Why are there no thirty winter months in some years? What caused it?

Because the moon's orbit around the earth is elliptical, it moves slowly when it is far away from the earth and faster when it is close to the earth. The fast and slow movement of the moon causes the overlapping time between the sun and the moon, so the period between the two moons cannot be completely fixed, but the duration is very short.

The longest lunar month is about 29 days 19 hours, and the shortest is about 29 days and 6 hours, with an average of 29.5306 days. For convenience, the lunar calendar is divided into a big moon and a small moon, in which the big moon is 30 days and the small moon is 29 days. Therefore, if the agricultural twelfth lunar month coincides with the small moon on the 29th, there will be no twelfth lunar month. If it coincides with the 30-day lunar month, then the 30th lunar month will not be absent.

Extended data:

The period of the earth's orbit around the sun is 365 days, 5 hours, 48 minutes and 46 seconds (365.438+09 days), which is the tropic of cancer. The Gregorian calendar has an average of only 365 days a year, which is about 0.2422 days shorter than the tropical year, and the remaining time is about one day every four years. Therefore, add 1 day at the end of February of the fourth year, so that the calendar year of that year is 366 days, and this year is a leap year.

The current Gregorian calendar has 97 leap years every 400 years. According to a leap year every four years, the average annual calculation will be 0.0078 days, so after 400 years, it will be about 3 days.

Therefore, three leap years should be reduced every 400 years. Therefore, the Gregorian calendar stipulates that when the current year is a whole hundred, it must be a multiple of 400 to be considered as a leap year; A century year that is not a multiple of 400 or even a multiple of 4 is not a leap year.