Transform.RotateAround

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.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    void Update() {
        transform.RotateAround(Vector3.zero, Vector3.up, 20 * Time.deltaTime);
    }
}