The Moon: Formation, Characteristics, Phases, Influence, and Exploration

The Moon is Earth’s natural satellite and the brightest celestial object in our night sky (surpassed only by the Sun during the day). Its almost constant presence has sparked human curiosity since ancient times, inspiring myths, calendars, and numerous scientific investigations. In this article, we explore five fundamental aspects of the Moon: how it formed, its physical characteristics, why it has phases, how it influences Earth, and an overview of its exploration from the Apollo missions to future plans.

Formation of the Moon: the Giant Impact Hypothesis

Planetary scientists have proposed several theories to explain the Moon’s origin. These include the fission hypothesis (the Moon broke away from a primordial Earth), the co-formation hypothesis (the Moon and Earth formed at the same time from the same disk of material), and the capture hypothesis (the Moon formed elsewhere in the Solar System and was later captured by Earth’s gravity). However, the most widely accepted theory today is the giant impact hypothesis. According to this theory, early in the Solar System’s history, a Mars-sized object collided with the young Earth about 4.5 billion years ago, ejecting a massive amount of debris from both bodies into space.

Formation of the Moon

Part of this debris remained orbiting Earth and eventually coalesced under gravity to form the Moon. This theory explains why the Moon is composed of material similar to Earth’s mantle (as it would have originated from the “young” Earth) and why it has a relatively small core. Samples of lunar rocks brought back by astronauts during the Apollo missions have supported this idea, indicating that the Moon indeed solidified after a major impact that occurred about 4.5 billion years ago.

Physical Characteristics: Size, Distance, Composition, Temperature, and Surface

Size and mass

The Moon has a diameter of approximately 3,474 km, about one quarter of Earth’s diameter. Its mass is only 1.23% of Earth’s mass (roughly 1/81), which means lunar gravity is about one-sixth that of Earth. Despite its smaller size, the Moon’s average density (3.34 g/cm³) is relatively high; in fact, it is the second densest satellite in the Solar System after Io (a moon of Jupiter). This suggests a primarily rocky composition with a metallic core.

The average distance between the Moon and Earth is about 384,400 km, equivalent to fitting roughly 30 Earths in a row between the two bodies. To visualize this: if Earth were the size of a basketball, the Moon would be about the size of a tennis ball located roughly 7.2 meters away.

Internal composition

The Moon is a differentiated body, with a well-defined crust, mantle, and core. Lunar rocks are similar to terrestrial igneous rocks and are rich in silicates (compounds of oxygen and silicon, along with magnesium, iron, calcium, and aluminum).

The surface crust, about 50 km thick, is composed mainly of anorthosite (rich in calcium and aluminum feldspar). Beneath the crust lies the lunar mantle, composed primarily of minerals such as olivine and pyroxene; the lunar mantle is richer in iron than Earth’s mantle and was the source of the basaltic lavas that once flooded low-lying regions of the Moon.

At the center, the Moon has a relatively small metallic core, with a radius of about 350 km (approximately 20% of the Moon’s radius). This core is thought to be composed mainly of iron, with small amounts of nickel and sulfur. Unlike Earth, the Moon’s outer core may be partially molten, but to a much lesser extent; most of the Moon’s mass resides in the crust and mantle rather than in the core.

Extreme temperatures

The Moon lacks a significant atmosphere to regulate temperature, so its surface experiences extreme thermal variations. During the lunar day (which lasts about 14 Earth days under sunlight), temperatures at the equator can reach around 120 °C, hot enough to boil water. During the long lunar night (another ~14 Earth days in darkness), the surface cools rapidly and temperatures drop to about −130 °C at the equator.

In polar regions, especially inside permanently shadowed craters, even lower temperatures have been recorded, approaching −250 °C. These conditions make the Moon a very hostile environment: there is no air, no liquid water, and heat gained during the day is quickly lost at night.

Surface and appearance

To the naked eye, we can distinguish light and dark regions on the Moon. The lighter areas correspond to the highlands or ancient mountainous terrain, while the darker zones are the famous lunar maria (or “seas”), which are actually basaltic plains formed by ancient lava flows. This contrast is due to differences in relief and composition: the highlands consist of lighter-colored anorthositic rocks at higher elevations, while the maria are low basins filled with dark volcanic rock.

The Moon lacks a dense atmosphere—there is no wind or water to erode its surface, nor weather or rain to erase marks. As a result, impact craters remain almost intact for millions of years, and even the footprints left by astronauts persist in the lunar regolith (dust).

Moon.

The lunar landscape is covered with craters of all sizes formed by meteorite impacts, including gigantic circular craters visible from Earth and microscopic craters on individual grains of dust. On its near side (the side we always see from Earth), large craters and dark, circular maria stand out, while the far side (not visible from Earth) is almost entirely covered with craters and has very few maria.

Lunar Phases: Why Do They Change Shape?

Sequence of the Moon’s main phases, from new moon to full moon, as seen from Earth.The Moon does not emit its own light; it reflects sunlight, and its phase depends on how much of the illuminated portion we can see at any given time.

Over the course of roughly one month, the Moon displays different phases or appearances as seen from Earth, gradually changing shape night after night. This happens because the Moon has no light of its own; what we see is sunlight reflected off its surface. At all times, half of the Moon is illuminated by the Sun (the lunar day side) and the other half is in shadow (the lunar night side). However, as the Moon orbits Earth, we see different portions of its illuminated half. In other words, lunar phases depend on the geometric relationship between the Sun, the Moon, and the Earth.

When the Moon is positioned approximately between Earth and the Sun (with the Sun behind it), we see a new moon. In this phase, the illuminated side faces away from us, so the Moon is nearly invisible in the sky. As the Moon continues along its orbit, a thin illuminated crescent becomes visible—this is the waxing crescent phase. About one week after the new moon, the Moon reaches the first quarter, showing half of its disk illuminated (often appearing in the shape of a “D”).

The Moon then continues to grow (the waxing gibbous phase) until about 14–15 days after the new moon it reaches the full moon, when the entire face of the Moon visible from Earth is illuminated. During a full moon, Earth lies approximately between the Moon and the Sun, allowing us to see the fully sunlit lunar hemisphere.

After the full moon, the illuminated portion begins to decrease: first the waning gibbous phase appears, followed by the last quarter (with half of the disk illuminated, often resembling a reversed “C”), and finally a waning crescent that becomes thinner and thinner until returning to the new moon. This complete cycle, from new moon to new moon, lasts about 29.5 days (a synodic month), which explains why calendar months have traditionally been associated with lunar phases.

In summary, the phases of the Moon are an effect of perspective: we see different amounts of the Moon’s illuminated half depending on its relative position with respect to the Sun and the Earth.

It is important to emphasize that the Moon always retains the same spherical shape and always has half of its surface illuminated by the Sun; what changes is how much of that illuminated half is visible from our vantage point on Earth. Finally, it is worth noting that during eclipses, the Moon’s appearance can also change temporarily (for example, turning reddish during a lunar eclipse), but these events occur only during specific phases: a lunar eclipse can occur only at full moon, and a solar eclipse only at new moon, when the Sun–Earth–Moon alignment is exact.

Influence of the Moon on Earth: Tides, Rotation, and Axial Stability

The Moon is not only a beautiful object in the sky; it also exerts significant physical influences on Earth due to the mutual gravitational interaction between the two bodies. The Moon’s gravitational pull is the primary cause of ocean tides on our planet. The Moon’s gravity pulls slightly more strongly on the side of Earth closest to it than on the far side, creating a stretching effect in the oceans. This produces two bulges of water: one on the side of Earth facing the Moon and another on the opposite side.

As Earth rotates on its axis, these water bulges manifest as two high tides and two low tides each day (with a tidal phase approximately every six hours). In simple terms, when the Moon is above the horizon at a given location, its gravity draws the ocean beneath it, causing high tide; the same occurs on the opposite side due to the inertia of the water, while low tide is observed at points in between. The Sun also influences tides through its gravity, though to a lesser extent. When the Sun and Moon are aligned (during new or full moon), spring tides occur and are more pronounced; when they are at right angles (during the first or last quarter), neap tides of smaller amplitude occur.

Another consequence of the Earth–Moon gravitational interaction is that the rotation of both bodies has become partially synchronized. The Moon is in synchronous rotation with Earth, meaning it rotates on its axis in the same amount of time it takes to orbit Earth. This is why we always see the same face of the Moon from Earth. In other words, the Moon’s rotational period matches its orbital period around Earth.

Rotation of the Moon

This “resonance” is the result of tidal forces: millions of years ago the Moon rotated more rapidly, but gravitational friction gradually slowed it down until it became locked into this stable configuration. As a curiosity, the side of the Moon that we cannot see from Earth is sometimes mistakenly called the “dark side” of the Moon, but it is not dark at all; that side also receives sunlight regularly—it is simply hidden from us due to synchronous rotation. In fact, the far side is illuminated when we see the new moon phase from Earth.

The Moon’s influence has been crucial for the stability of Earth’s rotational axis and, therefore, for our climate. Earth currently has an axial tilt of about 23.5°, which produces the seasons of the year. Scientists estimate that without the presence of the Moon, Earth would wobble chaotically on its axis, with much larger variations in tilt. Lunar gravity acts as a stabilizer, damping these oscillations. Thanks to the Moon, Earth’s tilt has remained relatively stable over time, allowing for a more regular and seasonal climate on geological timescales. Otherwise, Earth’s axis could vary uncontrollably (possibly tilting close to 0° or up to 45°), causing extreme climatic changes that would make life as we know it difficult.

In addition to stabilizing climate, the Moon sets important natural rhythms: the tidal cycle (two rises and falls of sea level each day) influences coastal ecosystems, navigation, and fishing since ancient times, and the monthly cycle of lunar phases has served as the basis for calendars in many cultures.

The gravitational interaction between Earth and the Moon also has very long-term effects. For example, Earth’s rotation is gradually slowing due to tides: days have been getting longer over time (by about 2 milliseconds per century today). This same interaction transfers energy to the Moon’s orbit, causing the Moon to move away from Earth at a rate of about 3.8 centimeters per year. Although this change is imperceptible on human timescales, over millions of years it alters the length of Earth’s day and the Earth–Moon distance. In about 600 million years, for instance, the Moon will be far enough away that total solar eclipses will no longer be visible (because its apparent size will be smaller than that of the Sun). These examples illustrate how the Moon and Earth form a closely linked, dynamic system.

Lunar Exploration: Apollo Missions, Scientific Discoveries, and Future Plans

Footprint left by astronaut Buzz Aldrin (Apollo 11) on the lunar surface in July 1969. Twelve men walked on the Moon between 1969 and 1972 during the Apollo program. Upcoming missions aim to return humans to the Moon, including the first woman to set foot on our natural satellite.

The Moon has been the only celestial body, aside from Earth, on which humans have landed and walked. Modern lunar exploration began in the mid-20th century in the context of the space race. In 1959, the Soviet Union achieved the first spacecraft impact on the Moon (Luna 2) and the first photograph of the far side (Luna 3). During the 1960s, both superpowers launched multiple probes. The Apollo program of the United States ultimately achieved the goal of landing humans on the lunar surface.

Man on the Moon

The first crewed mission to orbit the Moon was Apollo 8 in 1968. Then, on July 20, 1969, Apollo 11 landed in the Sea of Tranquility: astronaut Neil Armstrong became the first human to walk on the Moon, followed minutes later by Buzz Aldrin. In total, the Apollo program carried out six crewed lunar landings between 1969 and 1972, sending twelve astronauts (all men) to walk on the Moon. The final mission was Apollo 17 in December 1972, after which no humans have returned to the lunar surface.

Each Apollo mission brought back samples of rocks and regolith; collectively, the astronauts collected 382 kilograms of lunar material for analysis. These samples revolutionized our understanding of the Moon. For example, geochemical studies of Apollo samples confirmed that the Moon shares a common origin with Earth (supporting the giant impact hypothesis), revealed the age of the basaltic maria, and recorded the history of meteorite bombardment on the lunar surface. Previously unknown minerals were also discovered, and scientists studied the effects of the lunar environment on materials and life (some microorganisms were even brought back to Earth to examine their survival).

Parallel to Apollo, the Soviet Union continued lunar exploration through its Luna robotic program. It achieved the first successful soft uncrewed landing (Luna 9 in 1966), robotic sample returns (Luna 16, 20, and 24 between 1970 and 1976), and deployed the first remotely operated lunar rover, Lunokhod 1, in 1970. Lunokhod 1 traveled 10 km, transmitting images of the surface before its mission ended. These Soviet achievements pioneered the use of robots to explore another world.

Decades later, new nations joined lunar exploration. Japan, the European Space Agency (ESA), China, India, and once again the United States have sent orbiters and landers in the 21st century. Notably, China successfully landed robotic vehicles (the Yutu rovers of the Chang’e missions) and even achieved a landing on the far side of the Moon in 2019 (Chang’e 4). Many of these recent missions have enriched lunar mapping and led to significant discoveries, such as the detection of water ice in polar craters. Indeed, observations from orbiters (such as NASA’s Lunar Reconnaissance Orbiter, LRO) and deliberate impact experiments have confirmed the presence of water in the form of ice in permanently shadowed regions near the lunar poles. This ice is mixed with the regolith and could become a valuable resource for future missions—for example, to obtain drinking water, breathable oxygen, or rocket fuel through electrolysis.

Today, lunar exploration is experiencing a renaissance. The United States, through NASA’s Artemis program, plans to return humans to the Moon in the coming years. Artemis aims to land the first woman and the first person of color on the Moon during the crewed Artemis III mission (scheduled for the middle of this decade). Beyond repeating Apollo landings, Artemis seeks to establish a sustainable long-term human presence on the Moon. This includes building a lunar orbital station called Gateway, in collaboration with international agencies (NASA, ESA, JAXA, CSA), and creating surface bases or camps—possibly at the lunar south pole, where ice deposits are concentrated. The vision is for the Moon to serve as a testbed for technologies needed for life beyond Earth and as a stepping stone for deeper-space missions, including eventual crewed journeys to Mars.

Meanwhile, China and Russia have announced plans for an international lunar base in the next decade, and various private companies are also interested in the Moon (for space tourism or resource utilization). In 2023, missions such as Artemis I (uncrewed) successfully orbited the Moon, testing the new Orion capsule and SLS rocket and paving the way for astronauts to return. The renewed interest in our natural satellite is driven both by the desire for scientific exploration—studying its geology, searching for resources, testing technologies—and by its strategic value in the new era of space exploration.

The Moon has gone from being a mysterious point of light in the sky to a familiar world thanks to science. Its formation after a giant impact reminds us of our shared origins; its craters and rocks tell the history of the Solar System; its cycles and forces continue to influence Earth; and its gray deserts await humanity’s return. The Moon remains a beacon for space exploration and a source of wonder, research, and inspiration for present and future generations.