Android example source code file (TwilightCalculator.java)

This example Android source code file (TwilightCalculator.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Android by Example" ^TM.

Java - Android tags/keywords

altidute_correction_civil_twilight, android, c1, c2, c3, day, degrees_to_radians, format, j0, night, obliquity, text, twilightcalculator, utc_2000, util, utilities, utils

The TwilightCalculator.java Android example source code

/*
 * 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;
        }
    }

}