Illustrated Astronomy/The Moon
III
THE MOON
The Moon, that large object that we can see in the sky both at daytime and nighttime, is a natural satellite of the Earth. Its surface, with such areas of different shapes and various shadows, pop up immediately, and it has inspired practically all cultures, including artists, priests, warriors, and lovers. The Moon is, comparatively, the biggest natural satellite in the Solar System (if we measure its size in respect of the planet that orbits). In the Solar System, the only satellites which are larger than the Moon, in absolute terms, are Ganymede, Calisto and Europa in Jupiter, Titan in Saturn, and Triton in Neptune.
The Moon’s surface is covered in craters but, how were they created? Since its formation, different sizes of rocks have attacked the Moon, as the Earth and the rest of the planets have[1]. When these rocks collide with the surface at high speed, they melt the rock, turning it into some kind of lava; once the molten rock cools down, it leaves a region darker, quite flatter than its surroundings. These areas are called maria, whereas the neighboring space, lighter in color, is called plateau or ground.
During these collisions, large amounts of gases were released, but the Moon was not able to hold them back due to its low mass, and therefore, it doesn’t have an atmosphere.
Due to these impacts on the solid rock of the Moon, broken stones called regolith accumulated there. In the case of the Earth, the regolith exists mainly in tropical regions, but its origin is different from the Moon’s. Here, 1% of this material comes from meteorites, and it is thought that comes from the bombardment and accumulation of meteorites of different sizes. This regolith may vary its thickness between 2 to 20 meters, so every time a mission land on the Moon, it leaves a record of that event. Even though the Moon is covered in this dust, most of its surface (under this layer) has basalt, a volcanic rock, or, in this case, from a lunar core rock that resurfaced after meteors impacts.
The Moon, unlike stars like the Sun, but like the planets, doesn’t emit light itself but reflects the light that comes from the Sun, which happens due to the lack of mass to produce energy in its core.
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AT-HOME EXPERIMENT
How are craters formed?
You are going to see the differences between the impacts and recognize which ones were bigger meteors or where did they come from to create the many craters of the Moon.
Materials:
· 1 plastic bowl
· 1 strainer
· 3 kilos of flour
· 1 cup of chocolate powder
· You can find some other colored powder if you want
Step by step:
· Pour the 3 kg of powder into the bowl. Get at least 5 cm of thickness · Powder the chocolate over with a strainer · Throw different stones but in different sizes and with different speeds and angles. · Look at the results and compare them with real images of Moon’s craters. · You can try the experiment more times and add powders of different colors besides the chocolate.
This small rocky world, which tells us a story full of impacts, orbits the Earth in 27.32 days. It is called the sidereal period. As the Earth revolves around the Sun, the time between two identical lunar phases is known as the synodic period, and it lasts 29.53 days.
Perhaps, you may notice the shape or shapes of the Moon that we see are always the same way. That is because the Moon always shows the same face to Earth, so we always see the same half of it.
When a celestial object is showing the same face to another is called synchronous rotation, which means that its rotational and orbital periods are the same. In the case of the Earth with the Sun, it doesn’t happen because the Earth rotates 365.25 times per orbit.
The fact that we always see the same face of the Moon made us jump into the conclusion that it has a dark side. There is indeed a side we can’t see from Earth, but that doesn’t mean that it doesn’t get any light and it is always dark because of the Sun lights up every inch of the Moon in different moments of the lunar phase. For instance, during the New Moon phase, the Sun is entirely lightning up the side that we can’t see.
The ancient Greeks were right when they thought that the Moon phases could be explained through its relative motion around the Earth and that it is much closer than the Sun. The reason why we can see some brightness, even in those parts where the Sun can’t light directly, is because of the sunlight reflection on the Earth’s surface.
DID YOU KNOW THAT…
...we can see more than one face of the Moon?
In fact, we can see about 59 % of the Moon from Earth thanks to a phenomenon called libration. Libration is an oscillation of the Moon in relation to Earth, and it happens for three reasons: libration in longitude (results from the eccentricity of the lunar orbit); libration in latitude (results from a slight inclination of the rotation axis regarding its orbit with Earth); and diurnal libration, which is a consequence of the Earth’s rotation. So, if we see the Moon at breaking dawn, midday, or twilight, we have different perspectives of it, its position in the sky, and its size.
DID YOU KNOW THAT…
...as the Earth reflects the Sunlight onto the Moon’s surface, some astronomers have observed the dark side of it using terrestrial telescopes?
How does this work?
The Sunlight reflects itself on Earth, and gently lights up the dark side of the Moon. Such light has the characteristics, or chemical fingertips of the terrestrial atmosphere tattooed, allowing us to observe the Earth as if it were an exoplanet.
The origin of the Moon is still under scientific discussion.
The most frequent explanation involves an early catastrophic impact, when the Solar System was still in formation, about 4,5 billion years.
This “traditional theory” implies that a Mars-sized object called Theia, just like the Greek goddess -mother of Selene, goddess of the Moon- collided with young Earth (or at least 90 % of it), and this caused a considerable impact that melted and vaporized Theia, creating a significant damage on Earth. Part of the material coming from Theia stayed on Earth, and another part was lost to space and were accumulated, only by gravity, to create what we know today as the Moon. This theory offers reasonable explanations to understand some of the properties we see both in our planet and in the Moon, like its distance, circular orbit, and the fact that its rotation is in sync with its orbit rotation (and that is why it shows us the same face). However, considering the new analysis of lunar rocks brought by the Apollo mission, which shows that these rocks have an Earth-alike composition and that Theia having the same composition of former Earth is lower to 1 %, other theories have come up. One of them suggests that, if there were two planets, five times bigger than Mars each, would form a disk when they collide, so the formation of the Earth and Moon can be explained, and, in this case, both would share the same composition indeed. However, the last word has not been said yet, so it is essential to keep exploring our natural satellite.
Now, apart from the theory about the Moon’s formation, every one of them agrees on something: in the past, it was closer to Earth, and it is a phenomenon we can observe today. At the moment it was formed, the Moon was very close to us, around 25,000 kilometers away, which means, less than a tenth portion of the current distance, this situation caused massive tides, and distortions on Earth’s surface, producing at the same time the slower rotation of Earth. The energy that our planet lost by rotating so slow transferred to the Moon’s orbit and caused the Moon to move away, and consequently, to move slightly slower.
Today, our satellite moves away about four centimeters from us every year. Such is the case that in 100 years more, the day would last two thousandths of a second less, which means that in the not-too-distant future, the Moon is going to look smaller than we see it today, tides won’t change so much as they do currently, and the day is going to have more than 24 hours.
If the Moon produces a tidal force on Earth, does the Earth also cause a tidal force on the Moon?
The answer is yes, it does. The difference is that the Moon’s surface is solid, so the deformation is much lower, whereas the liquid water is easily deformable.
However, among all the things that stand out of the Moon, the most important one occurred in the late ’60s. The Apollo 11 mission landed on July 20th in 1969, and it turned the Moon into the first place outside the Earth in where humanity could put a foot on the ground. The astronauts, Neil Armstrong and Buzz Aldrin, were less than two hours on this surface, in the area called “mare tranquillitatis” (“sea of tranquility”). There, they installed some mirrors, still in use today to measure the exact distance to the Moon, they took walks, did some experiments, and gathered samples. They brought around 22 kg in stones! The third astronaut, who could not get off since someone had to fly the spaceship to come back to Earth, was Michael Collins.
That was the first time out of the six missions of the Apollo program that carry astronauts who landed successfully on the Moon between 1969 and 1972. The achievement obtained by the different spatial agencies, particularly from the United States and the former Soviet Union, marked a milestone in human history, such was the case that it had been mentioned on multiple occasions that today we are living in the Spatial Era or Spatial Age.
DID YOU KNOW THAT…
...the Lunar Orbiter Reconnaissance, a survey for lunar exploration, could record in a video the traces left by Apollo 11, 12, 14, 15, 16, and 17, besides capturing images from other satellites such as Chang’e 3 from China?
You can watch the rest of the descent stage of Apollo 11, 12, and 14, the footprints or traces of the Apollo 14’s astronauts, and the vehicle that Apollo 17 left, besides the many instruments used in the mentioned missions.
Watch the video, following https://moon.nasa.gov/resources/128/lro-explores-the-apollo-11-landing-site/ or scan the following QR code with your phone.
In different regions of the Moon, Apollo’s missions left a few instruments to study the inner core of this satellite, and to measure seismic movements in three dimensions. The so-called moonquakes are very mild and, even though near three thousand movements per year are registered, none of them exceed the 2nd grade in the Richter scale. They are very shallow and occur mainly due to temperature variations, the landing of spatial missions sent from Earth, or the hit of meteorites.
Another aspect that stands out of the Moon is its minimal core, compared with the rest of the rocky bodies in the Solar System. According to the last seismic measurements, its composition is mainly made up of partly molten iron.
The exploration of this satellite will keep providing data about its evolution and formation. It is also possible that the Moon is used as a basis to launch future exploration missions of the Solar System or as a host of future telescopes generations which can work together with some observatories on Earth.
DID YOU KNOW THAT…
...the Moon would be an ideal place to install a telescope due to its lack of atmosphere, and for that reason, it could be observed with a quality impossible to reach from Earth? Also, it could be observed during most of the lunar day since the sunlight doesn’t scatter, leaving skies in total darkness both at day and at night.
- ↑ We can see some craters on Earth. However, vulcanism, water (rain, rivers, glaciations), and wind have been in charge of erasing the majority of the craters, especially the smaller and older ones.