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determining time for given length of shadow
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@MenoData
MenoData committed Sep 8, 2020
1 parent 32aa072 commit 89a1b3a
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175 changes: 157 additions & 18 deletions time4j-android/src/main/java/net/time4j/calendar/astro/SolarTime.java
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Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,7 @@
import net.time4j.PlainDate;
import net.time4j.PlainTime;
import net.time4j.PlainTimestamp;
import net.time4j.base.MathUtils;
import net.time4j.base.ResourceLoader;
import net.time4j.engine.CalendarDate;
import net.time4j.engine.ChronoCondition;
Expand Down Expand Up @@ -205,6 +206,7 @@ public final class SolarTime
static final double STD_ZENITH = 90.0 + (SUN_RADIUS + STD_REFRACTION) / 60.0;
static final String DECLINATION = "declination";
static final String RIGHT_ASCENSION = "right-ascension";
static final double ARC_MIN = 1.0 / 60;

private static final Calculator DEFAULT_CALCULATOR;
private static final ConcurrentMap<String, Calculator> CALCULATORS;
Expand Down Expand Up @@ -936,6 +938,104 @@ public Moment apply(CalendarDate date) {

}

/**
* <p>Calculates the moment before noon when the given shadow occurs on given date
* at the location of this instance. </p>
*
* <p>The function is not supported in polar regions (+/- 66°) because the angle of sun
* is often changing too slowly to guarantee reliable results. Furthermore, the function
* might obtain a null result if the given shadow is too short for the date in question. </p>
*
* @param objectHeight the height of object in meters, must be positive
* @param shadowLength the length of shadow thrown by the object in meters, must not be negative
* @return function for calculating the time of shadow applicable on any calendar date in minute precision
* @throws IllegalArgumentException if one or both parameters are not finite or out of range
* @throws UnsupportedOperationException if this instance is within the arctic or antarctic circle
* @see SunPosition#getShadowLength(double)
* @see #timeOfShadowAfterNoon(double, double)
* @since 4.7
*/
/*[deutsch]
* <p>Berechnet den Moment in der ersten Tagesh&auml;lfte, zu dem der fragliche Schatten beobachtet
* wird, an der Position dieser Instanz. </p>
*
* <p>Die Funktion wird in Polargebieten (+/- 66°) nicht unterst&uuml;tzt, weil sich der Winkel der Sonne
* oft zu wenig ver&auml;ndert, um zuverl&auml;ssige Ergebnisse zu erm&ouml;glichen. Au&szlig;erdem kann
* die Funktion ohne Ergebnis sein, wenn der angegebene Schatten zum fraglichen Tag zu kurz ist. </p>
*
* @param objectHeight the height of object in meters, must be positive
* @param shadowLength the length of shadow thrown by the object in meters, must not be negative
* @return function for calculating the time of shadow applicable on any calendar date in minute precision
* @throws IllegalArgumentException if one or both parameters are not finite or out of range
* @throws UnsupportedOperationException if this instance is within the arctic or antarctic circle
* @see SunPosition#getShadowLength(double)
* @see #timeOfShadowAfterNoon(double, double)
* @since 4.7
*/
public ChronoFunction<CalendarDate, Moment> timeOfShadowBeforeNoon(
final double objectHeight,
final double shadowLength
) {

this.checkShadow(objectHeight, shadowLength);
return new ChronoFunction<CalendarDate, Moment>() {
@Override
public Moment apply(CalendarDate date) {
return SolarTime.this.timeOfShadow(date, false, objectHeight, shadowLength);
}
};

}

/**
* <p>Calculates the moment after noon when the given shadow occurs on given date
* at the location of this instance. </p>
*
* <p>The function is not supported in polar regions (+/- 66°) because the angle of sun
* is often changing too slowly to guarantee reliable results. Furthermore, the function
* might obtain a null result if the given shadow is too short for the date in question. </p>
*
* @param objectHeight the height of object in meters, must be positive
* @param shadowLength the length of shadow thrown by the object in meters, must not be negative
* @return function for calculating the time of shadow applicable on any calendar date in minute precision
* @throws IllegalArgumentException if one or both parameters are not finite or out of range
* @throws UnsupportedOperationException if this instance is within the arctic or antarctic circle
* @see SunPosition#getShadowLength(double)
* @see #timeOfShadowBeforeNoon(double, double)
* @since 4.7
*/
/*[deutsch]
* <p>Berechnet den Moment in der zweiten Tagesh&auml;lfte, zu dem der fragliche Schatten beobachtet
* wird, an der Position dieser Instanz. </p>
*
* <p>Die Funktion wird in Polargebieten (+/- 66°) nicht unterst&uuml;tzt, weil sich der Winkel der Sonne
* oft zu wenig ver&auml;ndert, um zuverl&auml;ssige Ergebnisse zu erm&ouml;glichen. Au&szlig;erdem kann
* die Funktion ohne Ergebnis sein, wenn der angegebene Schatten zum fraglichen Tag zu kurz ist. </p>
*
* @param objectHeight the height of object in meters, must be positive
* @param shadowLength the length of shadow thrown by the object in meters, must not be negative
* @return function for calculating the time of shadow applicable on any calendar date in minute precision
* @throws IllegalArgumentException if one or both parameters are not finite or out of range
* @throws UnsupportedOperationException if this instance is within the arctic or antarctic circle
* @see SunPosition#getShadowLength(double)
* @see #timeOfShadowBeforeNoon(double, double)
* @since 4.7
*/
public ChronoFunction<CalendarDate, Moment> timeOfShadowAfterNoon(
final double objectHeight,
final double shadowLength
) {

this.checkShadow(objectHeight, shadowLength);
return new ChronoFunction<CalendarDate, Moment>() {
@Override
public Moment apply(CalendarDate date) {
return SolarTime.this.timeOfShadow(date, true, objectHeight, shadowLength);
}
};

}

@Override
public boolean equals(Object obj) {

Expand Down Expand Up @@ -1235,54 +1335,100 @@ static Moment fromLocalEvent(
}

private static PlainTimestamp onAverage(Moment context, ZonalOffset offset) {

Moment ut =
Moment.of(
context.getElapsedTime(TimeScale.UT) + 2 * 365 * 86400,
context.getNanosecond(TimeScale.UT),
TimeScale.POSIX);
return ut.toZonalTimestamp(offset);

}

private static Moment transitAtNoon(
CalendarDate date,
double longitude,
String calculator
) {

Moment utc = fromLocalEvent(date, 12, longitude, calculator);
return utc.with(Moment.PRECISION, precision(calculator));

}

private static Moment transitAtMidnight(
CalendarDate date,
double longitude,
String calculator
) {

Moment utc = fromLocalEvent(date, 0, longitude, calculator);
return utc.with(Moment.PRECISION, precision(calculator));
}

private void checkShadow(
double objectHeight,
double shadowLength
) {
if (Double.isInfinite(objectHeight) || Double.isNaN(objectHeight) || (objectHeight <= 0.0)) {
throw new IllegalArgumentException("Object height must be finite and positive.");
} else if (Double.isInfinite(shadowLength) || Double.isNaN(shadowLength) || (shadowLength < 0.0)) {
throw new IllegalArgumentException("Length of shadow must be finite and not negative.");
} else if (Math.abs(this.latitude) > 66.0) {
throw new UnsupportedOperationException("Cannot calculate time of shadow for polar regions.");
}
}

private static TimeUnit precision(String calculator) {
private Moment timeOfShadow(
CalendarDate date,
boolean afterNoon,
double objectHeight,
double shadowLength
) {
PlainDate d = toGregorian(toLMT(date));
Moment riseset = afterNoon ? d.get(sunset()) : d.get(sunrise());
assert riseset != null;
Moment noon = d.get(transitAtNoon());
double maxE = SunPosition.at(noon, this).getElevation();

if (maxE <= ARC_MIN) {
return riseset;
} else {
double elevation = (
(shadowLength == 0.0)
? 90.0
: Math.toDegrees(Math.atan(objectHeight / shadowLength)));

return (calculator.equals(StdSolarCalculator.SIMPLE.name()) ? TimeUnit.MINUTES : TimeUnit.SECONDS);
if (elevation > maxE + ARC_MIN) {
return null;
} else {
return timeOfShadow(riseset.getPosixTime(), noon.getPosixTime(), elevation);
}
}
}

private Moment timeOfShadow(
long lowSun,
long highSun,
double elevation
) {
Moment center = Moment.of(MathUtils.safeAdd(lowSun, highSun) / 2, TimeScale.POSIX);
double sunpos = SunPosition.at(center, this).getElevation();

if (Math.abs(sunpos - elevation) < ARC_MIN) {
return center;
} else if (Double.compare(elevation, sunpos) > 0.0) {
return timeOfShadow(center.getPosixTime(), highSun, elevation);
} else {
return timeOfShadow(lowSun, center.getPosixTime(), elevation);
}
}

private static TimeUnit precision(String calculator) {
return (calculator.equals(StdSolarCalculator.SIMPLE.name()) ? TimeUnit.MINUTES : TimeUnit.SECONDS);
}

private double geodeticAngle() {

return this.getCalculator().getGeodeticAngle(this.latitude, this.altitude);

}

private double zenithAngle() {

return this.getCalculator().getZenithAngle(this.latitude, this.altitude);

}

private static void check(
Expand All @@ -1291,7 +1437,6 @@ private static void check(
int altitude,
String calculator
) {

if (Double.isNaN(latitude) || Double.isInfinite(latitude)) {
throw new IllegalArgumentException("Latitude must be a finite value: " + latitude);
} else if (Double.isNaN(longitude) || Double.isInfinite(longitude)) {
Expand All @@ -1309,11 +1454,9 @@ private static void check(
} else if (!CALCULATORS.containsKey(calculator)) {
throw new IllegalArgumentException("Unknown calculator: " + calculator);
}

}

private CalendarDate toLMT(CalendarDate input) {

if ((this.observerZoneID == null) || (Math.abs(this.longitude) < 150.0)) {
return input;
}
Expand All @@ -1328,22 +1471,19 @@ private CalendarDate toLMT(CalendarDate input) {

ZonalOffset lmtOffset = ZonalOffset.atLongitude(new BigDecimal(this.longitude));
return noon.inTimezone(this.observerZoneID).toZonalTimestamp(lmtOffset).getCalendarDate();

}

private static boolean equalZones(
TZID z1,
TZID z2
) {

if (z1 == null) {
return (z2 == null);
} else if (z2 == null) {
return false;
} else {
return z1.canonical().equals(z2.canonical());
}

}

/**
Expand All @@ -1358,7 +1498,6 @@ private void readObject(ObjectInputStream in)

in.defaultReadObject();
check(this.latitude, this.longitude, this.altitude, this.calculator);

}

//~ Innere Klassen ----------------------------------------------------
Expand Down
3 changes: 2 additions & 1 deletion time4j-android/src/main/java/net/time4j/calendar/astro/SunPosition.java
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Original file line number Diff line number Diff line change
Expand Up @@ -284,7 +284,8 @@ public double getShadowLength(double objectHeight) {
} else if (e == 90.0) {
return 0.0;
} else {
return objectHeight / Math.tan(Math.toRadians(e));
double len = objectHeight / Math.tan(Math.toRadians(e));
return (len < SolarTime.ARC_MIN) ? 0.0 : len;
}
} else {
throw new IllegalArgumentException("Object height must be finite and positive: " + objectHeight);
Expand Down

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