Since the moon is Earth's closest celestial neighbor, we've been able to learn more about it than any other in the solar system. There are surprising details about the moon, such as how in 1958, the US proposed exploding a nuclear bomb on its surface, for clout.
Here are 10 scientific and historical facts about the moon that might surprise you.
1. It hasn't been walked on by man since 1972
After astronaut Neil Armstrong landed on the moon as part of the Apollo 11 mission in 1969, astronaut Gene Cernan was the last man to do so in 1972 during the Apollo 17 mission. Twelve men walked on the moon between those years, and all spacecraft on the moon's surface since have been unmanned.
2. The moon could soon be visited by humans again
NASA is also planning for what would be the first strategic human presence on the moon. NASA's new rocket, the Space Launch System, will send astronauts aboard the Orion spacecraft a quarter million miles from Earth to a lunar orbit. Astronauts will dock Orion at the Gateway, a spacecraft where they will live and work around the moon. The crew will take trips from the Gateway to the moon's surface before returning to the orbital living station.
3. The first spacecraft to reach the moon was a Soviet craft
In the race to space, the former Soviet Union launched ahead of the US with its unmanned Luna 1 mission in 1959. The spacecraft didn't actually land on the moon, though. It was the first craft to reach the vicinity of the moon, missing the landing due to a timing issue during its launch. Luna 1 was also the first to orbit the sun. The Luna 2 craft was the first man-made, unmanned object to touch the moon later that year.
4. During the 1950s the US considered detonating a nuclear bomb on the moon
A top-secret project transpiring in 1958 -- Project A119, also known as A Study of Lunar Research Flights -- was planned as a show of strength and an opportunity to boost domestic confidence at a time when the US was losing to the the race to space to the Soviets. If the bomb exploded on the surface instead of in a crater, the flash of light would've been visible to people on Earth with their naked eye.
However, the plan was never carried out, due to concern about a negative public reaction, possible militarization of space and the higher probability that a manned moon landing would be more popular in the eyes of the world. The project documents actually remained a secret for over 40 years until former NASA executive Leonard Reiffel, who led the project, revealed the project in 2000. The US government has never officially recognized their involvement in the project, and there's no record of it on NASA's website.
5. There are rules for how the moon's craters are named
Brussels engineer Michael van Langren began the custom of applying personal names to lunar formations in 1645, starting with the names of kings and other distinguished people.
Six years later, Giovanni Battista Riccioli of Bologna created his own lunar map which rid of the names Van Langren used and instead chose names exclusively from those of famous astronomers -- the guideline for the system used today by the International Astronomical Union. The IAU tends to pull the names for craters from two groups: deceased scientists, scholars, explorers and artists; or deceased American astronauts and Russian cosmonauts.
6. The sun and the moon only appear to be the same size
The moon is 400 times smaller than the sun, but it looks the same size in Earth's sky because the moon is 400 times closer to Earth than the sun. When objects are closer to us, they appear bigger than they would far away. For example, many other stars are larger than the Sun, but they're a lot farther away from Earth.
7. The moon is a broken off piece of the Earth
Scientific consensus about how the moon formed is that billions of years ago, a protoplanet crashed into Earth and the pieces of it revolved around the Earth to form the moon.
By modeling the evolution of ejecta fragments from the collision, researchers of a 2015 study on lunar formation estimated the moon formed around 4.47 billion years ago.
8. Tides are caused by the moon
There are two bulges in Earth resulting from the gravitational pull of the moon. One bulge is on the side of Earth that faces the moon; the other is on the side that faces away. When the Earth rotates, the bulges move around in the oceans, causing the high tides and low tides in oceans all over the world.
There's a special type of tide when the sun, moon and Earth align together bi-monthly around the times of the New Moon and Full Moon -- a spring tide, which means high tides are higher and low tides are lower no matter the season.
9. The moon has quakes and is shrinking
The gravitational pull of the Earth causes small moonquakes several kilometers beneath the surface, causing ruptures and cracks.
"Thrust faults," where one section of crust is pushed up over a neighboring part, also develop when the moon shrinks as its interior cools. Th moon has gotten about 150 feet smaller over the last several hundred million years.
10. The diameter of the moon is almost the same distance from New York city to Phoenix, Arizona
The mean diameter of the moon is 2,159 miles, close to the 2,406 miles between Phoenix and New York City.
New facts about the moon -- and even Mars -- can be expected once NASA completes their mission to send the first woman and the next man to the moon by 2024.