地月关系统模拟图示，月关模拟图示,#!/usr/bin/e

地月关系统模拟图示，月关模拟图示,#!/usr/bin/e

#!/usr/bin/env python3"""       turtle-example-suite:        tdemo_planets_and_moon.pyGravitational system simulation using theapproximation 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 stableorbit around the planet.You can hold the movement temporarily bypressing the left mouse button with themouse over the scrollbar of the canvas."""from turtle import Shape, Turtle, mainloop, Vec2D as Vecfrom time import sleepG = 8class 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 planetsdef 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()