Please provide more details. How fast do you want it to move? Is it suspended. If it is suspended you can use a flying crank ghost type mechanism. Using pulleys you can mount the motor out of sight. The length of the crank arm needs to be 1/2 of the distance you want the saucer to move. You can use counter weights if the saucer is too heavy.
Thanks, probably too heavy for that, its eight feet across and probably 60lbs, at least.
I was just going to have it sit on landing gear, maybe two feet off the ground.
I'm just looking for a simple way to move it up and down maybe 18"
Scissor action pneumatic would work - that is a lot of weight. It would be difficult to balance and the action may be faster than you are looking for. The balancing issue could be solved if you put the saucer on some sort of guide. The speed could be adjusted by regulating the exhaust.
Sadly, at 60 lbs, you'll have a hard time finding a decent motor. You'll probably end up using a washing machine motor or a motor from one of those electric mobility scooters before you see satisfactory motion.
You will probably need some kind of weight to counter balance the saucer, almost like a balancing scale. Then any reasonably powerful motor, like a wiper motor, could handle the motion. Some sort of rocker arm assembly, hanging from a tree... something like that?