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      */
calculateTwilight(long time, double latiude, double longitude)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