Student or Learner
I donít believe that the orbital ellipses of its planets are unwavering or its suns inextinguishable.True, tumbles into the night are rare, as are those who know about them, but the crack threatening a cataclysm never closes up entirely; it threatens every instant to expand, to yawn into a world-enveloping abyss. -Autobiography of a Corpse, Krzhizhanovsky
What do you think 'tumble' means here?
I think it means something like this:
Planets generally stick to their orbits and, while they are not unwavering, they don't often break away from them suddenly.
Their orbit may change over time, but they don't suddenly tumble (fall) and disappear into the night.