NOT a Solar System xD

#!/usr/bin/env python
# -*- coding: utf8 -*-
dimension=400 #(0.01 of A.U.)
dt = 0.001
delay=1
import numpy as np
import Tkinter
import time
from matplotlib import pyplot as plt
class Planet:
    def __init__(self,canv,x,y,vx,vy,color):
        self.x = float(x)
        self.y = float(y)
        self.vx = float(vx)
        self.vy = float(vy)
        self.color = color
        self.canv = canv
        self.position=self.canv.create_oval(200+int(100*self.x)-5,200+int(100*self.y)-5,
        200+int(100*self.x)+5,200+int(100*self.y)+5,fill=self.color,outline='white')
    def time_step(self):
        canv.delete(self.position)
        self.canv.create_oval(200+int(100*self.x),200+int(100*self.y),
        200+int(100*self.x),200+int(100*self.y),fill=self.color,outline=self.color)
        self.vx = self.vx - np.sign(self.x)/np.sqrt(self.x**2 + self.y**2) * dt
        self.vy = self.vy - np.sign(self.y)/np.sqrt(self.x**2 + self.y**2) * dt
        self.x = self.x + self.vx * dt
        self.y = self.y + self.vy * dt
        self.position=self.canv.create_oval(200+int(100*self.x)-5,200+int(100*self.y)-5,
        200+int(100*self.x)+5,200+int(100*self.y)+5,fill=self.color,outline='white')
root = Tkinter.Tk()
canv = Tkinter.Canvas(root, width = dimension, height = dimension, bg = 'black')
canv.pack(expand='yes',fill='both')
mercury = Planet(canv,0.38710,0.,0.,1,"orange")
venus = Planet(canv,0.72334,0.,0.,1,"yellow")
earth = Planet(canv,1.00000,0.,0.,1,"blue")
mars = Planet(canv,1.52371,0.,0.,1,"red")
system=[]
system.append(mercury)
system.append(venus)
system.append(earth)
system.append(mars)
def on_timer():
    for p in system:
        p.time_step()
    root.after(delay, on_timer)
on_timer()
root.mainloop()