/// <summary>
/// Finds intersections (overlaps) of the range with second one.
/// </summary>
/// <param name="r">Second angle range to find overlaps with.</param>
/// <returns>Collection (max 2 items) of angle ranges which represent overlaps of two initial ranges.</returns>
/// <remarks>
/// The idea of this method is based on solution provided here:
/// <see href="https://stackoverflow.com/questions/48984436/finding-the-intersections-between-two-angle-ranges-segments"/>.
/// </remarks>
public ICollection <AngleRange> Overlaps(AngleRange r)
{
List <AngleRange> ranges = new List <AngleRange>();
double aStart = Start;
double aSweep = Range;
double bStart = r.Start;
double bSweep = r.Range;
double greaterAngle;
double greaterSweep;
double originAngle;
double originSweep;
if (aStart < bStart)
{
originAngle = aStart;
originSweep = aSweep;
greaterSweep = bSweep;
greaterAngle = bStart;
}
else
{
originAngle = bStart;
originSweep = bSweep;
greaterSweep = aSweep;
greaterAngle = aStart;
}
double greaterAngleRel = greaterAngle - originAngle;
if (greaterAngleRel < originSweep)
{
ranges.Add(new AngleRange(greaterAngle, Math.Min(greaterSweep, originSweep - greaterAngleRel)));
}
double rouno = greaterAngleRel + greaterSweep;
if (rouno > 360)
{
ranges.Add(new AngleRange(originAngle, Math.Min(rouno - 360, originSweep)));
}
return(ranges);
}
/// <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);
}
