program circle_anim
      integer pgopen
      if(pgopen('/xwin') <= 0) stop
      call pgpap(3.0,1.0)
      call pgenv(-2.0,2.0,-2.0,2.0,1,0)
      call pgsfs(2)
      x = 0.0
      y = 0.0
      r = 0.8
      call pgcirc(x,y,r)
      rold = r
      d_theta = 0.0001
      theta = 0.0
      do i =1,100000000
       theta = theta + d_theta
       r = 0.8 + 0.2*sin(theta)
       call pgbbuf
       call pgsci(0)
       call pgcirc(x,y,rold)
       call pgsci(1)
       call pgcirc(x,y,r)
       call pgebuf
       rold = r
      end do 

      call pgclos
      end