How did the flagpole shadow come from?

The flagpole is at the pole: the shadow of the flagpole is in the night hemisphere at the vernal equinox; The shadow of the flagpole faces south from summer to morning; The shadow of the flagpole at the autumn equinox is in the night hemisphere; There is no sunshine on the solstice in winter, so there is no shadow.

Winter solstice is an important solar term in the China lunar calendar, and it is also a traditional festival of the Chinese nation.

The winter solstice is the earliest of the 24 solar terms, which originated in the Spring and Autumn Period. On the solstice of winter, direct sunlight reaches the southernmost part of the year, and the northern hemisphere gets 50% less sunlight than the southern hemisphere, and the daytime in the northern hemisphere reaches the shortest. Winter solstice is regarded as a big holiday in winter. There is a folk saying that winter solstice is as big as a year, so the ancients called it "sub-year" or "off-year". Most northerners have the custom of eating jiaozi from winter solstice, while most southerners have the custom of eating sweets.