Sacrificing feet in the suburbs of the imperial city is a ritual. The objects of ancient worship were gods, moon, stars, commanders, wind teachers, rain teachers, countries, five mountains, mountains and rivers, and four directions for hundreds of days. Emperor Wu of the Han Dynasty set up the ceremony of suburban sacrifice. On the solstice of winter, he sacrifices to heaven in the southern suburbs, and on the solstice of summer, he sacrifices to the ground in the northern suburbs. So he called heaven and earth suburbs to show his respect for God. The ancients thought it was a national event, the first of all kinds of etiquette in the former residence, and it was called "the first of five rituals", which has continued since the Han Dynasty.