Find with tag not working

I am trying to make a solar system and i am trying to set the stars and find them with “FindWithTag”.
Every time run the game it generates this error:

NullReferenceException: Object reference not set to an instance of an object
ShowInfoScript.Update () (at Assets/Scripts/ShowInfoScript.cs:42)

It points to the place where i set the rotation of the star with a Quaternion.
Code:

using UnityEngine;
using System.Collections;

public class ShowInfoScript : MonoBehaviour {

	public GameObject camera;

	private GameObject mercury;
	private GameObject venus;
	private GameObject earth;
	private GameObject mars;
	private GameObject jupiter;
	private GameObject saturn;
	private GameObject uranus;
	private GameObject neptune;

	void start(){
		mercury = GameObject.FindWithTag ("Mercury");
		Debug.Log( "find");
		venus = GameObject.FindWithTag ("Venus");
		Debug.Log( "find");
		earth = GameObject.FindWithTag ("Earth");
		Debug.Log( "find");
		mars = GameObject.FindWithTag ("Mars");
		Debug.Log( "find");
		jupiter = GameObject.FindWithTag ("Jupiter");
		Debug.Log( "find");
		saturn = GameObject.FindWithTag ("Saturn");
		Debug.Log( "find");
		uranus = GameObject.FindWithTag ("Uranus");
		Debug.Log( "find");
		neptune = GameObject.FindWithTag ("Neptune");
		Debug.Log( "find");
	}
	void  Update (){
		if (Input.GetMouseButtonDown(0)) {
			RaycastHit hit;
			Ray ray= Camera.main.ScreenPointToRay(Input.mousePosition);

			if (Physics.Raycast(ray, out hit)) {
				if (hit.transform.name == "Earth" ){
					mercury.transform.rotation = Quaternion.AngleAxis(0, Vector3.up);
					venus.transform.rotation = Quaternion.AngleAxis(0, Vector3.up);
					earth.transform.rotation = Quaternion.AngleAxis(0, Vector3.up);
					mars.transform.rotation = Quaternion.AngleAxis(0, Vector3.up);
					jupiter.transform.rotation = Quaternion.AngleAxis(0, Vector3.up);
					saturn.transform.rotation = Quaternion.AngleAxis(0, Vector3.up);
					uranus.transform.rotation = Quaternion.AngleAxis(0, Vector3.up);
					neptune.transform.rotation = Quaternion.AngleAxis(0, Vector3.up);
					OrbitScript.exeleration = 0;
					Debug.Log( "Earth");
				}
			}
		}
	}
}

TLDR:
Can’t find my objects using FindWithTag.

Please help me i need it for school…

Ethan

Method naming is wrong in your code

     void start(){

Just rename your method like this:

     void Start(){

It must start with a capital letter.

Are the planets really tagged as such? Or are they just named like that?

Try GameObject.Find Instead of GameObject.FindWithTag