Earth's Moon: Formation, Composition and Orbit
The moon is the easiest celestial object to find in the night sky — when it's there. Moon phases and the moon's orbit are a mystery to many. Because it takes 27.3 days both to rotate on its axis and to orbit Earth, the Moon always shows us the same face. We see the Moon because of reflected sunlight. How much of it we see depends on its position in relation to Earth and the Sun.
Though a satellite of Earth, the Moon is bigger than Pluto. Some scientists think of it as a planet (four other moons in our solar system are even bigger), though that viewpoint has never caught on officially. There are various theories about how the Moon was created, but recent evidence indicates it formed when a huge collision tore a chunk of the Earth away.
Physical Characteristics of Earth's Moon
The leading explanation for how the moon formed was that a giant impact knocked off the raw ingredients for the moon off the primitive molten Earth and into orbit. Scientists have suggested the impactor was roughly 10 percent the mass of Earth, about the size of Mars.
- Internal Structure
The moon very likely has a very small core just 1 to 2 percent of the moon's mass and roughly 420 miles (680 km) wide. It likely consists mostly of iron, but may also contain large amounts of sulfur and other elements.
Its rocky mantle is about 825 miles (1,330 km) thick and made up of dense rocks rich in iron and magnesium. Magmas in the mantle made their 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 past.
The crust on top averages some 42 miles (70 km) deep. The outermost part of the crust is broken and jumbled due to all the large impacts it has received, a shattered zone that gives way to intact material below a depth of about 6 miles (9.6 km).
- Surface Composition
Like the four inner planets, the moon is rocky. It's pockmarked with craters formed by asteroid impacts millions of years ago. Because there is no weather, the craters have not eroded.
The average composition of the lunar surface by weight is roughly 43 percent oxygen, 20 percent silicon, 19 percent magnesium, 10 percent iron, 3 percent calcium, 3 percent aluminum, 0.42 percent chromium, 0.18 percent titanium and 0.12 percent manganese. [Photos: Our Changing Moon]
The Moon has almost no atmosphere, so a layer of dust — or a footprint — can sit undisturbed for centuries. And without an atmosphere, heat is not held near the planet, so temperatures vary wildly. Daytime temperatures on the sunny side of the Moon reach 273 degrees F; on the dark side it gets as cold as -243.
Orbital Characteristics of Earth's Moon
Average Distance from Earth
English: 238,855 miles
Metric: 384,400 km
English: 225,700 miles
Metric: 363,300 km
English: 252,000 miles
Metric: 405,500 km
- Tidal Effects
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 the Earth's crust.
High tides are when water bulges upward, and low tides are when water drops down. High tide results on the side of the Earth nearest the moon due to gravity, and it also happens 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 that 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 3.8 centimeters annually.
The moon's gravitational pull might 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 where life could flourish.
During eclipses, the moon, Earth and sun are in a straight line, or nearly so. A lunar eclipse takes place when Earth gets directly or almost directly between the sun and the moon, and Earth's shadow falls on the moon. A lunar eclipse can occur only during a full moon. A solar eclipse occurs when the moon gets directly or nearly directly between the sun and Earth, and the moon's shadow falls on us. A solar eclipse can occur only during a new moon. See: All about Lunar Eclipses, and also Pictures: Total Solar Eclipse of 2010.
The Earth's axis of rotation is tilted in relation to the ecliptic plane, an imaginary surface through Earth's orbit around the sun. This means the northern and southern hemispheres will sometimes point toward or away from the sun depending on the time of year, varying the amount of light they receive and causing the seasons.
The tilt of Earth's axis is about 23.5 degrees, but the tilt of the moon's axis is only about 1.5 degrees. As such, the moon virtually has no seasons. This means that some areas are always lit by sunlight, and other places are perpetually draped in shadow.
Exploration & Research
The moon, the brightest object in the night sky, has created a rhythm that 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. Some ancient peoples 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 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," which is Latin for seas and land, respectively.
The legendary scientist Galileo was the first to use a telescope to make scientific observations of the moon, describing a rough, mountainous surface in 1609 that was quite different from the popular beliefs of his day that the moon was smooth. In 1959, the Soviet Union sent the first spacecraft to impact the moon's surface and returned the first photographs of its far side. In 1969, the United States landed the first astronauts on the moon, undoubtedly the most famous of NASA's achievements, and their efforts returned 842 pounds (382 kg) of rocks and soil to Earth for study. It remains the only extraterrestrial body that humanity has ever visited.
After a long interlude, lunar exploration resumed in the 1990s with the U.S. robotic missions Clementine and Lunar Prospector. Both missions suggested water might be present at the lunar poles, hints the joint launch of the Lunar Reconnaissance Orbiter and the Lunar Crater Observation and Sensing Satellite (LCROSS) helped prove were real in 2009.
In 2011, NASA's Lunar Reconnaissance Orbiter (LRO) spacecraft sent back the best moon map ever.