Diagram 1

Relating Earth's rotation and revolution to day and night.

We start with our reference longitude (arrow) pointed directly at Sun, that is noon local time.  30 complete rotations later (about a month) the reference longitude is again pointing exactly the same direction as at the start.  Because Earth has revolved about 30 degrees in its orbit, pointing the same direction is no longer pointing at Sun.  The local time is 10 am, not noon.