Earth’s Closest Neighbour
Between 1969 and 1972, NASA’s Apollo program brought 12 astronauts to the Moon’s surface. Since then, the Moon has been untouched by human feet, standing as the only celestial body aside from Earth that we’ve set foot on.
The near side of the Moon (north at top) as seen from Earth
We call Earth’s natural satellite simply “the Moon” because, before Galileo Galilei’s discovery of Jupiter’s moons in 1610, no one was aware of other satellites. The Romans called it Luna, inspiring the term “lunar” for all moon-related matters.
Born from destruction, the Moon’s formation theories agree on a cataclysmic event around 4.5 billion years ago when the solar system was just forming. The young Earth was struck by a Mars-sized object—or possibly multiple objects—that sent molten rocks and dust into space, eventually merging into the Moon.
One of the Moon’s most fascinating characteristics is its synchronous rotation; it takes 27 days to both spin on its axis and orbit Earth. This means we always see the same side of the Moon, commonly referred to as the “near side,” while the “far side” remains somewhat mysterious even after the space age allowed us to glimpse it from orbit.
This graphic shows the position of the Moon and the Sun during each of the Moon’s phases and the Moon as it appears from Earth during each phase. Not to scale. Credit: NASA/JPL-Caltech
The Moon’s surface is a testament to its tumultuous past. Craters, some spanning hundreds of miles, tell stories of cosmic impacts. The dark, flat plains known as “maria,” Latin for “seas,” were once believed to be filled with water. In truth, they are basins created by ancient volcanic activity, filled with solidified lava.
Unlike Earth, the Moon’s surface remains largely unaltered over time due to its lack of erosive elements like water and wind. Its craters, cliffs, and even the legacy of Apollo astronauts’ footsteps endure for ages.
Temperatures on the Moon are extreme and fluctuate swiftly, lacking an atmospheric shield to protect it from the space environment. These temperature shifts, along with meteor impacts and solar radiation, all contribute to its harsh climate.
Moon’s internal structure: solid inner core (iron-metallic), molten outer core, hardened mantle and crust
The seismometers left by Apollo missions have taught us that the Moon is seismically active, with quakes originating from both tidal interactions with Earth and the contraction of the Moon as it still cools after its formation. This gradual cooling has caused the Moon to shrink by roughly 150 feet (50 meters) over hundreds of millions of years.
The Moon’s near side and far side. Scientists still puzzle over why its near side differs from the far, with variations in crust thickness, geology, and volcanic history. Credit: NASA LRO / Jatan Mehta
The Moon orbits our planet at an average distance of about 238,860 miles (382,500 km), which means a jet would take more than 17 days to reach the Moon. This proximity allows it to exert a significant influence on Earth, shaping the tides. Additionally, the Moon’s gravity is ever so slightly slowing our planet’s rotation—a phenomenon known as tidal braking. This interaction has been gently increasing the length of our days while incrementally distancing the Moon from Earth by 1.5 inches (3.8 centimeters) per year.
The Moon’s influence extends beyond tides to stabilizing Earth’s tilt, fostering a climate where life thrives.
The Moon boasts an impressive size, spanning about 2,159 miles (3,475 kilometers) in diameter, outclassing Pluto. It’s not alone in its grandeur—four other moons in our solar system are even more massive. At 1/4 of the size of Earth, the Moon’s substantial proportion relative to our planet is unmatched by any other moon-planet duo.
Apollo 15 astronaut Jim Irwin uses a scoop to collect soil samples on the lunar surface. Credit: NASA
Following Apollo 11’s historic 1969 landing with Neil Armstrong and Buzz Aldrin, six missions until 1972 brought a dozen astronauts to the Moon. There, they gathered samples, conducted experiments, traversed the terrain, and even played extraterrestrial golf.
Awaiting our return since those pioneering missions, the Moon will once again host human footsteps with NASA’s Artemis III mission, slated for a 2025 lunar landing.
Words of wisdom
“A lie can travel half way around the world while the truth is putting on its shoes.” —Mark Twain
“Never doubt that a small group of thoughtful, committed, citizens can change the world. Indeed, it is the only thing that ever has.” —Margaret Mead
“You have enemies? Good. That means you’ve stood up for something, sometime in your life.” —Winston Churchill
“He never went out without a book under his arm, and he often came back with two.” —Victor Hugo, Les Misérables