Solar system simulation

orbit simulation.py

# http://nssdc.gsfc.nasa.gov/planetary/factsheet/
import turtle
import math
turtle.bgcolor("black")

G = 6.67428e-11
AU = 149.6e9  # m
SCALE = 250 / (2 * AU)  # pixels/AU
TIMESTEP = 24 * 3600  # s

sun = turtle.Pen()
sun.shape("circle")
sun.turtlesize(5)
sun.color("yellow")
sun.mass = 1.989e30  # kg

# Earth
earth = turtle.Pen()
earth.shape("circle")
earth.turtlesize(1)
earth.color("blue")
earth.up()
earth.px, earth.py = 1*AU, 0
earth.vx, earth.vy = 0, -29.8e3  # m/s
earth.goto(earth.px*SCALE, earth.py*SCALE)

# Mars
mars = turtle.Pen()
mars.shape("circle")
mars.turtlesize(0.6)
mars.color("red")
mars.up()
mars.px, mars.py = 1.5*AU, 0
mars.vx, mars.vy = 0, -24.1e3  # m/s
mars.goto(mars.px*SCALE, mars.py*SCALE)

planets = [earth, mars]

while True:
    for planet in planets:
        r = math.sqrt(planet.px**2 + planet.py**2)
        a = G * sun.mass / r**2  # Thanks, Newton!
        angle = math.atan2(-planet.py, -planet.px)
        ax = a * math.cos(angle)
        ay = a * math.sin(angle)
        planet.vx += ax * TIMESTEP
        planet.vy += ay * TIMESTEP
        planet.px += planet.vx * TIMESTEP
        planet.py += planet.vy * TIMESTEP
        planet.goto(planet.px*SCALE, planet.py*SCALE)
        planet.down()