private void ComputeSunPosition()
{
// See http://en.wikipedia.org/wiki/Position_of_the_Sun. (But these formulas seem to be a bit
// more precise.)
double meanAnomaly = 6.24 + 628.302 * _epoch2000Centuries;
// Ecliptic longitude.
_sunEclipticLongitude = 4.895048 + 628.331951 * _epoch2000Centuries + (0.033417 - 0.000084 * _epoch2000Centuries) * Math.Sin(meanAnomaly)
+ 0.000351 * Math.Sin(2.0 * meanAnomaly);
// Distance from earth in astronomical units.
double geocentricDistance = 1.000140 - (0.016708 - 0.000042 * _epoch2000Centuries) * Math.Cos(meanAnomaly)
- 0.000141 * Math.Cos(2.0 * meanAnomaly);
// Sun position.
Vector3D sunPositionEcliptic = ToCartesian(geocentricDistance, 0, _sunEclipticLongitude);
Vector3D sunPositionEquatorial = EclipticToEquatorial * sunPositionEcliptic;
// Note: The sun formula is already corrected by precession.
SunPosition = _equatorialToWorldNoPrecession.TransformDirection(sunPositionEquatorial);
Vector3D sunDirection = SunPosition.Normalized;
// Convert from astronomical units to meters.
const double au = 149597870700; // 1 au = 149,597,870,700 m
SunPosition *= au;
// Account for atmospheric refraction.
double elevation = Math.Asin(sunDirection.Y);
elevation = Refract(elevation);
sunDirection.Y = Math.Sin(elevation);
sunDirection.Normalize();
SunDirectionRefracted = sunDirection;
}