The Moon has always played a central role in human civilizations, from being used to keeping track of time to inspire myths and music.
And you, how much do you know about the Moon?
The Moon In (Few) Numbers
- Age: 4.51 billion years
- Distance to Earth: 384 400 km (average)
- Diameter: 3474.8 km (Earth: 12742 km)
- Mass: 7.342 x 1022 kg (Earth: 5.972 x 1024 kg)
- Surface Gravity: 1.62 m/s2 (Earth: 9.81 m/s2)
- Temperature: from 120 ºC in full sunlight to -173 ºC at night and even -242 ºC in places never reached by sunlight
How Did The Moon Form?
The argument is still debated, but the most accredited hypothesis for the formation of the Moon is the Giant Impact Hypothesis.
The story goes like this: when the solar system was very, very young, the proto-Earth collided with Theia, another protoplanet about the size of Mars. In the aftermath of the collision, Theia was largely destroyed, together with a part of our planet.
In time, part of the debris of this cosmic collision “condensates” went to form the Moon, while a part fell back to Earth.
How Big Is The Moon?
The Moon is the fifth largest moon in the Solar System.
Since Pluto was demeaned and is not a planet anymore, our Moon also became the largest moon, relative to the size of its planet, in the entire solar system.
If our planet were hollow, it would be possible to fit in it about 50 Moons.
Did You Know?
According to the Rare Earth Hypothesis, our large Moon may have played a major role in the flourishing of life on our planet by helping to stabilize the Earth tilting (which affects climate), promoting tectonic plates, etc.
How Far Is The Moon From Earth?
The Moon sits at an average distance from Earth of 384 400km. At Perigee, though, the Earth-Moon distance is 362 600 km and it stretches to 405 400km when the Moon is at the Apogee.
How far is that? With an average speed of 5 500 km/h and peaks of 40 000 km/h, Apollo 11 entered lunar orbit after traveling in space for about 76 hours. Light from Earth, though, will arrive there in a little more than 1 second.
The combination of the actual size of the Moon and its distance to Earth is such that the apparent size of the Moon, i.e., how large it appears to us in the sky, matches that of our Sun.
This is why we can enjoy total solar eclipses.
As the Moon drifts away from us, it will also appear smaller in the sky and our distant descendants will only get to see partial lunar eclipses.
Did You Know?
The Moon hasn’t always been so far. In fact, it formed much closer to us, as close as orbiting at just 24000 km from Earth.
Since then, the Moon has been drifting away from us, and it is now receding at a speed of about 3.8 cm/year.
But don’t worry: even so, the Moon will never escape from Earth’s gravitational attraction. We will always have our companion.
What Is The Moon Made Of?
[…] “I’m at the foot of the ladder. The LM footpads are only depressed into the surface about one or two inches, although the surface appears to be very, very fine-grained, as you get close to it. It’s almost like a powder.
“Okay. I’m going to step off the LM now.
“That’s one small step for [a] man; one giant leap for mankind. […]”
Neil Armstrong, July 20, 1969
First man to walk on the Moon
The Moon is pretty much made by the same stuff all other rocky planets and asteroids in the solar system are made of. What really changes between them is their compositions, which depends on the formation process.
At its center, the Moon has a small solid iron core, surrounded by a partially melted outer core. Around the core there is the mantle, which makes up most of the Moon.
Finally, we reach the lunar crust, which, on average, has a thickness of about 50 km. The crust is composed of about 60% of oxygen, 17% of silicon, then aluminum, calcium, magnesium, iron, and titanium, plus traces of other elements.
The lunar surface is covered in regolith, a layer of dust, rocks and loose material above the solid crust.
Did You Know?
You can actually check the composition of the Moon’s surface by yourself with just your camera.
The Moon is not as gray as you may think. Instead, the lunar surface has subtle colors that locally depend on its mineral content: areas that are rich in iron look reddish, while parts rich in titanium look blueish.
By stacking images of the Moon, you can enhance those colors and create what is known as a “Mineral Moon”.
The Moon Is Actually “Dark”
The Moon is the second brightest object in the sky after the Sun, but how bright is it really?
A common way to compare the brightness of a celestial object is the apparent magnitude: Vega has an apparent magnitude 0, while Sirius, the brightest star in the night sky, has an apparent magnitude of -1.
In comparison, the apparent magnitude of the Sun is -26.74 and that of the full Moon is -12.74, with the Sun appearing to us 400 000 times brighter than the full Moon.
Still, if you ever observed the full Moon with a binocular or a telescope, you know it is almost blinding from how bright the view is.
So: why is the Moon dark?
The Moon does not emit light; instead it shines because it reflects sunlight. The ability of a surface to reflect light is a number called albedo, and for the Moon this number is a mere 0.12.
The Moon is dark as it reflects only 12% of all the sunlight it receives.
For comparison, Earth’s albedo is, on average, 0.30: it’s brighter, but not by much.
But Venus, being enveloped in thick clouds, has an albedo of 0.75: not surprisingly, Venus is the brightest planet in our sky.
Did You Know?
You are probably familiar with how blinding snow can get on a sunny day, but how bright is the fresh snow when compared to the full Moon?
Well, the albedo for fresh snow is 0.95, meaning it reflects 95% of all light it received. And that is why you need sunglasses when you go skiing.
Phases Of The Moon
The Moon always shows us the same face, as its rotation is almost in sync with its orbital period. The Moon, in fact, rotates once on itself every 27 days, and it takes 27.322 days to orbit Earth once.
But we get to see a bit more than you may think. Thanks to Moon libration, we get to see more than the closest 50% of the Moon: about 59% of the lunar surface is visible over time.
During its orbit around Earth, the Moon goes through a series of phases, where its illumination changes, as shown in the image above.
For example, during the full Moon, the light is frontal and casts no shadows on the lunar surface: the Moon looks very flat and lacks contrast.
But during the other phases, particularly near the lunar terminator (the line separating the illuminated surface by that in shadow), you can always see much more details: mountain peaks, mountain ranges, valleys, and craters become more three dimensional.
What Other Names Do We Call The Moon?
The Moon (with capital M) is the english name of our satellite, while the word moon, with the small “m”, indicates a celestial body orbiting around any planet.
The Moon inspired us since the first time our ancestors looked up at the night sky, often being identified with a goddess in the myths of ancient civilization. In greek mythology, Selene was the Moon goddess, while the ancient Roman personification of the Moon was the goddess, Luna.
But Full Moons also have traditional names that are inspired by a particular period of the year or agricultural activity based on a lunar calendar. You may have heard about the Wolf, Harvest and/or the Blue Moons.
Here is an interesting list of names for the full Moons throughout the year.
3 Cool Facts About The Moon
The Top 5 Extreme Features On The Moon
[…] Da osservazioni più volte ripetute di tali macchie fummo tratti alla convinzione che la superficie della Luna non è levigata, uniforme ed esattamente sferica, come gran numero di filosofi credette di essa e degli altri corpi celesti, ma ineguale, scabra e con molte cavità e sporgenze, non diversamente dalla faccia della Terra, variata da catene di monti e profonde valli. […]
Sidereus Nuncius – Galileo Galilei, 1610
First ever account from observing the Moon with a telescope
As Galileo noted 411 years ago, when first in history decided to point his telescope at the Moon, our satellite is not that featureless, smooth and perfect ball philosophers and theologians believed it was.
Instead, he saw mountain ranges, planes, valleys, cliffs, peaks and craters, many of which you can see for yourself even with a small binocular.
Here is a list of notable lunar features:
- Highest point on the Moon: Selenean Summit, 10.8 km (not a mountain);
- Deepest point on the Moon: a small unnamed crater on the floor of the larger Antoniadi crater, 9.2 km deep;
- Highest Mountain on the Moon: Mons Huygens, 5.5 km;
- Longest Mountain Range: Montes Rock, 791 km
- Largest Impact Crater: South Pole–Aitken basin, 2500 km in diameter;
That Time America Almost Nuked The Moon
Back in 1958, the US Air Force (USAF) planned to explode a nuclear bomb on the Moon, a little in the name of science and a lot for political reasons.
Caught in the midst of the cold war, the USAF considered exploding a nuke on the Moon as a strong way to establish America’s military and technological supremacy over the Soviet Union.
Luckily, fearing a bad reception from the general public and to start a race toward space militarisation, these plans were soon aborted.
In this article, you can read the story of Project A119.
Men On The Moon
If Earth is our home, the Moon is our porch.
To date, of the billions of individuals that lived and died throughout history, only 12 men ventured outside and stood there, on our porch.
12 men that walked, slept, drove and worked on the surface of the Moon because of NASA’s Apollo program.
A program packed with records and achievements:
- The Saturn V is still today the tallest and most powerful rocket ever used
- With Apollo 8, men orbited around the Moon for the first time
- Apollo 8 crew also were the first time men to witness an Earthrise and to see the far side of the Moon with their own eyes
- With Apollo 11, Neil Armstrong and Edwin “Buzz” Aldrin were the first men to land and set foot on the Moon
- Apollo 13 was the first and only aborted lunar mission. It was called a successful failure, as they didn’t land on the Moon, but returned safely back home
- While flying over the far side of the Moon for the free re-entry trajectory that would bring them home, Apollo 13 astronauts Jim Lowell, Fred Haise, and John Swigert went as far as 400171 km from Earth. Their record for the farthest a human being has ever been from Earth still stands today.
- With Apollo 15, Commander David Scott was the first man ever to drive a vehicle on the Moon
- With Apollo 17, geologist Harrison Schmitt became the first (and only) scientist to date to have walked on the Moon
Today, thanks to images from NASA’s Lunar Reconnaissance Orbiter (LRO) images of the Apollo landing sites, we can admire in great detail the remains of the Apollo vehicles and equipment left on the Moon, as well as the trails left by the astronauts’ activities.
If you are interested in a nice documentary about Apollo 11 moon landing, visit this page to watch MOONSCAPES by journalist Paolo Attivissimo, with restored original videos and audio synced.
USA president JF Kennedy set things in motion with this speech at the Congress in 1962:
“[…] First, I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the moon and returning him safely to the earth. No single space project in this period will be more impressive to mankind, or more important for the long-range exploration of space; and none will be so difficult or expensive to accomplish. […]”
And important and expensive the Apollo program was indeed. The entire program’s total cost is estimated to be about USD 260 billion in today’s money.
Because it is a familiar presence in the night sky, you may be tricked into the idea that the Moon has no more mysteries and gems to offer. Too often we barely take a glance at the Moon, vaguely acknowledging its beauty.
But the truth is that the Moon is still today a wonderful and interesting place to study, observe, and photograph.
And, to visit.
It really is about time for us to come back and walk again on the surface of our beloved Moon.