program moving_circ
      i_temp=pgopen('/xwin')
      call pgenv(-5.0,5.0,-5.0,5.0,1,0)
      x_old=0.0
      y=0.0
      r=1.0
      call pgsfs(2)
      theta=0.0
      d_theta=3.14/18000
      do i=1,100000
        theta = theta + d_theta
        x=3*sin(theta)
        call pgbbuf
         call pgsci(0)
         call pgcirc(x_old,y,r)
         call pgsci(1)
         call pgcirc(x,y,r)
        call pgebuf
        x_old=x
      enddo
      call pgclos
      end