What units do we often use? Year, month and day.

Year, month and day are our usual time units.

1, year: the year is the time unit in which events related to the earth's revolution around the sun occur repeatedly.

2. Month: Month is the time unit in the calendar. It is assumed that its length should be equivalent to the natural orbital period of the moon's revolution around the earth, but traditionally the period of the moon's phase change is regarded as the length of one month, that is, the length of one month (lunar month) is the rendezvous month (the first lunar month), which is about 29.53 days.

3. Day: The day is the time unit to measure the rotation of the planet. It is often called heaven in spoken language.

Extended data:

Calculation method

Calendar is to calculate the length of years, months and days and the relationship between them. The law of making time sequence is called "calendar". Taking the tropical year as the unit, how many integer months are arranged in a year, how many integer days are arranged in a month and how to choose the starting point of a year are called calendars.

The complexity of the calendar problem lies in the fact that the tropical year and the new moon are too fragmented, and their relationship with "day" is not as simple as the relationship between kilometers and meters; Besides, they can't communicate with each other

Therefore, the calendar always pays attention to one thing and loses another, so it is impossible to coordinate two cycles at the same time. For these two reasons, calendars are generally divided into three categories; Lunar calendar, solar calendar and lunar calendar. The solar calendar, which focuses on coordinating the relationship between Wang Shuo and the calendar, is referred to as the solar calendar for short; The lunar calendar considering the moon and the tropic year, calendar month and calendar year is called the Yin-Yang calendar.

No matter what kind of calendar, there is a problem of coordinating the relationship between calendar day period and astronomical period. In principle, the calendar month should be equal to the new moon and the calendar year should be equal to the Tropic of Cancer. However, because the new moon and the tropic of cancer are not all days, so.

Calendar months must be divided into large months and small months, and calendar years must be divided into normal years and leap years. Through the proper collocation and arrangement of big month and small month, flat year and leap year, the average calendar month is equal to the new moon, or the average calendar year is equal to the tropic year. This is the main content of the calendar.

References:

Baidu Encyclopedia-Year Month Day