根据经纬度和当前日期获取当地的日出日落时间，强大的Google搜索让我迅速找到了一个外国人写的一个java类，代码中的注释写得很清楚。
这里直接贴出代码来：

/******************************************************************************
*
*       SunriseSunset.java
*
*******************************************************************************
* 
* Java Class: SunriseSunset
* 
* This Java class is part of a collection of classes developed for the 
* reading and processing of oceanographic and meterological data collected 
* since 1970 by environmental buoys and stations.  This dataset is 
* maintained by the National Oceanographic Data Center and is publicly 
* available.  These Java classes were written for the US Environmental 
* Protection Agency's National Exposure Research Laboratory under Contract 
* No. GS-10F-0073K with Neptune and Company of Los Alamos, New Mexico.
* 
* Purpose:
* 
*  This Java class performs calculations to determine the time of 
* sunrise and sunset given lat, long, and date.
* 
* Inputs:
* 
*  Latitude, longitude, date/time, and time zone.
* 
* Outputs:
* 
*  Local time of sunrise and sunset as calculated by the 
*   program.
* If no sunrise or no sunset occurs, or if the sun is up all day 
*   or down all day, appropriate boolean values are set.
* A boolean is provided to identify if the time provided is during the day.
*
* The above values are accessed by the following methods:
*
*  Date getSunrise() returns date/time of sunrise
*  Date getSunset()  returns date/time of sunset
*  boolean isSunrise()  returns true if there was a sunrise, else false
*  boolean isSunset()  returns true if there was a sunset, else false
*  boolean isSunUp()  returns true if sun is up all day, else false
*  boolean isSunDown()  returns true if sun is down all day, else false
*  boolean isDaytime()  returns true if sun is up at the time 
*         specified, else false
* 
* Required classes from the Java library:
* 
*  java.util.Date
*  java.text.SimpleDateFormat
*  java.text.ParseException;
*  java.math.BigDecimal;
* 
* Package of which this class is a member:
* 
* default
* 
* Known limitations:
* 
* It is assumed that the data provided are within valie ranges
* (i.e. latitude between -90 and +90, longitude between 0 and 360,
* a valid date, and time zone between -14 and +14.
* 
* Compatibility:
* 
* Java 1.1.8
* 
* References:
* 
* The mathematical algorithms used in this program are patterned 
* after those debveloped by Roger Sinnott in his BASIC program, 
* SUNUP.BAS, published in Sky & Telescope magazine:
* Sinnott, Roger W. "Sunrise and Sunset: A Challenge"
* Sky & Telescope, August, 1994 p.84-85
* 
* The following is a cross-index of variables used in SUNUP.BAS.
* A single definition from multiple reuse of variable names in 
* SUNUP.BAS was clarified with various definitions in this program.
*
* SUNUP.BAS this class
* 
* A   dfA
* A(2)  dfAA1, dfAA2
* A0   dfA0
* A2   dfA2
* A5   dfA5
* AZ   Not used
* C   dfCosLat
* C0   dfC0
* D   iDay
* D(2)  dfDD1, dfDD2
* D0   dfD0
* D1   dfD1
* D2   dfD2
* D5   dfD5
* D7   Not used
* DA   dfDA
* DD   dfDD
* G   bGregorian, dfGG
* H   dfTimeZone
* H0   dfH0
* H1   dfH1
* H2   dfH2
* H3   dfHourRise, dfHourSet
* H7   Not used
* J   dfJ
* J3   dfJ3
* K1   dfK1
* L   dfLL
* L0   dfL0
* L2   dfL2
* L5   dfLon
* M   iMonth
* M3   dfMinRise, dfMinSet
* N7   Not used
* P   dfP
* S   iSign, dfSinLat, dfSS
* T   dfT
* T0   dfT0
* T3   not used
* TT   dfTT
* U   dfUU
* V   dfVV
* V0   dfV0
* V1   dfV1
* V2   dfV2
* W   dfWW
* Y   iYear
* Z   dfZenith
* Z0   dfTimeZone
* 
*  
* Author/Company:
* 
*  JDT: John Tauxe, Neptune and Company
* JMG: Jo Marie Green
* 
* Change log:
*  
* date       ver    by description of change
* _________  _____  ___ ______________________________________________
*  5 Jan 01  0.006  JDT Excised from ssapp.java v. 0.005.
* 11 Jan 01  0.007  JDT Minor modifications to comments based on 
*         material from Sinnott, 1994.
*  7 Feb 01  0.008  JDT Fixed backwards time zone.  The standard is that 
*         local time zone is specified in hours EAST of 
*         Greenwich, so that EST would be -5, for example.
*         For some reason, SUNUP.BAS does this backwards 
*         (probably an americocentric perspective) and 
*         SunriseSunset adopted that convention.  Oops.
*         So the sign in the math is changed.
*  7 Feb 01  0.009  JDT Well, that threw off the azimuth calculation...
*         Removed the azimuth calculations.
* 14 Feb 01  0.010  JDT Added ability to accept a time (HH:mm) in 
*         dateInput, and decide if that time is daytime 
*         or nighttime.
* 27 Feb 01  0.011  JDT Added accessor methods in place of having public 
*         variables to get results. 
* 28 Feb 01  0.012  JDT Cleaned up list of imported classes. 
* 28 Mar 01  1.10   JDT Final version accompanying deliverable 1b.
*    4 Apr 01  1.11   JDT Moved logic supporting .isDaytime into method.
*         Moved calculations out of constructor.
*   01 May 01  1.12   JMG   Added 'GMT' designation and testing lines.
*   16 May 01  1.13   JDT   Added setLenient( false ) and setTimeZone( tz )
*                           to dfmtDay, dfmtMonth, and dfmtYear in 
*       doCalculations.
*   27 Jun 01  1.14   JDT Removed reliance on StationConstants (GMT).
* 13 Aug 01  1.20   JDT Final version accompanying deliverable 1c.
*  6 Sep 01  1.21   JDT Thorough code and comment review.
* 21 Sep 01  1.30   JDT Final version accompanying deliverable 2.
* 17 Dec 01  1.40   JDT Version accompanying final deliverable.
*    
*----------------------------------------------------------------------------*/

// Import required classes and packages
import java.util.Date;
import java.text.SimpleDateFormat;
import java.text.ParseException;
import java.math.BigDecimal;
import java.util.TimeZone;

/******************************************************************************
* class:     SunriseSunset class
*******************************************************************************
*
*  This Java class performs calculations to determine the time of 
* sunrise and sunset given lat, long, and date.
*
* It is assumed that the data provided are within valie ranges
* (i.e. latitude between -90 and +90, longitude between 0 and 360,
* a valid date, and time zone between -14 and +14.
*  
*----------------------------------------------------------------------------*/
public class SunriseSunset
{

 // Declare and initialize variables
 private double dfLat;     // latitude from user
 private double dfLon;     // latitude from user
 private Date dateInput;    // date/time from user
 private double dfTimeZone;    // time zone from user
 private Date dateSunrise;   // date and time of sunrise
 private Date dateSunset;    // date and time of sunset
 private boolean bSunriseToday  = false; // flag for sunrise on this date
 private boolean bSunsetToday   = false; // flag for sunset on this date
 private boolean bSunUpAllDay   = false; // flag for sun up all day
 private boolean bSunDownAllDay = false; // flag for sun down all day
 private boolean bDaytime    = false; // flag for daytime, given 
           // hour and min in dateInput
 private boolean bSunrise = false;  // sunrise during hour checked
 private boolean bSunset  = false;  // sunset during hour checked
 private boolean bGregorian = false;  // flag for Gregorian calendar
 private int  iJulian;    // Julian day
 private int  iYear;     // year of date of interest
 private int  iMonth;     // month of date of interest
 private int  iDay;     // day of date of interest
 private int  iCount;     // a simple counter
 private int  iSign;     // SUNUP.BAS: S
 private double dfHourRise, dfHourSet; // hour of event: SUNUP.BAS H3
 private double dfMinRise, dfMinSet; // minute of event: SUNUP.BAS M3
 private double dfSinLat, dfCosLat;  // sin and cos of latitude
 private double dfZenith;    // SUNUP.BAS Z: Zenith
 private SimpleDateFormat dfmtDate;  // formatting for date alone
 private SimpleDateFormat dfmtDateTime; // formatting for date and time
 private SimpleDateFormat dfmtYear;  // formatting for year
 private SimpleDateFormat dfmtMonth;  // formatting for month
 private SimpleDateFormat dfmtDay;  // formatting for day
 // Many variables in SUNUP.BAS have undocumented meanings, 
 // and so are translated rather directly to avoid confusion:
 private double dfAA1 = 0, dfAA2 = 0; // SUNUP.BAS A(2)
 private double dfDD1 = 0, dfDD2 = 0; // SUNUP.BAS D(2)
 private double dfC0;     // SUNUP.BAS C0
 private double dfK1;     // SUNUP.BAS K1
 private double dfP;     // SUNUP.BAS P
 private double dfJ;     // SUNUP.BAS J
 private double dfJ3;     // SUNUP.BAS J3
 private double dfA;     // SUNUP.BAS A
 private double dfA0, dfA2, dfA5;  // SUNUP.BAS A0, A2, A5
 private double dfD0, dfD1, dfD2, dfD5; // SUNUP.BAS D0, D1, D2, D5
 private double dfDA, dfDD;    // SUNUP.BAS DA, DD
 private double dfH0, dfH1, dfH2;  // SUNUP.BAS H0, H1, H2
 private double dfL0, dfL2;    // SUNUP.BAS L0, L2
 private double dfT, dfT0, dfTT;  // SUNUP.BAS T, T0, TT
 private double dfV0, dfV1, dfV2;  // SUNUP.BAS V0, V1, V2

 private TimeZone tz = TimeZone.getTimeZone( "GMT" );

 
/******************************************************************************
* method:     SunriseSunset
*******************************************************************************
*
* Constructor for SunriseSunset class.
* 
*----------------------------------------------------------------------------*/
 SunriseSunset( 
      double dfLatIn,    // latitude 
      double dfLonIn,    // longitude 
      Date  dateInputIn,   // date
      double dfTimeZoneIn   // time zone
      ) 
 {
  // Copy values supplied as agruments to local variables.
  dfLat   = dfLatIn;
  dfLon   = dfLonIn;
  dateInput  = dateInputIn;
  dfTimeZone  = dfTimeZoneIn;

  // Call the method to do the calculations.
  doCalculations();
 } // end of class constructor

/******************************************************************************
* method:     doCalculations
*******************************************************************************
*
* Method for performing the calculations done in SUNUP.BAS.
* 
*----------------------------------------------------------------------------*/
 private void doCalculations()
 {
  try
  {
   // Break out day, month, and year from date provided.
   // (This is necesary for the math algorithms.)
   dfmtYear  = new SimpleDateFormat( "yyyy" );
   dfmtYear.setLenient( false );
   dfmtYear.setTimeZone( tz );
   dfmtMonth = new SimpleDateFormat( "M" );
   dfmtMonth.setLenient( false );
   dfmtMonth.setTimeZone( tz );
   dfmtDay   = new SimpleDateFormat( "d" );
   dfmtDay.setLenient( false );
   dfmtDay.setTimeZone( tz );

   iYear  = Integer.parseInt(  dfmtYear.format( dateInput ) );
   iMonth = Integer.parseInt( dfmtMonth.format( dateInput ) );
   iDay   = Integer.parseInt(   dfmtDay.format( dateInput ) );

   // Convert time zone hours to decimal days (SUNUP.BAS line 50)
   dfTimeZone = dfTimeZone / 24.0;
   // NOTE: (7 Feb 2001) Here is a non-standard part of SUNUP.BAS:
   // It (and this algorithm) assumes that the time zone is 
   // positive west, instead of the standard negative west.
   // Classes calling SunriseSunset will be assuming that 
   // times zones are specified in negative west, so here the 
   // sign is changed so that the SUNUP algorithm works:
   dfTimeZone = -dfTimeZone;
   // Convert longitude to fraction (SUNUP.BAS line 50)
   dfLon = dfLon / 360.0;
   // Convert calendar date to Julian date:
   // Check to see if it's later than 1583: Gregorian calendar
   // When declared, bGregorian is initialized to false.
   // ** Consider making a separate class of this function. **
   if( iYear >= 1583 ) bGregorian = true;
   // SUNUP.BAS 1210
   dfJ = -Math.floor( 7.0  // SUNUP used INT, not floor
        * ( Math.floor( 
              ( iMonth + 9.0 )
              / 12.0
             ) + iYear
           ) / 4.0
        )
    // add SUNUP.BAS 1240 and 1250 for G = 0
    + Math.floor( iMonth * 275.0 / 9.0 )
    + iDay
    + 1721027.0
    + iYear * 367.0;

   if ( bGregorian )
   {
    // SUNUP.BAS 1230
    if ( ( iMonth - 9.0 ) < 0.0 ) iSign = -1;
    else iSign = 1;
    dfA = Math.abs( iMonth - 9.0 );
    // SUNUP.BAS 1240 and 1250
    dfJ3 = -Math.floor(
          (
       Math.floor(
         Math.floor( iYear 
          + (double)iSign 
            * Math.floor( dfA / 7.0 )
            )
            / 100.0
           ) + 1.0
          ) * 0.75
         );
    // correct dfJ as in SUNUP.BAS 1240 and 1250 for G = 1
    dfJ = dfJ + dfJ3 + 2.0;
   }
   // SUNUP.BAS 1290
   iJulian = (int)dfJ - 1;

   // SUNUP.BAS 60 and 70 (see also line 1290)
   dfT = (double)iJulian - 2451545.0 + 0.5;
   dfTT = dfT / 36525.0 + 1.0;    // centuries since 1900

   // Calculate local sidereal time at 0h in zone time
   // SUNUP.BAS 410 through 460
   dfT0 = ( dfT * 8640184.813 / 36525.0
     + 24110.5
     + dfTimeZone * 86636.6
     + dfLon * 86400.0
      )
      / 86400.0;
   dfT0 = dfT0 - Math.floor( dfT0 ); // NOTE: SUNUP.BAS uses INT()
   dfT0 = dfT0 * 2.0 * Math.PI;
   // SUNUP.BAS 90
   dfT = dfT + dfTimeZone;
   // SUNUP.BAS 110: Get Sun's position
   for( iCount=0; iCount<=1; iCount++ ) // Loop thru only twice
   {
    // Calculate Sun's right ascension and declination
    //   at the start and end of each day.
    // SUNUP.BAS 910 - 1160: Fundamental arguments
    //   from van Flandern and Pulkkinen, 1979

    // declare local temporary doubles for calculations
    double dfGG;      // SUNUP.BAS G
    double dfLL;      // SUNUP.BAS L
    double dfSS;      // SUNUP.BAS S
    double dfUU;      // SUNUP.BAS U
    double dfVV;      // SUNUP.BAS V
    double dfWW;      // SUNUP.BAS W

    dfLL = 0.779072 + 0.00273790931 * dfT;
    dfLL = dfLL - Math.floor( dfLL );
    dfLL = dfLL * 2.0 * Math.PI;

    dfGG = 0.993126 + 0.0027377785 * dfT;
    dfGG = dfGG - Math.floor( dfGG );
    dfGG = dfGG * 2.0 * Math.PI;

    dfVV =   0.39785 * Math.sin( dfLL )
      - 0.01000 * Math.sin( dfLL - dfGG )
      + 0.00333 * Math.sin( dfLL + dfGG )
      - 0.00021 * Math.sin( dfLL ) * dfTT;

    dfUU = 1
         - 0.03349 * Math.cos( dfGG )
      - 0.00014 * Math.cos( dfLL * 2.0 )
      + 0.00008 * Math.cos( dfLL );

    dfWW = - 0.00010
      - 0.04129 * Math.sin( dfLL * 2.0 )
      + 0.03211 * Math.sin( dfGG )
      - 0.00104 * Math.sin( 2.0 * dfLL - dfGG )
      - 0.00035 * Math.sin( 2.0 * dfLL + dfGG )
      - 0.00008 * Math.sin( dfGG ) * dfTT;

    // Compute Sun's RA and Dec; SUNUP.BAS 1120 - 1140
    dfSS = dfWW / Math.sqrt( dfUU - dfVV * dfVV );
    dfA5 = dfLL 
        + Math.atan( dfSS / Math.sqrt( 1.0 - dfSS * dfSS ));

    dfSS = dfVV / Math.sqrt( dfUU );
    dfD5 = Math.atan( dfSS / Math.sqrt( 1 - dfSS * dfSS ));     

    // Set values and increment t
    if ( iCount == 0 )  // SUNUP.BAS 125
    {
     dfAA1 = dfA5;
     dfDD1 = dfD5;
    }
    else     // SUNUP.BAS 145
    {
     dfAA2 = dfA5;
     dfDD2 = dfD5;
    }
    dfT = dfT + 1.0;  // SUNUP.BAS 130
   } // end of Get Sun's Position for loop

   if ( dfAA2 < dfAA1 ) dfAA2 = dfAA2 + 2.0 * Math.PI;
               // SUNUP.BAS 150

   dfZenith = Math.PI * 90.833 / 180.0;   // SUNUP.BAS 160
   dfSinLat = Math.sin( dfLat * Math.PI / 180.0 ); // SUNUP.BAS 170
   dfCosLat = Math.cos( dfLat * Math.PI / 180.0 ); // SUNUP.BAS 170

   dfA0 = dfAA1;         // SUNUP.BAS 190
   dfD0 = dfDD1;         // SUNUP.BAS 190
   dfDA = dfAA2 - dfAA1;       // SUNUP.BAS 200
   dfDD = dfDD2 - dfDD1;       // SUNUP.BAS 200

   dfK1 = 15.0 * 1.0027379 * Math.PI / 180.0;  // SUNUP.BAS 330

   // Initialize sunrise and sunset times, and other variables
   // hr and min are set to impossible times to make errors obvious
   dfHourRise = 99.0;
   dfMinRise  = 99.0;
   dfHourSet  = 99.0;
   dfMinSet   = 99.0;
   dfV0 = 0.0;  // initialization implied by absence in SUNUP.BAS
   dfV2 = 0.0;  // initialization implied by absence in SUNUP.BAS

   // Test each hour to see if the Sun crosses the horizon
   //   and which way it is heading.
   for( iCount=0; iCount<24; iCount++ )   // SUNUP.BAS 210
   {
    double tempA;        // SUNUP.BAS A
    double tempB;        // SUNUP.BAS B
    double tempD;        // SUNUP.BAS D
    double tempE;        // SUNUP.BAS E

    dfC0 = (double)iCount;
    dfP = ( dfC0 + 1.0 ) / 24.0;    // SUNUP.BAS 220
    dfA2 = dfAA1 + dfP * dfDA;     // SUNUP.BAS 230
    dfD2 = dfDD1 + dfP * dfDD;     // SUNUP.BAS 230
    dfL0 = dfT0 + dfC0 * dfK1;     // SUNUP.BAS 500
    dfL2 = dfL0 + dfK1;       // SUNUP.BAS 500
    dfH0 = dfL0 - dfA0;       // SUNUP.BAS 510
    dfH2 = dfL2 - dfA2;       // SUNUP.BAS 510
    // hour angle at half hour
    dfH1 = ( dfH2 + dfH0 ) / 2.0;    // SUNUP.BAS 520
    // declination at half hour
    dfD1 = ( dfD2 + dfD0 ) / 2.0;    // SUNUP.BAS 530

    // Set value of dfV0 only if this is the first hour, 
    // otherwise, it will get set to the last dfV2 (SUNUP.BAS 250)
    if ( iCount == 0 )       // SUNUP.BAS 550
    { 
     dfV0 = dfSinLat * Math.sin( dfD0 )
       + dfCosLat * Math.cos( dfD0 ) * Math.cos( dfH0 )
       - Math.cos( dfZenith );   // SUNUP.BAS 560
    }
    else
     dfV0 = dfV2; // That is, dfV2 from the previous hour.

    dfV2 = dfSinLat * Math.sin( dfD2 )
       + dfCosLat * Math.cos( dfD2 ) * Math.cos( dfH2 )
       - Math.cos( dfZenith );   // SUNUP.BAS 570

    // if dfV0 and dfV2 have the same sign, then proceed to next hr
    if ( 
      ( dfV0 >= 0.0 && dfV2 >= 0.0 )  // both are positive
      ||        // or
      ( dfV0 < 0.0 && dfV2 < 0.0 )   // both are negative
       )
    {
     // Break iteration and proceed to test next hour
     dfA0 = dfA2;       // SUNUP.BAS 250
     dfD0 = dfD2;       // SUNUP.BAS 250
     continue;        // SUNUP.BAS 610
    }

    dfV1 = dfSinLat * Math.sin( dfD1 )
      + dfCosLat * Math.cos( dfD1 ) * Math.cos( dfH1 )
      - Math.cos( dfZenith );    // SUNUP.BAS 590

    tempA = 2.0 * dfV2 - 4.0 * dfV1 + 2.0 * dfV0;
               // SUNUP.BAS 600
    tempB = 4.0 * dfV1 - 3.0 * dfV0 - dfV2;  // SUNUP.BAS 600
    tempD = tempB * tempB - 4.0 * tempA * dfV0; // SUNUP.BAS 610

    if ( tempD < 0.0 ) 
    {
     // Break iteration and proceed to test next hour
     dfA0 = dfA2;       // SUNUP.BAS 250
     dfD0 = dfD2;       // SUNUP.BAS 250
     continue;        // SUNUP.BAS 610
    }

    tempD = Math.sqrt( tempD );     // SUNUP.BAS 620

    // Determine occurence of sunrise or sunset.

    // Flags to identify occurrence during this day are 
    // bSunriseToday and bSunsetToday, and are initialized false.
    // These are set true only if sunrise or sunset occurs 
    // at any point in the hourly loop. Never set to false.

    // Flags to identify occurrence during this hour:
    bSunrise = false;    // reset before test
    bSunset  = false;    // reset before test

    if ( dfV0 < 0.0 && dfV2 > 0.0 ) // sunrise occurs this hour
    {
     bSunrise = true;   // SUNUP.BAS 640
     bSunriseToday = true;  // sunrise occurred today
    }

    if ( dfV0 > 0.0 && dfV2 < 0.0 ) // sunset occurs this hour
    {
     bSunset = true;    // SUNUP.BAS 660
     bSunsetToday = true;  // sunset occurred today
    }

    tempE = ( tempD - tempB ) / ( 2.0 * tempA );
    if ( tempE > 1.0 || tempE < 0.0 ) // SUNUP.BAS 670, 680
     tempE = ( -tempD - tempB ) / ( 2.0 * tempA );     

    // Set values of hour and minute of sunset or sunrise
    // only if sunrise/set occurred this hour.
    if ( bSunrise )
    {
     dfHourRise = Math.floor( dfC0 + tempE + 1.0/120.0 );
     dfMinRise  = Math.floor( 
            ( dfC0 + tempE + 1.0/120.0 
               - dfHourRise 
            )
            * 60.0
           );
    }

    if ( bSunset )
    {
     dfHourSet  = Math.floor( dfC0 + tempE + 1.0/120.0 );
     dfMinSet   = Math.floor( 
            ( dfC0 + tempE + 1.0/120.0
               - dfHourSet 
            ) 
            * 60.0
           );
    }

    // Change settings of variables for next loop
    dfA0 = dfA2;        // SUNUP.BAS 250
    dfD0 = dfD2;        // SUNUP.BAS 250

   } // end of loop testing each hour for an event 

   // After having checked all hours, set flags if no rise or set
   // bSunUpAllDay and bSundownAllDay are initialized as false
   if ( !bSunriseToday && !bSunsetToday )
   {
    if ( dfV2 < 0.0 )
     bSunDownAllDay = true;
    else
     bSunUpAllDay = true;
   }

   // Load dateSunrise with data
   dfmtDateTime = new SimpleDateFormat( "d M yyyy HH:mm z" );
   if( bSunriseToday )
   {
    dateSunrise = dfmtDateTime.parse( iDay 
          + " " + iMonth 
          + " " + iYear 
          + " " + (int)dfHourRise
          + ":" + (int)dfMinRise 
          + " GMT" );
   }

   // Load dateSunset with data
   if( bSunsetToday )
   {
    dateSunset = dfmtDateTime.parse( iDay 
          + " " + iMonth 
          + " " + iYear 
          + " " + (int)dfHourSet
          + ":" + (int)dfMinSet 
          + " GMT" );
   }
  } // end of try
  // Catch errors
  catch( ParseException e )
  {
   System.out.println( "\nCannot parse date" );
   System.out.println( e );
   System.exit( 1 );
  } // end of catch
 }

 
/******************************************************************************
* method:     getSunrise()
*******************************************************************************
*
*   Gets the date and time of sunrise.  If there is no sunrise, returns null.
*    
* Member of SunriseSunset class
*    
* -------------------------------------------------------------------------- */
 public Date getSunrise()
 {
  if ( bSunriseToday )
   return( dateSunrise );
  else
   return( null );
 }

/******************************************************************************
* method:     getSunset()
*******************************************************************************
*
*   Gets the date and time of sunset.  If there is no sunset, returns null.
*    
* Member of SunriseSunset class
*    
* -------------------------------------------------------------------------- */
 public Date getSunset()
 {
  if ( bSunsetToday )
   return( dateSunset );
  else
   return( null );
 }

/******************************************************************************
* method:     isSunrise()
*******************************************************************************
*
*   Returns a boolean identifying if there was a sunrise.
*    
* Member of SunriseSunset class
*    
* -------------------------------------------------------------------------- */
 public boolean isSunrise()
 {
  return( bSunriseToday );
 }

/******************************************************************************
* method:     isSunset()
*******************************************************************************
*
*   Returns a boolean identifying if there was a sunset.
*    
* Member of SunriseSunset class
*    
* -------------------------------------------------------------------------- */
 public boolean isSunset()
 {
  return( bSunsetToday );
 }

/******************************************************************************
* method:     isSunUp()
*******************************************************************************
*
*   Returns a boolean identifying if the sun is up all day.
*    
* Member of SunriseSunset class
*    
* -------------------------------------------------------------------------- */
 public boolean isSunUp()
 {
  return( bSunUpAllDay );
 }

/******************************************************************************
* method:     isSunDown()
*******************************************************************************
*
*   Returns a boolean identifying if the sun is down all day.
*    
* Member of SunriseSunset class
*    
* -------------------------------------------------------------------------- */
 public boolean isSunDown()
 {
  return( bSunDownAllDay );
 }

/******************************************************************************
* method:     isDaytime()
*******************************************************************************
*
*   Returns a boolean identifying if it is daytime at the hour contained in 
* the Date object passed to SunriseSunset on construction.
*    
* Member of SunriseSunset class
*    
* -------------------------------------------------------------------------- */
 public boolean isDaytime()
 {
  // Determine if it is daytime (at sunrise or later) 
  // or nighttime (at sunset or later) at the location of interest
  // but expressed in the time zone requested.
  if ( bSunriseToday && bSunsetToday )  // sunrise and sunset
  {
   if ( dateSunrise.before( dateSunset ) ) // sunrise < sunset
   {
    if ( 
      ( 
      dateInput.after( dateSunrise ) 

      dateInput.equals( dateSunrise ) 
      )
      &&
      dateInput.before( dateSunset )
       )
     bDaytime = true;
    else
    bDaytime = false;
    }
   else  // sunrise comes after sunset (in opposite time zones)
   {
    if ( 
      ( 
      dateInput.after( dateSunrise ) 

      dateInput.equals( dateSunrise ) 
      )
      ||   // use OR rather than AND
      dateInput.before( dateSunset )
       )
     bDaytime = true;
    else
    bDaytime = false;
    }
  }
  else if ( bSunUpAllDay )     // sun is up all day
   bDaytime = true;
  else if ( bSunDownAllDay )    // sun is down all day
   bDaytime = false;
  else if ( bSunriseToday )     // sunrise but no sunset
  {
   if ( dateInput.before( dateSunrise ) )
    bDaytime = false;
   else
    bDaytime = true;
  }
  else if ( bSunsetToday )     // sunset but no sunrise
  {
   if ( dateInput.before( dateSunset ) )
    bDaytime = true;
   else
    bDaytime = false;
  }
  else bDaytime = false;     // this should never execute
  return( bDaytime );
 }

} // end of class 
/*-----------------------------------------------------------------------------
*       end of class
*----------------------------------------------------------------------------*/