The correct answer is obscene material.
Basically, when one side of the Earth has summer, the other has winter, because only one is directly facing the sun.
On March 21 and September 23, the sun is directly above both sides. So, these days are the equinoxes.
On June 21 and December 21, the sun gives maximum light to each side. So these days are the solstices.
It was Liu bang from the han dynasty i believe
