When a person starts from a point on the earth and travels in one direction, which direction may he return to the starting point?

No matter which direction you go, you will return to the starting point. Even after passing through the North Pole and the South Pole, because there will be two sign changes, similar to twice multiplying negative 1, the original number remains unchanged. That is, after passing through two poles and changing the direction twice, the direction remains unchanged, so it will return to the starting point. In fact, as long as it is not on the equator and along the equator, there will be two changes in direction. Otherwise, it's not a trip to the earth, but a trip to latitude, so it's just a trip around the North Pole or the South Pole.