The Moon

The moon is the easiest celestial object to find in the night sky — when it’s there. Earth’s only natural satellite hovers above us bright and round until it seemingly disappears for a few nights. The rhythm of the moon’s phases 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.

Moon phases and the moon’s orbit are mysteries to many. For example, the moon always shows us the same face. That happens because it takes 27.3 days both to rotate on its axis and to orbit Earth. We see either the full moon, half moon or no moon (new moon) because the moon reflects sunlight. How much of it we see depends on the moon’s position in relation to Earth and the sun.

Though a satellite of Earth, the moon, with a diameter of about 2,159 miles (3,475 kilometers), is bigger than Pluto. (Four other moons in our solar system are even bigger.). The moon is a bit more than one-fourth (27 percent) the size of Earth, a much smaller ratio (1:4) than any other planets and their moons. This means the moon has a great effect on the planet and very possibly is what makes life on Earth possible.

How did the moon form?

There are various theories about how the moon was created, but recent evidence indicates it formed when a huge collision tore a chunk of Earth away.

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. 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.)

Although the large impact theory dominates the scientific community’s discussion, another theory suggests that two young moons could have collided to form a single large one. Earth may even have stolen the moon from Venus, according to a recent theory.

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.

Long ago the moon had active volcanoes, but today they are all dormant and have not erupted for millions of years.

Exploration

Human beings have studied the moon for millennia, watching its phases change and observing eclipses — both solar and lunar. During a solar eclipse, our moon moves between Earth and the sun and blocks the sunlight. In a lunar eclipse, Earth blocks the sun’s light that normally lights up the moon, so we see Earth’s shadow over the face of the moon. From Earth, we see the moon get dark and often turn red. This happens because Earth’s atmosphere scatters blue and green light while it bends yellow, orange and red wavelengths toward the moon.

The moon is the most explored body in our solar system besides Earth, having been visited by numerous spacecraft from multiple space agencies around the world. It’s also the only place besides Earth where human beings have set foot.

Significant Dates:

• 1609: Thomas Harriot becomes the first person to use a telescope aimed at the sky and sketches the moon. Later he made the first maps of the moon.
• 1610: Galileo Galilei publishes scientific observations of the moon in Sidereus Nuncius (Starry Messenger).
• 1959-1976: The U.S.S.R.’s Luna program of 17 robotic missions achieves many „firsts“ — including the first glimpse of the far side of the moon — and three sample returns.
• 1961-1968: The U.S. Ranger, Lunar Orbiter, and Surveyor robotic missions pave the way for Apollo human lunar landings.
• 1969: Astronaut Neil Armstrong is the first human to walk on the moon’s surface.
• 1994-1999: Clementine and Lunar Prospector data suggest that water ice may exist at the lunar poles.
• 2003: The European Space Agency’s SMART-1 lunar orbiter inventories key chemical elements.
• 2007-2008: Japan’s second lunar spacecraft, Kaguya, and China’s first lunar spacecraft, Chang’e 1, both begin one-year missions orbiting the moon; India’s Chandrayaan-1 soon follows in lunar orbit.
• 2008: The NASA Lunar Science Institute is formed to help lead NASA’s research activities related to lunar exploration goals.
• 2009: NASA’s Lunar Reconnaissance Orbiter and LCROSS launch together, beginning the U.S. return to lunar exploration. In October, LCROSS was directed to impact a permanently shadowed region near the lunar south pole, resulting in the discovery of water ice. LRO is still exploring the moon from orbit.
• 2011: Twin GRAIL spacecraft launch to map the interior of the moon from crust to core, and NASA begins the ARTEMIS mission to study the moon’s interior and surface composition. After a successful mission, the twin GRAIL spacecraft were directed to impact the moon in 2012.
• 2013: NASA launches LADEE to gather detailed information about the structure and composition of the thin lunar atmosphere. The successful mission ended in April 2014.
• 14 December 2013: China becomes the third nation to safely land a robotic spacecraft on the moon with the touchdown and deployment of Chang’e 3’s Yutu rover.