How night and day happen?

Answer 1

Day and night occur because the Earth is a rotating sphere, only half of which can be illuminated by the Sun at any one time. The sun-lit side is the day side, while the darkened side is in night.

If the Earth did not rotate, it would still experience daylight over the entire surface (although for much longer extended periods) because the Earth also revolves around the Sun. This situation occurs at the poles due to the Earth's tilt with regard to its rotational axis. In any event, over a given year's time, all areas of the globe experience a total of 6 months of day and 6 months of night. At the poles, there is 6 months of continuous day followed by 6 months of continuous night.

*If the Earth were tidally locked with the Sun, as the Moon is with the Earth, and the rotation/revolution periods were the same, only one side of the Earth would have perpetual day, while the other had perpetual night. This scenario would be highly unlikely to occur for a major planet.