The moon is Earth's most constant companion and the easiest celestial object to find in the night sky.

The rhythm of the phases of the moon has guided humanity for millennia; for instance, calendar months are roughly equal to the time it takes to go from one full moon to the next. But the moon's orbit and phases can seem mysterious. for example, the moon always shows us the same face, but it's always changing size as how much of it we see depends on the moon's position in relation to Earth and the sun.

While it's a satellite of Earth, the moon, with a diameter of about 2,159 miles (3,475 kilometers), is bigger than Pluto. (And there are four other moons in our solar system even bigger than ours.)

The moon is a bit more than one-fourth (27%) the size of Earth, a much larger ratio (1:4) than any other moons to their planets. This means the moon has a great effect on our planet and may even be a major factor in making life on Earth possible.

There are various theories about how the moon was created, but recent evidence indicates it formed when a huge collision tore off a chunk of the primitive molten Earth, sending the raw ingredients for the moon into orbit.

Scientists have suggested the impacting object was likely about 10% the mass of Earth and about the size of Mars. Because Earth and the moon are so similar in composition, researchers have concluded that the impact must have occurred about 95 million years after the formation of the solar system, give or take 32 million years. (The solar system is roughly 4.6 billion years old.)

In 2015, new research gave further weight to this theory, using simulations of planetary orbits in the early solar system as well as newly uncovered differences in the abundance of the element tungsten-182 detected in the Earth and the moon.

While this theory, most commonly known as the large impact theory, is the dominant theory in the scientific community, there are several other ideas for the moon's formation. These include the concept that the Earth captured the moon, that the moon fissioned out of the Earth or even that Earth may even have stolen the moon from Venus.
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The moon's rocky mantle is about 825 miles (1,330 km) thick and made up of dense rocks rich in iron and magnesium. Magma in the mantle made its way to the surface in the past and erupted volcanically for more than a billion years — from at least four billion years ago to fewer than three billion years ago.

The crust that includes the lunar surface averages some 42 miles (70 km) deep. The outermost part of the crust is broken and jumbled due to all the large impacts the moon has endured , with the shattered zone giving way to intact material below a depth of about 6 miles (9.6 km).

Like our solar system's four innermost planets, the moon is rocky. It's pockmarked with craters created by asteroid impacts millions of years ago and, because there is no weather, the craters have not eroded.

Photos: Our changing moon

The average composition of the lunar surface by weight is roughly 43% oxygen, 20% silicon, 19% magnesium, 10% iron, 3% calcium, 3% aluminum, 0.42% chromium, 0.18% titanium and 0.12% manganese.

Orbiting spacecraft have found traces of water on the lunar surface that may have originated from deep underground. They have also located hundreds of pits that could one day house explorers living on the moon long-term.

Ongoing observations from the Lunar Reconnaissance Orbiter (LRO) have shown that water is more abundant on slopes facing the lunar south pole, although scientists do caution that the water quantity is comparable to an extremely dry desert. Meanwhile, a 2017 study suggested the moon's interior could be abundant in water too.

The moon has only a very thin atmosphere, so a layer of dust — or a footprint — can sit undisturbed for centuries. And without much of an atmosphere, heat is not held near the surface, so temperatures vary wildly. Daytime temperatures on the sunny side of the moon reach 273 degrees F (134 Celsius); on the night side, it gets as cold as minus 243 F (minus 153 C).

Here are some statistics from NASA:

Average distance from Earth: 238,855 miles (384,400 km)
Perigee (closest approach to Earth): 225,700 miles (363,300 km)
Apogee (farthest distance from Earth): 252,000 miles (405,500 km)
Orbit circumference: 1,499,618.58 miles (2,413,402 km)
Mean orbit velocity: 2,287 mph (3,680.5 kph)
The moon's gravity pulls at the Earth, causing predictable rises and falls in sea levels known as tides. To a much smaller extent, tides also occur in lakes, the atmosphere and within Earth's crust.

High tides refer to water bulging up from Earth's surface, and low tides when water levels drop. High tide occurs on the side of the Earth nearest the moon due to gravity, and on the side farthest from the moon due to the inertia of water. Low tides occur between these two humps.

The pull of the moon is also slowing the Earth's rotation, an effect known as tidal braking, which increases the length of our day by 2.3 milliseconds per century. The energy that Earth loses is picked up by the moon, increasing its distance from the Earth, which means the moon gets farther away by 1.5 inches (3.8 centimeters) annually.

The moon's gravitational pull may have been key to making Earth a livable planet by moderating the degree of wobble in Earth's axial tilt, which led to a relatively stable climate over billions of years in which life could flourish.

The moon doesn't escape from this interplay unscathed. A new study suggests that Earth's gravity stretched the moon into its oddly distorted shape early in its lifetime.

During eclipses, the moon, Earth and sun are in a straight line, or nearly so. A lunar eclipse takes place when Earth is directly between the sun and the moon, casting Earth's shadow onto the moon's face. A lunar eclipse can occur only during a full moon.

During a full lunar eclipse, because the moon is in shadow, it can appear blood red in the night sky.

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A solar eclipse occurs when the moon passes between the sun and the Earth, casting the moon's shadow onto Earth. A solar eclipse can occur only during a new moon.

Depending on the degree to which the moon blocks the sun as seen from a particular location on Earth, a solar eclipse can be total, annular or partial. Total solar eclipses are rare in a given location because the shadow of the moon is so small on the Earth's surface.

The last total solar eclipse visible from the United States happened in August 2017; the next one will take place in April 2024.

The Earth's axis of rotation is tilted about 23.5 degrees in relation to the ecliptic plane, an imaginary disk through Earth's orbit around the sun. This means the Northern and Southern hemispheres point somewhat toward or away from the sun depending on the time of year, varying the amount of solar radiation they receive and causing the seasons.

Early observations from Earth
Some ancient communities believed the moon was a bowl of fire, while others thought it was a mirror that reflected Earth's lands and seas, but ancient Greek philosophers knew the moon was a sphere orbiting the Earth whose moonlight reflected sunlight.

The ancient Greeks also believed the dark areas of the moon were seas while the bright regions were land, which influenced the current names for those places — "maria" and "terrae," Latin for seas and lands, respectively.

The Renaissance astronomer Galileo Galilei was the first to use a telescope to make scientific observations of the moon, describing in 1609 a rough, mountainous surface that was quite different from the popular beliefs of his day that the moon was smooth.

Race to the moon
In 1959, the Soviet Union sent the first spacecraft to impact the moon's surface and returned the first photographs of its far side. This spurred a series of Cold War-era uncrewed missions launched by both the Soviet Union and the United States to observe the moon's surface.

Many of these early moon probes were failures or only partially successful. However, over time, these missions began to return information about the moon's surface and geological history. The United States launched a series of missions dubbed Pioneer, Ranger and Surveyor, while the Soviet Union sent probes under the names Luna and Zond.