python中的动画加速

Question

我一直在用python编写一个轨道物体的简单模拟。我终于让它工作了，但随着时间的推移，动画似乎在加速。我可以看到，我的轨道物体的速度是周期性的和正确的，但在动画中似乎移动得越来越快。我不知道为什么。在

import numpy as np
import math
from matplotlib import pyplot as plt
from matplotlib import animation
from time import sleep
import sys

fig = plt.figure()
"Defines useful information such as masses and initial velocities"
AU= (149.6e6 * 1000)
smass=1.98892*10**30 #kg    mass of the sun
emass=5.9742*10**24 #kg     mass of the earth
evx=0                      #Earth velocity in the X direction 
evy=29.783 * 1000          #Earth velocity in the Y direction
r=-1*AU #m                 #Earth Radius
G= 6.67408*10**-11 #m^3*kg^-1*s^-2


"Defines animated area and bodies"
ax = plt.axes(xlim=(-2*AU,2*AU), ylim=(-2*AU,2*AU))
earth = plt.Circle((0, 0), 6371000000, fc='b')
sun = plt.Circle((0,0), 6371000000, fc='y')


"Defines the starting positions of the bodies"
def init():
    earth.center = (r,0)
    sun.center = (0,0)
    ax.add_patch(sun)
    ax.add_patch(earth)
    return earth,sun

"Calculates new positions of orbiting body (in this case Earth)"    
def animate(i):
    global evx
    global evy
    if i==0:

        pass  #this avoids a division by 0
    else:


        x, y = earth.center         #Defines x and y 
        t=i*240                     #Defines the time step
        d=math.sqrt(x**2+y**2)      #calculates distance between two objects
        f=-(G*emass*smass)/(d**2)   #calculates the force exerted on orbiting object
        theta = math.atan2(y,x)     #calculates the angle to the orbiting object
        fx=math.cos(theta)*f        #calculates the component of the force in the X direction on the orbiting object
        fy=math.sin(theta)*f        #calculates the component of the force in the Y direction on the orbiting object
        evx+=fx/emass*t             #calculates the velocity in the X direction of the orbiting object
        evy+=fy/emass*t             #calculates the velocity in the Y direction of the orbiting object
        x += evx*t                  #converts velocity to X position using the time step
        y += evy*t                  #converts velocity to Y position using the time step
        earth.center = (x, y)       #prints object at correct X and Y position
        "Debug information"
        #print ("distance is", d,"force is", f, "angle is", theta)
        #print ("Speed is", math.sqrt(mvx**2+mvy**2))
        #print ("x position is", x,"y position is",y,i)
        "Watches for a collision between the two bodies distance is arbitrary"
        if d<=6371000000:
            print("Collision")
            sys.exit()
        else:
            pass



    return earth,sun
"Defines run time and animation information"
anim = animation.FuncAnimation(fig, animate, 
                               init_func=init, 
                               frames=360000, 
                               interval=1,
                               blit=True)

plt.show()