Within a day, the time is divided as follows:

1, am

It refers to the period from midnight after 12 to noon at 1 1: 59. Internationally, it is generally marked by AM.

2. Afternoon

Compared with the morning, the time from noon 12 to 18 generally refers to the time from noon 12 to sunset. The English abbreviation is pm, p.m. or p.m.

3. At night

Evening refers to the period from 6 pm to 5 am the next day.

For example, it takes 23 hours, 56 minutes and 4 seconds for the earth to rotate once a day, while it takes 24 hours, 39 minutes and 35 seconds for Mars to rotate once a day. In physics, a day is a unit of time, and its value is fixed, that is, 24 hours. It can also be converted into time units with fixed values, such as seconds, minutes and hours.

Extended data:

The meaning of time

A time calendar unit that records the earth's rotation period. The concept of the universe has existed since ancient times. Huainanzi in the Western Han Dynasty: "The past is called the universe, and the four sides are called the universe." The early ancients recorded the periodic phenomenon that the sun rose and set once as a day, which was easy to remember. Later, the day was divided into twelve hours, and the earthly branch was used as the name of the hour, which had an ancient and established name.

The beginning of an hour is midnight. Gu "Rizhilu": "Since Han. The calendar is getting closer and closer, so the day is divided into twelve o'clock, and I don't know who started it, but it has not been abolished so far ... However, what is said about midnight is the so-called children's time, the crowing is ugly, the flat feet are ugly, and the sunrise and sunset are ugly. At noon, people eat in the corner, but not at noon. A day is divided into twelve, and only here. "