diff --git a/scripts/orbits/IMassive.cs b/scripts/orbits/IMassive.cs
index a7d5f80..f5fac4b 100644
--- a/scripts/orbits/IMassive.cs
+++ b/scripts/orbits/IMassive.cs
@@ -3,5 +3,5 @@ using System;
 
 public interface IMassive
 {
-    float Mass { get; }
+    double Mass { get; }
 }
diff --git a/scripts/orbits/IMassiveExtensions.cs b/scripts/orbits/IMassiveExtensions.cs
new file mode 100644
index 0000000..6f4cf1a
--- /dev/null
+++ b/scripts/orbits/IMassiveExtensions.cs
@@ -0,0 +1,4 @@
+public static class IMassiveExtensions
+{
+    public static double U(this IMassive m) => Kepler.U(m.Mass);
+}
\ No newline at end of file
diff --git a/scripts/orbits/Orbit.cs b/scripts/orbits/Orbit.cs
index 3ebab6c..9e74201 100644
--- a/scripts/orbits/Orbit.cs
+++ b/scripts/orbits/Orbit.cs
@@ -2,56 +2,42 @@ using Godot;
 using System;
 using Vim.Math3d;
 
-public class Orbit
+public class Orbit : IOrbit
 {
-    // The position of the primary body relative to the ellipse - the 
-    // orbit itself is presented as being centred on the primary, but
-    // all our math is elliptical.
-    private DVector2 PrimaryPosition => Ellipse.Focus0;
+    public IEllipse Ellipse { get; set; }
 
-    public Ellipse Ellipse { get; set; }
+    private readonly IMassive _massive;
+    private readonly ILocation _satellite;
 
-    public DVector3 GetPosition(float t)
+    /// <summary>
+    /// An orbit is treated as one stationary body at one focus of the orbit's ellipse. 
+    /// The satellite is treated as a massless point.
+    /// </summary>
+    /// <param name="massive">Mass of the object being orbited</param>
+    /// <param name="satellite">The orbiting body</param>
+    public Orbit(IMassive massive, ILocation satellite)
     {
-        // TODO: determine period from mass of orbiting bodies
-        var P = 10f;
+        // TODO: determine ellipse better
+        // semi-major axis is distance of satellite from origin
+        var a = satellite.Position.Magnitude();
+        var e = 0.8f;
+        this.Ellipse = new Ellipse(a, e);
 
+        _massive = massive;
+        _satellite = satellite;
+    }
+
+    public DVector3 GetPosition(double t)
+    {
         var a = Ellipse.a;
         var e = Ellipse.e;
 
+        var P = Kepler.GetPeriod(a, _massive.U());
+
         var M = Kepler.GetMeanAnomaly(t, P);
         var E = Kepler.GetEccentricAnomaly(e, M) + Math.PI;
 
         var p = Kepler.GetEccentricPosition2D(e, a, t);
         return new DVector3(p.X, 0, p.Y);
     }
-
-    public void Draw(ImmediateGeometry geo)
-    {
-        geo.Clear();
-        geo.Begin(Mesh.PrimitiveType.LineLoop);
-        DrawEllipse(geo);
-        geo.End();
-    }
-
-    private void DrawEllipse(ImmediateGeometry geo)
-    {
-        geo.SetColor(new Color(1, 0, 0));
-
-        int steps = 100;
-
-        for (int i = 0; i < steps; i++)
-        {
-            float t = i / (float)steps;
-            float a = t * Mathf.Tau;
-
-            var v2 = Ellipse.Focus0 + Ellipse.GetPosition(a);
-
-            geo.AddVertex(new Godot.Vector3
-            {
-                x = (float)v2.X,
-                z = (float)v2.Y
-            });
-        }
-    }
 }
diff --git a/scripts/orbits/Planet.cs b/scripts/orbits/Planet.cs
index b12c0bc..815a8f9 100644
--- a/scripts/orbits/Planet.cs
+++ b/scripts/orbits/Planet.cs
@@ -6,7 +6,7 @@ using Vim.Math3d;
 public class Planet : Spatial, IPointMass
 {
     [Export]
-    public float Mass { get; set; }
+    public double Mass { get; set; }
 
     private DVector3? _position;
     public DVector3 Position