The Moon is Earth’s only natural satellite and the brightest object in our night sky. Although it appears familiar, it is an entire rocky world with mountains, plains, enormous impact craters and deposits of water ice hidden in some of its coldest, darkest regions.
Orbiting at an average distance of about 384,400 km (238,855 miles), the Moon has accompanied Earth for roughly 4.5 billion years. Its gravity plays a major role in our ocean tides, while its changing illumination produces the familiar lunar phases. Its ancient surface also preserves evidence from a period of Solar System history that has largely been erased from our own geologically active planet.
The Moon is also the only world beyond Earth that humans have visited. Yet despite the Apollo landings and decades of robotic exploration, scientists are still discovering new details about how the Moon formed, evolved and continues to interact with Earth.
So what exactly is the Moon, why do we always see nearly the same side, and what would this seemingly tranquil world actually be like to visit?
What Is the Moon?
The Moon is a rocky body held in orbit around Earth by gravity and one of hundreds of natural satellites found throughout our Solar System.
Compared with many other planetary moons, ours is unusually large relative to the planet it accompanies. But the Moon is still a very different world from Earth. Its surface gravity is only about one-sixth as strong, meaning an astronaut would weigh about one-sixth as much there while retaining exactly the same mass. It has no oceans or substantial atmosphere, and its landscape has been shaped by billions of years of impacts and ancient volcanic activity.
The Moon is not simply a feature in Earth’s night sky, either. Earth and the Moon form a gravitationally interacting system. Each pulls on the other, producing effects ranging from Earth’s ocean tides to measurable distortions of the solid lunar surface.
The Moon at a glance
| Property | Moon |
|---|---|
| Average distance from Earth | 384,400 km (238,855 miles) |
| Diameter | About 3,475 km (2,159 miles) |
| Radius | About 1,740 km (1,080 miles) |
| Surface gravity | About 1/6 of Earth’s |
| Orbital period | About 27.3 Earth days |
| Rotation period | About 27.3 Earth days |
| New Moon to New Moon | About 29.5 days |
| Age | About 4.5 billion years |
| Atmosphere | Extremely thin exosphere |
| Natural satellites | None |
How Big Is the Moon and How Far Away Is It?
The Moon is about 27% as wide as Earth. That makes it much smaller than our planet, but large enough to appear surprisingly prominent in our sky.
The gap between the two worlds is much more impressive. At its average distance of about 384,400 km (238,855 miles), roughly 30 Earth-sized planets could fit side by side between Earth and the Moon.
Because the lunar orbit is slightly elliptical, the Moon’s distance from Earth varies considerably over the course of an orbit, typically ranging from roughly 356,000–407,000 km (221,000–253,000 miles).
That distance is also slowly increasing. Measurements using lasers reflected from mirrors left on the Moon by Apollo astronauts show that the Moon is moving away from Earth by about 3.8 cm (1.5 inches) per year.
Why Do We Always See the Same Side of the Moon?
One of the most persistent misconceptions about the Moon is that it doesn’t rotate.
It does.
The Moon takes about 27.3 days to complete one rotation on its axis and almost exactly the same amount of time to complete one orbit around Earth. As a result, the same hemisphere remains pointed roughly toward us throughout its orbit. This condition is known as synchronous rotation or tidal locking.
The hemisphere that usually faces Earth is called the near side, while the hemisphere facing mostly away is called the far side. The far side is sometimes incorrectly called the “dark side” of the Moon, but it receives just as much sunlight as the near side over the course of a lunar day. During New Moon, for example, much of the far side is illuminated while the near side faces away from the Sun.
The two hemispheres also look noticeably different. The near side contains extensive dark volcanic plains known as maria, while the far side is dominated much more strongly by ancient, heavily cratered highlands.
We don’t actually see precisely 50% of the lunar surface. Small apparent rocking motions known collectively as libration allow observers on Earth to glimpse slightly around the Moon’s edges at different times. Over time, this lets us see about 59% of its surface.
There is another useful lunar timescale to understand. Because the Moon moves along its orbit while it rotates, the Sun takes about 29.5 Earth days to return to the same position in the lunar sky. At many locations on the Moon, this produces roughly two weeks of daylight followed by roughly two weeks of darkness.
Why Does the Moon Have Phases?
The Moon does not produce its own visible light. What we see is sunlight reflected from its surface.
Half of the Moon is illuminated by the Sun at almost any given moment, but as the Moon orbits Earth our viewing angle changes. We therefore see different fractions of its sunlit hemisphere, producing the familiar lunar phases.
The cycle is usually divided into eight main phases: New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter and Waning Crescent.
One complete cycle from New Moon to New Moon takes about 29.5 days. This is longer than the Moon’s 27.3-day orbit around Earth because Earth and the Moon are also moving together around the Sun. The Moon must travel a little farther before the Sun, Earth and Moon return to the same relative geometry.
Earth’s shadow does not normally cause the phases. It becomes important during a lunar eclipse, when the Moon passes through Earth’s shadow. A solar eclipse occurs when the Moon passes between Earth and the Sun. Eclipses don’t happen every month because the Moon’s orbit is tilted by about 5° relative to Earth’s orbital plane around the Sun.
You can follow the complete cycle in our guide to the phases of the Moon.
How Did the Moon Form?
To understand where the Moon came from, we have to go back almost to the birth of the Solar System.
The Moon formed about 4.5 billion years ago, when the young Solar System was still a violent place filled with collisions between growing planetary bodies. The leading family of explanations involves an enormous impact between the young Earth and another large object.
In the classic version of this idea, a Mars-sized body often called Theia struck the early Earth. Material blasted into orbit eventually combined to form the Moon. Modern research suggests the precise event may have been more complicated, however, and scientists continue to investigate different impact scenarios and how rapidly the Moon assembled afterward.
Whatever happened, any successful theory has to explain the Moon we see today — including its composition, its unusually small iron-rich core and its relationship with Earth. Lunar samples returned by the Apollo missions revealed important similarities between rocks from Earth and the Moon, providing crucial evidence for reconstructing their shared early history.
After forming, much of the young Moon was probably molten. As this vast magma ocean cooled, denser materials sank while lighter minerals rose and helped form the early lunar crust.
Because Earth has since been transformed by erosion, volcanism and plate tectonics, much of the geological evidence from this distant era has disappeared here. The Moon preserves far more of that ancient record.
You can explore the evidence and competing ideas in more detail in our guide to how the Moon formed.
What’s Inside the Moon?
The Moon’s violent formation and early magma ocean also helped create the layered world hidden beneath the surface today.
Like Earth, the Moon is differentiated into a crust, mantle and core. Its crust and mantle are dominated by silicate rocks and minerals, while its relatively small core contains iron and other elements.
The lunar crust varies considerably in thickness, while the mantle extends deep into the Moon’s interior. At the center lies a small metallic core, much less dominant relative to the Moon’s size than Earth’s core is within our own planet.
Although the Moon has cooled enormously, it isn’t completely inert. Apollo seismometers detected moonquakes, while young fault scarps show that the Moon continues to contract slowly as its interior cools. Over the last several hundred million years, this contraction has reduced its diameter by about 50 meters (150 feet).
Moonquakes and the Moon’s continuing contraction are just two of the more surprising facts about the Moon revealed by modern exploration.
Why Is the Moon Covered in Craters and Dark Patches?
The Moon’s countless craters tell another part of its long history. For billions of years, asteroids, meteoroids and other objects have struck the lunar surface, leaving behind impact scars ranging from tiny pits to enormous basins hundreds of kilometers across.
Many of the Moon’s brighter regions are ancient, heavily cratered highlands. The darker areas visible even with the unaided eye are called maria, from the Latin word for “seas.” Early astronomers once thought these smooth dark regions might contain water.
They are actually vast plains of dark basaltic rock. Huge impacts created enormous basins early in lunar history, and some were later flooded by lava rising from the Moon’s interior. When the lava cooled and solidified, it produced the dark plains we see today.
Craters can survive for billions of years because the Moon has no thick atmosphere, rain, rivers or oceans to erode them and no Earth-like system of active plate tectonics continually recycling its crust. Its surface therefore preserves a much longer visible impact record than Earth’s.
Much of the surface is covered by regolith, a layer of broken rock, dust and debris produced by countless impacts over immense periods of time.
If you’d like to explore the lunar surface yourself, see our guide to the best features on the Moon for stargazers.
Does the Moon Have an Atmosphere or Water?
The Moon is often described as having no atmosphere, but that isn’t quite true. It possesses an extremely thin layer of gases called an exosphere.
The particles in this exosphere are so widely separated that they rarely collide with one another. It is nothing like Earth’s dense atmosphere and provides almost no protection or insulation at the surface.
Without a substantial atmosphere to distribute heat, lunar surface temperatures vary dramatically between sunlight and darkness. Near the lunar equator, daytime temperatures can reach around 127°C (260°F), while nighttime temperatures can fall to roughly −173°C (−280°F).
At the poles, however, some deep craters never receive direct sunlight at all. These permanently shadowed regions can remain extraordinarily cold for billions of years, creating cold traps where volatile substances can survive.
Water ice has been detected in some of these permanently shadowed polar regions. This does not mean the Moon contains frozen lakes like those found in familiar environments on Earth. Much of the ice is mixed with lunar soil or trapped within extremely cold terrain.
Evidence has also revealed water and hydroxyl in other forms on and within lunar material. Together, these discoveries transformed our picture of the Moon from the almost completely dry world once imagined and made its polar regions especially important targets for scientific exploration.
How Does the Moon Affect Earth?
The most familiar effect of the Moon on Earth is the tides.
The Moon’s gravity pulls more strongly on the side of Earth closest to it than on the far side. This difference in gravitational pull helps produce tidal bulges in Earth’s oceans. As Earth rotates through these tidal bulges, sea levels along our coastlines rise and fall, producing the familiar cycle of high and low tides.
The Sun also contributes to Earth’s tides. When the Sun, Earth and Moon are aligned around New Moon and Full Moon, their effects combine to produce larger tidal ranges known as spring tides. Around First and Last Quarter, their gravitational effects partly oppose one another, producing smaller neap tides.
In practice, the timing and height of tides vary from place to place because coastlines, ocean depth and other local conditions complicate this simple picture.
The effect works both ways. Earth’s gravity slightly stretches and deforms the solid Moon, producing measurable solid-body tides in the lunar surface.
Over immense periods of time, this tidal interaction transfers angular momentum between Earth and the Moon. The result is that Earth’s rotation gradually slows while the Moon slowly moves farther away. This tidal exchange is the reason, mentioned earlier, that the Moon recedes from Earth by about 3.8 cm (1.5 inches) each year.
The Earth-Moon system is therefore not static. The relationship between the two worlds has been changing for billions of years and continues to change today.
How Have Humans Explored the Moon?
For most of human history, the Moon could only be studied from Earth. That changed dramatically during the Space Age, when robotic spacecraft began photographing, orbiting and eventually landing on our nearest celestial neighbor.
In 1959, the Soviet spacecraft Luna 3 returned the first images of the lunar far side, revealing terrain that had never before been seen by human eyes.
A decade later, Apollo 11 transformed lunar exploration forever. On July 20, 1969, Neil Armstrong and Buzz Aldrin became the first humans to land on and walk across the Moon while Michael Collins remained in lunar orbit.
Altogether, 24 different astronauts traveled from Earth to the vicinity of the Moon during the Apollo program, and 12 of them walked on its surface. By the time Apollo 17 departed in December 1972, humans had completed six successful crewed lunar landings.
The Apollo missions also returned about 382 kg (842 lb) of lunar rocks, soil and other material to Earth. Scientists continue studying these samples today, and they have profoundly influenced our understanding of the Moon’s age, composition and formation.
Discover more about the spacecraft, astronauts and extraordinary achievements behind the Apollo missions to the Moon.
Robotic exploration continued after Apollo and has expanded considerably in the 21st century. Orbiters have mapped the Moon in extraordinary detail, while landers and rovers from several nations have investigated its surface and polar regions.
In 2024, China’s Chang’e 6 mission returned the first samples ever collected from the lunar far side, providing scientists with material from a region geologically distinct from the familiar near side.
Human exploration also resumed in the lunar vicinity with NASA’s Artemis II mission in April 2026, which carried four astronauts on a journey around the Moon and safely back to Earth — the first crewed voyage to the Moon since Apollo 17.
The Moon is therefore once again an active destination for human and robotic exploration, with new missions adding to a story that began more than six decades ago.
How to Observe the Moon Yourself
You don’t need a telescope to begin exploring the Moon.
With the unaided eye, you can already distinguish the contrast between its brighter highlands and darker maria. Their patterns have inspired familiar cultural images such as the “Man in the Moon” for centuries.
Binoculars are an excellent next step. Even a modest pair can reveal large craters, mountain ranges and the boundaries of the dark lunar maria much more clearly.
A small telescope takes the view much further, revealing crater walls, mountain peaks, valleys and countless smaller features that change appearance as the angle of sunlight shifts.
Surprisingly, Full Moon is not usually the best time to see lunar relief. When sunlight strikes the surface almost directly from our viewpoint, shadows become shorter and many features appear flatter.
Instead, look near the terminator — the boundary between lunar day and night. Here, low-angle sunlight casts long shadows across crater walls and mountains, making the landscape appear especially dramatic.
Because the terminator moves across the surface as the Moon changes phase, different features become prominent from night to night. The Moon can therefore reward repeated observation even with very simple equipment.
It is also one of the easiest astronomical targets to observe from a city. Unlike faint galaxies and nebulae, the Moon remains spectacular even under substantial light pollution.
Explore More About the Moon
The Moon may be our nearest celestial neighbor, but its story reaches back almost to the birth of the Solar System. Its ancient surface records billions of years of impacts and geological change, while its continuing interaction with Earth reminds us that the two worlds are still evolving together. And because the Moon is visible from almost anywhere on Earth, much of that story begins with something as simple as stepping outside and looking up.
