Machine Problem in Fortran 90

program star
implicit none
real (kind=8) :: x
integer :: i

dt=0.001
pi=4*(atan1.0)

do i=1, Nsteps
     t = n*dt
     !calculate acceleration
     ax= 
     ay=...
     !calculate values at next step
     vy=vy+ay*dt
     vx=vx+ax*dt
     x=x+vx*dt
     y=y+vy*dt
end do

print *, "vx= ", vx
     



end program star