Tomb-Sweeping Day, why is the time not fixed?

Because Tomb-Sweeping Day is the only day that is both a solar term and a festival, its date is not calculated according to time, but according to the ecliptic.

When Tomb-Sweeping Day is angry, the earth revolves around the sun 15 degrees. However, because the time of the earth's revolution is not necessarily 365 days, there may be a phenomenon that the date is not fixed in the division of solar terms.

Under normal circumstances, the Qingming Festival is about 15 days after the vernal equinox every year, and the specific date changes no more than three days.

Because the length of a year is determined by the movement of the sun, and the 24 solar terms are also determined by the position of the sun, basically the date of each solar term will not change much every year.

However, the time that the earth orbits the sun is called the "tropic year". The actual length of a tropical year is 365.2422 days, and the Gregorian calendar year is 365 days, so there will be 0.2422 days more each year, and the specific time of solar terms will be "delayed" by 0.2422 days each year.

In this way, the cumulative time after 4 years is 0.9688 days, almost one day. In order to correct mistakes, there is a "leap year" system in the Gregorian calendar, with one more day on February 29th every four years.

If the solar term is delayed from midnight to the next day, or advanced to the previous day due to leap year, the date of the solar term will change, but the date difference will only be within three days.

According to astronomical experts, in recent decades, the date of Qingming has been changing from April 4 to 6.

References:

Why is the date in Tomb-Sweeping Day not fixed? How to calculate the date in Tomb-Sweeping Day? _ Calendar.