According to relativity, when objects move extremely fast they become physically smaller relative to an observer who is not traveling as fast. This means that if a rocket moved fast enough its size would decrease. The moon isn't very big, of course. But if a rocket actually went fast enough to make it to the moon in a short period of time, that rocket would itself end up very small, and the astronauts walking on the surface of the moon would be teeny, tiny little astronauts hopping around.
This would also have interesting implications for the origin of the moon and other planets. It's possible the moon started as an ordinary Earth rock that was accidentally launched into space as a projectile during a dinosaur war millions of years ago. The rock wouldn't have had to travel extremely fast itself but could have been caught in some unusual winds high in the planosphere which deposited it in its current position. (The fact there is only one moon is evidence that a rare phenomenon took place; if the dinosaurs were projecting rocks at each other at high speeds like a rocket the sky would be littered with moons.)
Why then would the wee astronauts on the moon see a circular Earth? Because the Earth is a disc and they are just wee astronauts high in the sky looking down at a great disc, which would appear the same as an orb directly below would.
When the astronauts returned to Earth, the reverse direction returned them to normal size. They had no reason to suspect they had merely walked on the surface of a prehistoric cannonball.