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 21 /** @hide */ 22 public class TwilightCalculator { 23 24 /** Value of {@link #mState} if it is currently day */ 25 public static final int DAY = 0; 26 27 /** Value of {@link #mState} if it is currently night */ 28 public static final int NIGHT = 1; 29 30 private static final float DEGREES_TO_RADIANS = (float) (Math.PI / 180.0f); 31 32 // element for calculating solar transit. 33 private static final float J0 = 0.0009f; 34 35 // correction for civil twilight 36 private static final float ALTIDUTE_CORRECTION_CIVIL_TWILIGHT = -0.104719755f; 37 38 // coefficients for calculating Equation of Center. 39 private static final float C1 = 0.0334196f; 40 private static final float C2 = 0.000349066f; 41 private static final float C3 = 0.000005236f; 42 43 private static final float OBLIQUITY = 0.40927971f; 44 45 // Java time on Jan 1, 2000 12:00 UTC. 46 private static final long UTC_2000 = 946728000000L; 47 48 /** 49 * Time of sunset (civil twilight) in milliseconds or -1 in the case the day 50 * or night never ends. 51 */ 52 public long mSunset; 53 54 /** 55 * Time of sunrise (civil twilight) in milliseconds or -1 in the case the 56 * day or night never ends. 57 */ 58 public long mSunrise; 59 60 /** Current state */ 61 public int mState; 62 63 /** 64 * calculates the civil twilight bases on time and geo-coordinates. 65 * 66 * @param time time in milliseconds. 67 * @param latiude latitude in degrees. 68 * @param longitude latitude in degrees. 69 */ 70 public void calculateTwilight(long time, double latiude, double longitude) { 71 final float daysSince2000 = (float) (time - UTC_2000) / DateUtils.DAY_IN_MILLIS; 72 73 // mean anomaly 74 final float meanAnomaly = 6.240059968f + daysSince2000 * 0.01720197f; 75 76 // true anomaly 77 final double trueAnomaly = meanAnomaly + C1 * Math.sin(meanAnomaly) + C2 78 * Math.sin(2 * meanAnomaly) + C3 * Math.sin(3 * meanAnomaly); 79 80 // ecliptic longitude 81 final double solarLng = trueAnomaly + 1.796593063d + Math.PI; 82 83 // solar transit in days since 2000 84 final double arcLongitude = -longitude / 360; 85 float n = Math.round(daysSince2000 - J0 - arcLongitude); 86 double solarTransitJ2000 = n + J0 + arcLongitude + 0.0053d * Math.sin(meanAnomaly) 87 + -0.0069d * Math.sin(2 * solarLng); 88 89 // declination of sun 90 double solarDec = Math.asin(Math.sin(solarLng) * Math.sin(OBLIQUITY)); 91 92 final double latRad = latiude * DEGREES_TO_RADIANS; 93 94 double cosHourAngle = (Math.sin(ALTIDUTE_CORRECTION_CIVIL_TWILIGHT) - Math.sin(latRad) 95 * Math.sin(solarDec)) / (Math.cos(latRad) * Math.cos(solarDec)); 96 // The day or night never ends for the given date and location, if this value is out of 97 // range. 98 if (cosHourAngle >= 1) { 99 mState = NIGHT; 100 mSunset = -1; 101 mSunrise = -1; 102 return; 103 } else if (cosHourAngle <= -1) { 104 mState = DAY; 105 mSunset = -1; 106 mSunrise = -1; 107 return; 108 } 109 110 float hourAngle = (float) (Math.acos(cosHourAngle) / (2 * Math.PI)); 111 112 mSunset = Math.round((solarTransitJ2000 + hourAngle) * DateUtils.DAY_IN_MILLIS) + UTC_2000; 113 mSunrise = Math.round((solarTransitJ2000 - hourAngle) * DateUtils.DAY_IN_MILLIS) + UTC_2000; 114 115 if (mSunrise < time && mSunset > time) { 116 mState = DAY; 117 } else { 118 mState = NIGHT; 119 } 120 } 121 122 } 123
