Question

How does Earth's rotation on its axis cause day and night?

Answer 1

Light rays from the sun only strikes the surface of the Earth facing it, and since the Earth is not flat, but geoid, or spherical, then there will always be places backing the light rays of the sun. Now, had the Earth not been rotating about its axis, the place facing the sun would forever remain facing the sun, likewise for the other part backing it. So, there would never be night for those living on the side of Earth facing the sun, and there would never be day for those backing it.

However, since the Earth rotates, then the part facing the sun would slowly move away from it, and the part of the sun backing it would slowly move towards it, causing night time for the former, and day time to the latter.

Now, the place once backing the sun, B, is now facing the sun, and the place once facing the sun, A, is now backing it. But since the Earth continuously rotates, then B would now be slowly inching away from the sun, and A would be slowly inching towards it.

Thus, causing night at B, and day at A, and thus the circle continues, until some external force slows the Earth's rotation to a halt.