Have you not considered your Lord - how He extends the shadow, and if He willed, He could have made it stationary? Then We made the sun for it an indication.1
Translation: Saheeh International (English)
1 i.e., showing the existence of a shadow or making it apparent by contrast.
لَجَعَلَهُۥsurely He (could) have made itlajaʿalahu
سَاكِنٗاstationarysākinan
ثُمَّThenthumma
جَعَلۡنَاWe madejaʿalnā
ٱلشَّمۡسَthe sunl-shamsa
عَلَيۡهِfor itʿalayhi
دَلِيلٗاan indicationdalīlan
Tafsir
Have you - O Messenger - not seen how your Lord extends the shade on the surface of the earth. If He had willed to make it stand still without moving He could have done so. Then I made the sun an indicator for it, becoming long and short in accordance to it.