124 lines
4.2 KiB
Java
124 lines
4.2 KiB
Java
/*
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* Copyright (C) 2010 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package com.android.server;
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import android.text.format.DateUtils;
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import android.util.FloatMath;
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/** @hide */
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public class TwilightCalculator {
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/** Value of {@link #mState} if it is currently day */
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public static final int DAY = 0;
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/** Value of {@link #mState} if it is currently night */
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public static final int NIGHT = 1;
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private static final float DEGREES_TO_RADIANS = (float) (Math.PI / 180.0f);
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// element for calculating solar transit.
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private static final float J0 = 0.0009f;
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// correction for civil twilight
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private static final float ALTIDUTE_CORRECTION_CIVIL_TWILIGHT = -0.104719755f;
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// coefficients for calculating Equation of Center.
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private static final float C1 = 0.0334196f;
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private static final float C2 = 0.000349066f;
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private static final float C3 = 0.000005236f;
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private static final float OBLIQUITY = 0.40927971f;
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// Java time on Jan 1, 2000 12:00 UTC.
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private static final long UTC_2000 = 946728000000L;
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/**
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* Time of sunset (civil twilight) in milliseconds or -1 in the case the day
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* or night never ends.
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*/
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public long mSunset;
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/**
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* Time of sunrise (civil twilight) in milliseconds or -1 in the case the
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* day or night never ends.
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*/
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public long mSunrise;
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/** Current state */
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public int mState;
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/**
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* calculates the civil twilight bases on time and geo-coordinates.
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*
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* @param time time in milliseconds.
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* @param latiude latitude in degrees.
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* @param longitude latitude in degrees.
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*/
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public void calculateTwilight(long time, double latiude, double longitude) {
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final float daysSince2000 = (float) (time - UTC_2000) / DateUtils.DAY_IN_MILLIS;
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// mean anomaly
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final float meanAnomaly = 6.240059968f + daysSince2000 * 0.01720197f;
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// true anomaly
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final float trueAnomaly = meanAnomaly + C1 * FloatMath.sin(meanAnomaly) + C2
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* FloatMath.sin(2 * meanAnomaly) + C3 * FloatMath.sin(3 * meanAnomaly);
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// ecliptic longitude
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final float solarLng = trueAnomaly + 1.796593063f + (float) Math.PI;
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// solar transit in days since 2000
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final double arcLongitude = -longitude / 360;
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float n = Math.round(daysSince2000 - J0 - arcLongitude);
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double solarTransitJ2000 = n + J0 + arcLongitude + 0.0053f * FloatMath.sin(meanAnomaly)
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+ -0.0069f * FloatMath.sin(2 * solarLng);
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// declination of sun
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double solarDec = Math.asin(FloatMath.sin(solarLng) * FloatMath.sin(OBLIQUITY));
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final double latRad = latiude * DEGREES_TO_RADIANS;
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double cosHourAngle = (FloatMath.sin(ALTIDUTE_CORRECTION_CIVIL_TWILIGHT) - Math.sin(latRad)
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* Math.sin(solarDec)) / (Math.cos(latRad) * Math.cos(solarDec));
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// The day or night never ends for the given date and location, if this value is out of
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// range.
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if (cosHourAngle >= 1) {
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mState = NIGHT;
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mSunset = -1;
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mSunrise = -1;
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return;
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} else if (cosHourAngle <= -1) {
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mState = DAY;
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mSunset = -1;
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mSunrise = -1;
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return;
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}
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float hourAngle = (float) (Math.acos(cosHourAngle) / (2 * Math.PI));
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mSunset = Math.round((solarTransitJ2000 + hourAngle) * DateUtils.DAY_IN_MILLIS) + UTC_2000;
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mSunrise = Math.round((solarTransitJ2000 - hourAngle) * DateUtils.DAY_IN_MILLIS) + UTC_2000;
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if (mSunrise < time && mSunset > time) {
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mState = DAY;
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} else {
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mState = NIGHT;
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}
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}
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}
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