making a platform that moves in an arc

I am trying to make a moving platform that will move in an arc from its starting position to ending position, and then back the other direction to the start. I want it do this in a loop. So it would start out at ground level on one side of a pit, move in a curved trajectory over the pit to the other side, and then move back on the same trajectory. Any ideas how to do this?
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Good Luck