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/*
* Copyright (C) 2010 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.android.server;
import android.text.format.DateUtils;
import android.util.FloatMath;
/** @hide */
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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