!/usr/bin/env python3

“”” turtle-example-suite:

    tdemo_planets_and_moon.py

Gravitational system simulation using the
approximation method from Feynman-lectures,
p.9-8, using turtlegraphics.
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 temporarily by
pressing the left mouse button with the
mouse over the scrollbar of the canvas.
“””
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(10000):
        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.[商品期货](https://www.gendan5.com/futures/cf.html)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()