/// <summary>
/// Calculates an intermediate point at any fraction along the great circle path
/// between two points with geographical coordinates
/// </summary>
/// <param name="p1">Geographical coordinates of the first point</param>
/// <param name="p2">Geographical coordinates of the second point</param>
/// <param name="fraction">Fraction along great circle route (f=0 is point 1, f=1 is point 2).</param>
/// <returns>
/// The intermediate point at specified fraction
/// </returns>
/// <remarks>
/// Formula is taken from <see href="http://www.movable-type.co.uk/scripts/latlong.html"/>
/// that is originally based on <see cref="http://www.edwilliams.org/avform.htm#Intermediate"/>.
/// </remarks>
public static CrdsGeographical Intermediate(CrdsGeographical p1, CrdsGeographical p2, double fraction)
{
if (fraction < 0 || fraction > 1)
{
throw new ArgumentException("Fraction value should be in range [0, 1]", nameof(fraction));
}
double d = ToRadians(Separation(p1, p2));
double a, b;
if (d <= 1e-6)
{
a = 1 - fraction;
b = fraction;
}
else
{
a = Math.Sin((1 - fraction) * d) / Math.Sin(d);
b = Math.Sin(fraction * d) / Math.Sin(d);
}
double alt1 = ToRadians(p1.Latitude);
double alt2 = ToRadians(p2.Latitude);
double az1 = ToRadians(p1.Longitude);
double az2 = ToRadians(p2.Longitude);
double x = a * Math.Cos(alt1) * Math.Cos(az1) + b * Math.Cos(alt2) * Math.Cos(az2);
double y = a * Math.Cos(alt1) * Math.Sin(az1) + b * Math.Cos(alt2) * Math.Sin(az2);
double z = a * Math.Sin(alt1) + b * Math.Sin(alt2);
double lat = Math.Atan2(z, Math.Sqrt(x * x + y * y));
double lon = Math.Atan2(y, x);
return(new CrdsGeographical(ToDegrees(lon), ToDegrees(lat)));
}
/// <summary>
/// Calculates distance in kilometers between 2 points on the Earth surface.
/// </summary>
/// <param name="g1">First point.</param>
/// <param name="g2">Second point.</param>
/// <returns>Distance in kilometers between 2 points.</returns>
/// <remarks>
/// The method is taken from https://www.movable-type.co.uk/scripts/latlong.html
/// </remarks>
public static double DistanceTo(this CrdsGeographical g1, CrdsGeographical g2)
{
const double R = 6371;
double phi1 = Angle.ToRadians(g1.Latitude);
double phi2 = Angle.ToRadians(g2.Latitude);
double deltaPhi = Angle.ToRadians(g2.Latitude - g1.Latitude);
double deltaLambda = Angle.ToRadians(g2.Longitude - g1.Longitude);
double a = Math.Sin(deltaPhi / 2) * Math.Sin(deltaPhi / 2) + Math.Cos(phi1) * Math.Cos(phi2) * Math.Sin(deltaLambda / 2) * Math.Sin(deltaLambda / 2);
double c = 2 * Math.Atan2(Math.Sqrt(a), Math.Sqrt(1 - a));
return(R * c);
}
/// <summary>
/// Calculates angular separation between two points with geographical coordinates
/// </summary>
/// <param name="p1">Geographical coordinates of the first point</param>
/// <param name="p2">Geographical coordinates of the second point</param>
/// <returns>Angular separation in degrees</returns>
public static double Separation(CrdsGeographical p1, CrdsGeographical p2)
{
double a1 = ToRadians(p1.Latitude);
double a2 = ToRadians(p2.Latitude);
double A1 = To360(p1.Longitude);
double A2 = To360(p2.Longitude);
double a = Math.Acos(
Math.Sin(a1) * Math.Sin(a2) +
Math.Cos(a1) * Math.Cos(a2) * Math.Cos(ToRadians(A1 - A2)));
return(double.IsNaN(a) ? 0 : ToDegrees(a));
}
/// <summary>
/// Finds horizon circle point for a given geographical longitude
/// for an instant of lunar eclipse, where Moon has zero altitude.
/// </summary>
/// <param name="e">Besselian elements of the Moon for the given instant.</param>
/// <param name="L">Geographical longitude, positive west, negative east, from -180 to +180 degrees.</param>
/// <returns>
/// Geographical coordinates for the point.
/// </returns>
/// <remarks>
/// The method core is based on formulae from the book:
/// Seidelmann, P. K.: Explanatory Supplement to The Astronomical Almanac,
/// University Science Book, Mill Valley (California), 1992,
/// Chapter 8 "Eclipses of the Sun and Moon"
/// https://archive.org/download/131123ExplanatorySupplementAstronomicalAlmanac/131123-explanatory-supplement-astronomical-almanac.pdf
/// </remarks>
private static CrdsGeographical Project(InstantLunarEclipseElements e, double L)
{
CrdsGeographical g = null;
// Nutation elements
var nutation = Nutation.NutationElements(e.JulianDay);
// True obliquity
var epsilon = Date.TrueObliquity(e.JulianDay, nutation.deltaEpsilon);
// Greenwich apparent sidereal time
double siderealTime = Date.ApparentSiderealTime(e.JulianDay, nutation.deltaPsi, epsilon);
// Geocenric distance to the Moon, in km
double dist = 358473400.0 / (e.F3 * 3600);
// Horizontal parallax of the Moon
double parallax = LunarEphem.Parallax(dist);
// Equatorial coordinates of the Moon, initial value is geocentric
CrdsEquatorial eq = new CrdsEquatorial(e.Alpha, e.Delta);
// two iterations:
// 1st: find geo location needed to perform topocentric correction
// 2nd: correct sublunar point with topocentric position and find true geoposition
for (int i = 0; i < 2; i++)
{
// sublunar point latitude, preserve sign!
double phi0 = Sign(e.Delta) * Abs(eq.Delta);
// sublunar point longitude (formula 8.426-1)
double lambda0 = siderealTime - eq.Alpha;
// sublunar point latitude (formula 8.426-2)
double tanPhi = -1.0 / Tan(ToRadians(phi0)) * Cos(ToRadians(lambda0 - L));
double phi = ToDegrees(Atan(tanPhi));
g = new CrdsGeographical(L, phi);
if (i == 0)
{
// correct to topocentric
eq = eq.ToTopocentric(g, siderealTime, parallax);
}
}
return(g);
}
/// <summary>
/// Converts local horizontal coordinates to equatorial coordinates.
/// </summary>
/// <param name="hor">Pair of local horizontal coordinates.</param>
/// <param name="geo">Geographical of the observer</param>
/// <param name="theta0">Local sidereal time.</param>
/// <returns>Pair of equatorial coordinates</returns>
public static CrdsEquatorial ToEquatorial(this CrdsHorizontal hor, CrdsGeographical geo, double theta0)
{
CrdsEquatorial eq = new CrdsEquatorial();
double A = Angle.ToRadians(hor.Azimuth);
double h = Angle.ToRadians(hor.Altitude);
double phi = Angle.ToRadians(geo.Latitude);
double Y = Math.Sin(A);
double X = Math.Cos(A) * Math.Sin(phi) + Math.Tan(h) * Math.Cos(phi);
double H = Angle.ToDegrees(Math.Atan2(Y, X));
eq.Alpha = Angle.To360(theta0 - geo.Longitude - H);
eq.Delta = Angle.ToDegrees(Math.Asin(Math.Sin(phi) * Math.Sin(h) - Math.Cos(phi) * Math.Cos(h) * Math.Cos(A)));
return(eq);
}
/// <summary>
/// Converts equatorial coodinates to local horizontal
/// </summary>
/// <param name="eq">Pair of equatorial coodinates</param>
/// <param name="geo">Geographical coordinates of the observer</param>
/// <param name="theta0">Local sidereal time</param>
/// <remarks>
/// Implementation is taken from AA(I), formulae 12.5, 12.6.
/// </remarks>
public static CrdsHorizontal ToHorizontal(this CrdsEquatorial eq, CrdsGeographical geo, double theta0)
{
double H = Angle.ToRadians(HourAngle(theta0, geo.Longitude, eq.Alpha));
double phi = Angle.ToRadians(geo.Latitude);
double delta = Angle.ToRadians(eq.Delta);
CrdsHorizontal hor = new CrdsHorizontal();
double Y = Math.Sin(H);
double X = Math.Cos(H) * Math.Sin(phi) - Math.Tan(delta) * Math.Cos(phi);
hor.Altitude = Angle.ToDegrees(Math.Asin(Math.Sin(phi) * Math.Sin(delta) + Math.Cos(phi) * Math.Cos(delta) * Math.Cos(H)));
hor.Azimuth = Angle.ToDegrees(Math.Atan2(Y, X));
hor.Azimuth = Angle.To360(hor.Azimuth);
return(hor);
}
/// <summary>
/// Calculates topocentric equatorial coordinates of celestial body
/// with taking into account correction for parallax.
/// </summary>
/// <param name="eq">Geocentric equatorial coordinates of the body</param>
/// <param name="geo">Geographical coordinates of the body</param>
/// <param name="theta0">Apparent sidereal time at Greenwich</param>
/// <param name="pi">Parallax of a body</param>
/// <returns>Topocentric equatorial coordinates of the celestial body</returns>
/// <remarks>
/// Method is taken from AA(II), formulae 40.6-40.7.
/// </remarks>
public static CrdsEquatorial ToTopocentric(this CrdsEquatorial eq, CrdsGeographical geo, double theta0, double pi)
{
double H = Angle.ToRadians(HourAngle(theta0, geo.Longitude, eq.Alpha));
double delta = Angle.ToRadians(eq.Delta);
double sinPi = Math.Sin(Angle.ToRadians(pi));
double A = Math.Cos(delta) * Math.Sin(H);
double B = Math.Cos(delta) * Math.Cos(H) - geo.RhoCosPhi * sinPi;
double C = Math.Sin(delta) - geo.RhoSinPhi * sinPi;
double q = Math.Sqrt(A * A + B * B + C * C);
double H_ = Angle.ToDegrees(Math.Atan2(A, B));
double alpha_ = Angle.To360(theta0 - geo.Longitude - H_);
double delta_ = Angle.ToDegrees(Math.Asin(C / q));
return(new CrdsEquatorial(alpha_, delta_));
}
private static LunarEclipseLocalCircumstancesContactPoint GetLocalCircumstancesContactPoint(double jd, LunarEclipse eclipse, PolynomialLunarEclipseElements e, CrdsGeographical g)
{
if (!double.IsNaN(jd))
{
return(new LunarEclipseLocalCircumstancesContactPoint(e.GetInstantBesselianElements(jd), g));
}
else
{
return(null);
}
}
public static LunarEclipseLocalCircumstances LocalCircumstances(LunarEclipse eclipse, PolynomialLunarEclipseElements e, CrdsGeographical g)
{
return(new LunarEclipseLocalCircumstances()
{
Location = g,
PenumbralBegin = GetLocalCircumstancesContactPoint(eclipse.JulianDayFirstContactPenumbra, eclipse, e, g),
PartialBegin = GetLocalCircumstancesContactPoint(eclipse.JulianDayFirstContactUmbra, eclipse, e, g),
TotalBegin = GetLocalCircumstancesContactPoint(eclipse.JulianDayTotalBegin, eclipse, e, g),
Maximum = GetLocalCircumstancesContactPoint(eclipse.JulianDayMaximum, eclipse, e, g),
TotalEnd = GetLocalCircumstancesContactPoint(eclipse.JulianDayTotalEnd, eclipse, e, g),
PartialEnd = GetLocalCircumstancesContactPoint(eclipse.JulianDayLastContactUmbra, eclipse, e, g),
PenumbralEnd = GetLocalCircumstancesContactPoint(eclipse.JulianDayLastContactPenumbra, eclipse, e, g),
});
}
/// <summary>
/// Calculates instants of rising, transit and setting for non-stationary celestial body for the desired date.
/// Non-stationary in this particular case means that body has fastly changing celestial coordinates during the day.
/// </summary>
/// <param name="eq">Array of three equatorial coordinates of the celestial body correspoding to local midnight, local noon, and local midnight of the following day after the desired date respectively.</param>
/// <param name="location">Geographical location of the observation point.</param>
/// <param name="theta0">Apparent sidereal time at Greenwich for local midnight of the desired date.</param>
/// <param name="pi">Horizontal equatorial parallax of the body.</param>
/// <param name="sd">Visible semidiameter of the body, expressed in degrees.</param>
/// <returns>Instants of rising, transit and setting for the celestial body for the desired date.</returns>
public static RTS RiseTransitSet(CrdsEquatorial[] eq, CrdsGeographical location, double theta0, double pi = 0, double sd = 0)
{
if (eq.Length != 3)
{
throw new ArgumentException("Number of equatorial coordinates in the array should be equal to 3.");
}
double[] alpha = new double[3];
double[] delta = new double[3];
for (int i = 0; i < 3; i++)
{
alpha[i] = eq[i].Alpha;
delta[i] = eq[i].Delta;
}
Angle.Align(alpha);
Angle.Align(delta);
List <CrdsHorizontal> hor = new List <CrdsHorizontal>();
for (int i = 0; i <= 24; i++)
{
double n = i / 24.0;
CrdsEquatorial eq0 = InterpolateEq(alpha, delta, n);
var sidTime = InterpolateSiderialTime(theta0, n);
hor.Add(eq0.ToTopocentric(location, sidTime, pi).ToHorizontal(location, sidTime));
}
var result = new RTS();
for (int i = 0; i < 24; i++)
{
double n = (i + 0.5) / 24.0;
CrdsEquatorial eq0 = InterpolateEq(alpha, delta, n);
var sidTime = InterpolateSiderialTime(theta0, n);
var hor0 = eq0.ToTopocentric(location, sidTime, pi).ToHorizontal(location, sidTime);
if (double.IsNaN(result.Transit) && hor0.Altitude > 0)
{
double r = SolveParabola(Math.Sin(Angle.ToRadians(hor[i].Azimuth)), Math.Sin(Angle.ToRadians(hor0.Azimuth)), Math.Sin(Angle.ToRadians(hor[i + 1].Azimuth)));
if (!double.IsNaN(r))
{
double t = (i + r) / 24.0;
eq0 = InterpolateEq(alpha, delta, t);
sidTime = InterpolateSiderialTime(theta0, t);
result.Transit = t;
result.TransitAltitude = eq0.ToTopocentric(location, sidTime, pi).ToHorizontal(location, sidTime).Altitude;
}
}
if (double.IsNaN(result.Rise) || double.IsNaN(result.Set))
{
double r = SolveParabola(hor[i].Altitude + sd, hor0.Altitude + sd, hor[i + 1].Altitude + sd);
if (!double.IsNaN(r))
{
double t = (i + r) / 24.0;
eq0 = InterpolateEq(alpha, delta, t);
sidTime = InterpolateSiderialTime(theta0, t);
if (double.IsNaN(result.Rise) && hor[i].Altitude + sd < 0 && hor[i + 1].Altitude + sd > 0)
{
result.Rise = t;
result.RiseAzimuth = eq0.ToTopocentric(location, sidTime, pi).ToHorizontal(location, sidTime).Azimuth;
}
if (double.IsNaN(result.Set) && hor[i].Altitude + sd > 0 && hor[i + 1].Altitude + sd < 0)
{
result.Set = t;
result.SetAzimuth = eq0.ToTopocentric(location, sidTime, pi).ToHorizontal(location, sidTime).Azimuth;
}
if (!double.IsNaN(result.Transit) && !double.IsNaN(result.Rise) && !double.IsNaN(result.Set))
{
break;
}
}
}
}
return(result);
}
/// <summary>
/// Calculates visibity details for the celestial body,
/// </summary>
/// <param name="eqBody">Mean equatorial coordinates of the body for the desired day.</param>
/// <param name="eqSun">Mean equatorial coordinates of the Sun for the desired day.</param>
/// <param name="minAltitude">Minimal altitude of the body, in degrees, to be considered as approproate for observations. By default it's 5 degrees for planet.</param>
/// <returns><see cref="VisibilityDetails"/> instance describing details of visibility.</returns>
// TODO: tests
public static VisibilityDetails Details(CrdsEquatorial eqBody, CrdsEquatorial eqSun, CrdsGeographical location, double theta0, double minAltitude = 5)
{
var details = new VisibilityDetails();
// period when the planet is above the horizon and its altitude is larger than "minAltitude"
RTS body = RiseTransitSet(eqBody, location, theta0, minAltitude);
// period when the Sun is above the horizon
RTS sun = RiseTransitSet(eqSun, location, theta0);
// body reaches minimal altitude but Sun does not rise at all (polar night)
if (body.TransitAltitude > minAltitude && sun.TransitAltitude <= 0)
{
details.Period = VisibilityPeriod.WholeNight;
details.Duration = body.Duration * 24;
}
// body does not reach the minimal altitude during the day
else if (body.TransitAltitude <= minAltitude)
{
details.Period = VisibilityPeriod.Invisible;
details.Duration = 0;
}
// there is a day/night change during the day and body reaches minimal altitude
else if (body.TransitAltitude > minAltitude)
{
// "Sun is below horizon" time range, expressed in degrees (0 is midnight, 180 is noon)
var r1 = new AngleRange(sun.Set * 360, (1 - sun.Duration) * 360);
// "body is above horizon" time range, expressed in degrees (0 is midnight, 180 is noon)
var r2 = new AngleRange(body.Rise * 360, body.Duration * 360);
// find the intersections of two ranges
var ranges = r1.Overlaps(r2);
// no intersections of time ranges
if (!ranges.Any())
{
details.Period = VisibilityPeriod.Invisible;
details.Duration = 0;
details.Begin = double.NaN;
details.End = double.NaN;
}
// the body is observable during the day
else
{
// duration of visibility
details.Duration = ranges.Sum(i => i.Range / 360 * 24);
// beginning of visibility
details.Begin = ranges.First().Start / 360;
// end of visibility
details.End = (details.Begin + details.Duration / 24) % 1;
// Evening time range, expressed in degrees
// Start is a sunset time, range is a timespan from sunset to midnight.
var rE = new AngleRange(sun.Set * 360, (1 - sun.Set) * 360);
// Night time range, expressed in degrees
// Start is a midnight time, range is a half of timespan from midnight to sunrise
var rN = new AngleRange(0, sun.Rise / 2 * 360);
// Morning time range, expressed in degrees
// Start is a half of time from midnight to sunrise, range is a time to sunrise
var rM = new AngleRange(sun.Rise / 2 * 360, sun.Rise / 2 * 360);
foreach (var r in ranges)
{
var isEvening = r.Overlaps(rE);
if (isEvening.Any())
{
details.Period |= VisibilityPeriod.Evening;
}
var isNight = r.Overlaps(rN);
if (isNight.Any())
{
details.Period |= VisibilityPeriod.Night;
}
var isMorning = r.Overlaps(rM);
if (isMorning.Any())
{
details.Period |= VisibilityPeriod.Morning;
}
}
}
}
return(details);
}
/// <summary>
/// Calculates instants of rising, transit and setting for stationary celestial body for the desired date.
/// Stationary in this particular case means that body has unchanged (or slightly changing) celestial coordinates during the day.
/// </summary>
/// <param name="eq">Equatorial coordinates of the celestial body.</param>
/// <param name="location">Geographical location of the observation point.</param>
/// <param name="theta0">Apparent sidereal time at Greenwich for local midnight of the desired date.</param>
/// <param name="minAltitude">Minimal altitude of the body above the horizon, in degrees, to detect rise/set. Used only for calculating visibility conditions.</param>
/// <returns>Instants of rising, transit and setting for the celestial body for the desired date.</returns>
public static RTS RiseTransitSet(CrdsEquatorial eq, CrdsGeographical location, double theta0, double minAltitude = 0)
{
List <CrdsHorizontal> hor = new List <CrdsHorizontal>();
for (int i = 0; i <= 24; i++)
{
double n = i / 24.0;
var sidTime = InterpolateSiderialTime(theta0, n);
hor.Add(eq.ToHorizontal(location, sidTime));
}
var result = new RTS();
for (int i = 0; i < 24; i++)
{
double n = (i + 0.5) / 24.0;
var sidTime = InterpolateSiderialTime(theta0, n);
var hor0 = eq.ToHorizontal(location, sidTime);
if (double.IsNaN(result.Transit) && hor0.Altitude > 0)
{
double r = SolveParabola(Math.Sin(Angle.ToRadians(hor[i].Azimuth)), Math.Sin(Angle.ToRadians(hor0.Azimuth)), Math.Sin(Angle.ToRadians(hor[i + 1].Azimuth)));
if (!double.IsNaN(r))
{
double t = (i + r) / 24.0;
sidTime = InterpolateSiderialTime(theta0, t);
result.Transit = t;
result.TransitAltitude = eq.ToHorizontal(location, sidTime).Altitude;
}
}
if (double.IsNaN(result.Rise) || double.IsNaN(result.Set))
{
double r = SolveParabola(hor[i].Altitude - minAltitude, hor0.Altitude - minAltitude, hor[i + 1].Altitude - minAltitude);
if (!double.IsNaN(r))
{
double t = (i + r) / 24.0;
sidTime = InterpolateSiderialTime(theta0, t);
if (double.IsNaN(result.Rise) && hor[i].Altitude - minAltitude < 0 && hor[i + 1].Altitude - minAltitude > 0)
{
result.Rise = t;
result.RiseAzimuth = eq.ToHorizontal(location, sidTime).Azimuth;
}
if (double.IsNaN(result.Set) && hor[i].Altitude - minAltitude > 0 && hor[i + 1].Altitude - minAltitude < 0)
{
result.Set = t;
result.SetAzimuth = eq.ToHorizontal(location, sidTime).Azimuth;
}
if (!double.IsNaN(result.Transit) && !double.IsNaN(result.Rise) && !double.IsNaN(result.Set))
{
break;
}
}
}
}
return(result);
}
/// <summary>
/// Creates new instance
/// </summary>
public LunarEclipseLocalCircumstancesContactPoint(InstantLunarEclipseElements e, CrdsGeographical g)
{
CrdsEquatorial eq = new CrdsEquatorial(e.Alpha, e.Delta);
// Nutation elements
var nutation = Nutation.NutationElements(e.JulianDay);
// True obliquity
var epsilon = Date.TrueObliquity(e.JulianDay, nutation.deltaEpsilon);
// Greenwich apparent sidereal time
double siderealTime = Date.ApparentSiderealTime(e.JulianDay, nutation.deltaPsi, epsilon);
// Geocenric distance to the Moon, in km
double dist = 358473400.0 / (e.F3 * 3600);
// Horizontal parallax of the Moon
double parallax = LunarEphem.Parallax(dist);
// Topocentrical equatorial coordinates
var eqTopo = eq.ToTopocentric(g, siderealTime, parallax);
// Horizontal coordinates
var h = eqTopo.ToHorizontal(g, siderealTime);
// Hour angle
double H = Coordinates.HourAngle(siderealTime, g.Longitude, eqTopo.Alpha);
// Position angles
// Parallactic angle - AA(II), p.98, formula 14.1
double qAngle = ToDegrees(Atan2(Sin(ToRadians(H)), Tan(ToRadians(g.Latitude)) * Cos(ToRadians(eqTopo.Delta)) - Sin(ToRadians(eqTopo.Delta)) * Cos(ToRadians(H))));
// Position angle, measured from North to East (CCW)
double pAngle = To360(ToDegrees(Atan2(e.X, e.Y)));
// Position angle, measured from Zenith CCW
double zAngle = To360(pAngle - qAngle);
JulianDay = e.JulianDay;
LunarAltitude = h.Altitude;
QAngle = qAngle;
PAngle = pAngle;
ZAngle = zAngle;
X = e.X;
Y = e.Y;
F1 = e.F1;
F2 = e.F2;
F3 = e.F3;
}
public CrdsGeographical(CrdsGeographical other)
: this(other.Longitude, other.Latitude, other.UtcOffset, other.Elevation, other.TimeZoneId, other.LocationName)
{
}
/// <summary>
/// Creates new point with Julian Day value and coordinates
/// </summary>
/// <param name="jd">Julian Day value</param>
/// <param name="c">Coordinates of the point</param>
public SolarEclipsePoint(double jd, CrdsGeographical c)
{
JulianDay = jd;
Coordinates = c;
}