本來想要用Inkscape自己一條一條慢慢畫，但是又覺得那樣很蠢，一定畫不好。
要畫這種規律的圖，應該要用程式來畫，於是我想起之前玩arduino的時候，認識Processing這套軟體，可以寫程式來畫圖。
寫程式本身不難，難是難在要用什麼語法和結構，我找了一個Processing 基本教學的講義，和一個範例，花了一個晚上修改那個範例變成我要的東西。
果然啊，用日影是可以畫出太極圖的！
我的作法是直接到網路天文館找太陽表，把全年過中天時的仰角都匯整起來，藉此來換算成日影。
我覺得如果太極圖真是古人觀測日影所畫出來的，那麼他的作法可能是拿根竹枝去折成當日最短日影的長度，每隔幾天就會做一次，一年下來就會蒐集到很多根竹枝。這這些竹枝按照順序排一排，就可以排出上圖下方的類似鐘型曲線的圖。後來古人可能又發現把這上百根竹枝排列成一個圓形，就會自然形成一個特別的弧度，也就變成太極的模樣。
那麼古人用來排太極的竹枝有沒有可能每天都測日影呢？我覺得不大可能，因為如果有360根竹枝，那會很難排成一個圓。
為了避免以後我忘記怎麼弄這個程式，我特地把程式貼在底下。以下是我參考其他範例所寫的程式，複製後貼到Processing 裡的PDE執行環境，就可以了
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float px, py;
float px2,py2;
float shadowLength;
float radius = 500;
float x;
int[] sunangle = {88,88,88,88,88,88,88,88,88,88,88,88,88,88,88,88,87,87,87,87,87,87,87,87,86,86,86,86,86,85,85,85,85,85,84,84,84,84,84,83,83,83,83,82,82,82,82,81,81,81,80,80,80,79,79,79,79,78,78,78,77,77,77,76,76,76,75,75,74,74,74,73,73,73,72,72,72,71,71,70,70,70,69,69,69,68,68,67,67,67,66,66,65,65,65,64,64,64,63,63,62,62,62,61,61,60,60,60,59,59,59,58,58,57,57,57,56,56,56,55,55,54,54,54,53,53,53,52,52,52,51,51,51,50,50,50,49,49,49,49,48,48,48,47,47,47,47,46,46,46,46,45,45,45,45,45,44,44,44,44,44,43,43,43,43,43,43,43,42,42,42,42,42,42,42,42,42,42,42,42,42,42,41,41,41,41,42,42,42,42,42,42,42,42,42,42,42,42,42,42,42,43,43,43,43,43,43,44,44,44,44,44,44,45,45,45,45,46,46,46,46,47,47,47,47,48,48,48,49,49,49,49,50,50,50,51,51,51,52,52,52,53,53,53,54,54,54,55,55,56,56,56,57,57,57,58,58,59,59,59,60,60,61,61,61,62,62,63,63,63,64,64,64,65,65,66,66,66,67,67,68,68,68,69,69,70,70,70,71,71,71,72,72,73,73,73,74,74,74,75,75,76,76,76,77,77,77,78,78,78,79,79,79,79,80,80,80,81,81,81,82,82,82,82,83,83,83,83,84,84,84,84,85,85,85,85,85,86,86,86,86,86,86,87,87,87,87,87,87,87,87,88,88,88,88,88,88,88,88,88,88,88,88,88,88,88};
void setup(){
size(600, 800);
}
void draw(){
background (0);
stroke(100);
fill(255);
ellipse(300, 300, radius, radius);
for(int i =0 ;i <180;i=i+3){
stroke(200,200,200);
px = 300 + cos(radians(i))*(radius/2);
py = 300 + sin(radians(i))*(radius/2);
line(300,300,px,py);
stroke(0,0,5);
shadowLength = radius/2 * cos(radians(sunangle[i])) / sin(radians(sunangle[i])) ;
px2 = px + cos(radians(i-180))*(shadowLength);
py2 = py + sin(radians(i-180))*(shadowLength);
line(px,py,px2,py2);
stroke(255,255,255);
line(100+i,750,100+i,750-shadowLength/2);
}
for(int i =180 ;i <360;i=i+3){
stroke(200,200,200);
px = 300 + cos(radians(i))*(radius/2);
py = 300 + sin(radians(i))*(radius/2);
line(300,300,px,py);
stroke(0,0,5);
shadowLength = radius/2 * cos(radians(sunangle[i])) / sin(radians(sunangle[i])) ;
px2 = 300 + cos(radians(i))*(shadowLength);
py2 = 300 + sin(radians(i))*(shadowLength);
line(300,300,px2,py2);
stroke(255,255,255);
line(100+i,750,100+i,750-shadowLength/2);
}
}