draw circle

scenes/OrbitSystem.tscn

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+[gd_scene load_steps=2 format=2]
+
+[ext_resource path="res://scripts/orbits/OrbitSystem.cs" type="Script" id=1]
+
+[node name="Orbit System" type="Spatial"]
+script = ExtResource( 1 )
+_a = NodePath("A")
+_b = NodePath("B")
+_barycenter = NodePath("Barycenter")
+
+[node name="A" type="Spatial" parent="."]
+
+[node name="B" type="Spatial" parent="."]
+
+[node name="Barycenter" type="Spatial" parent="."]

scripts/orbits/ILocation.cs

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+using Godot;
+using System;
+
+public interface ILocation
+{
+    Vector3 Position { get; }
+}

scripts/orbits/IMassive.cs

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+using Godot;
+using System;
+
+public interface IMassive
+{
+    float Mass { get; }
+}

scripts/orbits/OrbitSystem.cs

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+using Godot;
+using System;
+
+public class OrbitSystem : Node, IMassive, ILocation
+{
+    [Export] private NodePath _a;
+    [Export] private NodePath _b;
+    [Export] private NodePath _barycenter;
+
+    private readonly PointMass[] _pointMasses = new PointMass[2];
+
+    public Vector3 Position => Barycenter;
+
+    [Export]
+    public Vector3 Barycenter
+    {
+        get
+        {
+            var p0 = _pointMasses[0].Position;
+            var p1 = _pointMasses[1].Position;
+
+            return p0.LinearInterpolate(p1, .5f);
+        }
+        set => _ = value;
+    }
+
+    public float Mass { get; } = 1;
+
+    private ImmediateGeometry _orbit;
+
+    public override void _Ready()
+    {
+        InitPointMasses();
+        InitGeometry();
+    }
+
+    public override void _Process(float delta)
+    {
+        DrawOrbit();
+    }
+
+    private void DrawOrbit()
+    {
+        int steps = 100;
+
+        _orbit.Clear();
+        _orbit.Begin(Mesh.PrimitiveType.LineLoop);
+        _orbit.SetColor(new Color(1, 0, 0));
+        for (int i = 0; i < steps; i++)
+        {
+            float t = i / (float)steps;
+            float a = t * Mathf.Tau;
+
+            _orbit.AddVertex(new Vector3
+            {
+                x = Mathf.Sin(a),
+                z = Mathf.Cos(a)
+            });
+        }
+        _orbit.End();
+    }
+
+    private void InitGeometry()
+    {
+        _orbit = new ImmediateGeometry();
+        var m = new SpatialMaterial();
+        m.VertexColorUseAsAlbedo = true;
+        m.FlagsUnshaded = true;
+        _orbit.MaterialOverride = m;
+        AddChild(_orbit);
+    }
+
+    private void InitPointMasses()
+    {
+        var a = GetPointMass(_a);
+        var b = GetPointMass(_b);
+
+        if (a.Mass > b.Mass)
+        {
+            _pointMasses[0] = a;
+            _pointMasses[1] = b;
+        }
+        else
+        {
+            _pointMasses[0] = b;
+            _pointMasses[1] = a;
+        }
+    }
+
+    private PointMass GetPointMass(NodePath path)
+    {
+        var spatial = GetNode<Spatial>(path);
+        //var massive = node.GetChild<IMassive>(0);
+
+        return new PointMass(spatial, 1f);
+    }
+}

scripts/orbits/Planet.cs

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+using Godot;
+using System;
+
+public class Planet : Node
+{
+    [Export]
+    public float Mass { get; set; }
+
+    public override void _Ready()
+    {
+
+    }
+
+    //  public override void _Process(float delta)
+    //  {
+    //
+    //  }
+}

scripts/orbits/PointMass.cs

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+using Godot;
+
+struct PointMass : IMassive, ILocation
+{
+    public float Mass => _massive == null ? _mass : _massive.Mass;
+    public Vector3 Position => _spatial.Translation;
+
+    private IMassive _massive;
+    private Spatial _spatial;
+    private float _mass;
+
+    public PointMass(Spatial spatial, IMassive massive)
+    {
+        _spatial = spatial;
+        _massive = massive;
+        _mass = 0;
+    }
+
+    public PointMass(Spatial spatial, float mass)
+    {
+        _spatial = spatial;
+        _massive = null;
+        _mass = mass;
+    }
+}