Transform.RotateAround
RotateAround(point: Vector3, axis: Vector3, angle: float): void;
Description

Rotates the transform about axis passing through point in world coordinates by angle degrees.

This modifies both the position and the rotation of the transform.
	function Update() {
		// Spin the object around the world origin at 20 degrees/second.
		transform.RotateAround (Vector3.zero, Vector3.up, 20 * Time.deltaTime);
	}