1 /* 2 * Copyright (C) 2010 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 package com.android.server; 18 19 import android.text.format.DateUtils; 20 import android.util.FloatMath; 21 22 /** @hide */ 23 public class TwilightCalculator { 24 25 /** Value of {@link #mState} if it is currently day */ 26 public static final int DAY = 0; 27 28 /** Value of {@link #mState} if it is currently night */ 29 public static final int NIGHT = 1; 30 31 private static final float DEGREES_TO_RADIANS = (float) (Math.PI / 180.0f); 32 33 // element for calculating solar transit. 34 private static final float J0 = 0.0009f; 35 36 // correction for civil twilight 37 private static final float ALTIDUTE_CORRECTION_CIVIL_TWILIGHT = -0.104719755f; 38 39 // coefficients for calculating Equation of Center. 40 private static final float C1 = 0.0334196f; 41 private static final float C2 = 0.000349066f; 42 private static final float C3 = 0.000005236f; 43 44 private static final float OBLIQUITY = 0.40927971f; 45 46 // Java time on Jan 1, 2000 12:00 UTC. 47 private static final long UTC_2000 = 946728000000L; 48 49 /** 50 * Time of sunset (civil twilight) in milliseconds or -1 in the case the day 51 * or night never ends. 52 */ 53 public long mSunset; 54 55 /** 56 * Time of sunrise (civil twilight) in milliseconds or -1 in the case the 57 * day or night never ends. 58 */ 59 public long mSunrise; 60 61 /** Current state */ 62 public int mState; 63 64 /** 65 * calculates the civil twilight bases on time and geo-coordinates. 66 * 67 * @param time time in milliseconds. 68 * @param latiude latitude in degrees. 69 * @param longitude latitude in degrees. 70 */ 71 public void calculateTwilight(long time, double latiude, double longitude) { 72 final float daysSince2000 = (float) (time - UTC_2000) / DateUtils.DAY_IN_MILLIS; 73 74 // mean anomaly 75 final float meanAnomaly = 6.240059968f + daysSince2000 * 0.01720197f; 76 77 // true anomaly 78 final float trueAnomaly = meanAnomaly + C1 * FloatMath.sin(meanAnomaly) + C2 79 * FloatMath.sin(2 * meanAnomaly) + C3 * FloatMath.sin(3 * meanAnomaly); 80 81 // ecliptic longitude 82 final float solarLng = trueAnomaly + 1.796593063f + (float) Math.PI; 83 84 // solar transit in days since 2000 85 final double arcLongitude = -longitude / 360; 86 float n = Math.round(daysSince2000 - J0 - arcLongitude); 87 double solarTransitJ2000 = n + J0 + arcLongitude + 0.0053f * FloatMath.sin(meanAnomaly) 88 + -0.0069f * FloatMath.sin(2 * solarLng); 89 90 // declination of sun 91 double solarDec = Math.asin(FloatMath.sin(solarLng) * FloatMath.sin(OBLIQUITY)); 92 93 final double latRad = latiude * DEGREES_TO_RADIANS; 94 95 double cosHourAngle = (FloatMath.sin(ALTIDUTE_CORRECTION_CIVIL_TWILIGHT) - Math.sin(latRad) 96 * Math.sin(solarDec)) / (Math.cos(latRad) * Math.cos(solarDec)); 97 // The day or night never ends for the given date and location, if this value is out of 98 // range. 99 if (cosHourAngle >= 1) { 100 mState = NIGHT; 101 mSunset = -1; 102 mSunrise = -1; 103 return; 104 } else if (cosHourAngle <= -1) { 105 mState = DAY; 106 mSunset = -1; 107 mSunrise = -1; 108 return; 109 } 110 111 float hourAngle = (float) (Math.acos(cosHourAngle) / (2 * Math.PI)); 112 113 mSunset = Math.round((solarTransitJ2000 + hourAngle) * DateUtils.DAY_IN_MILLIS) + UTC_2000; 114 mSunrise = Math.round((solarTransitJ2000 - hourAngle) * DateUtils.DAY_IN_MILLIS) + UTC_2000; 115 116 if (mSunrise < time && mSunset > time) { 117 mState = DAY; 118 } else { 119 mState = NIGHT; 120 } 121 } 122 123 } 124
