Why does the time in Tomb-Sweeping Day change?

As a traditional solar term in China, Tomb-Sweeping Day is calculated according to the annual vernal equinox, which has a solar term every15th day, so Qingming is between April 4th and 6th in the solar calendar, mostly on the 5th. It is not fixed on April 5 of the solar calendar, nor is it determined by the lunar calendar.

The vernal equinox changes with the rotation of the earth. The rotation of the earth every day is not 24 hours, but 23 hours and 56 minutes, which will decrease by one day every four years, thus affecting the time of solar terms. According to this rule, next year's Tomb-Sweeping Day falls on April 4th, so the calendars on the market are not misprinted.

On April 4th, Tomb-Sweeping Day appeared in 2004 and 2000, and on April 6th, Tomb-Sweeping Day appeared in earlier years such as 1939 and 1935.