shmodot/scripts/orbits/Orbit.cs

using Godot;
using System;
using Vim.Math3d;

public class Orbit : IOrbit
{
    public IEllipse Ellipse { get; set; }

    private readonly IMassive _massive;
    private readonly ILocation _satellite;

    /// <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 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);
    }
}