code: def step(self):
      dt = self.gravSys.dt
      self.setpos(self.pos() + dt * self.v)
      if self.gravSys.planets.index(self) != 0:
          self.setheading(self.towards(self.gravSys.planets[0]))
      self.a = self.acc()
      self.v = self.v + dt * self.a
  
file_path: cpython/Lib/turtledemo/planet_and_moon.py
imports: from turtle import Shape, Turtle, mainloop, Vec2D
label_desc: Use image node_id 4 for calling the Star obj's underlying member method code with example usage: obj.step() without return types