The Moon is Earth’s only natural satellite. It is mentioned in many ancient civilizations as a deity, and throughout history, has been the inspiration in many forms of art. The Moon is the closest celestial object to our planet and has been part of every space exploration program. There are plenty of facts that may classify as “fun facts Moon”, so let’s see these below.
Moon in numbers
The mean Earth-Moon distance is 384,402 km, but the distance between the two is increasing by 3.8 cm a year. This will continue for the next 50 billion years, and as a result, a lunar month will be 47 days instead of the current 27.3.
It has a radius of 1734 km (the fifth largest satellite in the Solar system), so the Moon is 400 times smaller than the Sun. The distance between the Sun and the Earth is almost 390 times greater than the distance between the Earth and the Moon. This is the reason that the Moon and the Sun have almost the same apparent size in the sky.
The Moon has a mass of 7.342×1022 kg and a mean density of 3.334 g/cm3, which makes the Moon the second densest satellite in the Solar system after Jupiter’s Io. The gravitational force of the Moon is 16.5% compared with the Earth. So if you weigh 100 kg on Earth, you would weigh a mere 16.5 kg on the Moon.
The mass ratio of the Moon-Earth system is the highest in our Solar system with a value of 0.0123 (1.2%). The second highest is Triton-Neptune at a value of ~0.0002 (0.02%).
The mean temperature at Moon’s surface is -53°C (-64°F), while the minimum is -173°C (-280°F), and the maximum 117°C (242°F).
Formation of the Moon
The Moon formed around 4.5 billion years ago, around 60 million years after the Sun. For the formation of the Moon, various models exist. These vary tremendously, suggesting that Earth’s gravity captured the Moon, or material of a rapidly rotating early Earth was expelled forming the Moon, or a common formation of the two. All these models have weaknesses in explaining the nature of the Moon.
For example, a close encounter of two planets usually results in a collision, or altering trajectories. The early Earth would need a large atmosphere for braking the passing Moon. Earth’s crust is young (200 million years old), and the oceanic crust is very thin, thus the missing mass could have been expelled from Earth forming the Moon. But the lunar material is old, and it matches more mantle material. Finally, the common formation model cannot explain why the core of the Moon is only 25% of the Moon, while Earth’s core is almost 50%.
The most commonly accepted model suggests that the Moon resulted from the collision between a large planetary object (roughly the size of Mars), and the early Earth, with the resulting debris, forming the Moon.
There is no dark side of the Moon
Both sides of the Moon receive the same amount of sunlight. But from Earth, we only see one side of the Moon. This happens because the Moon needs the same time to complete an orbit around Earth and a rotation around its own axis. Thus, the only way to see the other side of the Moon comes from pictures taken from probes.
Structure and atmosphere of the Moon
Just like Earth, the Moon has a crust, a mantle, and a core. Starting from the center, the Moon has an iron-rich inner core with a radius around 240 km. The inner core is surrounded by a shell of liquid iron that is around 90 km thick.
A partially fluid layer comes up next that is 150 km thick, surrounding the iron core which is part of the inner mantle. The main mantle zone of the Moon extends for around 1200 km. It is solid, and its composition is similar to the crust.
The Lunar crust has a variable thickness. It is about 70 km on the near-side of the Moon, and nearly 150 km on the far-side. It is composed mostly of oxygen, silicon, magnesium, calcium, and aluminum. The surface of the Moon is the one that most of us are familiar with. The near side of the Moon is divided into two distinct parts. The highlands, which are the old and heavily cratered regions, and the Maria, which are darker in appearance, smoother, with fewer craters, and younger. But why does the Moon’s crust have this anomaly?
As mentioned above, the Moon resulted from a collision between the Earth and a planet the size of Mars. The products of this collision were a scorched Earth, and the newly formed Moon. The near side of the Moon was the side facing Earth, and it was basically cooked. As a result, part of the lunar crust melted and condensed to the far side of the Moon, explaining the thicker crust on the far-side. Bombardment from meteorites broke the thinner crust on the near side, allowing lava flows from the interior, forming the Maria regions.
The Moon has no atmosphere. This means that its surface is not protected from the Sun’s radiation, meteorites, and cosmic rays. It has tremendous temperature variations, and the sky there always appears dark. The lack of air means that if you visit the Moon, you cannot listen to anything.
The Moon causes the tides on Earth
A tide is the rise or fall of sea level that is caused by the gravitational effect of the Moon, the Sun, and Earth’s rotation. When a body the size of Earth, faces a body like the Moon, the gravitational effect of the side facing the Moon varies significantly from the side away. These forces are called tidal forces, and in extreme cases, they can distort or even rip apart a body.
As you can see on the figure to the right, the gravitational effect on Earth is stronger on the side that faces the Moon, than the other. The tidal force distorts Earth’s shape, and the effect of force F1 is greater than force F2, thus the water mass bulges towards the point that faces the Moon. But, the force F2 is stronger than F3, thus the center of the Earth is also pulled by the Moon, creating another water bulge on the point further away from the Moon. This is a periodic phenomenon, and its time scale is 24 hours and 50 minutes.
Tides reach their maximum when the Sun and the Moon are aligned. These tides are called spring tides (not to be confused with the season). When the Sun and the Moon are not aligned, the gravitational forces cancel each other out, and the tides are much lower. These are known as neap tides.
Lunar phases and eclipses
The Moon reflects the light it receives from the Sun, thus it is visible to us. This amount of light always remains constant, but because the Moon orbits Earth, and Earth orbits the Sun, the amount of reflected light that we see varies. A new Moon happens when the sunlight hits the far side of the Moon, while when the sunlight hits the near-side we have a full Moon.
Between these two we see only a part of the Moon. These are known as Lunar phases, and their order is: new Moon, waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, last quarter, waning crescent, and new Moon. The cycle repeats once a month (every 29.5 days).
An eclipse can only happen when the Sun, the Earth, and Moon are all in a straight line. A Solar eclipse occurs at new Moon, when the Moon is between the Sun and the Earth, while lunar eclipses are possible at full Moon, when the Earth is between the Moon and the Sun. The Sun and Moon have almost the same apparent size on the sky, but because of the non-circular orbits, there are occasions that they can be the same, but take place in different cycles. Due to this, we can observe both total and annular Solar eclipses.
During a total Solar eclipse, the Moon covers the Sun completely, and the Solar corona becomes visible. But this will not last forever. As the Earth-Moon distance increases, the apparent size of the Moon will decrease. Also, as the Sun evolves towards a red giant, his apparent size will increase. Thus, total Solar eclipses in the future (add hundreds of millions of years) will no longer be possible.
Lunar exploration facts
- The first probe to reach the Moon was Luna 1 in 1959. It was a probe from Soviet Union, which passed just shy of 6000 km from the Moon, before going into orbit around the Sun.
- The first probe that crashed landed on the Moon, and the first probe that took images from “the dark side” of the Moon, were Luna 2 and 3, both from the Soviet Union.
- Luna 9 was the first soft lander, while Luna 10 was the first probe that orbited the Moon, both in 1966.
- Between 1969 and 1972, 12 men have walked on the surface of the Moon, all part of the Apollo program and all American. The Moon is the only other place that a human has driven a vehicle.
- Between 1970 and 1976 Soviet probes, part of the Luna program, landed on the Moon, collected samples, and returned to Earth. Also, in the same period, two robotic rovers were sent to the Moon, as part of the Soviet Lunokhod program.
- If you are wondering why the US flag was waving on the Lunar surface when Buzz Aldrin planted it, the answer is simple. The flag was already in motion, and because of the lack of atmosphere on the Moon, there was no resistance to dissipate its motion. Also, vibrations from the pole assured that the flag would appear waving.
- There is water on the Moon – Water has been detected on areas of the Moon where sunlight cannot reach, thus it is there in the form of ice. Water was delivered to the Moon by comets. This hypothesis is realistic since the lunar surface is heavily contaminated by craters.
- There are Moonquakes – Quakes on the moon are caused by Earth’s gravitational influence. On Earth, quakes are short, but on the Moon can last for up to 30 minutes. Another difference is that Moonquakes are much weaker than what we experience here on Earth.
- The Moon is shrinking – The interior of the Moon is cooling down, so its interior becomes smaller. This also affects the surface, which shrivels just like a fruit that is drying. The “shrinking” process is slow, but over the last couple of hundred million years, the Moon has shrunk around 50 meters.