==== Procedural (Script-based) Sun-Earth-Moon Model Creation ====
++++ Click here to see detailed step-by-step procedure to create a Sun-Earth-Moon model |

<callout type="danger">
Remember that in Unity, the Y-axis is chosen to be the vertical axis unlike the astronomical convention, Blender, and even Unreal Engine.
</callout>

The following C# code is to implement a rotation of a GameObject (e.g., planet). Just attach this script to an object and set two parameters (rotation w.r.t and speed).
<code csharp "Rotate.cs">
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Rotate : MonoBehaviour
{
    public Transform target;
    public int speed;

    // Update is called once per frame
    void Update()
    {
        transform.RotateAround(target.transform.position, target.transform.up, speed * Time.deltaTime);
    }
}
</code>
This example creates a model of Sun-Earth-Moon with realistic surface textures and rotational/orbital motions.
  - Create a new project from Unity Hub (Choose the Universal 3D Core [Not the one with Build-in Render Pipeline])
  - <label type='primary'>In the Unity Editor</label>
  - Configure Build parameters: File -> Build Settings: Check if the platform is correct.
  - Configure Project settings: Edit -> Project Settings; 
    * import 'TextMesh Pro' if you plan to use texts
    * import 'XR Plugin' if you are building a VR model
  - Import relevant assets: Window -> Package Manager, select 'My Assests' and import the following assets (these assets can be purchased for free in Unity Asset Store)
    * Milky Way Skybox
    * Planets of the Solar System 3D
    * Simpler Kepler Orbits
  - Create a folder to store C# scripts: In the Project > Assests, right-click on the background and create a folder named "Scripts".
  - Simulation background setting: Click the "main camera", then Component > Rendering > Skybox. Then choose the Milky Way skybox in the "Custrom Skybox" panel.
  - <label type='primary'>Make the Sun (and set its rotate it)</label>
  - <label type='primary'>Prpare for the Solar System Container</label>: Since objects in this example are automatically created through the script "JPLOrbitLoader.cs", we need to prepare a container and attach this script to the container.
    - Create an empty, "SEM Container" and make it inactive (click the check mark off from the inspector panel).
    - Create an empty under SEM_Container, "BodyTemplate", with two child objects: "Velocity" empty and "View" sphere mesh.
    - Create an empty under SEM_Container, "ScreenSpaceOrbitLineTemplate".
      - Attach "JPLOrbitLoader.cs" to SEM_Container and set the followings:
        - set G-constant (6.67E-11)
        - Scale_per_diameter (0.1)
        - Body Template = 'BodyTemplate'
        - Jason Data = <yourinput.json>
      - Attch "TimeController.cs" and set default date/time
      - Attach "ScreenSpaceOrbitsDisplay" and set
        - Target Camera -> Main Camera
        - Line Template -> "ScreenSpaceOrbitLineTemplate"
        - Cam Distance = 0.7
    - Attach "KeplerOrbitMover.cs" to "BodyTemplate" and set
      - Velocity Handle = "Velocity" <-- the empty under BodyTemplate
      - Time Scale = 1
      - Lock Orbit Editing = True
    - Attach "KeplerOrbitLineDisplay" to "BodyTemplate" and set
      - Line Renderer Reference = "ScreenSpaceOrbitLineTemplate"
      - Select desired Gizmo options
    - Create UI canvas and panel with Input fields for YYYY/MM/DD HH:MM:SS, buttons (reset and set_time_now), and timescale input field and slider.
      - Attach "TimeControlPanel.cs" to panel and set corresponding items to the UI objects. 
      - Check the Canvas Scaler setting: Resolution = 1280x1024
  - Click "Play" and you will see an animated Solar System objects showing orbital motions.
    - Drag and drop 'Rotate.cs' on Sun Empty: set target (Sun Empty) and speed.

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<col md="5">
<callout type="warning">
Future Improvements:
  - Add "Rotate.cs" to a certain script so that we can set the rotation of each object (with correct tilt of the rotation axis)
  - Add realistic texture material for each object. 
  - Add additional camera views
    - top-view camera
    - Earth-following camera (same orbital speed but slightly displace from the Earth so that one can see the shadowing of the Earth or Moon (i.e., eclipses)). 
    - Add shadow cones to see the relationship among Sun, Earth, and Moon's position and shadows
    - Mars-tracking camera (to demonstrate the retrograde motion of Mars)
    - Moon-tracking camera (to see the phase change and libration)
</callout>
</col>
<col md="5">
<thumbnail>
{{ tech:tutorials:earth_moon_simulation.mp4 }}
<caption>
This is a screen recording of the rendered model from the above procedure.

And this is the link to [[{{ tech:tutorials:sun_earth_moon.unitypackage |the Unity package file for this model}}.
</caption>
</thumbnail>
</col>
</row>
</grid>
++++