| 첨 | 성 | 대 |
| (瞻, cheom) | (星, seong) | (臺, Dae) |
| (2)to observe | (3) Stars. | (1)Place |
A tower to observe stars built in AD 647 in Silla kingdom [1, 55].
It is called 'Cheom Seong Dae'.
This is the oldest surviving astronomical observatory in the world among the one without any reconstruction [2].
Despite experiencing several earthquakes, wars, and natural disasters, it still maintains its original appearance.
It was built by King Seonduk.
She was the 27th ruler and the first female ruler in Silla(ruled 632~647).
Interestingly, It was located in a palace [44,55].
This explans that she didn't build this CheomSeongDae only for the observatory, but also for her royal power.
There is more story about the location of Cheomsungdae.
People that time believed the cosmos and earth exist harmonious way.
The constellations were another kingdom ruled by a Polaris in their imagination.
So, there were a kitchen, a garden, a girl who weaves a loom, a boy who drives a cow, a river, a fortress, a captain called Hago and so on. the same like the world on earth.
The location of Cheomseongdae is consistent with 'Hago star(now 'Altair'), which is located on the river of the universe(now 'Milky way').
Well! Well! Well.
She had to emphasize that she was descended from parents who founded the kingdom to convince her power to the male-centered society.
'Well' was a symbolic icon for the king Seonduk.
From the location of the Cheomsungdae, we can read that one function of the Cheomsungdae was a well reflecting the constellation the 'Hago star' that placed on the river of the universe.
Its neighboring locations and their shapes on the site are also match with constellations.
To be more clear with it, she pluged code of Well on top of the building. by placing bar stones to shape the letter 'well(井)'.
In order to perform the observatory's function well, the direction of the bar had to be considered.
The vertices of the squares are directed to where sun rise and set on certain important days like a winter solstice . the shadow of the vertex of the sharp(#: or 'well') part will reach a certain level of the building on that day.
It can be used as an instrument to observe solstices and equinoxes.
In addition, 'Well' contains an important birth story of Silla.
Story goes like this.
There was a boy hatched from an egg near a well,
and also a girl who was born from the side of a chicken head-dragon.
She had a bird's beak. After washed her beak in the well, her beak fell off and she looked like a normal human.
Later on, the boy and the girl got married and founded their own kingdom Silla(B.C 57)[4].
 |
 |
| http://contents.history.go.kr/mobile/eh/view.do?levelId=eh_n0020_0010&code=eh_age_10 | |
Well also roled as a Buddhist symbol that serves as an important passage connecting the underground, earth, and universe.
To empower the Buddhist royal authority, she built many other architectures such as a Whangryong temple[8,9].
(The height was about 82m.)
Those architectures can fulfill religious, mythological and political functions as a way for her to prove her credentials and abilities towards a male-centered society.
Therefore, we can see how important it was for her to build the Cheomseongdae, following the founder's birth story and the authority of Buddhism, in addition to considering constellations.
The Harmony of a square and a circle.
Circles and squares were important shapes representing philosophy about parents at that time, with the circle symbolizing the father and sky, and the square symbolizing the mother and earth.
World only works beautifully with the harmony of parents(+,-). This philosophy continues throughout Korea's history.
We assume that Zhōubí Suànjīng(周髀算经), books of mathematics and astronomy from the Zhou dynasty(BC. 1046~AD.256/ ancient China), contributed a lot to Silla in building Cheomseongdae[12, 30]. This is because the world view of seeing the world as a circle and a square is very similar.
Circle
If we look closer to the building,
We can see that the surfaces of the stones that make up Cheomseongdae are polished into curved surfaces. It is not stacking uniform bricks to form a circle(n-gon polygon).
It was important for them to trim the arranged stones into perfect circles to reflect their belief about the world, (circle as a father or sky, square as a mother or earth).
Enduring about 1500 years, the balance of those circles has a bit broken. but still it maintains its original shape.
A Square
The platform in the bottom part of this building is a square prism.
Also the entrance does. It is a square with side length of 91cm.
The square entrance is rotated 16 degrees to east from south. So, sun light can reache deep inside on the equinoxes of march.
Based on the measure of the width of the square entrance, a sun beam will enter the room for two hours per day around noon. and the depth of the sun can reflect seasonal changes.
When the light does not enter the room at noon, it announces the summer solstice.
in March and September, when the sum beam touches the stepping stone of the door, it tells the equinox.
 |
 |
| Sun beam on summer solstice | Sun beam on equinox |
The sum beam enters deeply to the room touching inside the stone and tells the winter solstice.
S curve : Bottle body
The Top part was well shaped.
The platform part was a square prism.
What about the middle part?
How would you like to describe the shape of the body?
Doesn't it look like a bottle?
Let's take a tour to other country.
[Observatory in Mexico]
CheomSeongDae is not the only observatory that shapes bottle(or bell).
There are at least three more in the world. In Mexico, three astronomical observatories are known with the bottle shaped but with an internal curve. Among them only Xochicalco is open to the public. It was founded in about 650 AD.
Almost the same time Cheomsungdae was built [19, 28, 29].
 |
 |
| Xochicalco [28] | Xochicalco view [28] |
 |
 |
| Hexagonal hole from outside floor [28,29] | Hexagonal hole looked up from the cave [28,29] |
From the bell shaped tunnel in Xochicalco, Mayan people observed an important day for them through the movement of the sunlight reaching to the bottom.
 |
 |
| bottle shape turnel in Xochicalco [28,29] | Cave in Xochicalco |
Sun beam can be observed on May and July when the sun is at its zenith[34].
 |
 |
| Xochicalco sun light(01) observed [28] | Xochicalco sun light(02) observed [29] |
It's very interesting that two very distant civilizations used similar designs to figure out certain days of the year.
[Mathematical Curve]
Why those observatory needed to shape a bottle?
We can have another imagination.
The body curve of the Cheomseongdae is representing the annual change of sun in the zenith. For a country where agriculture is limited by four distinct seasonal changes, March through September is a very important season.
From the 1st to the 24th stage, it follows a curve, and above it, it forms a cylindrical shape[35].
The path of the sun over the season fits nicely with the cheomsungdae body [6].
 |
|
| cosin curve fiting test by considering the degree of 23.5 |
Is there any other mathematical story hidden?
It's a lot of fun to find a code hidden in a building, solve a riddle, and learn other people's discoveries.
Numbers and Mathematical symbols in the building.
Zhōubí Suànjīng(周髀算经), is a series of books of mathematics and astronomy from Zhou dynasty.
(BC.1046~AD.256/ Ancient China, the title 'Zhōubí' means the ratio of the circumference. this book contains math about squares or circles).
Kim(1974) gave a new interpretation connecting the 7 circles of cosmos explained from the book with a Cheomseongdae [31, 32].
The 7 circles represent the knowledge of that time about a cosmos. The smallest circle as a cycle of a winter solstice, middle part equinox, the largest circle as for summer. Cheomseungdae reflect the understanding of the cosmos at that time [12, 32].
As a mathematician, he likes to enjoy stories about numbers on Cheomseongdae.
let's find more number from Cheomseongdae.
(these kinds of numbers exist in other astronomical buildings too(such as Xochikalco like 7, 12, 364, 365, 91. etc.)).
Let's take a look those numbers[12 ,32].
| 365 | Cheomseongdae used 365 stones for the body part. 364 stones on the surface of the body, 1 on top of the inner body. It is days of an year. (vs. Xochicalco : 91 steps * 4 faces = 364 steps, + 1 top covering stage tone of the 4 faces = 365 steps in total) |
| 27 | The body consists of 27th layers. this number is the orbital period of the moon(27.3) and also the order of King Seondeok's ruling. 3 layers in the middle divide the lower part and the upper part of the body (12+3+12=27). number of layers on CheomSeongDae can be varied adding the bottom 2 layers or top 2 layers(28~31). |
| 12 | 12 under layers consist of 16, 15, 15, 16, 16, 15, 18, 15, 15, 14, 14, and 13 stones counted from the bottom. In total the number of stones is 182. The upper 15 layers consist of a total of 184 stones. |
| 12 |
How many stones were used in the platform?
We don't have an exact picture of the inside of the bottom platform.
However, based on the world view at the time, it is natural to reason that 12 stones were used for the platform.
| 24 | Apart from the division of 12 months based on the motion of the moon, 24 periodic divisions of the year were considered to understand seasonal changes. It is based on solar rotation dividing it by 15 degrees. with this division 16, 15, 15, 16, 16, 15 days were counted for that period. Those numbers of stones are represented on the lower layer of the CheomSeongDae. |
Kim (1974) has found interesting mathematical relationship between 27, 364 and 365[12].
27 as for the number of King Seonduk., 364 is the number of CheomSeongDae, and 365 is the number of a year.
We never know whether they considered this relationship of numbers.
But we can tell that they used the knowledge of sides of the right triangle in the construction.
The book Zhōubí Suànjīng(周髀算经) introduced a visual prove of a theorem called 'gu-go-hyun' (base-hight-chord). Familiar to a Pythagora's theorem. the special numbers 3, 4, 5 are also introduced to explain this relationship of the sides.
We can also find those numbers from the ratio of the CheomSeongDae[12, 32]. The Earth's axis is tilted 23.5 degree from the plane of its orbit around the sun. this angle is also reflected on the CheomseongDae.
ratio on front side, (top width):(bottom width) = 3:5,
ratio on half rotated side (bottom width): (height) = 4 : 5
Earthquake resistance
Gyeongju is an unstable stratum region, so earthquakes have occurred frequently. Still, one to three intensity earthquakes occur every year in the region. The highest-intensity earthquake recorded recently was 5.8 on Sep. 12, 2016.
However, Cheomseongdae has endured over 1500 years and still maintains its original form at the time of its construction[46].
| (Earthquake of Gyeongju on 2016). => https://youtu.be/8fgK0XHrcfI?si=-Ew8c5jJb9BgqGz7 |
What are the secrets of the construction for the earthquake resistance?
1) Materials :
Granite stones used and stacked without using mortar. the lower 12 layers used high hard Granite stones, other parts with standard granite. Granite has a hardness of 6.5 to 8. generally it is harder than a steel knife or scissor.
The weight of exposed stones upper from the 13th layer is estimated to be 50 kg to 870kg from the measure of the volume. The stones used for under 12 layers are even denser, harder, and way bigger than the stones above exposed. So we can easily guess how heavy it could be. The most heaviest one could weight about 2000kg [12, 35, 36]. For the lower layer, the space between the stones be filled with soil to increase the load on the lower part allowing the gravity point to be stably placed low enough.
2) Each layer is trimmed in parallel, so it can help to maintain the balance of the building.
3) Stones of each layer placed slightly inward from the lower layer. and the edges are trimmed to protrude slightly upward to block the force of pushing outward.
4) What about inside of the bottom stage?
We can assume the following arrangement of 12 stones for the bottom platform.
This platform is stacked on the first platform. the first layer platform is hidden and burried underground.
The center hole will function to lock upper stones from the vibraion of earthquake.
stones of the first layer of the curvy body must be stacked on the square platform locking its edge using protrusion to the inner space. so to prevent its force to outward.
5) Rubble and soil filled the space between stones and inner area and minced to absorb vibrations and collisions.
6) Interlocked bar on the upper layer
Upper layer has no filled soil to protect this building from collision, vibration or any other kind of obstacles.
Instead, 8 crossbars are used.
This serves as physical support of the bottle body and helps maintain the placement of other stones of the body.
The arrangement and direction is following.
| on 19th layer, two bars located in the east-west direction. on the 20th layer, two bars located to the south-north direction, but not penetrated to outside. on the 25th layer, two bars located to the south-north direction, penetrated to the outside. on the 26th layer, two bars located to the east-west direction, but not penetrated to the outside. |
 |
7) It is leaning or leaned.
The recent earthquake caused the building to tilt a little more.
Measure of CheomSeongDae
Numbers give us more imagination about the model and architecture.
If you want to model CheomseongDae using Tinkercad or other modeling tools, you may need information on measurement[42, 43]. Each level has a height of about 31 centimeters.
Lengths
Diameter and numbers of stones for each level.
Overview of the number of stones
Correspond to a constellation. :
Expansion of the cosmos on earth[39,40]
The location of the Cheomseongdae means something.
Cheomseongdae is located at an intersection of important locations.
The latitude of the Seondo Mountain base and the path of the winter solstice were considered [39, 40].
Three Fortress on the mountain and one giant mountain shaped tomb
form the rhombus's vertex of the site.
Cheomseongdae is located at the center of them.
The location of Cheomseongdae is correspomd with 'Hago star(Altair)'
It is located on the river of the sky(Milky way).
and the same way, Cheonjeon star(Field of sky) and Gugam(stream of sky) star are also consistent with 'garden of Banwol' and 'Banwol Fortress'.
 |
 |
| Cheonjeon(Field of sky) [51] | Gugam(Stream) [51] |
 |
 |
| Moon pond(Worlji) on the Constellation map[51] | Top view of the moon pond (googe map)[51] |
It is clear that there was an attempt to match the constellations of the sky with the constellations of the earth.
The arrangement of ancient tombs in the Daerungwon is correspond with the constellation of Cheonsiwon.
to understand and enjoy more about the Cheomseongdae, we have to see it globally including its nature and architectures nearby. that might have story connected with it.
The location of ancient tombs and other important architectures of that time reflect the constellation map. The size of tomb reflect the brightness of stars.
Amazing.
Thanks very much for reading my story.
I hope this story will help further STEAM education.
especially
In-depth appreciation of Cheomseongdae through modeling activities
References
[1] https://encykorea.aks.ac.kr/Article/E0002925
[2] https://en.wikipedia.org/wiki/Cheomseongdae
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[34] https://en.wikipedia.org/wiki/Xochicalco
[35] 송민구, 1981, '경주 첨성대 실측 및 복원도에 의한 비례분석'. <한국과학사학회지>, 3(1), pp. 52~75.
[36] 이동우, 1986. '경주첨성대의 형태와 구조 - 왜 첨성대는 오늘날까지 그 형태를 유지해 왔는가?', <건설기술 연구속보>, 4(10). 한국건설기술연구원, pp. 4~18.
[37] https://images.app.goo.gl/iheP7KHfhHAJYzCg6 우리역사넷
[38] https://www.mk.co.kr/news/culture/4634675 첨성대 논란
[39] https://www.pressian.com/pages/articles/4862
[40] https://blog.naver.com/saearm21/223338948043
[41] https://blog.naver.com/PostView.nhn?blogId=gne_education&logNo=222070203582 견우성과 직녀성
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[47] https://m.blog.naver.com/jw1405/220828547050
[48] https://cm.asiae.co.kr/article/2019080616422719937
[49] https://www.mk.co.kr/news/culture/4634675
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[51] https://lionetshomeworld.tistory.com/751(첨성대 별기 영상)
[52] https://6042cjm.tistory.com/15692076 (첨성대에서 문무대왕릉까지)
[53] https://blog.naver.com/ohyh45/222838917038 (첨성대에 대한 깊이 있는 분석)
[54] https://youtu.be/BKRD4WxNPtM (viedo reading link)
[55] https://namu.wiki/w/%EC%B2%A8%EC%84%B1%EB%8C%80
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