If a flagpole is erected on the equator, its shadow will change from south to north.

If a flagpole is erected on the equator, the shadow of the flagpole will change from south to north at the autumn equinox.

The autumnal equinox is the sixteenth solar term in the twenty-four solar terms of the lunar calendar, usually on September 23rd or 24th every year. On this day, the sun reached longitude 180 degrees and hit the equator of the earth directly. So a day is divided into 24 hours, and each hour has 12 hours. There is no extreme day and night phenomenon in the world. After the autumnal equinox, the direct point of the sun continues to move south, and the shadow of the flagpole will move north.