When the day becomes night, that is, when the sun goes down, it is called night.

A day usually refers to the changing law of this astronomical phenomenon, which is easy to be grasped by people and used as the basic unit for calculating time.

A day is the first unit in the calendar.

The day set by the sun is called "solar day"

There are two kinds of sun days: true sun day and peaceful sun day.

Real solar days are long and short, because the earth's orbit around the sun is elliptical. In winter, the earth is farther away from the sun, so the revolution speed is slower.

The speed of the earth's revolution is fast or slow, so the time of the day is long or short.

Therefore, the time of "true sun day" is not exactly the same, and it is very difficult to use.

For convenience, the calendar is not a real solar day, but a day with a constant length. This day is the average length of the real solar day in a year, which is called "flat solar day"

According to the current practice, a day is divided into 24 equal parts, which is 24 hours.