Another not good solar system

#!/usr/bin/env python
# -*- coding: utf8 -*-
dimension=800 #(0.01 of A.U.)
dt = 0.0001
delay=1
import numpy as np
import Tkinter
import itertools
class Planet:
    def __init__(self,canv,x,y,vx,vy,color,index):
        self.x = float(x)
        self.y = float(y)
        self.vx = float(vx)
        self.vy = float(vy)
        self.color = color
        self.canv = canv
        self.index=index
        self.position=self.canv.create_oval(400+int(200*self.x)-5,400+int(200*self.y)-5,
        400+int(200*self.x)+5,400+int(200*self.y)+5,fill=self.color,outline='white')
        self.energy = self.canv.create_text(100,100*self.index,text=str(0.5 * (self.vx**2+self.vy**2)),fill=self.color)
    def time_step(self):
        canv.delete(self.position)
        canv.delete(self.energy)
        self.energy = self.canv.create_text(100,100*self.index,text=str(0.5 * (self.vx**2+self.vy**2)),fill=self.color)
        self.canv.create_oval(400+int(200*self.x),400+int(200*self.y),
        400+int(200*self.x),400+int(200*self.y),fill=self.color,outline=self.color)
        self.vx = self.vx - np.sign(self.x)/(self.x**2 + self.y**2) * dt
        self.vy = self.vy - np.sign(self.y)/(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(400+int(200*self.x)-5,400+int(200*self.y)-5,
        400+int(200*self.x)+5,400+int(200*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')
canv.create_oval(400-10,400-10,400+10,400+10,fill="white",outline="white")
mercury = Planet(canv,0.38710,0.,0.,2,"orange",1)
venus = Planet(canv,0.72334,0.,0.,1.6,"yellow",2)
earth = Planet(canv,1.00000,0.,0.,1.3,"blue",3)
mars = Planet(canv,1.52371,0.,0.,1.0,"red",4)
system=[]
system.append(mercury)
system.append(venus)
system.append(earth)
system.append(mars)
def on_timer():
    for p in system:
        for _ in itertools.repeat(None, 100):
            p.time_step()
    root.after(delay, on_timer)
on_timer()
root.mainloop()