Why do people's birthdays go back and forth on 19?

19 Lunar calendar and solar calendar overlap once.

There are some exceptions to birthdays.

This situation is mainly the result of the leap of the lunar calendar. 19 Why is there a coincidence? The number of days in each month in the lunar calendar is determined according to the cycle of the moon. In fact, the average number of days per month is 29.5306. If multiplied by 12, there are only 354.3672 days in a year. One revolution of the earth around the sun (tropical year and solar year) is 365.2422 days, so the lunar year 1 the actual number of days is less than that of the sun 10.875 days, almost 1 1 day, 19 is 209 days less.

209 days divided by 30 is 6.9666, that is to say, 19 years, the lunar calendar is almost 7 months shorter than the solar calendar. In order to make the difference between the lunar calendar and the solar calendar not too much, it is necessary to make up these seven months.

China's lunar calendar is actually a typical lunar calendar, which is formulated in consideration of the movement cycle of the moon around the earth and the movement cycle of the earth around the sun. In order to make up for these seven months, people adopt the method of setting leap months in the lunar calendar-seven leap months in nineteen years, that is, nineteen years of the lunar calendar, and 12 is a normal year, every 65438. Plus seven leap years (3, 5, 8, 1 1, 14, 17, 19), every 1 leap year has 13 months.

In this way, the number of days in the 19 solar year is almost equal to the number of days in the 19 lunar year. Therefore, whenever the birthday is 19 or 19 multiplied by the age, there is a chance that the solar calendar birthday and the lunar calendar birthday are on the same day.