EARTH'S MOON

   The brightest and largest object in our night sky, the Moon makes Earth a more livable planet by moderating our home planet's wobble on its axis, leading to a relatively stable climate. It also causes tides, creating a rhythm that has guided humans for thousands of years.

Size and Distance

 With a radius of about 1,080 miles (1,740 kilometers), the Moon is less than a third of the width of Earth. If Earth were the size of a nickel, the Moon would be about as big as a coffee bean.

  The Moon is an average of 238,855 miles (384,400 kilometers) away. That means 30 Earth-sized planets could fit in between Earth and the Moon.

 The Moon is slowly moving away from Earth, getting about an inch farther away each year.

 Orbit and Rotation

 The Moon is rotating at the same rate that it revolves around Earth (called synchronous rotation), so the same hemisphere faces Earth all the time. Some people call the far side – the hemisphere we never see from Earth – the "dark side" but that's misleading. As the Moon orbits Earth, different parts are in sunlight or darkness at different times. The changing illumination is why, from our perspective, the Moon goes through phases. During a "full moon," the hemisphere of the Moon we can see from Earth is fully illuminated by the Sun. And a "new moon" occurs when the far side of the Moon has full sunlight, and the side facing us is having its night.

Structure

 Earth's Moon has a core, mantle, and crust.

 The Moon’s core is proportionally smaller than other terrestrial bodies' cores. The solid, iron-rich inner core is 149 miles (240 kilometers) in radius. It is surrounded by a liquid iron shell 56 miles (90 kilometers) thick. A partially molten layer with a thickness of 93 miles (150 kilometers) surrounds the iron core.

 The mantle extends from the top of the partially molten layer to the bottom of the Moon's crust. It is most likely made of minerals like olivine and pyroxene, which are made up of magnesium, iron, silicon, and oxygen atoms.

 The crust has a thickness of about 43 miles (70 kilometers) on the Moon’s near-side hemisphere and 93 miles (150 kilometers) on the far-side. It is made of oxygen, silicon, magnesium, iron, calcium, and aluminum, with small amounts of titanium, uranium, thorium, potassium, and hydrogen.

 Formation

 The leading theory of the Moon's origin is that a Mars-sized body collided with Earth about 4.5 billion years ago. The resulting debris from both Earth and the impactor accumulated to form our natural satellite 239,000 miles (384,000 kilometers) away. The newly formed Moon was in a molten state, but within about 100 million years, most of the global "magma ocean" had crystallized, with less-dense rocks floating upward and eventually forming the lunar crust.

 Surface

  With too sparse an atmosphere to impede impacts, a steady rain of asteroids, meteoroids, and comets strikes the surface of the Moon, leaving numerous craters behind. Tycho Crater is more than 52 miles (85 kilometers) wide.

 The light areas of the Moon are known as the highlands. The dark features, called maria (Latin for seas), are impact basins that were filled with lava between 4.2 and 1.2 billion years ago. These light and dark areas represent rocks of different compositions and ages, which provide evidence for how the early crust may have crystallized from a lunar magma ocean. The craters themselves, which have been preserved for billions of years, provide an impact history for the Moon and other bodies in the inner solar system.

 If you looked in the right places on the Moon, you would find pieces of equipment, American flags, and even a camera left behind by astronauts. While you were there, you'd notice that the gravity on the surface of the Moon is one-sixth of Earth's, which is why in footage of moonwalks, astronauts appear to almost bounce across the surface.

  Water on the Moon

 During the initial exploration of the Moon, and the analysis of all the returned samples from the Apollo and the Luna missions, we thought that the surface of the Moon was dry.

 The first definitive discovery of water was made in 2008 by the Indian mission Chandrayaan-1, which detected hydroxyl molecules spread across the lunar surface and concentrated at the poles. Missions such as Lunar Prospector, LCR OSS, and Lunar Reconnaissance Orbiter, have not only shown that the surface of the Moon has global hydration, but there are actually high concentrations of ice water in the permanently shadowed regions of the lunar poles.

  Atmosphere

  The Moon has a very thin and weak atmosphere, called an exosphere. It does not provide any protection from the Sun's radiation or impacts from meteoroids.

 Magnetosphere

 The early Moon may have developed an internal dynamo, the mechanism for generating global magnetic fields for terrestrial planets, but today, the Moon has a very weak magnetic field. 

 Tides of the Moon

  The lunar solid crust experiences tides of around 10 cm (4 in) amplitude over 27 days, with three components: a fixed one due to Earth, because they are in synchronous rotation, a variable tide due to orbital eccentricity and inclination, and a small varying component from the Sun.^[160] The Earth-induced variable component arises from changing distance and lib ration, a result of the Moon's orbital eccentricity and inclination (if the Moon's orbit were perfectly circular and inclined, there would only be solar tides).^[160]

The cumulative effects of stress built up by these tidal forces produces moon quakes. Moon quakes are much less common and weaker than are earthquakes, although moon quakes can last for up to an hour – significantly longer than terrestrial quakes – because of scattering of the seismic vibrations in the dry fragmented upper crust. The existence of moon quakes was an unexpected discovery from seismometers placed on the Moon by Apollo astronauts from 1969 through 1972.^[161]