Python_Turtle_Planets_and_Moons.py

# Gravitational system simulation using the approximation method using turtle graphics. 
# Example: heavy central body, light planet, very light moon. 
# Planet has a circular orbit, moon a stable orbit around the planet. 
# You can hold the movement temoporarily by pressing the left mouse button with the mouse over the scrollbar of the canvas. 

from os import access
from turtle import shape, Turtle, mainloop, Vec2D as Vec

G = 8

class GravSys(object):
    def __init__(self):
        self.planets = []
        self.t = 0
        self.dt = 0.01
    def init(self):
        for p in self.planets:
            p.init()
    def start(self):
        for i in range(100000):
            self.t += self.dt 
            for p in self.planets:
                p.step()

class Star(Turtle):
    def __init__(self, m, x, v, gravSys, shape):
        Turtle.__init__(self, shape=shape)
        self.penup()
        self.m = m
        self.setpos(x)
        self.v = v
        gravSys.planets.append(self)
        self.gravSys = gravSys
        self.resizemode("user")
        self.pendown()
    def init(self):
        dt = self.gravSys.dt
        self.a = self.acc()
        self.v = self.v + 0.5*dt*self.a
     def acc(self):
        a = Vec(0,0)
        for planet in self.gravSys.planets:
            if planet != self:
                v = planet.pos()-self.pos()
                a += (G*planet.m/abs(v)**3)*v
        return a
    def step(self):
        dt = self.gravSys.dt
        self.setpos(self.pos() + dt*self.v)
        if self.gravSys.planets.index(self) != 0:
            self.setheading(self.towards(self.gravSys.planets[0]))
        self.a = self.acc()
        self.v = self.v + dt*self.a
        
# Create compound yellow/blue turtleshape for planets

def main():
    s = Turtle()
    s.reset()
    s.getscreen().tracer(0,0)
    s.ht()
    s.pu()
    s.fd(6)
    s.lt(90)
    s.begin_poly()
    s.circle(6, 180)
    s.end_poly()
    m1 = s.get_poly()
    s.begin_poly()
    s.circle(6,180)
    s.end_poly()
    m2 = s.get_poly()

    planetshape = Shape("compound")
    planetshape.addcomponent(m1,"orange")
    planetshape.addcomponent(m2,"blue")
    s.getscreen().register_shape("planet", planetshape)
    s.getscreen().tracer(1,0)

# Setup gravitational system
    gs = GravSys()
    sun = Star(1000000, Vec(0,0), Vec(0,-2.5), gs, "circle")
    sun.color("yellow")
    sun.shapesize(1.8)
    sun.pu()
    earth = Star(12500, Vec(210,0), Vec(0,195), gs, "planet")
    earth.pencolor("green")
    earth.shapesize(0.8)
    moon = Star(1, Vec(220,0), Vec(0,295), gs, "planet")
    moon.pencolor("blue")
    moon.shapesize(0.5)
    gs.init()
    gs.start()
    return "Done!"

if __name__ == '__main__':
    main()
    mainloop()