Wilhelms, D. E. (1987). The Geologic History of the Moon. U.S. Geological Survey Professional Paper 1348.

^[Moon Formation] Wiechert, U., et al. (2001). Oxygen isotopes and the moon-forming giant impact. Science, 294(5543), 345-348.