Lunar and Planetary Institute

Marvel Moon - Background
EXPLORE! To the Moon and Beyond with NASA's LRO Mission

Marvel Moon

What does the Moon mean to you? Does it call up stories of werewolves or memories of moonlit walks? Have you imagined friendly shapes, like the "Man in the Moon," a rabbit, or a tree in its bright, full face? Cultures throughout the centuries have created stories about the night's brightest visitor and occasional companion of the daytime Sun. Poets have extolled their personal connection to the romantic Moon. It inspires songwriters ("Fly Me to the Moon") and plays a key role in children's stories ("Hey Diddle Diddle" and Goodnight Moon). Ancient and contemporary astronomers made scientific breakthroughs by studying its motions and appearance. The Moon marked a major landmark in exploration with the Apollo landings when humans first stepped on another world in 1969. Scientists now study the Moon because it records the history of our early solar system — and Earth — and it will play a pivotal role in our future exploration of the solar system.

Turbulent Beginnings

The tranquil, friendly Moon in our sky today that inspires cultural stories, urban legends, and romance was forged through a 4.5-billion-year history of turmoil.

The current scientific hypothesis holds that our Moon was born of a calamitous giant impact. Shortly after the planets in our solar system formed, 4.5 billion years ago, Earth did not have a Moon. The moonless Earth was a very different place. The surface was hot, and nothing lived there. Earth's distinctive oceans and continents had not yet formed. Instead of a blue-green planet, Earth was glowing red with rivers and seas of lava.


The debris of formation still littered the solar system. For millions of years, Earth and another small planetary body orbited the Sun in the same region of our solar system. The small planetary body's orbit brought it across Earth's path and they collided. It smashed into Earth at twenty-five thousand miles an hour and plunged toward Earth's core.

The collision occurred within the space of about ten minutes, and left both Earth and the impactor changed forever. The impactor shattered, and its remains were blown into space or incorporated into the Earth. The core of the colliding object combined with the Earth and its own dense core. Earth's upper layers were melted. The small planetary body didn't hit the center of Earth, like a bulls-eye, but off to the side, pushing Earth into a faster spin. It may also have knocked Earth so that it tipped over a little, helping to give Earth the 23.5° tilt of its axis that it has today.
Giant impact

A giant impact is believed to have occurred between the early Earth and a small planetary body, which measured half its width, 4.5 billion years ago.

Credit: Lunar and Planetary Institute.

Shattered pieces of the Earth’s outer layers together with debris from outer layers of the impactor, were thrown into orbit around the Earth. For a time, Earth had a ring around it.

Shattered pieces of the Earth's outer layers together with debris from outer layers of the impactor, were thrown into orbit around the Earth. For a time, Earth had a ring around it.

Earth ring Credits: Lunar and Planetary Institute.


Over a short time — perhaps a hundred years or less — the ring of vapor, dust, and molten rock clumped — accreted. The pieces collided, and if they collided slowly enough, they stuck together. The collisions were random; sometimes particles stuck and sometimes they did not — sometimes collisions caused particles to break apart! As the clumps grew larger, they eventually reached a point where they were large enough to have their own field of gravity. The largest attracted more and more particles, growing larger faster and faster to form our Moon. Just after its birth, the Moon was 15 times closer to Earth and Earth's day was only six hours long!

Credit: Lunar and Planetary Institute.

Moon forms

The Giant Impact Story Is an Unfolding Tale
The story of the Moon's birth continues to intrigue scientists. Some pages of this story are missing.

The Moon's characteristics provide clues of its origins, which are best explained by the current leading scientific model of the Moon's origin — the Giant Impact hypothesis. The motions and orientation of the Earth and Moon record the physics that must have led to their particular configuration. The Earth-Moon pair has an unusually large amount of rotational energy, which is combined in both the Moon's orbit around the Earth and Earth's spin. The Moon's orbit and Earth's spin are consistent with the Giant Impact hypothesis — the impact added rotational energy to the Earth-Moon system. The Moon has a much smaller core than our Earth. This, too, is consistent with the model; the glancing blow of the impact stripped part of the outer layers of the impacting object and Earth to form the Moon. The core of the colliding object combined with the Earth and its own dense core. The Moon was formed with much less iron and other heavy elements to form its core. Lunar rock samples and meteorites contain the chemistry of the Moon and support the Giant Impact hypothesis. The searing heat of the impact drove away most of the gases and liquids, leaving a relatively dry world. Moon rocks have very little of the water and gases found in Earth's rocks.

The Giant Impact hypothesis is the leading framework for explaining the current scientific evidence, but there are many unanswered details. To untangle the complex physics behind the Giant Impact hypothesis, the NASA Lunar Science Institute (NLSI) Center for Lunar Origin and Evolution (CLOE) team is using powerful computer models and information about the chemistry of early Earth and Moon rocks from other NLSI teams. With their models, the CLOE team will help determine how — or even if it is possible — the disk evolved into the Moon we see today. Future planetary scientists — the children of today — may continue to refine this story or even propose new models as new data are collected!

The Moon's Childhood of Turmoil

Infancy: Differentiation — The slamming together of debris to form the Moon might have created enough heat to melt the outer surface of the Moon, or perhaps the entire Moon! Like all terrestrial planetary bodies, the Moon underwent a process of differentiation early in its history. Its bulk, brought together by the process of accretion, settled into layers. The heavier iron separated from the less-dense rock in the mantle and sank, forming a small core. The Moon's oldest rocks record the slower processes that shaped its mantle and crust. These oldest rocks could only have formed if the infant Moon had been covered by an ocean of liquid rock — a magma ocean.

Moon's interior

The Moon was originally a mixture of materials ejected by the giant impact that created it. Through the process of planetary differentiation, its interior organized into layers of different densities: a dense metallic core, a rocky mantle of intermediate density, and a rocky crust that is comprised of the least dense materials.

Credit: Lunar and Planetary Institute.

Differentiation within the magma ocean produced the features we still see on the Moon today. The uppermost part of the Moon's crust is mainly the rock anorthosite, which is primarily made of a single mineral: low-density, aluminum-rich, plagioclase feldspar. This mineral floated to the top of the magma ocean and crystallized to form a light-colored rock, anorthosite. This slow crystallization from magma is the only known way to produce the single-mineral crustal igneous rock, anorthosite. This rock forms the "lunar highlands," the brighter, light-colored, heavily cratered regions we see on the Moon. Deeper parts of the Moon's crust and mantle include larger amounts of other minerals, such as pyroxene and olivine. These denser materials sank out of the magma ocean, and were the source, in part, for the basalt lavas that later erupted from lunar volcanos and filled in low-lying areas to form the darker-colored plains.

60025 76535
Apollo Rock Sample 76535
Age: 4.2-4.3 billion years
Apollo Rock Sample 60025
Age: 4.44-4.51 billion years

These are two examples of anorthosite rocks that crystallized from the lunar magma ocean. They are made of the less-dense, light gray minerals that can be seen in the brighter regions of the Moon. They provide geologists with a snapshot of the Moon's infancy.

Credit: NASA.

Moon enveloped by a deep ocean of molten rock The magma ocean cooled and crystallized over a period of 50–100 million years to form a lunar crust that is, on average, about 42 miles (68 kilometers) thick. The oldest rocks collected by Apollo astronauts are 4.5 billion years old, which is thought to indicate when the Moon's crust solidified. CLOE scientists are working to determine how hot and deep the lunar magma ocean was, which in turn will allow them to understand how long it took to cool and crystallize.
When the Moon formed, it was enveloped by a deep ocean of molten rock.

Credit: Lunar and Planetary Institute.


Young Moon: Big Impacts Form Big Basins — The Moon was born into a solar system still in the throes of formation. Debris orbited the Sun alongside the nascent planets and our Moon. For the first 600 million years of its existence, large asteroids and comets continued to strike the Moon and the planets in our solar system — including Earth. These impacts are recorded as the largest gouges on the Moon, including the large circles that have since been filled in with darker rock to form the familiar "Man in the Moon." By about 3.8 billion years ago, much of the debris in the solar system had been swept up into the planets and their moons, corralled into the asteroid belt, or relegated to the outer reaches of our solar system, and impact strikes were smaller and less frequent.

Imbrium Rim

Imbrium Rim

Orientale Basin

Orientale Basin

Both Earth and Moon were struck by numerous large asteroids and comets in their early history, until 3.8 billion years ago. These impacts produced deep basins up to 1000 km across surrounded by high rings of mountains on the Moon and are visible to the human eye as prominent circular structures.

Left: A view of the mountains that surround the Imbrium impact basin. The smooth, dark regions on the right side of the image are younger lava flows. Right: Three mountain rings surround the Orientale impact basin. Both the Imbrium and the Orientale impacts occurred around 3.8 billion years ago. Impact events continue on all moons and planets today, but on a much smaller scale.

Credit: NASA.

The details of this rain of planetary debris are a mystery. Lunar scientists at the Center for Lunar Origin and Evolution are investigating whether this intense bombardment occurred in several waves, one massive storm, or spread out over a long period of time.

Number of Impacting Asteroids

Lunar scientists are currently addressing several outstanding questions about this major event in the Moon's — and our solar system's — history: How long did these massive impacts continue? How many massive objects struck the Moon— and Earth? Were the impacts spread out over a period of time (as depicted in yellow in the graph) as solar system formation drew to a close?

Some scientific evidence suggests a more intense storm of impacts, which peaked 3.9 billion years ago (as shown in red line.) Scientists call this event the Late Heavy Bombardment or the Lunar Cataclysm. What could have caused this downpour of asteroids or comets?

Credit: Lunar and Planetary Institute.

The answers to this puzzle from ancient solar system history lie on the Moon. Impacts threw out — ejected — the rocks far from the crater, where they were collected by Apollo astronauts and brought back to Earth. Scientists continue to study the many pieces contained in each rock. Breccias provide scientists with the timing of basin formation, ranging from 3.8 to 4.0 billion years ago, and help indicate whether the impacts were caused by asteroids and comets. Computer models trace the motions of asteroids and comets in the early solar system to determine how they might have moved — and when — to strike the Moon and other planets.

Apollo Rock Sample 14306

Apollo Rock Sample 14306, viewed
through a microscope

The force of large impacts fragmented the original lunar rocks and compressed them into new, complex rocks known as breccias.

Credit: NASA


Moon craters Many of the Moon's largest craters have been dated using rocks from the Moon. Sometimes, portions of breccias melt and resolidify, which allows the age of the impacts to be measured using radiometric dating methods.

Credit: Planetary Science Research Discoveries/NASA.

The answers will also lead to other questions. The first evidence of life on our planet appears in the geologic record at the very end of the heavy impacts. What effect did this downpour of asteroids have on our own planet? Did it prevent life from taking hold earlier? Did impacts play a role in the evolution of life itself by delivering trace elements that gave a boost to primitive life?

Teenage Angst: Lunar Volcanism – While cool on the outside, portions of the Moon's interior were still hot, heated by radioactive decay of unstable isotopes of elements, such as uranium and thorium, and the processes of accretion and differentiation. Isolated pockets of hot mantle material slowly rose to the surface, melting at lower pressures. Molten rock flowed onto the lunar surface through cracks in the lunar surface — fissures — many of which were created by the earlier impacts. The magma flooded across the lowest regions on the lunar surface to fill the impact basins. It crystallized quickly, forming basalt, a dark, fine-grained, volcanic rock. The composition of the basalt varies because the magma formed in different places in the lunar interior. Some basalts have more titanium; others are more enriched in other elements such as potassium and aluminum.

Lunar volcanic rocks

Orange soil

These rocks are typical of lunar volcanic rocks. Both are 3.3 billion year old basalts, similar to those produced by volcanos such as Hawaii on Earth. The lower image (sample 15016) contained some type of gas, possibly carbon monoxide, which formed the round holes known as vesicles.

Credit: NASA

This "orange soil" crystallized when it erupted from a fire fountain 3.64 billion years ago.

Credit: NASA.

Imagine standing on the Moon at this time. Hot basalt lava flowed from long fissures, filling regions of low elevation. Fountains of lava sporadically erupted along the fissures, spewing molten rock high above the lunar surface. Chilled magma droplets fell back as beads of colored volcanic glass, later sampled by Apollo astronauts. Flowing lava cut channels into the landscape. In a few locations, small volcanic domes built up on the surface. Gradually, as the Moon's interior cooled, volcanism ceased. Lunar volcanism decreased significantly by 3 billion years ago and ceased completely by about 1 billion years ago as the interior cooled.

The large, smooth, dark regions we see on the Moon are the basaltic "lunar maria." "Maria" is Latin for "seas," as these areas looked like seas to early astronomers. They are smooth because they are less cratered than the lunar highlands. The smaller number of craters in the maria suggests that these regions have not been impacted as much and therefore are younger. Mare basalts have been radiometrically dated to be between 3.0 and 3.8 billion years old.

Lunar impact basins

The Moon bears large, somewhat circular, dark basins across its face. These are the basalt-filled ancient impact basins. In spite of this exciting history, the Moon has been geologically inactive for at least the last billion years.

Pronunciation Guide

Mare (pronounced mahr-ay) is the singular term for one of these features. Maria (pronounced mahr-ee-uh) is the plural form.



Mare Imbrium Southwestern Mare Imbrium
Mare Imbrium Southwestern Mare Imbrium

Spacecraft have glimpsed the plains of dark basaltic rock from orbit, such as that which fills the circular Imbrium Basin (left image). At right, the edges of a long lava flow form distinctive lines from the lower left to the upper right of the image. The volcanism that formed Mare Imbrium occurred about 3.3 billion years ago.

Credit: NASA.

Adulthood: Ongoing Impacts and Human Exploration — For the last one billion years, our Moon has been geologically inactive except for small meteoroids pummeling its surface, breaking the rocks and gradually adding to the layer of fine lunar dust — regolith — that covers the surface. In some places, the regolith may be thicker than 50 feet (15 meters). The Moon has no atmosphere, flowing water, or life to erode or disturb its surface features. Other than impactors, only a few spacecraft, and the footsteps of 12 humans, have reshaped its landscape. Even the most ancient craters are preserved — although they may be covered by other craters and the debris from impacts!

Credit: NASA.


Vredefort crater

The Earth has also been hit by asteroids and comets throughout its history — and the Moon is too far away to serve as a shield from these impacts .In fact, Earth's greater gravitational pull causes it to attract more impactors than the Moon. The Vredefort crater in South Africa was probably about 90 miles (140 kilometers) across when it first formed about 2 billion years ago. Erosion and sediment cover have reduced the exposed crater to about (50 miles) 80 kilometers in diameter.

Space shuttle image STS51I-33-56AA, courtesy of NASA.

Scientists determine the relative ages of cratered surfaces, such as those on the Moon, by counting craters! The more craters a surface has, the older it is — just like the more candles you have on your birthday cake, the older you are. Scientists also use the same observations the children did — superposition and crosscutting of features — to determine which crater happened first, second, third, and so on.

The Lunar Reconnaissance Orbiter, which is currently orbiting the Moon, is sending back high-resolution photographs of the lunar surface. Scientists use these images, and images from other lunar orbiters, to understand which areas of the Moon formed as part of its original crust and the relative ages of younger surfaces.

While craters allow scientists to determine the relative ages of surfaces on the Moon, the Apollo astronauts brought back rock samples that scientists radiometrically dated to determine the absolute ages. These samples come from only six places on the Moon's surface; lunar researchers now know the absolute ages of the basins in these places. With these specific sample ages, the scientists can extrapolate across the lunar surface. In this way they can determine the ages of other features and they can calibrate the ages of surfaces they dated by crater counting.

The data returned by orbiting spacecraft and the Apollo program reveal much about the formation and evolution of our Moon and, in turn, of our own Earth. Resurfacing processes active on Earth have obscured our planet's early history of formation, differentiation, and asteroid bombardment. Because the Moon is relatively quiescent, it holds the best-preserved record of our solar system's early history and the processes of formation, differentiation, and bombardment which shaped all terrestrial planets. New missions are helping scientists piece together details of the history and evolution of the Moon (and Earth) and will help us better understand lunar processes and the distribution of resources.

Earth's Companion

Earth and Moon

Credit:  NASA.

Throughout this long history, the Moon has been Earth's companion in space. They shaped each other's destinies — and perhaps created an environment suitable for burgeoning life — through the invisible — but powerful — connection of their gravitational pull on each other. The Moon's gravitational pull is relatively weak compared to Earth's. (Apollo astronauts were able to leap across the lunar surface because of this weaker pull.) Yet, the Moon's gravitational pull is responsible for Earth's current length of day, stable seasons, and tides.

Length of Day – Earth and the Moon are gravitationally linked in an eternal dance. These partners have a lot of mass, spin, and orbital motion — angular momentum — compared to other planets and moons. Tracing the Moon's motion back over the eons brings it closer to the Earth. Imagining the Moon "rewind" to its infancy is like watching a dancer pull in her arms:  The Earth and Moon spin faster in their dance.

During the Moon's infancy, the Earth was spinning at a much faster rate. Computer models can trace the Moon's and Earth's motions backward in time. According to these models, the infant Earth had a six-hour day 4.5 billion years ago! Since then, with the help of our Moon, Earth has been slowing down and our days have been getting longer. Scientists count growth rings in fossil corals and shells and ancient photosynthetic bacteria layers, called stromatolites, like tree rings. Stromatolites living 850 million years ago record a day length that was about 21 hours long. Fossil corals from 400 million years were living on an Earth with 22-hour days.

Over time, the Moon's gravitational pull on the Earth "stole" some of Earth's spin energy, launching the Moon slowly into higher and higher orbits. (The Apollo laser experiments confirmed that the Moon is moving away at the rate of two inches (five centimeters) per year.) The distance between Earth and Moon increased and the spins of both decreased. Today, Earth spins around once in a relatively sedate 24 hours.

The more distant Moon of today takes over 27 days to complete one full orbit around Earth. Just like Earth, our Moon rotates on its own axis and experiences daylight and dark cycles. Our Moon's day and night cycles are longer than Earth's — the Moon spins on its axis once every 27.3 days. The Moon takes the same amount of time to spin around once as it does to orbit completely around Earth. This means that Earth observers always see the same side of the Moon (called the "nearside"). The side we do not see from Earth, called the "farside," has been mapped during lunar missions.

Credit: NASA.


Nearside view of the Moon

Nearside view of Earth's Moon as seen
by the Galileo spacecraft.

Farside view of Moon

Farside view of Earth's Moon as seen
by the Clementine spacecraft.

Stable Seasons – The giant impact was a catastrophic event for Earth. The collision changed Earth's orbit around the Sun — contributing to its present-day position — and altered its orientation in space. The impact may have tipped the Earth a little and contributed to the 23.5° tilt of our North Pole away from "straight up." This tilt gives us our seasons — and the very presence of the Moon helps to keep Earth's climate relatively stable.

As Earth orbits the Sun, its axis points to the same fixed location in space. By a convenient coincidence, Earth's northern axis points almost directly toward Polaris, the North Star.

Earth as it orbits the Sun

This picture shows Earth from its side as it orbits our Sun. The axis is tilted and points to the North Star no matter where Earth is in its orbit. Because of this, the distribution of the Sun's rays changes. In June, the Sun's rays warm the northern hemisphere all the way to the north pole. In December, in the northern hemisphere winter, the north pole points away from the incoming sunshine.

Credit: Lunar and Planetary Institute.

The "fixed" tilt means that, during our orbit around our Sun each year, different parts of Earth receive sunlight for different lengths of time.  It also means that the angle at which sunlight strikes different parts of Earth's surface changes through the year. Sunlight striking the surface at an angle is "spread" across a wider area compared to sunlight striking perpendicular to Earth's surface. Areas that receive sunlight that is more scattered receive less energy from our Sun.  All of these factors combine to give Earth its annual cycle of seasons!

For part of our orbit, the northern half of Earth is pointed toward the Sun. This is summer in the northern hemisphere; there are longer periods of daylight, the Sun is higher in the sky, and the Sun's rays strike the surface more directly, giving us warmer temperatures.  The north pole is in constant daylight!

When the northern half of Earth is pointed toward the Sun, the southern hemisphere is tilted away. People in the southern hemisphere experience the shorter day lengths and colder temperatures of winter.

During winter in the northern hemisphere, our northern axis continues to point to the North Star, but, because we have moved in our orbit around the Sun, our northern hemisphere now points away from our Sun. The north pole is completely dark and other places in the northern hemisphere experience the shorter day lengths and colder temperatures of winter as the Sun traces a lower arc across the southern sky and the Sun's rays strike the surface at a lower angle. When it is winter in the northern half of Earth, the southern hemisphere, tilted toward our Sun, has summer. 

During fall and spring, some locations on Earth experience similar, milder, conditions. Earth has moved to a position in its orbit where its axis is more or less perpendicular to the incoming rays of the Sun. The durations of daylight and darkness are more equally distributed across all latitudes of the globe.

The seasons are not caused by how far Earth is from our Sun. Earth's orbit around our Sun has a slightly elliptical path (very slight!), and the Sun is not exactly in the center of the ellipse. This means that, during the year, Earth is sometimes farther from our Sun, and sometimes closer — but the difference is small (not so for some other planets!). Earth is closest to our Sun in January (perihelion) and the farthest away in July (Earth is 147.5 million kilometers from the Sun when it reaches aphelion). If distance were the most important factor, the entire Earth would have summer in January when we are closest to our Sun and winter in July when we are farthest away!

The Moon's gravitational pull acts like training wheels for Earth on its journey around the Sun. It keeps Earth’s axis pointed roughly in a consistent direction — although it does drift slightly over a period of about 26,000 years. Without the Moon, the Earth's stately progression through spring, summer, fall, and winter would have fluctuated widely over time. Some eons would have enjoyed the relatively stability of our current seasonal pattern, while others would have experienced the heat and ice of one pole facing continually toward the Sun and the other left in icy darkness.

Tides – The Moon's gravitational pull tugs on Earth — especially the portion that is nearest to it as it travels in orbit around Earth. Earth's crust rises slightly (several centimeters) due to this force. Ponds and lakes — such as the Great Lakes — experience small tides, as well. Earth's oceans, however, are free to lift many feet in response this tug. As the Moon orbits the Earth, it drags along behind it a "bulge" in the oceans.

If you've ever tried to push a stalled car in neutral from a dead stop, you've experienced the same force that counteracts the Moon's gravitational pull:  inertia. On the side of Earth opposite the Moon, the universal tendency for objects to resist a change in its movement wins out over the gravitational pull. The ocean here can keep going in its original direction. Near the Moon, the oceans are pulled upward and along. Opposite the Moon, the oceans are "left behind."

Earth's oceans from space

In this very, very exaggerated image of Earth's oceans from space, water bulges upward from the surface in two opposite locations on Earth's surface. Earth continues to spin, bringing different coastlines under the bulge throughout the course of a day. Most coastlines experience two tides each day as they pass through the two bulges.

Credit: National Oceanic and Atmospheric Administration.

The Moon's contribution to Earth's tides is significant because it is so close. The Sun, of course, also exerts a gravitational pull on Earth — that's what keeps Earth in a steady orbit. Earth's oceans are pulled toward the Sun, but the Sun's gravitational pull contributes only about a third of the tides’ height.

Early in Earth's history, the Moon was even closer to Earth. Billions of years ago, the Moon was 10 times closer and tides were 1000 times higher. Scientists believe that these extreme tides occurred once every three hours because the Earth was spinning more rapidly. The tides eroded the coastal areas, adding minerals to the oceans. These minerals may have been essential for life to evolve as quickly as it did.

Human Inspiration

Humans have long had a personal, cultural, and scientific connection to the Moon. Stories were told about its changes in apparent shape over time as it cycled through its lunar phases. Many words in our language contain references to the Moon: moonbeam, moonwalk, loony and lunatic, Monday and month. Our calendars are divided into months that reflect the length of the lunar cycle, and some calendars continue to be based on the lunar cycle rather than on our yearly orbit around the Sun. The Moon inspired myths and urban legends. Its surface features and motions were a relatively nearby opportunity for scientific scrutiny.

Shining Light – Aside from the gravitational influences described above, the Moon's only other influence on Earth is moonlight. Just like the planets, our Moon does not produce its own light. It "shines" because it reflects the Sun's light. Careful statistical studies have shown no correlation between the full Moon and any number of affects highlighted in urban legends, such as the birth rates of babies or the menstrual cycle, or human behaviors, including homicide rates and traffic accidents. The full Moon's light does make it easier for humans and other animals to see — and be seen. Studies have documented changes in the success rates of predators and foraging patterns of prey animals due to this added nighttime illumination. Corals time their mating events by the light of the Moon. Most other animal behaviors relating to the Moon are because the tides change the coastal environment, alternately exposing and covering it.

Despite the exaggerated size of the Moon in movies, books, and art, the Moon's apparent size is relatively small — and constant — in the sky. Nearly everyone has experienced the illusion that the Moon's appearance is magnified on the horizon. However, it takes up about the same number of degrees in the sky when it is near the horizon and straight overhead! Psychologists don't yet have a precise explanation for the "Moon illusion," but it may relate to our perception of the sky as an inverted bowl rather than a hemisphere. The dome of the sky directly overhead seems closer than the edge of the bowl, near the horizon. In reality, the sky is not a shallow bowl but a hemisphere. As the night progresses, the Moon rises to the top of the hemisphere, rather than along the bowl we perceive. The brain may perceive the Moon overhead as smaller than it was on the horizon.

Changing Shape – The changing appearance of the Moon has inspired stories, songs, poems and words (e.g. "month"). It is used to keep time (e.g. lunar calendar), especially historically, when nighttime activities like farming and hunting were illuminated only by natural lighting. The repeating pattern inspired astronomers to think scientifically to explain the phenomenon and challenged incorrect theories.

Our Moon's shape doesn't really change — it only appears that way! The "amount" of Moon that we see as we look from Earth changes in a cycle that repeats about once a month (29.5 days). The relative positions of our Sun, Earth, and Moon, cause these changes.

As our Moon orbits around Earth, the side facing the Sun is always illuminated, just as Earth's daylight side is illuminated by the Sun. What we see from Earth, however, is a different story. Starting with the dark new Moon, we see the light part of the Moon "grow" from a sliver to a half to a full Moon — and then the illuminated part decreases, becoming thinner until there is no visible Moon in the sky and we are at the new Moon part of the cycle again.


Phases of the Moon

Credit: Lunar and Planetary Institute

We have a "new Moon" when our Moon's orbit around Earth moves it between Earth and the Sun. From Earth, the Moon's surface looks dark because the illuminated side is facing away from Earth. As our Moon continues its orbit counterclockwise around Earth (viewed from above the North Pole), more and more of the illuminated part of the Moon becomes visible to us, until it reaches the "full Moon" stage. A full Moon occurs when the Moon has moved in its orbit so that Earth is “between” the Moon and the Sun.

Between the new and full Moon, the amount of Moon we see grows — or waxes from its right side toward its left side. As it passes the full Moon stage, the amount of illumination decreases — or wanes — from right to left. Finally, the Moon returns to its position between the Earth and the Sun, and on Earth, we observe the new Moon again.

In the southern hemisphere, illumination of the Moon increases from the left to the right side in the waxing phase and the dark part increases in coverage from left to right in the waning phase, which is opposite of the northern hemisphere. No matter where on Earth an observer is, however, the phases of the Moon occur at the same time.


Nearby Stepping Stone to the Cosmos – The Moon has sparked revolutions in scientific thought throughout history. Before Galileo and his contemporaries turned their telescopic sights on the Moon, they expected to find a smooth sphere that fit in with the established view of the heavens as the realm of perfection. The reality of its rough surface helped change that long-held hypothesis. Astronauts left Earth's cradle, bridged the intervening 238,500 miles to land on its surface, and safely returned with the aid of revolutionary new technologies. The Apollo Moon rocks unlocked the origin of craters — not only on the Moon, but on Mercury, Venus, and Mars — from impacts rather than volcanos. They also overturned existing hypotheses of the Moon's formation and led to the Giant Impact hypothesis as the predominant explanation of its beginnings.

Scientists continue to look to the Moon to understand Earth. Its ancient rocks provide scientists with new views of early Earth, how the Earth-Moon system and the solar system formed and evolved, and the role of asteroid impacts in influencing Earth's history — and possibly future! The Moon is the only place in our inner solar system that is relatively unaltered since its formation; the surfaces of Mercury, Venus, Earth, and Mars have all been altered by weathering or volcanic and tectonic activity. The Moon's surface preserves evidence that has been lost on these planets, including a record of the Late Heavy Bombardment from 4.0 to 3.8 billion years ago. Lunar scientists hope to answer questions about that solar-system wide rain of asteroids or comets by studying the Moon. The Moon also offers a relatively near-by opportunity to study the processes of planetary differentiation, volcanism, and ongoing impacts. A more thorough collection of samples from its surface may find rocks that were blown off of Earth during impacts and landed on the Moon. Earth's distant past — including, perhaps, a fossil record of early life — may be stored on the Moon in these ejected Earth rocks.

The Moon presents numerous exciting engineering challenges. It is an excellent place to test technologies, flight capabilities, life support systems, and exploration techniques to reduce the risks and increase the productivity of future missions. It offers a nearby base for future colonies, where humans can experience the challenges of living and working on another world and test advanced materials and equipment in the temperature and radiation extremes of space. Establishing an outpost on the Moon would be like camping in the backyard before venturing out on a major expedition. Using the Moon as a test bed would enable Earth's adventurers to extend exploration and settlement to planets and moons beyond Earth.

Many astronomers, geologists, planetary scientists, engineers, and astronauts owe their productive careers to childhood memories of the Moon. What other great personal, cultural, and scientific achievements will the Moon inspire next?

Thanks to the Moon

What if the giant impact had not occurred? How would Earth — and life itself — be different without the Moon?


On Companion Earth

On Moonless Earth


Length of day

24 hours

8 hours

Life has altered patterns of waking, sleeping, mating, and hunger
Winds blow at 100 mph due to Earth’s faster rotation


Earth’s large Moon stabilizes the variation of its tilt so that it “wobbles” only about 3° over a cycle of about 41,000 years

Gravitational pull of the giant planets causes Earth's tilt to vary wildly over the eons between 0 and 80°

Only the hardiest bacteria or other simple life forms can endure the drastic climate changes  


Governed by Moon's gravitational pull

Caused only by the Sun and rise only about 1/3 current height

Life takes longer to evolve without the benefit of minerals mixed into the global ocean by tides


The Moon changes its appearance in the sky through a predictable pattern

Venus and Mercury undergo phases, but these are not discovered before the invention of a telescope

Science advances more slowly without the repeating pattern of lunar phases to inspire astronomers to explain the phenomenon scientifically


Sunlight reflects off of the lunar surface, making the Moon visible in the sky

The Sun, other stars, and planets are the only celestial objects in the sky

Some unique animals  behaviors (e.g. coral spawning events, moon wrasse foraging, salmon biological changes) do not occur or are tied to some other external stimulus


Astronauts walk on the Moon during the Apollo missions, 1969-72

There are no objects within the technological reach of current rockets and radiation shielding that are appropriate for manned explorations

Humans are not able to visit another world so early in their technological lifetime and may not be inspired to do so without a nearby goal


Last updated
May 9, 2011