diff --git a/scripts/orbits/math/Ellipse.cs b/scripts/orbits/math/Ellipse.cs
index e1fff2e..22292c1 100644
--- a/scripts/orbits/math/Ellipse.cs
+++ b/scripts/orbits/math/Ellipse.cs
@@ -2,7 +2,7 @@ using Godot;
 using System;
 using Vim.Math3d;
 
-public struct Ellipse
+public struct Ellipse : IEllipse
 {
     public Ellipse(double a = 1, double e = 0)
     {
@@ -12,6 +12,16 @@ public struct Ellipse
         // TODO: this is an immutable struct, so initialise everything else 
         // in the constructor to avoid recalculating properties whenever
         // they are accessed
+
+        Apisides = new Apisides(a, e);
+        b = Math.Sqrt(Apisides.max * Apisides.min);
+
+        var d = Math.Sqrt(a * a - b * b);
+        Foci = new DVector2[]
+        {
+            new DVector2(-d, 0),
+            new DVector2(d, 0)
+        };
     }
 
     /// <summary>
@@ -29,8 +39,7 @@ public struct Ellipse
     /// <summary>
     /// Semi-minor axis
     /// </summary>
-    public double b => SemiMinorAxis;
-    public double SemiMinorAxis => Math.Sqrt(Apisides.max * Apisides.min);
+    public double b { get; }
 
     /// <summary>
     /// Get a position on the auxiliary circle
@@ -40,13 +49,10 @@ public struct Ellipse
     public DVector2 GetAuxiliaryPosition2D(double t = 0) =>
         new DVector2(Math.Cos(t), Math.Sin(t)) * a;
 
-    private Apisides Apisides => new Apisides(a, e);
+    private Apisides Apisides { get; }
 
-    public DVector2 GetPosition(double t) =>
-        new DVector2(Math.Cos(t) * a, Math.Sin(t) * b);
+    public DVector2[] Foci { get; }
 
-    public DVector2 Focus0 => new DVector2(-GetFocusDistance(), 0);
-    public DVector2 Focus1 => new DVector2(GetFocusDistance(), 0);
     private double GetFocusDistance() => Math.Sqrt(a * a - b * b);
 }
 
diff --git a/scripts/orbits/math/IEllipse.cs b/scripts/orbits/math/IEllipse.cs
new file mode 100644
index 0000000..c7b3d7b
--- /dev/null
+++ b/scripts/orbits/math/IEllipse.cs
@@ -0,0 +1,20 @@
+using Godot;
+using System;
+using Vim.Math3d;
+
+public interface IEllipse
+{
+    /// <summary>
+    /// Semi-major axis
+    /// </summary>
+    double a { get; }
+    double b { get; }
+    /// <summary>
+    /// Eccentricity
+    /// </summary>
+    double e { get; }
+    /// <summary>
+    /// Positions of the two foci of the ellipse
+    /// </summary>
+    DVector2[] Foci { get; }
+}
diff --git a/scripts/orbits/math/IEllipseExtensions.cs b/scripts/orbits/math/IEllipseExtensions.cs
new file mode 100644
index 0000000..40f1137
--- /dev/null
+++ b/scripts/orbits/math/IEllipseExtensions.cs
@@ -0,0 +1,8 @@
+using Vim.Math3d;
+using System;
+
+public static class IEllipseExtensions
+{
+    public static DVector2 GetPosition(this IEllipse e, double t) =>
+        new DVector2(Math.Cos(t) * e.a, Math.Sin(t) * e.b);
+}
\ No newline at end of file