What are some algorithms that will allow me to simulate planetary physics?

Question

I\'m interested in doing a \"Solar System\" simulator that will allow me to simulate the rotational and gravitational forces of planets and stars.

I\'d like to be able t

Answer 1

Algorithms to simulate planetary physics.

Here is an implementation of the Keppler parts, in my Android app. The main parts are on my web site for you can download the whole source: http://www.barrythomas.co.uk/keppler.html

This is my method for drawing the planet at the 'next' position in the orbit. Think of the steps like stepping round a circle, one degree at a time, on a circle which has the same period as the planet you are trying to track. Outside of this method I use a global double as the step counter - called dTime, which contains a number of degrees of rotation.

The key parameters passed to the method are, dEccentricty, dScalar (a scaling factor so the orbit all fits on the display), dYear (the duration of the orbit in Earth years) and to orient the orbit so that perihelion is at the right place on the dial, so to speak, dLongPeri - the Longitude of Perihelion.

drawPlanet:

public void drawPlanet (double dEccentricity, double dScalar, double dYear, Canvas canvas, Paint paint, 
            String sName, Bitmap bmp, double dLongPeri)
{
        double dE, dr, dv, dSatX, dSatY, dSatXCorrected, dSatYCorrected;
        float fX, fY;
        int iSunXOffset = getWidth() / 2;
        int iSunYOffset = getHeight() / 2;

        // get the value of E from the angle travelled in this 'tick'

        dE = getE (dTime * (1 / dYear), dEccentricity);

        // get r: the length of 'radius' vector

        dr = getRfromE (dE, dEccentricity, dScalar);

        // calculate v - the true anomaly

        dv = 2 * Math.atan (
                Math.sqrt((1 + dEccentricity) / (1 - dEccentricity))
                *
                Math.tan(dE / 2)
                ); 

        // get X and Y coords based on the origin

        dSatX = dr / Math.sin(Math.PI / 2) * Math.sin(dv);
        dSatY = Math.sin((Math.PI / 2) - dv) * (dSatX / Math.sin(dv));

        // now correct for Longitude of Perihelion for this planet

        dSatXCorrected = dSatX * (float)Math.cos (Math.toRadians(dLongPeri)) - 
            dSatY * (float)Math.sin(Math.toRadians(dLongPeri));
        dSatYCorrected = dSatX * (float)Math.sin (Math.toRadians(dLongPeri)) + 
            dSatY * (float)Math.cos(Math.toRadians(dLongPeri));

        // offset the origin to nearer the centre of the display

        fX = (float)dSatXCorrected + (float)iSunXOffset;
        fY = (float)dSatYCorrected + (float)iSunYOffset;

        if (bDrawOrbits)
            {
            // draw the path of the orbit travelled
            paint.setColor(Color.WHITE);
            paint.setStyle(Paint.Style.STROKE);
            paint.setAntiAlias(true);

            // get the size of the rect which encloses the elliptical orbit

            dE = getE (0.0, dEccentricity);
            dr = getRfromE (dE, dEccentricity, dScalar);
            rectOval.bottom = (float)dr;
            dE = getE (180.0, dEccentricity);
            dr = getRfromE (dE, dEccentricity, dScalar);
            rectOval.top = (float)(0 - dr);

            // calculate minor axis from major axis and eccentricity
            // http://www.1728.org/ellipse.htm

            double dMajor = rectOval.bottom - rectOval.top;
            double dMinor = Math.sqrt(1 - (dEccentricity * dEccentricity)) * dMajor;

            rectOval.left = 0 - (float)(dMinor / 2);
            rectOval.right = (float)(dMinor / 2);

            rectOval.left += (float)iSunXOffset;
            rectOval.right += (float)iSunXOffset;
            rectOval.top += (float)iSunYOffset;
            rectOval.bottom += (float)iSunYOffset;

            // now correct for Longitude of Perihelion for this orbit's path

            canvas.save();
                canvas.rotate((float)dLongPeri, (float)iSunXOffset, (float)iSunYOffset);
                canvas.drawOval(rectOval, paint);
            canvas.restore();
            }

        int iBitmapHeight = bmp.getHeight();

        canvas.drawBitmap(bmp, fX - (iBitmapHeight / 2), fY - (iBitmapHeight / 2), null);

        // draw planet label

        myPaint.setColor(Color.WHITE);
        paint.setTextSize(30);
        canvas.drawText(sName, fX+20, fY-20, paint);
}

The method above calls two further methods which provide values of E (the mean anomaly) and r, the length of the vector at the end of which the planet is found.

getE:

public double getE (double dTime, double dEccentricity)
    {
    // we are passed the degree count in degrees (duh) 
    // and the eccentricity value
    // the method returns E

    double dM1, dD, dE0, dE = 0; // return value E = the mean anomaly
    double dM; // local value of M in radians

    dM = Math.toRadians (dTime);

    int iSign = 1;

    if (dM > 0) iSign = 1; else iSign = -1;

    dM = Math.abs(dM) / (2 * Math.PI); // Meeus, p 206, line 110
    dM = (dM - (long)dM) * (2 * Math.PI) * iSign; // line 120
    if (dM < 0)
        dM = dM + (2 * Math.PI); // line 130
    iSign = 1;
    if (dM > Math.PI) iSign = -1; // line 150
    if (dM > Math.PI) dM = 2 * Math.PI - dM; // line 160

    dE0 = Math.PI / 2; // line 170
    dD = Math.PI / 4; // line 170

    for (int i = 0; i < 33; i++) // line 180 
        {
        dM1 = dE0 - dEccentricity * Math.sin(dE0); // line 190
        dE0 = dE0 + dD * Math.signum((float)(dM - dM1));
        dD = dD / 2; 
        }

    dE = dE0 * iSign;

    return dE;
    }

getRfromE:

public double getRfromE (double dE, double dEccentricty, double dScalar)
{
    return Math.min(getWidth(), getHeight()) / 2 * dScalar * (1 - (dEccentricty * Math.cos(dE)));
}