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Compute sun and moon phases
There is a newer version: 3.11
/*
 * Shredzone Commons - suncalc
 *
 * Copyright (C) 2017 Richard "Shred" Körber
 *   http://commons.shredzone.org
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 */
package org.shredzone.commons.suncalc;

import static java.lang.Math.*;

import javax.annotation.ParametersAreNonnullByDefault;
import javax.annotation.concurrent.Immutable;

import org.shredzone.commons.suncalc.param.Builder;
import org.shredzone.commons.suncalc.param.TimeParameter;
import org.shredzone.commons.suncalc.util.BaseBuilder;
import org.shredzone.commons.suncalc.util.JulianDate;
import org.shredzone.commons.suncalc.util.Moon;
import org.shredzone.commons.suncalc.util.Sun;
import org.shredzone.commons.suncalc.util.Vector;

/**
 * Calculates the illumination of the moon.
 * 
 * NOTE: This implementation will change in one of the next versions. Methods
 * may give other results, or disappear completely.
 *
 * @see Formulas the
 *      calculations base on
 * @see Formulas used for moon
 *      calculations
 * @see "Astronomical Algorithms, 2nd edition by Jean Meeus (Willmann-Bell,
 *      Richmond) 1998, Chapter 48"
 */
@ParametersAreNonnullByDefault
@Immutable
public class MoonIllumination {

    private final double fraction;
    private final double phase;
    private final double angle;

    private MoonIllumination(double fraction, double phase, double angle) {
        this.fraction = fraction;
        this.phase = phase;
        this.angle = angle;
    }

    /**
     * Starts the computation of {@link MoonIllumination}.
     *
     * @return {@link Parameters} to set.
     */
    public static Parameters compute() {
        return new MoonIlluminationBuilder();
    }

    /**
     * Collects all parameters for {@link MoonIllumination}.
     */
    public static interface Parameters extends
            TimeParameter,
            Builder {
    }

    /**
     * Builder for {@link MoonIllumination}. Performs the computations based on the
     * parameters, and creates a {@link MoonIllumination} object that holds the result.
     */
    private static class MoonIlluminationBuilder extends BaseBuilder implements Parameters {
        @Override
        public MoonIllumination execute() {
            JulianDate t = getJulianDate();
            Vector s = Sun.position(t);
            Vector m = Moon.position(t);

            double phi = acos(sin(s.getTheta()) * sin(m.getTheta()) + cos(s.getTheta()) * cos(m.getTheta()) * cos(s.getPhi() - m.getPhi()));
            double inc = atan2(s.getR() * sin(phi), m.getR() - s.getR() * cos(phi));
            double angle = atan2(cos(s.getTheta()) * sin(s.getPhi() - m.getPhi()), sin(s.getTheta()) * cos(m.getTheta()) -
                    cos(s.getTheta()) * sin(m.getTheta()) * cos(s.getPhi() - m.getPhi()));

            return new MoonIllumination(
                            (1 + cos(inc)) / 2,
                            360.0 * (0.5 * inc * signum(angle) / PI),
                            toDegrees(angle));
        }
    }

    /**
     * Illuminated fraction. {@code 0.0} indicates new moon, {@code 1.0} indicates full
     * moon.
     */
    public double getFraction() {
        return fraction;
    }

    /**
     * Moon phase. Starts at {@code -180.0} (new moon, waxing), passes {@code 0.0} (full
     * moon) and moves toward {@code 180.0} (waning, new moon).
     */
    public double getPhase() {
        return phase;
    }

    /**
     * Midpoint angle in degrees of the illuminated limb of the moon reckoned eastward
     * from the north point of the disk; the moon is waxing if the angle is negative, and
     * waning if positive.
     * 
     * By subtracting {@link MoonPosition#getParallacticAngle()} from {@link #getAngle()},
     * one can get the zenith angle of the moons bright limb (anticlockwise). The zenith
     * angle can be used do draw the moon shape from the observers perspective (e.g. moon
     * lying on its back).
     */
    public double getAngle() {
        return angle;
    }

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append("MoonIllumination[fraction=").append(fraction);
        sb.append(", phase=").append(phase);
        sb.append("°, angle=").append(angle);
        sb.append("°]");
        return sb.toString();
    }

}