Tafsir of Al-Anbiya' 21:33

وهو الذي خلق الليل والنهار

And He it is Who has created the night and the day, meaning, the one with its darkness and stillness, and the other with its light and human interaction; sometimes the one is longer while the other is shorter, then they switch.

والشمس والقمر

and the sun and the moon, the sun with its own light and its own path and orbit and allotted time, and the moon which shines with a different light and travels on a different path and has its own allotted time.

كل في فلك يسبحون

each in an orbit floating. means, revolving. Ibn Abbas said, "They revolve like a spinning wheel, in a circle.'' This is like the Ayah:

فالق اﻻصباح وجعل اليل سكنا والشمس والقمر حسبانا ذلك تقدير العزيز العليم

(He is the) Cleaver of the daybreak. He has appointed the night for resting, and the sun and the moon for reckoning. Such is the measuring of the All-Mighty, the All-Knowing. (6:96)