ASTR-201 Lessons

2.9: Origin of the Moon: 

Folktales and Scientific Theories

Welcome back to the final lesson of Year Two! I’m glad to see so many of you as excited to finish this year of lunar lessons as you were to start it. Today we have a fun discussion planned about the origin of the Moon.  I have some interesting folktales told by three different civilizations to start us off, and then I will share with you the more scientific theories about the origin of the Moon. Without any further ado, let’s jump right into it!

Folktales

Since the beginning of time, or at least coherent thought and reason, humans (and other beings) have sought to give meaning to the strange events they saw happening around them. Every civilisation from every corner of the globe has their own story of the “why” behind natural phenomena like thunder, the stars, or the divide between land and water. The origin of the Moon is no exception! In fact, the Moon might be one of the most popular topics of mythology and folktales. 

Of course, as you’ll see once we get to the various scientific theories, these stories are quite far from reality, but the tales are valuable in other ways. For example, they give us present-day folks the ability to travel back in time a bit to see how people used to think about the world. In many of these tales, you’ll see examples of impossible feats. Remember that in days of old, the strict separation between magic and Muggle was nonexistent, and even magical folk did not understand the limits of their abilities. So, to many, it seemed like anything was possible! 

I’ve chosen a few tales for today, each from a different area of the world, that cover the origin of the celestial body we’ve studied this year. In fact, you’ll see all of the myths here have different origins, though they do have their similarities. For starters, all of them involve the Sun in some way. If you were to research in depth, you would find that to be a very common trend. Secondly, every myth has at least one fantastical or supernatural component. So, keep an eye out for any sightings of magical influence, and let’s begin!

The Creation of the Moon

We start our tour of the Moon’s beginnings in India, among the Santali people from the eastern region of the country. Identifying the time period when the tale originated, however, is much more difficult, as Santal, the language of the original myth, lacked a written script. Because of this, the story was completely oral until 1925. But, sit back and relax while I recount a story, possibly thousands of years old.

In the beginning, the Sun shone all the time, and there was no day or night. This caused many problems: there was no set time to sleep or stop work, and so people worked until they couldn’t anymore.  The myth then goes on to say that the Creator, Marang Buru, took the Sun aside and told it to set in the evening and rise again in the morning. 

Unused to the total darkness, people got scared.  Marang Buru had assumed that the people would simply know to rest when it was dark, but instead they tried to continue on with their tasks and stumbled about, bumping into each other and tripping over things.  The leaders saw that this was foolish and managed to persuade the people to rest when it was dark.  Most people were happy with this arrangement, but a problem remained: those who didn’t want to rest for the entire period between sunset and sunrise still stumbled about, bumping into each other and so on.  At the behest of the people, the Creator of the world made the Moon, which wasn’t bright enough to prevent people from resting if they wanted to, but was enough to keep those who were still up and about from hurting themselves.  A children's version of that story can be found here .

Sister Sun and Brother Moon

This particular story dates at least as far back as the 13th century in traditional Korean folklore. As such, there are many recorded versions of this story, much like some Muggle European folktales (such as Little Red Riding Hood or Cinderella) that can vary widely! For example, the original version of this tale is actually quite grim and gruesome but over the centuries has changed into something a bit more family friendly, which is the version you’ll be learning today. 

It starts in the Korean countryside with a poor woman and her family. She  was on her way home from work -- selling rice cakes at the market --  when she met a ferocious beast. In most versions, this creature is a tiger, called Holangi. However, other versions have included a wolf, a wild dog, and even a Nianshu (a magical creature more commonly found in China, but also found in Korea). Regardless of what form the creature takes, it is always ferocious and, problematically, very hungry. Holangi threatened to eat the woman unless she gave him one of her rice cakes. 

Understandably, she obliged, but Holangi would never be satisfied. He returned multiple times on her way back home, each time demanding more and more rice cakes until, the last time, she had no more. She attempted to plead with the tiger and play on his sympathy, telling him she had two young children at home. But rather than be swayed, this only enticed him more. He ate her, disguised himself with her clothes and the leftover rice flour, and finished the journey to her home with the intention of tricking her son and daughter and consuming them as well.  

To make a long story short, a tiger does not look much like a woman. In most versions it’s the tail that gives it away. So, naturally, when Holangi arrived at the woman’s home, her children were not fooled.  They ran out of their house and climbed a tree, with the tiger in hot pursuit.  The two children, Haesik and Dalsun, prayed to the heavens for a rope, asking for one that was as strong as they were pure of heart. Their prayers were heard, and a thick sturdy rope (or in some versions an iron chain) appeared, which they climbed up into the sky, and then pulled up the rope.  The tiger did the same, also praying to the heavens for a rope so that he could devour the two siblings, but it was rotten, and before he got halfway up, it broke and he fell back to Earth.  Once in the sky, the brother, Haesik, became the Sun, and the sister, Dalsun, became the Moon, and they have been there ever since. Interestingly, in some traditions the two siblings later switch positions (for a number of reasons depending on the version you know), but the language has remained the same, with words similar to Haesik being related to the Sun and associated characteristics, and the same true of Dalsun and the Moon.  That story with pictures can be found here .

Why the Sun and the Moon Live in the Sky

We’ll finish up the folktale section with a more friendly story hailing from Nigeria, specifically from the Efik people. Again, because of a lack of a known writing system to accompany Efik until 1812, it is unclear how old this story really is. 

Before today, the Sun and his wife, the Moon, lived together on Earth.  But they were not alone. In fact, the Sun was friends with the Water and would often visit. However, the Water never visited the Sun, who noticed this and asked why. The Water, very concerned, said that the Sun’s house wasn’t big enough for the Water and all his people (or, all of the creatures of the sea).  The Sun and the Moon then agreed to build a grand house to welcome the Water, and when they had finished, the Water agreed to visit. 

However, the Sun was wrong in his calculations. When the Water came, the house was nearly full. Add on that he brought with him the fish and all the other living things of the sea and they filled it completely, bursting through the roof and carrying the Sun and the Moon into the sky, where they remain to this day.  That story can be found here.

Scientific theories

Quite fantastical, no? We’ll leave the impossible feats aside now and focus more on science, reason, and fact. However, you’ll find that there are still many different theories, just like there are many different origin stories. Let’s look at a few of the most popular. 

The Capture Theory

Originating in 1909 from the mind of one Thomas Jefferson Jackson See, capture theory states that the Earth “captured” another celestial body in its gravitational field as it was passing by. That body, trapped by Earth’s gravitational pull, began orbiting the planet and is now what we call the Moon.  This theory stands up upon first glance - after all, it’s the leading theory for the origin of other moons in our solar system - and was popular until the 1980’s, when scientists analysed lunar material brought back from the Apollo missions and found that it was too similar to material on the Earth to have come from elsewhere in the universe.  In particular, both astronomical bodies have the same ratio of isotopes of oxygen, whereas material from other bodies, including meteorites that didn’t come from the Moon, have different ratios.  We defined isotopes last year, but for the sake of this lesson, it suffices to say that isotopes are different forms of the same element.  The ratio of the number of each kind of isotope of the same element varies from one body to another; so if they’re the same for two bodies, the two bodies probably had a common origin.

In addition to this piece of evidence against the theory, the Moon displays other properties that are irregular when compared to other captured bodies in the solar system - namely its round shape and circular orbit. Since a captured body is generally pulled while it is moving past a planet, it exhibits an eccentric orbit. They are also generally mis-shapen, similar to asteroids, rather than round. 

So if the Moon is likely not a captured body, what else could it be, you ask? Well, let’s move on to the fission theory!

The Fission Theory

According to this theory, the Earth was once spinning so fast that part of it – from the crust beneath the Pacific Ocean - was thrown off and became the Moon.  This is quite a fascinating situation to picture, if you ask me, and was originally posited by George Darwin in 1878. 

Unfortunately it was also discredited by the material brought back by the Apollo astronauts from the Moon.  While the lunar rock was too similar to Earth’s for capture theory, analysis showed that it was too different from oceanic crust to support this model. Rather, it is quite similar to material in the Earth’s mantle, which is much older than the oceanic crust.  Having dispensed with this theory, let’s move on to the accretion theory.

The Accretion Theory

Picture this: it’s the beginning of the solar system and there is a massive cloud of dust orbiting a brand new Sun. As small dust particles run into each other, slightly larger particles begin to form. These larger pieces pull other small pieces in with their stronger gravity, eventually forming larger and larger rocks. According to accretion theory, the Moon and the Earth formed together in this fashion as a double system - that is, two bodies orbiting each other.  Proposed by Edouard Roche in 1873, this theory has also held popularity for quite some time, but alas, also has some holes in it. The problem with this theory is that it doesn’t explain what made the Moon revolve around the Earth - as opposed to their original co-orbit - or why the Moon and the Earth exhibit different densities. After all, the radius of the Moon’s iron core is only 25% of its total radius compared with 50% for the Earth. Had these two bodies originated at the same time and from the same material, we would expect their composition to be mostly the same.

On then to our last, and most accepted, theory of the day!

The Initial Giant Impact Theory

According to the impact theory, a Mars-sized planet named Theia dealt the Earth a glancing blow four and a half billion years ago. The impact of this collision dislodged a lot of the Earth’s material that came to form the Moon.  This is our current leading theory for a reason, as it explains why the Moon revolves around the Earth – the glancing blow of Theia and Earth provided the material it dislodged with the necessary speed to obtain revolution.  Additionally, it explains the variation in density between Earth and the Moon that disproved the accretion theory. You see, when Theia and Earth collided, their cores likely merged, leaving the pieces that split off to form the Moon lacking in iron and leading to its relatively small metallic core.

Should it be true, this theory leads to an interesting question about the core of the Moon and what magic it might contain. Remember that the Earth’s core contains some level of raw and untamed magic from back when the planet first formed and releases this magic little by little. If the Moon is truly a piece of broken off Earth or Earth/Theia combo, its stored magic is likely of the same or a similar “flavour” as that of the Earth. This would mean that its influence on spells and magic cast on its surface is also likely similar to that which we experience here at home. While experimenting to confirm such a thing would be rather difficult, due to the multitude of other factors that would affect lunar spellcasting, it is an interesting concept to think about!

Collision between Earth and Theia. 

Source: here

That said, the theory doesn’t completely explain why the Earth and the Moon have the same ratio of isotopes (suggesting a common origin), since some of the material that formed the Moon must have come from Theia, whose isotope composition was unlikely to be identical to that of the Earth. To account for this, some have speculated Theia and Earth may have formed in the same area of the universe and therefore had similar makeups to begin with, or perhaps the other planet was made of ice and melted.

As I mentioned earlier, this theory remains popular, and several other attempts have been made to modify it to resolve its problems.   One such modification was proposed in 2012 by Robin M. Canup of the Southwest Research Institute, San Antonio, Texas.  She suggested that two bodies, each one five times the size of Mars, collided and then re-collided, forming a large disc of debris that eventually accreted to form Earth and the Moon.  That would explain why the Earth and the Moon have the same ratio of isotopes, since both bodies would contain a similar mixture of material from the two bodies that collided. Additionally, if the collisions were not head on, this theory also explains why the Moon revolves around the Earth.

However, as she originally invented it, it has a fatal flaw: it predicts that Earth now rotates twice as fast as it does.  This flaw was corrected by two of her colleagues: Matija Ćuk and Sarah Stewart.  This version is now compatible with everything we have so far observed about the Moon, but that doesn’t necessarily mean that the Moon was actually created the way the theory describes.  In the absence of a Time Turner powerful enough to enable us to observe the Moon’s creation, the only way we could be sure would be if an ancient alien civilization had made such observations and communicated them to us Earthlings.  However, a theory can be made more plausible if it predicts something that hasn’t yet been observed but that is observed later on.  Perhaps her revised theory will eventually pass that test.  Perhaps too, such a theory will be invented by one of you.

Robin M. Canup.  

Source: here

Concluding Remarks

Now that we’ve had a chance to look at both the factual and the mythical, all that remains are your assignments! There is not only a ten-question quiz, but also a 25-question final exam, and an optional essay to write. The final exam will focus a bit more on the content not covered on the midterm (Lessons Six through Nine), but will touch on material from the entire year. Once you’ve completed the required assignments, you can finally enjoy a well-deserved break before you come back for Year Three Astronomy.  Next year is all about Earth, which is, after all, a planet.  I hope to see you all then.  For now, enjoy your vacation!

Lesson written by Professor Plumb.