Created
November 13, 2016 15:20
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Twilight calculator for Android
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import android.text.format.DateUtils; | |
import android.util.FloatMath; | |
public class TwilightCalculator { | |
/** Value of {@link #mState} if it is currently day */ | |
public static final int DAY = 0; | |
/** Value of {@link #mState} if it is currently night */ | |
public static final int NIGHT = 1; | |
private static final float DEGREES_TO_RADIANS = (float) (Math.PI / 180.0f); | |
// element for calculating solar transit. | |
private static final float J0 = 0.0009f; | |
// correction for civil twilight | |
private static final float ALTIDUTE_CORRECTION_CIVIL_TWILIGHT = -0.104719755f; | |
// coefficients for calculating Equation of Center. | |
private static final float C1 = 0.0334196f; | |
private static final float C2 = 0.000349066f; | |
private static final float C3 = 0.000005236f; | |
private static final float OBLIQUITY = 0.40927971f; | |
// Java time on Jan 1, 2000 12:00 UTC. | |
private static final long UTC_2000 = 946728000000L; | |
/** | |
* Time of sunset (civil twilight) in milliseconds or -1 in the case the day | |
* or night never ends. | |
*/ | |
public long mSunset; | |
/** | |
* Time of sunrise (civil twilight) in milliseconds or -1 in the case the | |
* day or night never ends. | |
*/ | |
public long mSunrise; | |
/** Current state */ | |
public int mState; | |
/** | |
* calculates the civil twilight bases on time and geo-coordinates. | |
* | |
* @param time time in milliseconds. | |
* @param latiude latitude in degrees. | |
* @param longitude latitude in degrees. | |
*/ | |
public void calculateTwilight(long time, double latiude, double longitude) { | |
final float daysSince2000 = (float) (time - UTC_2000) / DateUtils.DAY_IN_MILLIS; | |
// mean anomaly | |
final float meanAnomaly = 6.240059968f + daysSince2000 * 0.01720197f; | |
// true anomaly | |
final float trueAnomaly = meanAnomaly + C1 * FloatMath.sin(meanAnomaly) + C2 | |
* FloatMath.sin(2 * meanAnomaly) + C3 * FloatMath.sin(3 * meanAnomaly); | |
// ecliptic longitude | |
final float solarLng = trueAnomaly + 1.796593063f + (float) Math.PI; | |
// solar transit in days since 2000 | |
final double arcLongitude = -longitude / 360; | |
float n = Math.round(daysSince2000 - J0 - arcLongitude); | |
double solarTransitJ2000 = n + J0 + arcLongitude + 0.0053f * FloatMath.sin(meanAnomaly) | |
+ -0.0069f * FloatMath.sin(2 * solarLng); | |
// declination of sun | |
double solarDec = Math.asin(FloatMath.sin(solarLng) * FloatMath.sin(OBLIQUITY)); | |
final double latRad = latiude * DEGREES_TO_RADIANS; | |
double cosHourAngle = (FloatMath.sin(ALTIDUTE_CORRECTION_CIVIL_TWILIGHT) - Math.sin(latRad) | |
* Math.sin(solarDec)) / (Math.cos(latRad) * Math.cos(solarDec)); | |
// The day or night never ends for the given date and location, if this value is out of | |
// range. | |
if (cosHourAngle >= 1) { | |
mState = NIGHT; | |
mSunset = -1; | |
mSunrise = -1; | |
return; | |
} else if (cosHourAngle <= -1) { | |
mState = DAY; | |
mSunset = -1; | |
mSunrise = -1; | |
return; | |
} | |
float hourAngle = (float) (Math.acos(cosHourAngle) / (2 * Math.PI)); | |
mSunset = Math.round((solarTransitJ2000 + hourAngle) * DateUtils.DAY_IN_MILLIS) + UTC_2000; | |
mSunrise = Math.round((solarTransitJ2000 - hourAngle) * DateUtils.DAY_IN_MILLIS) + UTC_2000; | |
if (mSunrise < time && mSunset > time) { | |
mState = DAY; | |
} else { | |
mState = NIGHT; | |
} | |
} | |
} |
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