Have you not considered (the work of) your Lord, how He extends the shade? And if He had pleased He would certainly have made it stationary; then We make the sun its guide.
Meaning:-
Stationary: standing still; not moving.
Explanation of the Ayat: The morning shadows are long but more definite, and their length and direction are seen to be guided by the sun. But they change insensibly every second or fraction of a second.