def setup():
    size(900, 450)
    
def draw():
    t = 0.01*frameCount
    xsun = 0.5*width
    ysun = 0.5*height
    # clear screen before each new frame
    background(128)
    # draw sun
    ellipse(xsun, ysun, 20, 20)
    rplanet = 200
    xplanet = xsun + rplanet*cos(t)
    yplanet = ysun + rplanet*sin(t)
    # draw planet
    ellipse(xplanet, yplanet, 10, 10)
    rmoon = 30
    xmoon = xplanet + rmoon*cos(t*365/27.3)
    ymoon = yplanet + rmoon*sin(t*365/27.3)
    # draw moon
    ellipse(xmoon, ymoon, 5, 5)
    # add second planet
    year_mercury_days = 115.88  # from Wikipedia
    T_ratio = year_mercury_days/365.25
    R_ratio = T_ratio**(2.0/3)
    xplanet = xsun + R_ratio*rplanet*cos(t/T_ratio)
    yplanet = ysun + R_ratio*rplanet*sin(t/T_ratio)
    ellipse(xplanet, yplanet, 7, 7)