✔ 最佳答案
mple answer is gravity!
The earth has an enormous amount of gravity, and its pulling everything around it towards the center of the earth. No matter where you are around the earth, you're always pulled towards it.
The directions up and down dont really make sense when you think of the world as a whole. Down is always facing towards the center of the earth, up is always facing away from it.
This also works for the moon, if you're near the moon you're being pulled towards the center of it.