孝經緯曰。春分役十五日。斗指乙為清明。言萬物至此齊潔而清明也。太陽在戌，婁降(星)之次。表影長六尺六寸五分。
According to 月令采奇，the description about the 3rd month of the year, it is the Ching Ming festival. At this time, the tail of the big dipper star is pointing the direction Yi 乙, then it is the time for Ching Ming festival.
During this time, all the winter stored growing energy in the soil has been wakened and bursted out above ground. Weather is warm and clear, thats why it’s called Ching (clear) Ming (warm). The sun is in direction Xu 戌 and the shadow length is 6ft 6inches.
