from visual import *

#Calculates Earth's orbit about the Sun, v^2=G Msun/r, gives v=29900 m/s,
#using G=6.7e-11 Nm^2/kg^2,Msun=2e30 kg, r=1AU=1.5e11 m

scene.width=800
scene.height=800

factor=1.0
# To change initial speed of the Earth from its circ.value 29900 m/s,
# factor=1.4142 makes parabolic orbit (E=0)

sun = sphere()
sun.pos = vector(0,0,0)
sun.radius = 3e10
sun.color = color.yellow
sun.mass = 2e30
sun.p = vector(0, 0, 0) * sun.mass

earth = sphere()
earth.pos = vector(1.5e11,0,0)
earth.radius = 2e10
earth.color = color.green
earth.mass = 6e24
earth.p = vector(0, factor*29900, 0) * earth.mass

dist = sun.pos - earth.pos
force = 6.7e-11 * sun.mass * earth.mass * dist /mag(dist)**3

earth.orbit = curve(color=earth.color, radius = 2e9)
farrow=arrow(pos=earth.pos,axis=2.0e-12*force)

dt = 86400
t=0
while 1:
  rate(100)
  t=t + dt

  dist = sun.pos - earth.pos
  force = 6.7e-11 * sun.mass * earth.mass * dist /mag(dist)**3

  earth.p = earth.p + force*dt
  earth.pos=earth.pos + (earth.p/earth.mass)*dt
  earth.orbit. append(pos=earth.pos)
  farrow.pos=earth.pos
  farrow.axis=2.0e-12*force
  print "DAY:",t/86400