How the Moon Works

As we mentioned, the first thing that you'll notice when you look at the moon's surface are the dark and light areas. The dark areas are called maria. There are several prominent maria.

The maria cover only 15 percent of the lunar surface.

The remainder of the lunar surface consists of the bright highlands, or terrae. Highlands are rough, mountainous, heavily cratered regions. The Apollo astronauts observed that the highlands are generally about 4 to 5 km (2.5 to 3 miles) above the average lunar surface elevation, while the maria are low-lying plains about 2 to 3 km (1.2 to 1.8 miles) below average elevation. These results were confirmed in the 1990s, when the orbiting Clementine probe extensively mapped the lunar surface.

The moon is littered with craters, which are formed when meteors hit its surface. They may have central peaks and terraced walls, and material from the impact (ejecta) can be thrown from the crater, forming rays that emanate from it. Craters come in many sizes, and you'll see that the highlands are more densely cratered than the maria.

Another type of impact structure is a multi-ringed basin. These structures were caused by huge impacts that sent shockwaves outward and pushed up mountain ranges. The Orientale Basin is an example of a multi-ringed basin.

Besides craters, geologists have noticed cinder cone volcanoes, rilles (channel-like depressions, probably from lava), lava tubes and old lava flows, which indicate that the moon was volcanically active at some point.

The moon has no true soil because it has no living matter in it. Instead, the "soil" is called regolith. Astronauts noted that the regolith was a fine powder of rock fragments and volcanic glass particles interspersed with larger rocks.

Upon examining the rocks brought back from the lunar surface, geologists found the following characteristics:

Astronauts placed other scientific packages on the moon to collect data:

Let's look at what all of this information tells us about the formation of the moon.

Where did the moon come from?

At the time of Project Apollo in the 1960s, there were basically three hypotheses about how the moon formed.

But based on the findings of Apollo and some scientific reasoning, none of these hypothesis worked very well.

The Giant Impact Theory

Because none of these hypotheses was satisfactory, scientists looked for another explanation.

In the mid-1970s, scientists proposed a new idea called the Giant Impactor (or Ejected Ring) hypothesis. According to this hypothesis, about 4.45 billion years ago, while t­he Earth was still forming, a large object (about the size of Mars) hit the Earth at an angle. The impact threw debris into space from the Earth's mantle region and overlying crust. The impactor itself melted and merged with the Earth's interior, and the hot debris coalesced to form the moon.

The Giant Impactor hypothesis explains why the moon rocks have a composition similar to the Earth's mantle, why the moon has no iron core (because the iron in the Earth's core and impactor's core remained on Earth), and why moon rocks seem to have been baked and have no volatile compounds. Computer simulations have shown that this hypothesis is feasible.

­Based on analyses of the rocks, crater densities and surface features, geologists came up with the following geologic history of the moon:

Let's look at some of the phenomena involving the moon's orbit.

Every night, the moon shows a different face in the night sky. On some nights we can see its entire face, sometimes it's partial, and on others it isn't visible at all. These phases of the moon aren't random — they change throughout the month in a regular and predictable way.

As the moon travels in its 29-day orbit, its position changes daily. Sometimes it's between the Earth and the sun and sometimes it's behind us. So a different section of the moon's face is lit up by the sun, causing it to show different phases.

Over the billions of years of the moon's existence, it has moved farther away from the Earth, and its rate of rotation has also slowed. The moon is tidally locked with the Earth, which means that the Earth's gravity "drags" the moon to rotate on its axis. This is why the moon rotates only once per month and why the same side of the moon always faces the Earth.

The moon has eight phases in each lunar cycle: four primary phases and four secondary phases. The four primary phases are:

These occur at a specific time each month, and you can check the exact time to see if the moon will be risen and visible in your location as it reaches that phase.

The four secondary phases are not at a specific time; they are transitions that the moon goes through between the primary phases. For instance, the moon may stay in one of these phases for seven days. They are:

"Waxing" means the illuminated part of the moon is growing and "waning" means the illuminated part is shrinking. The "crescent" moon is also a familiar concept, as the moon appears in the shape of a crescent during this time. Perhaps the most novel term in the lunar calendar is "gibbous," which broadly means convex, but in astronomy specifically refers to the time when the moon is more than half but not wholly illuminated.

The eight phases in order are: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter and waning crescent. For more details, see our article on moon phases.

Tides

Every day, the Earth experiences tides, or changes in the level of its oceans. They're caused by the pull of the moon's gravity. There are two high tides and two low tides every day, each lasting about six hours.

The moon's gravitational force pulls on water in the oceans and stretches the water out to form tidal bulges in the ocean on the sides of the planet that are in line with the moon. The moon pulls water on the side nearest it, which causes a bulge toward the moon. The moon pulls on the Earth slightly, which drags the Earth away from the water on the opposite side, making another tidal bulge there. So, the areas of the Earth under the bulge experience high tide, while the areas on the thin sides have low tide. As the Earth rotates underneath the elongated bulges, this creates high and low tides about 12 hours apart.

The moon also stabilizes the Earth's rotation. As the Earth spins on its axis, it wobbles. The moon's gravitational effect limits the Chandler wobble to a small degree. If we had no moon, the Earth might move almost 90 degrees off its axis, with the same motion that a spinning top has as it slows down.

Since 1972, no one has set foot on the moon. However, there is a renewed effort for a lunar return. Why? In 1994, the orbiting Clementine probe detected radio reflections from shadowed craters at the moon's South Pole. The reflections were consistent with the presence of ice. Later, the orbiting Lunar Prospector probe detected hydrogen-rich signals form the same area, possibly hydrogen from ice.

Where could water on the moon have come from? It was probably carried to the moon by the comets, asteroids and meteors that have impacted the moon over its long history. Water was never detected by the Apollo astronauts because they didn't explore that region of the moon. If there is indeed water on the moon, it could be used to support a lunar base. The water could be split by electrolysis into hydrogen and oxygen -- the oxygen could be used to support life and both gases could be used for rocket fuel. So, a lunar base could be a staging point for future exploration of the solar system (Mars and beyond). Plus, because of the moon's lower gravity, it is cheaper and easier to lift a rocket off of its surface than from Earth.

It might tricky to get back there though, at least for U.S. astronauts. In 2010, President Barack Obama decided to cancel the Constellation program, the intent of which was to get Americans back on the moon by 2020. That means U.S. astronauts may have to hitch a ride with private space companies, which will receive some funding from NASA.

In September 2022, NASA attempted to launch megarocket Artemis 1 on the first mission to the moon since 1972, but technical difficulties have put this on hold for now.