set the speed in respect of time

@meat5000 thank you that save me a lot of time, the sun cycle is not working perfectly, but the color lerp is not perfectly synchronized

code : sun Slerp :

 using UnityEngine;
 using System.Collections;
 
 public class sunSet : MonoBehaviour
 {
     //sunrise position
     public Transform sunrise;
     //sunset position
     public Transform sunset;
     //total time  
     public float journeyTime = 1.0f ;
     //counter
     private float startTime;
     //materials 
     public Material sun, moon;
     
     void Start ()
     {
         //init
         startTime = Time.time;
         gameObject.renderer.material = sun;
         
     }
 
     void Update ()
     {
         journey ();
         
     }
 
     void journey ()
     {
         //Slerp code from unity script doc 
         Vector3 center = (sunrise.position + sunset.position) * 0.5F;
         center -= new Vector3 (0, 1, 0);
         Vector3 riseRelCenter = sunrise.position - center;
         Vector3 setRelCenter = sunset.position - center;
         float fracComplete = (Time.time - startTime) / journeyTime;
         transform.position = Vector3.Slerp (riseRelCenter, setRelCenter, fracComplete);
         transform.position += center;
         
         //loop the cycle and change material 
         if (transform.position == sunset.position) {
             transform.position = sunrise.position;
             startTime = Time.time;
             if (gameObject.renderer.material.name == "sun (Instance)") {
                 gameObject.renderer.material = moon;
             } else {
                 gameObject.renderer.material = sun;
             }
             
         }
     }
 }

SkyColor lerp :

 using UnityEngine;
 using System.Collections;
 
 public class SkyTrans : MonoBehaviour
 {
     //day & night colors
     public Color day = new Color (0.01f, 0.4f, 0.8f);
     public Color night = new Color (0.01f, 0.01f, 0.02f);
     //main color
     public Color skyColor ;
     //total time 
     public float journeyTime;
     //lerp speed 
     public float lerpSpeed ;
     //checking if it's day or night 
     bool isDay ;
     //instance of the sun object 
     sunSet sun;
 
     void Start ()
     {
         
         //init 
         sun = GameObject.Find ("sun").GetComponent<sunSet> ();
         lerpSpeed = 0; 
         journeyTime = sun.journeyTime;
         skyColor = day;
         isDay = true;
     }
     
     void Update ()
     {
         //setting the color to skyColor 
         gameObject.renderer.material.color = skyColor;
     
         
         if (lerpSpeed < 1) {
             lerpSpeed += Time.deltaTime / journeyTime;
         }
         
         //if it's day then lerp to night 
         if (isDay) {
             skyColor = Color.Lerp (day, night, lerpSpeed / 2);
         }
         //if it's night then lerp to day 
         else {
             skyColor = Color.Lerp (night, day, lerpSpeed / 2);
         }
         //day or night 
         if (skyColor == day) {
             isDay = true;
             lerpSpeed = 0;
         } else if (skyColor == night) {
             isDay = false;
             lerpSpeed = 0;
         }
         
     }    
 }

thank you

You should post your script! Anyway, I suppose that both things must cycle from day to night and night to day - am I right? If so, the empty could be Rotate'd at the right speed, while the color should Lerp from blue to black during the sunset, then back to blue during the dawn. My suggestion is to control both with the same variable in order to avoid loss of synchronicity over time. Basically, the code could be something like this:

 public var sunMoon: Transform; // drag here the empty that contains sun and moon
 public var duration: float = 3600; // day duration in seconds (1 hour, in this example)
 public var transitionTime: float = 0.2; // transition duration control
 public var nightColor = Color.black;
 public var dayColor = Color.blue;
 
 private var curTime: float = 0;
 
 function Update(){
   curTime += Time.deltaTime; // update curTime
   var sunAngle: float = 360*curTime/duration; // calculate rotation angle
   sunMoon.eulerAngles = Vector3(0,0,sunAngle); // set rotation angle about Z
   // transition goes 0->1 when sunAngle near to zero, 1->0 when around 180
   var transition: float = Mathf.Sin(sunAngle*Mathf.Deg2Rad)/transitionTime + 0.5;
   var skyColor: Color = Color.Lerp(nightColor, dayColor, transition);
   // apply skyColor to the appropriate object
 }