1#!/usr/bin/env python
2"""       turtle-example-suite:
3
4        tdemo_planets_and_moon.py
5
6Gravitational system simulation using the
7approximation method from Feynman-lectures,
8p.9-8, using turtlegraphics.
9
10Example: heavy central body, light planet,
11very light moon!
12Planet has a circular orbit, moon a stable
13orbit around the planet.
14
15You can hold the movement temporarily by
16pressing the left mouse button with the
17mouse over the scrollbar of the canvas.
18
19"""
20from turtle import Shape, Turtle, mainloop, Vec2D as Vec
21from time import sleep
22
23G = 8
24
25class GravSys(object):
26    def __init__(self):
27        self.planets = []
28        self.t = 0
29        self.dt = 0.01
30    def init(self):
31        for p in self.planets:
32            p.init()
33    def start(self):
34        for i in range(10000):
35            self.t += self.dt
36            for p in self.planets:
37                p.step()
38
39class Star(Turtle):
40    def __init__(self, m, x, v, gravSys, shape):
41        Turtle.__init__(self, shape=shape)
42        self.penup()
43        self.m = m
44        self.setpos(x)
45        self.v = v
46        gravSys.planets.append(self)
47        self.gravSys = gravSys
48        self.resizemode("user")
49        self.pendown()
50    def init(self):
51        dt = self.gravSys.dt
52        self.a = self.acc()
53        self.v = self.v + 0.5*dt*self.a
54    def acc(self):
55        a = Vec(0,0)
56        for planet in self.gravSys.planets:
57            if planet != self:
58                v = planet.pos()-self.pos()
59                a += (G*planet.m/abs(v)**3)*v
60        return a
61    def step(self):
62        dt = self.gravSys.dt
63        self.setpos(self.pos() + dt*self.v)
64        if self.gravSys.planets.index(self) != 0:
65            self.setheading(self.towards(self.gravSys.planets[0]))
66        self.a = self.acc()
67        self.v = self.v + dt*self.a
68
69## create compound yellow/blue turtleshape for planets
70
71def main():
72    s = Turtle()
73    s.reset()
74    s.tracer(0,0)
75    s.ht()
76    s.pu()
77    s.fd(6)
78    s.lt(90)
79    s.begin_poly()
80    s.circle(6, 180)
81    s.end_poly()
82    m1 = s.get_poly()
83    s.begin_poly()
84    s.circle(6,180)
85    s.end_poly()
86    m2 = s.get_poly()
87
88    planetshape = Shape("compound")
89    planetshape.addcomponent(m1,"orange")
90    planetshape.addcomponent(m2,"blue")
91    s.getscreen().register_shape("planet", planetshape)
92    s.tracer(1,0)
93
94    ## setup gravitational system
95    gs = GravSys()
96    sun = Star(1000000, Vec(0,0), Vec(0,-2.5), gs, "circle")
97    sun.color("yellow")
98    sun.shapesize(1.8)
99    sun.pu()
100    earth = Star(12500, Vec(210,0), Vec(0,195), gs, "planet")
101    earth.pencolor("green")
102    earth.shapesize(0.8)
103    moon = Star(1, Vec(220,0), Vec(0,295), gs, "planet")
104    moon.pencolor("blue")
105    moon.shapesize(0.5)
106    gs.init()
107    gs.start()
108    return "Done!"
109
110if __name__ == '__main__':
111    main()
112    mainloop()
113