The size of the space capsule is inconsistent throughout the entire feature. It is, apparently, big enough to carry multiple people, yet when a Selenite is seen on the back of the capsule following its return to the Earth, he is almost as big as the entire capsule.
A pendulum clock is visible within the spacecraft as the capsule is being readied for takeoff. A pendulum and the clock that it works depend upon gravitation for accurate time, so it will be worthless during the flight. The effects of gravitation were well known at the time.
Pocket watches then readily available were reliable enough.
When the umbrella is growing in the mushroom garden, you can see the edge of the first Selenite, off camera to the right, waiting for his cue to enter the scene. May not be visible in all versions of the film.