“Square the Circle” The Moon is the perfect size to solve the riddle. It’s huge in fact, larger than any other in proportion to its planet. Some say we have a double planet system. The Moon is 27.3% the size of Earth. It also happens to take 27.3 days to complete an orbit around Earth. Need I point out the fact that the Moon influences our emotions. Women’s menstruation cycles last one lunar period on average.