from __future__ import print_function, division
from visual.graph import *
# Angular momentum in an elliptical orbit

print("""click to specify location A.""")

scene.y = 0
scene.width = 1024
scene.height = 768
scene.background = color.white
L = 1.5e11
##scene.center = (L/2,0,0)
sun = sphere(pos=(0,0,0), color=color.yellow, radius=1e10)
sun.mass = 2e30
earth = sphere(pos=(L,0,0), radius=2e9, color=color.red)
earth.mass = 6e24
ev = 2.*pi*L/(365.*24.*60.*60.)
earth.p = vector(0, 0, -earth.mass*ev)
earth.trail = curve(color=earth.color)
dt = 60.*60.
G = 6.7e-11
scene.autoscale = 0
earth.p = earth.p/1.5
r = sun.pos-earth.pos
F = (G*earth.mass*sun.mass/r.mag**2)*norm(r)
flag=0
t=0
L1scale = 4e10/1e40
L2scale = 4e10/1e40
L1a = arrow(pos=sun.pos, shaftwidth=earth.radius,axis=(0,0,0), color=color.green,
            fixedwidth=1, visible=0)
L2a = arrow(pos=sun.pos, shaftwidth=earth.radius, axis=(0,0,0), color=color.green,
            fixedwidth=1, visible=0)
while 1:
    rate(1000)
    if scene.mouse.clicked:
        mm = scene.mouse.getclick()
        L2a.visible=1
        if mm.pick is sun:
            L2a.pos = sun.pos
        else:
            L2a.pos = mm.pos
    r = sun.pos-earth.pos
    F = (G*earth.mass*sun.mass/r.mag**2)*norm(r)
    earth.p = earth.p + F*dt
    earth.pos = earth.pos + (earth.p/earth.mass)*dt
    t=t+dt
    earth.trail.append(pos=earth.pos)
    L1 = cross (-r, earth.p)
    L2 = cross ((earth.pos-L2a.pos), earth.p)
    L1a.axis = L1*L1scale
    L2a.axis = L2*L2scale